1

STRUCTURE OF BONE

BY

Dr. Bhuvan Nagpal B.D.S. (Hons.), M.D.S. (Oral Pathology)

(Gold Medalist)

Consulting Oral & Maxillofacial Pathologist

Ex. Post Graduate Resident,

Dept. of Oral Pathology & Microbiology,

JSS Dental College & Hospital,

JSS University,

Mysuru, Karnataka, India

Dr. Archana S. B.D.S., M.D.S. (Oral Pathology)

Consulting Oral & Maxillofacial Pathologist

Ex. Post Graduate Resident,

Dept. of Oral Pathology & Microbiology,

JSS Dental College & Hospital,

JSS University,

Mysuru, Karnataka, India

Dr. Anupam Nagpal B.D.S. (Hons.)

(Gold Medalist)

House Surgeon

Teerthanker Mahaveer Dental College & Research Centre,

Teerthanker Mahaveer University,

Moradabad, Uttar Pradesh, India

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S.No CONTENTS Page No.

1. INTRODUCTION 3-6

2. CLASSIFICATION OF BONE 7-14

3. MECHANICAL PROPERTIES OF BONE 15

4. COMPOSITION OF BONE 16-22

5. ANATOMY OF BONE 23-34

6. MICROSCOPIC STRUCTURE OF BONE 34-53

7. HISTOGENESIS OF BONE 53-64

8. BONE DYNAMICS 64-97

9. FUNCTIONS OF BONE 98-99

10. CALCIUM & PHOSPHATE METABOLISM 99-110

11. BONE MINERALISATION 111-117

12. BLOOD SUPPLY & NUTRITION OF BONE 118-121

13. APPLIED ANATOMY 122-124

14. VENOUS & LYMPHATIC DRIANAGE OF BONE 125

15. NERVE SUPPLY OF BONE 125

16. DEVELOPMENT OF FACIAL BONES 126-132

17. MAXILLA 133-145

18. MANDIBLE 145-156

19. ALVEOLAR BONE 157-166

20. REFERENCES 167-169

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INTRODUCTION

The human skeleton is bilaterally symmetrical with the typical vertebrate

pattern of an axis, divided into segments for flexibility, and of two pairs of limbs,

pectoral and pelvic, also divided into jointed parts for locomotion, grasping etc. The

skull is the expanded and modified cranial end of the axis.Osseocartiliginous sesamoid

bones develop in some tendons and ligaments. All these elements are collectively

termed the’skeleton’.The skeletal system is composed of 206 bones that vary in size

and shape. The bones are interconnected by a variety of joints that allow for a wide

range of movement while maintaining stability1,2

The human skeleton is internal to the muscles with which it has evolved called

as ‘endoskeleton’.which has got a protective role except in the vault of skull and

spinal cord.The maxilla, mandible, clavicle and dentine of teeth are dermal

derivatives, All are vestiges of more extensive assemblies of dermal bones from which

they have been modified to form a human ‘exoskeleton’.1,2

Teeth is hardest and most stable of tissues.In mammals, where skeletal growth

is typically limited to an early period of life, there are generally two dentitions, the

first deciduous and other permanent, the condition is known as diphyodonty.The

evolution of mammals was associated with the posterosuperior downgrowth of

dentary bone, rearrangement of jaw muscles to move the mandible transversely and

change in the shape of the teeth. Teeth are selectively fossilized and preserved and

thus provide best evolutionary record. They are the excellent models of studying the

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relations between ontogeny and progeny.The durability of teeth to fire and bacterial

decomposition makes them invalulable in identification of otherwise unrecognizable

bodies, a point of great forensic importance.1,2,3

Bone is a specialized mineralized connective tissue made up of an organic

matrix of collagen fibrils embedded in an amorphous substance with mineral crystals

precipitated within the matrix. It is highly vascular, living, constantly changing

mineralized connective tissue with complicated hierarchical architecture. It is

remarkable for its hardness, resilience, regenerative property as well as its

characteristic growth mechanisms. Morphologically bone tissue appears to be under

the control of bone cells. Its surfaces are enveloped by active and resting osteoblasts

and osteoclasts, and it is permeated by an interconnected canalicular system in which

osteocytes are found. These cells control the composition of the extracellular fluids of

mineralized bone matrix within very narrow limits, and at the same time they can

remove and replace the mineralized tissue to meet the anatomical needs of a mature

skeleton.1,2,3

Throughout life, bones change in size, shape, and position. Two processes guide

these changes—modeling and remodeling. When a bone is formed at one site and

broken down in a different site, its shape and position is changed. This is called

modeling. This process allows individual bones to grow in size and to shift in

space.Remodeling repairs the damage to the skeleton that can result from repeated

stress by replacing small cracks or deformities in areas of cell damage. Remodeling

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also prevents the accumulation of too much old bone, which can lose its resilience and

become brittle. Remodeling is also important for the function of the skeleton as the

bank for calcium and phosphorus.4

Bone health is critically important to the overall health and quality of life of an

individual. Bones serve as a storehouse for minerals that are vital to the functioning of

many other life-sustaining systems in the body.. Both genes and the environment

contribute to bone health. External factors, such as diet and physical activity, are

critically important to bone health throughout life and can be modified. The

mechanical loading of the skeleton is essential for maintenance of normal bone mass

and architecture. In addition, the skeleton needs certain nutritional elements to build

tissue.4

The growth of the skeleton, its response to mechanical forces, and its role as a

mineral storehouse are all dependent on the proper functioning of a number of

systemic or circulating hormones produced outside the skeleton that work in concert

with local regulatory factors. Eg calcium regulating hormone (parathyroid, calcitonin,

calcitriol),Sex hormones (estrogen, testosterone), growth hormone, thyroid hormone,

cortisol. This complex system of regulatory hormones responds to changes in blood

calcium and phosphorus, acting not only on bone but also on other tissues such as the

intestine and the kidney.4

Genetic abnormalities can produce weak, thin bones, or bones that are too

dense.eg osteogenesis imperfecta, osteoporosis. Nutritional deficiencies, particularly

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of vitamin D, calcium, and phosphorus, can result in the formation of weak, poorly

mineralized bone.eg vitamin D deficiency. Many hormonal disorders can also affect

the skeleton. Overactive parathyroid glands or hyperparathyroidism can cause

excessive bone breakdown and increase the risk of fractures. In severe cases, large

holes or cystic lesions appear in the bone, which makes them particularly fragile. A

deficiency of the growth hormone/IGF-1 system can inhibit growth, leading to short

stature. Many bone disorders are local, affecting only a small region of the skeleton 4

Inflammation can lead to bone loss, probably through the production of local

resorbing factors by the inflammatory white cells. Bacterial infections, such as severe

gum inflammation or periodontal disease, can produce loss of the bones around the

teeth, and osteomyelitis can produce a loss of bone at the site of infection. This type of

bone loss is due to the direct damaging effect of bacterial products as well as

production of resorbing factors by white blood cells.

Thus bones have fascinated human beings since the dawn of time. Much of

what is known about the evolution of vertebrates is based on recovery of bones &

teeth from the soil. Remarkable progress has been gained in our understanding of the

cellular, molecular biology, and genetics of skeletal tissues in the last quarter century.

This has lead to new approaches to diagnosis, prevention, and treatment.1,2,4

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CLASSIFICATION:1,5.,6

I) Based on location, bones can be classified as follows:

� Axial skeleton - Bones of the skull, vertebral column, sternum, and ribs

� Appendicular skeleton - Bones of the pectoral, pelvis girdles, and limbs

� Acral bone - Part of the appendicular skeleton, including bones of the hands

and feet

2) Based on shape, bones can be classified as follows:

� Flat bone - Bones of the skull, sternum, pelvis, and ribs

� Tubular bone - Long tubular bone, including bones of the limbs; short tubular

bone, including bones of the hands and feet, such as phalanges, metacarpals,

and metatarsals

� Irregular bone - Bones of the face and vertebral column

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� Sesamoid bone - Bones developing in specific tendons, the largest example of

which is the patella

� Accessory bone or supernumerary bone - Extra bones developing in additional

ossification centers

3) Based on size, bones can be classified as follows:

� Long bone - Tubular in shape with a hollow shaft and two ends, including

bones of the limbs

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� Short bone - Cuboidal in shape, located only in the foot (tarsal bones) and wrist

(carpal bones)

4) Based on texture of cross sections, bone tissue can be classified as follows:

� Compact bone (dense bone, cortical bone): Compact bone is ivorylike and is

dense in texture without cavities. It is the shell of many bones, surrounding the

trabecular bone in the center. It consists mainly of Haversian systems or

secondary osteons

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� Sponge bone (trabecular bone, cancellous bone): Sponge bone is spongelike

with numerous cavities and is located within the medullary cavity. It consists of

extensively connected bony trabeculae, oriented along the lines of stress.

5) Based on matrix arrangement, bone tissue can be classified as follows:

� Lamellar bone (secondary bone tissue/Bundle bone): Lamellar bone is mature

bone with collagen fibers arranged in lamellae. In sponge bone, lamellae are

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arranged parallel to each other, whereas in compact bone, they are

concentrically organized around a vascular canal, termed a Haversian canal.

� Woven bone (primary bone tissue): Woven bone is immature bone with

collagen fibers arranged in irregular random arrays, containing smaller amounts

of mineral substance and a higher proportion of osteocytes than lamellar bone.

Woven bone is temporary and eventually is converted to lamellar bone. Woven

bone is pathologic tissue in adults, except in a few places, such as areas near

sutures of the flat bones of the skull, tooth sockets), and the insertion site of

some tendons

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� Composite bone: Formed by the deposition of lamellar bone within a woven

bone lattice- Cancellous Compaction .It Is the predominant osseous tissue for

stabilizing during early phases of retention or post operative healing

6) Based on maturity, bone tissue can be classified as follows:

� Immature bone (primary bone tissue): Immature bone is woven bone.

� Mature bone (secondary bone tissue): Mature bone characteristically is

lamellar bone. Almost all bones in adults are lamellar bones.

7) Based on developmental origin, bones can be classified as follows:

� Intramembranous bone (mesenchymal bone): Intramembranous bone develops

from direct transformation of condensed mesenchyme. Flat bones are formed in

this way.

� Intracartilaginous bone (cartilage bone, endochondral bone): Intracartilaginous

bone forms by replacing a reformed cartilage model. Long bones are formed in

this way.

8) Paired and Unpaired bones

Paired Cranial Bones:

� Parietals - the two parietal bones each have a superior and inferior temporal

line, to which the temporal muscle is attached.

� Temporals - the two Temporal bones (near the temples) each have two major

portions, the squamous (flat) portion, and the petrosal portion.

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Unpaired Cranial Bones:

� Frontal - roughly, the forehead and upper part of the eye orbit.

� Occipital - the flat, concave base, which rests upon the first vertebrae. The

occipital bone has a hole, the Foramen Magnum , through which the blood

vessels and nerves of the spine connect with the base of the brain.

� Sphenoid - difficult to describe; winged, with many fissures and protusions. It

roughly leads from the sinuses to the eyes.

� Ethmoid - also hard to describe; cannot be seen from every angle of the skull.

Like the sphenoid, it is located in the the mid-sagittal plane and helps connect

the cranial skeleton to the facial skeleton. It consists of various plates and

paired projections. The most superior projection is the Crista Galli , (or "cocks

comb," owing to its appearance), which helps divides the left and right frontal

lobes of the brain.

Paired Facial Bones:

� Lacrimals - these two are the smallest and most fragile of the facial bones,

forming the front of the side wall of each eye orbit. Basically rectangular with

two surfaces and four borders. Each of the four borders articulate with the

bones that surround them.

� Nasals - two small rectangular bones which form the bridge of the nose above

the nasal cavity

� Zygomatics - the cheekbones, running from the maxilla to the wall of the eye

orbit.

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� Maxillae - the paired upper jaw bones. They are nearly hollow, each with a

large cavity called a maxillary sinus.

� Palatines - wing-shaped. They assist in forming the rear of the hard palate and

part of the nasal cavity.

� Inferior Nasal Conchae - small and complicated. These conchae are thin,

porous, and fragile. They are elongated and curled in on themselves. They lay

horizontally and are attached to the side wall of the nasal cavity. They increase

the surface area inside the cavity and increases the amount of mucus membrane

and olfactory nerve endings exposed to the air.

Unpaired Facial Bones:

� Vomer - forms the nasal septum, creating the left and right nasal passages. The

part of the nose that most often gets broken.

� Mandible - the lower jaw. The formal anatomical name for its tip (the chin) is

the mental protuberence.

� Hyoid - the small U-shaped bone in the front of the throat, under the jaw but

above the larynx. (the "Adam's Apple.")

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MECHANICAL PROPERPERTIES OF BONE1,5

AGE (in years)

Property 10-20

(years)

20-30

(years)

30-40

(years)

40-50

(years)

50-60

(years)

60-70

(years)

70-80

(years)

ULTIMATE STRENGTH (MPa)

Tension 114 123 120 112 93 86 86

Compression - 167 167 161 155 145 -

Bending 151 173 173 162 154 139 139

Torsion - 57 57 52 52 49 49

ULTIMATE STRENGTH (%)

Tension 1.5 1.4 1.4 1.3 1.3 1.3 1.3

Compression - 1.9 1.8 1.8 1.8 1.8 -

Torsion - 2.8 2.8 2.5 2.5 2.7 2.7

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Composition of Bone7,8,9

Bone is a specialized mineralized connective tissue .Aboutx 60%.of its wet weight is

inorganic material, about 25% organic material and about 15% water. By volume,

about 36% is inorganic, 36% is organgic and 28% is water. .The association of

organic and inorganic substances gives bone its hardness and resistance.

Bone

Inorganic Organic

hydroxy apatite 28% 5%

collagen Osteo-calcin

(type I) Sialo-protein

Osteonectin

Osteopontin

Phospo-Protein

Biglycan, Decorin

Growth Factors

Bone specific Protein

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Organic Matrix

Collagen is defined as a molecule composed of three polypeptide chains termed

� chains which associate into a triple helical molecule. Bone consists predominantly

of type I collagen with traces of type III, V & XI collagen. Type I collagen comprising

about 90% of bone matrix is a complex molecule that consists of a heterotrimer of two

pro - �1(I) and pro – α2(I) polypeptide chains. These peptide chains are structurally

similar but genetically distinct. Type I procollagen is characterized by the repeated

triplet, Gly – x – y where glycine is frequently followed by a proline & hydroxy

proline. The post translational modifications of Type I collagen specific for bone

include hydroxylation of some proline & lysine residues and glycosylation of

hydroxylysyl residues to form galactosyl - hydroxy lysyl residues. Interstitial collagen

is composed of these rod shaped molecules that associate both end to end and laterally

in a quarter stagerred fashion to form fibrils.

Transmission electron micrographs of individual mineralized collagen fibrils show

that hydroxyapatite crystals are located mainly within the fibrils at the level of the gap

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regions. The plate-shaped crystals are observed to be more or less uniformly stacked

across the fibril diameter.Collagen fibers are synthesized from osteoblasts,

polymerizing tropocollagen extracellularly & becoming progressively more cross

linked as they mature. The formation of collagen cross-links is attributable to the

presence of two aldehyde-containing amino acids which react with other amino acids

in collagen to generate difunctional, trifunctional, and tetrafunctional cross-links. A

necessary prerequisite for the development of these cross-links is that

the collagen

molecules be assembled in the naturally occurring

fibrous polymer. Once this

condition is met, cross-linking occurs in a spontaneous, progressive fashion. The

chemical structures

of the cross-links dictate that very precise intermolecular

alignments must occur in the collagen polymer. This seems to be a function of each

specific collagen because the relative abundance of the different cross-links varies

markedly, depending upon the tissue of origin of the collagen.In primary bone they

form a complex interwoven meshwork (non-lamellar or woven bone) which is later

replaced by the regular arrays of nearly parallel collagen fibers (lamellar bone).

Collagen fibers from the periosteum are incorporated in cortical bone (extrinsic or

sharpey’s fibers) , anchoring this fibrocellular layer at its surface.

Non-collagenous proteins

In total 10% of organic phase is made of a variety of non collagenous proteins

including proteoglycans, glyco-proteins, � - carboxy glutamic acid containing proteins

and proteolipids.

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Proteoglycan & Glycoproteins

Osteonectins

Also known as SPARC (secreted protein, acidic and rich in cysteine)This acidic

glycoprotein is highly enriched in bone matrix and is synthesized by osteoblasts, skin

fibroblasts, tendon cells and odontoblasts. Its function is unknown. It is highly cross

linked and binds strongly to type I collagen and hydoxy apatite and may have a

function in acalcium –mediated organization of extracellular matrices..

RGD containing proteins (Arg – Gly – ASP) Fibronectin, Thrombospondin,

Osteopontin, Bone Sialo Protein as they contain specific amino acid sequence

arginie-glycine-aspartic acid.The bone matrix contains four proteins that contain the

amino acid sequence RGD (Arg – Gly – ASP) which binds to cell surface receptors

thereby mediating cell attachment.

Fibronectin is produced by osteoblastic cells and mediates cell attachment and

spreading of bone cells.

Thrombospondin an endogenous product mediates only adhesion of cells but

not spreading.

Osteopontin is produced by certain bone cells & mono nuclear cells and

mediates cell attachment. It is similar to bone sialoprotein, that is expressed in

differentiating bone cells. Osteopontin binds to the osteoclast integrin receptor and

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leads to activation of the phospholipase C pathway in osteoclasts and increases in

intracellular calcium.

Bone sialoprotein is found only in osteoblasts and osteocytes.

The other bone sialo protein, Bone acidic glycoprotein (B A G) is similar to B S

P and osteopontin. Their functions are not clear and they more likely helps to increase

bone formation. These cell attachment proteins in bone can maintain osteoclasts or

other bone cells in particular location.

γ-Carboxy Glutamic Acid (Gla) – containing protein.

Matrix Gla Protein and Osteocalcin are two proteins bearing modified amino

acid, �-carboxy glutamic acid (gla) generated by Vitamin K dependent enzymes.

Osteocalcin appears late in bone development . Osteocalcin is regulated by 1,25 –

dihydroxy Vitamin D3. Osteocalcin is produced by osteoblasts and osteocytes and is

used as a marker of osteoblast activity in clinical states. It is postulated that it could

retard mineralisation. It also acts as chemo attractant for osteoclast progenitors

attracting them towards bone surfaces.

Proteoglycan

They are composed of a central protein core to which glycosoaminoglycans are

attached. Decorin and biglycan comprise < 10% of the noncollagenous proteins in

bone, but this decreases with maturation of bone.A third small proteoglycan

(chondroitin sulfate proteoglycan) has been found entirely associated with mineral

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crystals. Biglycan is more prominent in developing bone and has been mineralized to

pericellular areas.It can bind TGF-β and extracellular matrix macromolecules,

including collagen and thereby regulate fibrillogenesis. Decorin binds mainly within

the gap region of collagen fibrils and decorates the fibril surface. The primary

calcification in bones is reported to follow the removal of decorin and fusion of

collagen fibrils.The other proteoglycans are biglycan, versican and serum proteins.

Lysyl oxidase and tyrosine rich acidic matrix proteins (TRAMP) are

components of demineralized bone and dentin matrix. Lysyl oxidase is a critical

enzyme for collagen crosslinking. TRAMP, also known as dermatopontin, binds

decorin and TGF-β and together these proteins regulate the cellular response to TGF-

β.

Other protein constituents

Growth Factors

Many growth regulatory factors that influence cell proliferation and/ or

differentiation is found in bone. They include transforming growth factor � - I, TGF �

- II, bone morphogenetic proteins, platelet derived growth factor, fibroblast growth

factors and insulin like growth factors.

Bone morphogenetic proteins are members of the extended transforming growth factor

� (TGF - � ) family and are synthesized by bone cells locally. They help in regulation

of normal bone remodeling.

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Inorganic Component

The inorganic component of bone consists of calcium hydroxy apatite which is

represented as Ca10(PO4)6(OH)2. The unit cell of the apatite has the shape of a rhombic

prism when stacked together, these prisms form the lattice of a crystal. A layer of

water called the hydration shell, exists around each crystallite, thereby there are 3

surfaces to an apatite crystal – the crystal interior, the crystal surface and the hydration

shell, all of which are available for the exchange of ions.

Thus the major ions are calcium, phosphate, hydroxyl, carbonate. Less numerous ions

are citrate, magnesium, sodium , potassium, fluoride, chloride, iron, zinc, copper,

aluminium, lead, strontium, silicon , boron,carbonate and lead. The percentage of

calcium in bones is 99% and phosphate is 85%. The relative ratio of calcium to

phosphorus can vary markedly under different nutritional conditions, the Ca/P ratio on

a weight basis varying between 1.3 and 2.0

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ANATOMY OF BONE1,2,5

Epiphysis

In long bones, the epiphysis is the region between the growth plate and the

expanded end of bone, covered by articular cartilage. An epiphysis in a skeletally

mature person consists of abundant trabecular bone and a thin shell of cortical bone.

While an epiphysis is present at each end of long limb bones, it is found at only one

end of metacarpals (proximal first and distal second through the fifth metacarpals),

metatarsals (proximal first and distal second through fifth metatarsals), phalanges

(proximal ends), clavicles, and ribs.

The epiphysis is the location of secondary ossification centers during

development. The structure of the epiphysis is more complex in bones that are fused

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from more than one part during development. Examples include proximal and distal

ends of the humerus, femur, and vertebrae. For instance, the proximal end of the

humerus is developed from 3 separate ossification centers, which later coalesce to

form a single epiphyseal mass. In the proximal humeral epiphysis, one of the centers

forms the articular surface, and the other two become the greater and lesser

tuberosities. Carpal bones, tarsal bones, and the patella are also called epiphysioid

bone and are developmentally equivalent to the epiphyses of long bones.

Knowledge of the location of the epiphysis and its equivalents in various bones

aids in recognition of the origin of bone lesions and further facilitates the diagnostic

consideration, as some bone tumors such as chondroblastoma have a strong

predilection for the epiphysis or epiphysioid bones.

Metaphysis

The metaphysis is the junctional region between the growth plate and the

diaphysis. The metaphysis contains abundant trabecular bone, but the cortical bone

thins here relative to the diaphysis. This region is a common site for many primary

bone tumors and similar lesions. The relative predilection of osteosarcoma for the

metaphyseal region of long bones in children has been attributed to the rapid bone

turnover due to extensive bone remodeling during growth spurts.

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Diaphysis

The diaphysis is the shaft and the region between metaphyses, composed

mainly of compact cortical bone. The medullary canal contains marrow and a small

amount of trabecular bone.

Physis (epiphyseal plate, growth plate)

The physis is the region separating the epiphysis from the metaphysis. It is the

zone of endochondral ossification in an actively growing bone or the epiphyseal scar

in a fully-grown bone.

Bone Surface markings

Depressions and openings that allow blood vessels and nerves to pass:

Foramen:

Round or oval opening through a bone through which blood vessels nevers or

ligaments pass. Eg optic foramen of the sphenoid bone.( foramen= hole)

Fissure:

Narrow, slitlike opening betweenadjacent parts of bone through which blood

vessels or nerves pass. Eg Superior orbital fissure

Fossa:

Shallow, basin like depression in a bone, often serving as an articular surface.

(Foramen=trench)Eg Coronoid fossa of the humerous.

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Meatus:

Canal-like or tube like opening.(Meatus= passageway) Eg external auditory

meatus of the temporal bone.

Sulcus:

Furrow along bone surface that accommodates a blood vessel, nerve or tendon.

(Sulcus =groove).Eg intertubercular sulcus of the humerous.

Sinus:

Cavity within a bone, filled with air and lined with mucous membrane.

Processes: Projections or outgrowths on bone that form joints or attachment points for

connective tissue, such as ligaments and tendons.

Process that form joints:

Condyle:

Rounded articular projection or round protuberance at the end of a

bone.(Condyle= knuckle) Eg lateral condyle of the femur.

Head:

Rounded articular projection.supported on the neck (constricted portion) of a

bone. Eg head of the Femur.

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Facet:

Smooth, nearly flat articular surface.Eg Superior articular surface of the

vertebra

Processes that form attachment points for connective tissue:

Crest:

Narrow, usually prominent, ridge of bone. Eg Illiac crest of the hip bone.

Epicondyle:

Raised area on or above a condyle Eg Medial condyle of the femur.

Line:

Narrow ridge of bone that is less prominent than a crest. Eg Linea aspera of the

femur.

Spinous process:

Sharp, slender, often pointed projection eg spinous process of a vertebra.

Trochanter:

Very large, blunt, irregularly shaped process.Eg greater trochanter of femur.

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Tubercle:

Small rounded process.(tuber=knob) eg greater tubercle of the humerous.

Tuberosity:

Large rounded projection that may be roughened eg ischial tuberosity of the hip

bone.

Marrow Cavity:

The marrow not only fills up the cylindrical cavities in the bodies of the long

bones, but also occupies the spaces of the cancellous tissue and extends into the larger

bony canals (Haversian canals) which contain the bloodvessels. It differs in

composition in different bones. In the bodies of the long bones the marrow is of a

yellow color, and contains, in 100 parts, 96 of fat, 1 of areolar tissue and vessels, and 3

of fluid with extractive matter; it consists of a basis of connective tissue supporting

numerous bloodvessels and cells, most of which are fat cells but some are “marrow

cells,” such as occur in the red marrow to be immediately described. In the flat and

short bones, in the articular ends of the long bones, in the bodies of the vertebræ, in

the cranial diploë, and in the sternum and ribs the marrow is of a red color, and

contains, in 100 parts, 75 of water, and 25 of solid matter consisting of cell-globulin,

nucleoprotein, extractives, salts, and only a small proportion of fat. The red marrow

consists of a small quantity of connective tissue, bloodvessels, and numerous cells,

some few of which are fat cells, but the great majority are roundish nucleated cells,

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the true “marrow cells” of Kölliker. These marrow cells proper, or myelocytes,

resemble in appearance lymphoid corpuscles, and like them are ameboid; they

generally have a hyaline protoplasm, though some show granules either oxyphil or

basophil in reaction. A number of eosinophil cells are also present. Among the

marrow cells may be seen smaller cells, which possess a slightly pinkish hue; these

are the erythroblasts or normoblasts, from which the red corpuscles of the adult are

derived, and which may be regarded as descendants of the nucleated colored

corpuscles of the embryo. Giant cells (myeloplaxes, osteoclasts), large,

multinucleated, protoplasmic masses, are also to be found in both sorts of adult

marrow, but more particularly in red marrow. They were believed by Kölliker to be

concerned in the absorption of bone matrix, and hence the name which he gave to

them—osteoclasts. They excavate in the bone small shallow pits or cavities, which are

named Howship’s foveolæ, and in these they are found lying.

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SUTURES:

A suture (sutur=seam) is a fibrous joint composed of a thin layer of dense

fibrous connective tissue that unites bones of the skull.eg suture between the parietal

and frontal bones. The irregular, interlocking edges of the suture give them strength

and decrease their chance of fracturing.Because the suture is immovable, it is

classified as synarthrosis.

Some suture, although present during childhood,are replaced by bone in the adult.

Such a suture is called as synostosis in which there is complete fusion of the bone

across the suture line. Eg frontal suture between the left and right sides of thr frontal

bone that begins to fuse during infancy.

Cranial sutures:

Norma Verticalis

Sagittal suture,Coronal suture,Lambdoid suture,Metopic suture

Norma Occipitalis

Occipitomastoid suture,Parietomastoid suture

Norma Frontalis

Internasal Frontonasal, Naso-maxillary, Lacrimo-maxillary, Frontomaxillary,

Inermaxillary, Zygomaticomaxiilary,Zygomaticofrontal

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Norma Lateralis

Zygomatico temporal, Squamomastoid

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Periosteum

The periosteum is composed of an inner cambium layer immediately adjacent to

the bone surface and an outer dense fibrous layer. The cambium layer consists of

osteoprogenitor cells, which are flat and spindle shaped and are capable of

differentiating into osteoblasts and forming bones in response to various stimulations

.The collagen fibers in the outer layer are contiguous with the joint capsule, ligament,

and tendons. The periosteum is thicker and is more loosely attached to the cortex in

children but is thinner and more adherent in adults. The periosteum completely covers

a bone except in the region of the articular cartilage and at sites of muscle

attachments. It is somewhat anchored to the cortex by Sharpey fibers that penetrate

into the bone. The periosteum carries a dense network of blood, lymphatic vessels,

and predominantly sensory nerves for maintenance of the bone structure.

Different patterns of periosteal stimulation result in different patterns of periosteal

bone formation. Continual insult results in streams of periosteal bone perpendicular to

the bone surface, resulting in a hair-on-end appearance on radiographs eg Sickle cell

anemia, Thalassemia. Intermittent periosteal stimulation results in multiple partially

separated streams of periosteal bone parallel to the bone surface, giving an onion skin

appearance on radiographs.eg Ewing’s Sarcoma. As opposed to osseous tissue,

periosteum has nociceptors nerve endings, making it very sensitive to

manipulation.The periosteum has an osteogenic role. In the adult, the osteogenic role

is demonstrated during fracture repair. In addition, periosteum is becoming

33

increasingly attractive for the treatment of certain clinical problems: cleft palate

repair, treatment of severely comminuted fractures ,pseudoarthrosis of the tibia and

for the repair of tracheal defects.

The functions are follows:

� Provides attachment to muscles, tendons and ligament

� Nourishes the underlying bone with the help of blood vessels.

� Helps in bone formation during growth period

� Repair of fractures because of the presence of osteoprogenitor cells

� Prevents overgrowth of bone by acting as a limiting membrane

Mucoperiosteum:

Mucoperiosteum is a compound structure consisting of mucous membrane and

of periosteum. In regions such as the gingiva and parts of the hard palate, oral mucosa

is attached directly to the periosteum of underlying bone, with no intervening

submucosa. This arrangement is called a mucoperiosteum and provides a firm

inelastic attachment.

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INDICATIONS:

� Areas with irregular bony contours, deep craters and other defects

� Pockets on teeth for which a complete removal of root irritant is not possible

� In cases of furcations involvement

� Intrabony pockets on distal to last molars, frequently associated with

mucogingival problems

� Persistent inflammation in areas of moderate to deep periodontal pockets

Endosteum

The endosteum is composed of osteoprogenitor cells and only a small amount

of connective tissue, covering the surface of bone trabeculae and the medullary

surface of cortical bone and Haversian canals. It serves as one of the functional

surfaces for bone remodeling.

MICROSCOPIC STRUCTURE OF BONE:

BONE CELLS

35

Osteoprogenitor cells:1,7,8,9

Are derived from the pleuripotent stromal stem cells present in the bone

marrow & other connective tissues which can proliferate & differentiate into

osteoblasts before bone formation. They are mesenchymal in origin. In

intramembranous bone they aggregate & undergo proliferation before differentiating

into osteoblasts while in endochondral bone formation.,similar cells migrate with the

ingrowth of blood vessels from the perichondrium into areas of degenerating cartilage

& differentiate into osteoclasts.

There are two types or stages of osteoprogenitor cells, one totally committed to

bone formation (committed osteoprogenitor cells),found associated with bone & the

other, (inducible osteoprogenitor cells) widely present in connective tissue,& probably

able to differentiate into various connective tissue cells depending on the nature of

inducer.

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Osteoblast1,7,8,9

Any cell that forms bone whether during growth or remodeling or during

fracture healing is an osteoblast.

Osteoblasts are large non dividing cells with a rounded to polygonal shape with an

eccentrically placed nucleus. Cytoplasm is deeply basophilic and exhibits a distinct

negative Golgi image. The cytoplasmic processes are in contact with one another and

also the processes of osteocytes in the lacunae beneath them. Gap junctions do form

between adjacent cells, preosteoblasts to osteoblasts, osteoblasts to osteocytes and

osteocytes to osteocytes.Preosteoblasts and Osteoblasts exhibit high levels of alkaline

phosphatase on the outer surface of their plasma membranes.

Differentiation of Osteoblasts

Mesenchymal stem cells differentiate into osteoblasts when they are

exposed to bone morphogenic proteins (BMP). BMPs are part of the

transforming growth factor (TGF) superfamily.

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Regulation of osteoblast differentiation10

Bone morphogenic protein

They regulate osteoblast and chondrocyte differentiation during skeletal

development

Smads

SMAD's (Small Mothers Against Decapentaplegic) are a class of proteins that

modulate the activity of transforming growth factor beta ligands.Smad transcription

factors are substrates of the activated type I receptor kinases in the cytoplasm. The

phosphorylated Smad proteins move into the nucleus, bind to the regulatory regions of

target genes, and regulate their transcription. Thus, Smad proteins are key molecules

in the transduction of signals from the cell membrane to the nucleus. There are three

classes of SMAD:

� The receptor-regulated Smads(R-SMAD) which include SMAD1, SMAD2,

SMAD3, SMAD5 and SMAD9

� The common-mediator Smad(co-SMAD) which include only SMAD4,

� The antagonistic or inhibitory Smads(I-SMAD) which include SMAD6 and

SMAD7.

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Runx2 and Osterix

Runx2 interacts tightly with BMP signaling through Smads in osteoblast

differentiation.Osterix acts downstream of Runx2 during bone development

Factors affecting bone resorption and formation, directly and indirectly9

Systemic hormones:

PTH, 1,25(OH)2D3, Calcitonin, Sex steroids, Glucocorticoids, Growth hormone,

Thyroid hormone.

Cytokines growth factors

Prostaglandins,Interleukin 1, Tumor necrosis factor, Interferon Y, Insulin-like

growth factors, Macrophage colony stimulating factor, Epidermal growth factor,

Transforming growth factor, Bone morphogenic proteins, Platelet-derived growth

factor, Fibroblast growth factor, Vasoactive intestinal peptide, PTH-related peptide,

Osteoprotegerin ligand, Osteoprotegerin,Calcitonin gene- related peptide.

Miscellaneous agents

Immobilisation, weightlessness, Stress/exercise, Protons, Calcium, Phosphate,

Fluoride, Bisphosphonates, Alcohol/tobacco.

39

Functions of osteoblasts

� Osteoblasts are responsible for production of the proteins of bone matrix type I

and IV collagen and other non collagenous proteins like osteocalcin,

osteopontin, bone sialoprotein and osteonectin.

� Osteoblasts secrete the growth factors which are stored in bone matrix such as

transforming growth factor �, bone morphogenetic protein, platelet – derived

growth factor and the insulin – like growth factor.

� Osteoblasts mineralize newly formed bone matrix which maybe mediated in

part by sub-cellular particles known as matrix vesciles enriched in alkaline

phosphatase which are generated from the osteoblast cytoplasm. Osteoblasts

also produce phospholipids and proteoglycans which may be important in the

mineralization process.

� Osteoblast may be required for normal bone resorption to occur. Under

physiologic conditions that support resorption, the osteoblasts are stimulated by

lymphokines to produce interleukin-6 which in turn stimulate the osteoblast to

produce proteolytic enzymes which prepare the bone surface for osteoclastic

resorption. Functional lifespan of osteoblasts may range from 3 – 4 months to

1-5 years with an average of about 5 – 6 months.

� Osteoblasts has a controlling influence in activating the bone-resorbing cells,

the osteoclasts.Itis the source of factors involved in this process (colony-

stimulating factors,prostaglandins, osteoprotegerin ligand).

40

� Osteoblasts contains receptors for parathyroid hormone and regulates the

osteoclastic response to this hormone.

Lining Cells1,2

Lining Cells are remnants of osteoblasts that previously laid down bone matrix.

The cells have thin flat nuclear profiles. Cytoplasmic organelles are few and these

cells retain their gap junctions with osteocytes creating a network that functions to

control mineral homeostasis and ensure bone vitality. It also manages bone

maintenance by forming a bone membrane that controls ion fluxes into and out of

bone and by secreting additional phosphoproteins and glycoproteins.

Osteocytes1,7,8,9

Osteocytes constitute the major cell type of mature bone, lyingscattered within

its matrix, but interconnected by numerous cellular extensions to form a complex

cellular network. They are derived from osteoblasts which have reduced or ceased

matrix formation and become enclosed in matrix, but retain contact with each other

and with cells at the surfaces of bone (osteoblasts and bone lining cells) throughout

their lifespan.

Mature, relatively inactive osteocytes possess a cell body which has the shape

of a three-axis ellipsoid, the longer axis(about 25 μm) parallel to the surrounding body

lamella and its shortest axis perpendicular to the plane of the lamella. The cytoplasm

is faintly basophilic and contains few organelles. Numerous fine processes emerge

41

from the cell body and branch a number of times to form an extensive tree. Such

processes contain bundles of microfilaments and some smooth endoplasmic reticulum.

At their distal tips they contact the processes of adjacent cells (other osteocytes and at

surfaces, osteoblasts and bone lining cells)

Embryonic (woven) bone and repair bone have more osteocytes than does

lamellar bone. After their formation the osteocytes become reduced in size and the

space in the matrix occupied by an osteocyte is called the osteocytic lacuna.

Functions of osteocytes

Their normal functions are not clearly known. Functions may include :

� Maintenance of bone matrix – Osteocytes possess enough organelles to

continue producing relatively small amounts of matrix constituents throughout

life.

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� Release of calcium ions – osteocytes may have the capacity to transfer calcium

ions from bone mineral to the blood plasma.

� Osteocytes may play a role in sensing strain resulting from mechanical force

applied to the skeleton during mechanical usage or they could act as part of a

transducer mechanism that converts changes in the strain environment into

organized bone cell work. Plays the role of mechanoreceptor of bone.

Osteoclasts1,7,8,9

Osteoclasts are multi nucleated giant cells which resorb bone. They range from 20 �m

to over 100 �m and contain 2–50 nuclei. They occupy shallow pits called ‘Howship’s

lacunae’ on flat bone surfaces, and they are present in the leading edge of cutting

cones in haversian bone. Features seen on light microscopy include a foamy

acidophilic cytoplasm, a striated or brush border appearance at the site of attachment

to the bone due to the projecting free collagen fibrils and a positive staining for

tartarate - resistant acid phosphatase. The part of an osteoclast that is directly

responsible for carrying out bone resorption is a transitory and highly motile structure

called its ruffled border seen on electron microscopy. Encircling the periphery of the

ruffled border is the clear zone a ring shaped region devoid of organelles. This region

is also called the podosome or filamentous zone where the ruffled border is sealed to

the bone surface. This seal apparently localizes the highly acidic micro environment,

which is conducive to resorption of bone. Farthest away from bone lies the basal

region of the cell containing multiple nuclei, golgi saccules, numerous mitochondria,

43

some secretary vesicles and lysosomes. Their lifespan is uncertain, though it may be

as long as 7 weeks.

Origin and Cell Lineage1,7,11

They come from mononuclear precursors i.e. blood monocytes. The proposed

model for the formation of osteoclast is that the colony stimulating factors (CSFs)

stimulate the proliferation and differentiation of the granulocyte – macrophage

committed progenitor cells (CFU-GM). The CFU–GM stimulated by CSF to form

pro-monocytes which are immature non-adherent progenitors of mono nuclear

phagocytes and osteoclasts. The pro-monocyte proliferate and differentiate along the

macrophage pathway or along the osteoclast pathway. The first osteoclast to form the

early pre-osteoclasts proliferates and circulate in blood. They contain non specific

44

esterase and not tartarate resistant. The early pre-osteoclast gives rise to a late pre-

osteoclast regulated by 1,25 – dihydroxy vitamin D, PTH. Then the late pre-osteoclast

attaches to bone, expresses osteoclast specific antigens and then fuses with other cells

to form a multinucleated osteoclast.

Differentiation of Osteoclasts10

The molecule that inhibits osteoclastogenesis is known by two different

names, OPG (osteopotegerin) and OCIF (osteoclastogenesis inhibit ing factor).

OPG is secreted by osteoblasts and functions to block the formation of

osteoclasts as well as bone resorption.

45

Regulation of osteoclast differentiation

TNF receptor-ligand family members

Osteoblasts/stromal cells regulate osteoclast differentiation and function

through TNF receptor –ligand family members.

RANKL–RANK interaction

Activation of NF-jB and JNK through the RANK mediated signaling system

appears to be involved in the differentiation and activation of osteoclasts.

Inflammatory cytokines

Interleukin-1 directly stimulates osteoclast function through the IL-1 type 1

receptor expressed by osteoclasts. LPS and some inflammatory cytokines such as

TNFa and IL-1 are directly involved in osteoclast differentiation and function through

a mechanism independent of RANKL–RANK interaction.

RECEPTOR ACTIVATION

Osteoclasts also express integrin receptors including the vitronectin receptor

which plays an important role in the adhesion of osteoclasts to bone

surface. Peptides containing the RGD motif have been shown to inhibit

osteoclast-mediated bone resorption in vitro and prevent osteoporosis in vivo

46

Osteoclastogenesis5

PTH stimulates bone resorption by osteoclasts, but it does so indirectly.

Receptors for PTH are located on osteoblasts, which then signal to bone marrow-

derived osteoclast precursors to stimulate their fusion, differentiation and activation.

Osteoclast precursors express a cell-surface receptor known as RANK (Receptor

Activator of Nuclear factor-Kappa B). Osteoblasts express RANKL (RANK Ligand)

on the extracellular surface of their plasma membrane.

When they are stimulated by PTH, osteoblasts up-regulate expression of

RANKL, which binds to RANK, activating signaling pathways that promote

osteoclast differentiation and survival. Osteoblasts also express a secreted factor

called osteoprotegerin. As its name implies, osteoprotegerin "protects bone" by

preventing bone resorption. Osteoprotegerin works as a decoy receptor for RANKL: it

binds RANKL and therefore prevents binding to RANK and stimulation of

osteoclastogenesis. The ratio of osteoprotegerin:RANKL produced by osteoblasts will

determine the extent of bone resorption.

47

Interrelationship between osteoblasts and osteoclasts:5

There is close relationshiop between bone deposition and bone

resorption.During the growing phase of a child, the amount of deposition exceeds that

of resorption, giving an increase in bone mass. During the adult phase, the amount of

bone deposition is equivalent to that of bone resorption and bone masses more or less

constant. In old age, the amount of bone deposition is generally less than that of bone

resorption and there is overall decrease in bone mass.In postmenopausal women

particularly this loss may be sufficient to lead to clinical condition of osteoporosis.

Many of the factors that result in bone resorption are known to have no direct

effect on osteoclasts, but act indirectly through osteoblasts. Most of the receptors to

bioactive molecules that cause bone resorption are present on osteoblasts.There are

several mechanisms whereby osteoblasts might promote bone resorption:

� By the local release of substances such as cytokines and growth factors

(macrophage colony-stimulating factor, osteoprotegrein and interleukins),

osteoblasts could stimulate the production of osteoclasts.

� By releasing enzymes (such as MMPs) to degrade the unminerialised osteoid

layer covering forming bone, osteoblasts could help expose mineralized matrix

on which osteoclasts could attach and commence resorption.

� By bioactive molecules present within bone (cytokines, BMPs, TGF-β) that

could be activated as a result of osteoclastic bone resorption and subsequently

have an effect on remodelling.

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Reversal lines mark the position where bone activity changes from resorption to

deposition. Such lines are darkly stained and irregular in outline, being composed of a

series of concavities that were once the sites of the resorptive Howship’s lacunae.

They may be seen to contain the enzyme acid phosphatase.

BONE MATRIX

Haversian system 1,7,8

The primary structural unit of compact bone is the Haversian system. Each

Haversian system is a long, often bifurcated, cylinder parallel to the long axis of bone,

formed by successive deposition of 4-20 (average 6) concentric layers of lamellae.

49

Collagen fibers are parallel to each other within each lamella but are oriented

perpendicularly to those in the neighboring lamellae. Such an arrangement can be

highlighted as alternating bright and dark layers in polarized microscopy.

Lamellar deposition starts from the periphery, so that younger lamellae are

closer to the center of the system, and the younger systems have larger canals.

Between the lamellae are lacunae containing the cell bodies and canaliculi holding the

cytoplasmic processes of osteocytes.

In the center of each Haversian system is a Haversian canal, which is lined by

endosteum and contains a neurovascular bundle and loose connective tissue.

The Haversian canals connect with each other by transverse or oblique

Volkmann canals, communicating with the marrow cavity and the periosteum to

provide channels for the neurovascular system. Volkmann canals are not surrounded

by concentric lamellae; rather, they perforate the lamellae. They contain blood vessels,

nerves and lymphatics and connect haversian canals with the medullary cavity and the

surface of the bone

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Interstitial lamellae

Interstitial lamellae are incomplete or fragmented osteons located between the

secondary osteons. They represent the remnant osteons left from partial resorption of

old osteon during bone remodeling.

The mixture of interstitial lamellae and complete osteons produces a mosaic

pattern. Thus, the age of the bone can be deduced from the proportion of interstitial

lamellae and intact osteons. Younger bone has more complete osteons and less

interstitial lamellae in between the osteons.

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Circumferential lamellae

Circumferential lamellae are circular lamellae lining the external surface of the

cortex adjacent to the periosteum and lining the inner surface of the cortex next to the

endosteum. There are more outer than inner circumferential lamellae.

Bony trabeculae

Bony trabeculae are seen as a system of plates, rods, arches and struts traversing

the medullary cavity and attached to the cortex endosteum. The internal surface of the

bone is covered by a single layer of bone cells, the endosteum which physically

separates the bone surface from the bone marrow within.

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Bone Marrow

The term bone marrow is usually restricted to the soft red or yellow tissue

occupying the macroscopically visible cavities in a fresh bone. It is essentially a frame

work of reticular tissue (reticulum cells & fibres) supporting blood vessels especially

venous sinusoids and either colonies of developing blood cells or large fat cells.

Reticulum cells readily turn into osteoprogenitor cells.

Bone marrow is considered one of the most valuable diagnostic tools to

evaluate hematologic disorders. Indications have included the diagnosis, staging, and

therapeutic monitoring for lymphoproliferative disorders such as chronic lymphocytic

leukemia (CLL), Hodgkin and Non-Hodgkin lymphoma, hairy cell leukemia,

53

myeloproliferative

disorders,andmultiplemyeloma. Furthermore, evaluationof cytopenia, thrombocytosis,

leukocytosis, anemia, and iron status can be performed. The bone marrow analysis has

also been used to evaluate nonhematologic, conditions. For example, in the

investigation for fever of unknown origin (FUO), specifically in those patients with

(AIDS), the marrow may reveal the presence of microorganisms, such as tuberculosis,

Mycobacterium avium intracellulare (MAI) infections, histoplasmosis, leishmaniasis,

and other disseminated fungal infections. Furthermore, the diagnosis of storage

diseases (eg. Niemann-Pick disease and Gaucher disease), as well as the assessment

for metastatic carcinoma and granulomatous diseases (eg, sarcoidosis) can be

performed.

In a bone marrow biopsy, a sample of solid bone marrow material is taken. A

bone marrow aspiration is usually done at the same time as a biopsy. In an aspiration,

a sample of the liquid portion of your marrow is withdrawn. Together, a bone marrow

biopsy and aspiration are often called a bone marrow exam. "Dry tap" is a term used

to describe failure to obtain bone marrow on attempted marrow aspirations. Extensive

marrow fibrosis and hypercellularity have been proposed as mechanisms to account

for the inability to withdraw marrow by aspiration.

HISTOGENESIS OF BONE:1,2,7,9

The bone is of mesodermal origin. The process of bone formation is called

OSSIFICATION. Formation of most bones is preceded by the formation of a

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cartilaginous model which is subsequently replaced by bone. This kind of ossification

is called ENDOCHONDRAL OSSIFICATION and the bones formed are called

CARTILAGE BONES.

Bone formation can take place in the mesenchymal blastema of some bones like

the bones of the skull cap. This is called INTRAMEMBRANOUS OSSIFICATION.

The 2 main forms of ossification are:

� Intramembranous ossification

� Endochondral ossification

Bone forms only by appositional deposition of matrix on the surface of a

preformed tissue. Woven bone is formed initially and is later converted to lamellar

bone by subsequent remodeling.

INTRA MEMBRANOUS OSSIFICATION:

Intramembranous ossification is the formation of bone directly on or within

fibrous connective tissue membranes formed by condensed mesenchymal cells. Such

bones form directly from mesenchyme without first going through a cartilage stage.It

begins approximately towards the end of second month of gestation. The process

involves the following steps:

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Formation of bone matrix within the fibrous membrane:

At the site where a bone will develop, there is initially loose mesenchyme,

which appears as widely separated, pale-staining, stellate cells with interconnecting

cytoplasmic processes. Then a center of osteogenesis develop in association with

capillaries that grow into the mesenchyme. The mesenchymal cells proliferate and

condense into compact nodules.Some these cells develop into capillaries.The

mesenchymal cells in the center become round and basophilic with thick

interconnecting processes. These cells differentiate into osteoblasts. These cells

secrete the organic matrix. Once surrounded by bone matrix, these are called

osteocytes. The matrix soon begins to calcify. The osteocytes obtain nutrients and

oxygen by diffusion along bone canaliculi. The organic matrix is also formed around

their interconnecting processes. The first small of newly formed bone matrix is an

irregular spicule.

Formation of woven bone

The bony spicules gradually lengthen into longer anastomosing structures called

trabeculae.The trabeculae extend in radial pattern. These trabeculae extend the local

blood vessels. This early membranebone is termed as woven bone.External to woven

bone, there is condensation of vascular mesenchyme called the periosteum.At this

stage, few mesenchymal cells remain undifferentiated.But, before these cells

disappear, they leave a layer of flat cells called as osteogenic cells or trabeculae which

do not have osteoblasts.In richly vascular areas, these osteogenic cells give rise to

56

osteoblasts that form the bone matrix. In areas,with no capillary blood supply, they

from chondroblasts which lay down cartilage.

Appositional growth mechanism and formation of compact bone plates:

Osteoblasts and osteogenic cells cover the spicules and trabeculae of

bone.These osteogenic cells proliferate in a richly vascularised environment and give

rise to osteoblasts that deposit new layers of bone matrix on preexisting bone surface.

They are always in a superficial position repeating the process again and again. This is

appositional growth, which results in build up of bone tissue one layer at a time.Every

generation of osteoblasts produce their own canaliculi. Hence, all the new osteocytes

remain linked throughout canaliculi to bone surface above and to osteocytes below.

As the trabeculae increase in width due to appositional growth, neighbouring

capillaries are incorporated to provide nutrition to osteocytes in deeper layers.New

bone is deposited on some surfaces and resorbed at other sites leading to remodelling

of trabeculae. This remodelling maintains shape and size of bone throughout life.

Continued appositional growth and remodelling of trabeculae converts

cancellous bone to compact bone. Cancellous bone is in the central part of the bone as

the trabeculae do not increase in size.The vascular tissue in cancellous bone

differentiate into red marrow.

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Formation of osteon:

As cancellous bone gets converted into compact bone, a number of narrow

channels are formed lined by osteogenic cells.These cells enclose vessels that were

present in soft tissue spaces of cancellous network.The consecutive lamellae of bone

added to the bony walls of spaces in cancellous bone, which is called osteon or

Haversian system.These osteons are called as primitive osteons as they are short,

compared to those in long bones.

The mechanism of intramembrasnous ossification involves bone morphogenetic

proteins and activation of transcription factor called cbfa1. BMP activate cbfa1 gene

in mesenchymal cells.The cbfa1 transcription factor transforms mesenchymal cells

into osteoblasts.It is believed that the proteins activate the genes for osteocalcin,

osteopontin and other bone specific extracellular matrix proteins.

58

59

ENDOCHONDRAL OSSIFICATION:

Endochondral ossification is the formation of bone within hyaline cartilage. In

this ossification process, mesenchymal cells are transformed into chondroblasts,

which initially produce a hyaline cartilage ‘model’ of the bone. Subsequently,

osteoblasts gradually replace the cartilage with bone.

Formation of cartilaginous model:

This process begins late in the second month of development At the site where

bone is going to form, mesenchymal cells crowd together in the shape of the future

bone. The mesenchymal cells differentiate into chondroblasts that produce a cartilage

matrix, hence the model consists of hyaline cartilage.In addition, a membrane called

perichondrium develops around the cartilage model consisting of outer fibrous layer

and inner chondrogenic layer. No osteoblasts are produced by the cells in the

chondrogenic layer, because differentiation is taking place in an avascular

environment. Fibroblasts in the fibrous layer produce collagen and a dense fibrous

covering is formed.

Growth of the cartilaginous model:

Growth of the cartilage model is by interstitial and appositional growth.

Increase in length is by interstitial growth due to repeated division of chondrocytes,

along with production of additional matrix by the daughter cells.Widening of the

model is due to further addition of matrix to its periphery by new chondroblasts,

60

derived from chondrogenic layer of the perichondrium.This is called appositional

growth. As the differentiation of cartilage cells moves towards the metaphysis, the

cells organize into longitudinal columns which are subdivided into three zones:

Zone of proliferation: The cells are small and flat, and constitute a source of new

cells.

Zone of hypertrophy and maturation: This is the broadest zone.The chondrocytes

hypertrophy, and in the early stages secrete Type II collagen. As hypertrophy

proceeds, proteoglycans are secreted.The increased cell size and cell secretion, lead to

an increase in the size of the cartilaginous model. As the chondrocytes reach the

maximum size, they secrete type X collagen and noncollagenous proteins.

Subsequently, there is partial breakdown of proteoglycans, creating a matrix

environment receptive for mineral deposition.

Zone of provisional mineralization: Matrix mineralization begins in the zone of

mineralization by formation of matrix vesicles. These membrane bound vesicles bud

off from the cell and form independent units in the longitudinal septa of the cartilage.

Formation of bone collar:

The capillaries grow into the perichondrium that surrounds midsection of the

model. The cells in the inner layer of the perichondrium differentiate into osteoblasts

in a vascular environment and form a thin collar of bone matrix around the midregion

of the model. At this stage, perichondrium is reffered to as periosteum as the

61

differentiation of cells from the inner layer of the perichondrium is giving rise to bone.

Vascularisation of the middle of the cartilage occurs, and chondroclasts resorb most of

the mineralized cartilage matrix. The bone collar holds together the shaft, which has

been weakened by disintegration of the cartilage. Hence, more space is created for

vascular ingrowth.

Formation of periosteal bud:

Periosteal capillaries accompanied by osteogenic cells invade the calcified

cartilage in the middle of the model and supply its interior.The osteogenic cells and

the vessels comprise a structure called the periosteal bud. The periosteal capillaries

grow into the cartilage model and initiate the development of a primary ossification

center. Osteogenic cells in the periosteakl bud give rise to osteoblasts that deposit

bone matrix on the residual calcified cartlage. This results in the formation of

cancellous bone that has remanents of calcified cartilage. This is the mixed spicule.

Formation of medullary cavity:

As the primary ossifications centre enlarges, spreading proximally and distally,

osteoclasts break down the newly formed spongy bone and open up a medullary

cavity in the center of the shaft. Hematopoietic stem cells enter the medullary cavity

giving rise to myeloid tissue.

The two ends of the developing bone are at this stage still composed entirely of

cartilage. The midsection of the bone becomes the diaphysis and the cartilaginous

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ends of bone become the epiphysis.Hence, the primary center of ossification is the

diaphyseal center of ossification.

Formation of secondary ossification center:

A birth, most of the long bones have a bony diaphysis surrounding remanents of

spongy bone, widening medullary cavity, and two cartilaginous epiphysis. Shortly

befor or after birth, secondary ossification centers appear in one or both epiphysis.

Initially chondrocytes in the middle of the epiphysys hypertrophy and mature, and the

matrix partitions between their lacunae calcify.

Periosteal buds carry mesenchymal cells and blood vessels and here spongy

bone I sretained and no medullary cavity forms in the epiphysis. The ossification

spreads from secondary center in all directions. Eventually, the cartilage in the middle

of epiphysis gradually gets replaced by cancellous bone. When secondary ossification

is complete hyaline cartilage remains at two places-on the epiphyseal surface as

articular surface and at the junction of the diaphysis and epiphyseal plates.this plate

continues to form new cartilage, which is replaced by bone, a process that increases

the length of the bone. Long bones have one or two secondary ossification centers.

Short bones have one ossification center. The union of primary and secondary center

is called epiphyseal line.

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64

BONE DYNAMICS:13

The dynamics of bone involve three different processes:

� Growth

� Modelling

� Remodelling

BONE GROWTH:1,14,15

Bone growth can be of two tyes:

� Appositional = bone growth on pre-existing bone surface

� Interstitial = bone growth via new cartilage formation within pre-existing

cartilage mass

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Long Bone Growth:

WIDTH --> Appositional (bone)

LENGTH --> Interstitial (cartilage)

During childhood, bones throughout the body grow in thickness by appositional

growth, & long bones lengthen by the addition of bone material the epiphyseal plate.

Bones stop growing in length at about age 25, although they may continue to thicken.

GROWTH IN LENGTH:

The epiphyseal growth plate is made up of three tissue types: the cartilage

component divided into distinct zones, the bony tissue of the metaphysis and the

fibrous tissue that surrounds the growth plate. The cartilage matrix is primarily

composed of collagens and proteoglycans. These macromolecules play a critical role

in the development and maintenance of a variety of functions including tissue

strength, architecture, and cell to cell interactions. Type II collagen is the most

abundant of the collagens in the growth plate, and since it is found almost exclusively

in cartilage it is a specific phenotype marker for chondrocytes. Type II collagen is

composed of three identical chains that are wound into the characteristic triple helix of

the collagen molecule. Type II collagen molecules form banded fibers seen with the

electron microscope and are therefore classified as fiber forming (class I) collagen. In

the developing limb and in models of endochondral ossification, type II collagen

synthesis can be correlated with chondrogenesis.Type II procollagen may be

66

expressed in two forms, IIA or IIB, due to differential splicing of recently transcribed

RNA. In embryonic human vertebral column, type IIB mRNA expression is correlated

with cartilage matrix synthesis, whereas IIA is expressed in pre-chondrocytes, the

cells surrounding the cartilage. Type XI collagen, also a class I collagen, is present in

cartilage matrix and is integrated into the interior of type II collagen fibrils. Its

function is not known. Type IX collagen is also found in cartilage, but is not a fiber

forming collagen since it will not form supramolecular aggregates alone. Type IX is

associated with the exterior of the type II collagen molecules and, since it has a single

glycosaminoglycan side chain, it is also a proteoglycan.

Type X collagen is a short chain, non-fibril forming collagen with a restricted

tissue distribution within the hypertrophic calcifying region of growth plates in fetal

and developing bone, where it makes up 45% of total collagen . It has been proposed

that type X collagen may play a role in regulating mineralisation of cartilage

calcification, however, this remains to be proven.

The other main structural component of cartilage is proteoglycan.

Proteoglycans are proteins with one or more attached glycosaminoglycan side chains,

e.g. chondroitin sulphate, heparan sulphate, dermatan sulphate. These sulphated side

chains occupy approximately two thirds of the C terminus region of the molecule,

while the other third, the carbohydrate-rich portion, binds to hyaluronic acid.. The

main proteoglycan of cartilage is aggrecan, a large proteoglycan composed of

approximately 90% chondroitin sulphate chains. Aggrecan is found as multi-

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molecular aggregates composed of many proteoglycan monomers (up to 100) bound

to hyaluronan. A small link protein helps to stabilize the aggregate. Synthesis of

aggrecan is another specific marker of the chondrocyte phenotype.

Another important matrix component is the enzyme alkaline phosphatase

(ALP). ALP is abundant in matrix vesicles and on the plasma membrane of the

maturing chondrocytes, and is required in the calcification process although the

precise mechanism of action remains unclear Growth plate chondrocytes are organised

into different zones with each cell population being part of a different stage of

maturation in the endochondral sequence,.

ZONE OF RESTING CARTILAGE;1,16,17

The resting zone lies immediately adjacent to the secondary bony epiphysis.

Various terms have been applied to this zone, including resting zone, zone of small-

size cartilage cells, and germinal zone. They appear to store lipid and other materials

and perhaps are held in reserve for later nutritional requirements. The cells in this

zone are spherical, exist singly or in pairs, are relatively few when compared with the

number of cells in other zones, and are separated from each other by more

extracellular matrix than are cells in any other zone. Electron microscopy reveals

these cells to contain abundant endoplasmic reticulum, a clear indication that they are

actively synthesizing protein. They contain more lipid bodies and vacuoles than do

cells in other zones but contain less glucose-6-phosphate dehydrogenase, lactic

dehydrogenase, malic dehydrogenase, and phosphoglucoisomerase. The zone also

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contains the lowest amount of alkaline and acid phosphatase, total and inorganic

phosphate, calcium, chloride, potassium, and magnesium. The matrix in the reserve

zone contains less lipid, glycosaminoglycan, protein polysaccharide, moisture, and ash

than the matrix in any other zone. It exhibits less incorporation of radiosulfur (35S)

than any other zone and also shows less Iysozyme activity than the other zones. It

contains the highest content of hydroxyproline of any zone in the plate.Collagen

fibrils in the matrix exhibit random distribution and orientation. Matrix vesicles are

also seen in the matrix, but they are fewer than in other zones. The matrix shows a

positive histochemical reaction for the presence of a neutral mucopolysaccharide or an

aggregated proteoglycan.

ZONE OF PROLIFERATING CARTILAGE;1,16,17

The spherical, single or paired chondrocytes in the reserve zone give way to

flattened chondrocytes in the proliferative zone. They are aligned in longitudinal

columns with the long axis of the cells perpendicular to the long axis of the bone.

The zone of proliferation contains the highest content of hexosamine,inorganic

pyrophosphate, and sodium, chloride, and potassium. It also has the highest level of

Iysozyme activity.

The chondrocytes in the proliferative zone are, with few exceptions, the only

cells in the cartilage portion of the growth plate that divide. The top cell of each

column is the true "mother" cartilage cell for each column, and it is the beginning or

the top of the proliferating zone that is the true germinal layer of the growth plate.

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Longitudinal growth in the growth plate is equal to the rate of production of new

chondrocytes at the top of the proliferating zone multiplied by the maximum size of

the chondrocytes at the bottom of the hypertrophic zone.

The matrix of the proliferating zone contains collagen fibrils, distributed at

random, and matrix vesicles, confined mostly to the longitudinal septa. The matrix

shows a positive histochemical reaction for a neutral mucopolysaccharide or an

aggregated proteoglycan.

Thus the function of the proliferative zone is twofold: matrix production and

cellular proliferation. The combination of these two functions equals linear or

longitudinal growth. It is a paradox that while this chondrogenesis or cartilage growth

is solely responsible for the increase in linear growth of the long bone, the cartilage

portion of the plate itself does not increase in length. This, of course, is due to the

vascular invasion that occurs from the metaphysis with the resultant removal of

chondrocytes at the bottom of the hypertrophic zone, events that, in the normal growth

plate, exquisitely balance the rate of cartilage production.

ZONE OF HYPERTROPHIC CARTILAGE:1,16,17

The flattened chondrocytes in the proliferative zone become spherical and

greatly enlarged in the hypertrophic zone. These changes in cell morphology are quite

abrupt, and one can usually determine the end of the proliferative zone and the

beginning of the hypertrophic zone within an accuracy of one to two cells. By the time

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the average chondrocyte reaches the bottom of the hypertrophic zone, it has enlarged

some five times over what its size was in the proliferative zone.

On light microscopy, the chondrocytes in the hypertrophic zone appear

vacuolated. Toward the bottom of the zone, such vacuolation becomes extensive,

nuclear fragmentation occurs, and the cells appear nonviable. At the very bottom of

each cell column the lacunae appear empty and are devoid of any cellular content.

On electron microscopy the chondrocytes in the top half of the hypertrophic

zone appear normal and contain the full complement of cytoplasmic components.

However, in the bottom half of the zone, the cytoplasm contains holes that occupy

over 58% of the total cytoplasmic column. Obviously, it is holes and not vacuoles that

account for the "vacuolation" seen on light microscopy. Electron microscopy also

shows that glycogen is abundant in the chondrocytes in the top half of the zone,

diminishes rapidly in the middle of the zone, and disappears completely from the cells

in the bottom portion of the zone. The last cell at the base of each cell column is

clearly nonviable and shows extensive fragmentation of the cell membrane and the

nuclear envelope with loss of all cytoplasmic components except a few mitochondria

and scattered remnants of endoplasmic reticulum. Clearly, the ultimate fate of the

hypertrophic chondrocyte is death.

ZONE OF CALCIFIED MATRIX :1,16,17

Matrix calcification occurs in longitudinal septae between the columns of

chondrocytes, and this calcified matrix becomes the scaffolding for bone deposition in

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the metaphysis. The hypertrophic zone contains the highest levels of alkaline

phosphatase. The traditional view was that these cells were metabolically very

inactive, and that increasing vacuolation indicated death by hypoxia. However, these

cells are clearly actively involved in the synthesis of type X and type II collagen.

Improvements in techniques of growth plate fixation that retain chondrocyte

morphology have led to the proposal that a terminal chondrocyte spends most of its

life as a fully viable cell indistinguishable from hypertrophic chondrocytes positioned

further proximally in the growth plate. The cells then die by apoptosis, a distinct

biological form of cell death, lasting approximately 18% of a terminal chondrocyte's

life span .Apoptosis may be triggered by the metaphyseal vasculature beyond the last

intact cartilage septum

ZONE OF JUNCTION OF GROWTH PLATE WITH METAPHYSIS:1,17

The region where the transition from cartilage to bone occurs. Chondrocyte

lysis is evident from empty lacunae invaded by vascular endothelial loops. The

vascular region of calcified cartilage is the primary spongiosum, upon which

osteoblasts lay down unmineralised bone, the osteoid. Metaphyseal bone formation is

associated with type I procollagen mRNA expression in the empty lacunae, osteoid,

bone and perichondrium .Type I collagen, a marker of the osteoblast phenotype, is

immunolocalised to the same areas, while types II and X collagen have restricted

immunolocalisation to calcified cartilage trabecular remnants within spongy

bone.Newly formed woven metaphyseal bone is gradually replaced by lamellar bone

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following osteoclastic degradation of bony matrix and chondroclastic removal of

remaining cartilage trabeculae. At the same time external reshaping of the bone is

brought about by surface osteoclastic bone resorption and appositional bone formation

by periosteally derived osteoblasts.

FIBROUS AND FIBROCARTILAGINOUS COMPONENTS1

Encircling the typical long-bone growth plate at its periphery are a wedge-

shaped groove of cells, termed the ossification groove, and a ring or band of fibrous

tissue and bone, termed the perichondrial ring. Ranvier, the first to describe these

structures, concentrated his study on the cells in the groove.

The ossification groove contains round to oval cells that, on light microscopy,

seem to flow from the groove into the cartilage at the level of the beginning of the

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reserve zone. .The function of the groove of Ranvier is to contribute chondrocytes to

the growth plate for the growth in diameter, or latitudinal growth, of the plate.

Three groups of cells were identified in the ossification groove: a group of

densely packed cells that seemed to be progenitor cells for osteoblasts that form the

bony band in the perichondrial ring; a group of undifferentiated cells and fibroblasts

that contribute to appositional chondrogenesis and, hence, growth in width of the

growth plate; and fibroblasts amid sheets of collagen that cover the groove and firmly

anchor it to the perichondrium of the hyaline cartilage above the growth plate.

The perichondrial ring is a dense fibrous band that encircles the growth plate at

the bone-cartilage junction and in which collagen fibers run vertically, obliquely, and

circumferentially. It is continuous at one end with the group of fibroblasts and

collagen fibers in the ossification groove and at the other end with the periosteum and

subperiosteal bone of the metaphysis

Hence the function of the ossification groove is to provide chondrocytes for the

growth in width of the growth plate, and the function of the perichondrial ring is to act

as a limiting membrane that provides mechanical support to the growth plate.

The activity of the epiphyseal plate is the only way that the diaphysis can

increase in length. As a bone grows, chondrocytes proliferate on the epiphyseal side of

the plate. New chondrocytes cover older ones, which are then destroyed by the

process of calcification. Thus the cartilage Is thus replaced by bone on the diaphyseal

side of the plate .In this way thickness remains constant but increases in length.

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Between the ages of 18 & 25, the epiphyseal plates close, they stop dividing &

bone replaces cartilage. It fades leaving a bony feature called the epiphyseal line.

GROWTH IN THICKNESS:1,4

Bone can grow in thickness by appositional growth. At the bone surface,

periosteal cells differentiate into osteoblasts, which stores collagen fibers & other

organic molecules that form bone matrix.The osteoblasts become surrounded by

matrix & develop into osteocytes. This process form bone ridges on either side of a

periosteal blood vessel. The ridges slowly enlarge & create a groove on the periosteal

blood vessel.Eventually, the ridges fold together & fuse, & groove becomes a tunnel

that encloses the blood vessel. The former periosteum now becomes the endosteum

that encloses the blood vessel.Bone deposition by osteoblasts from the endosteum

forms, new concentric lamellae. The formation of addition concentric lamellae

proceeds inward towards the periosteal blood vessel. In this way, the tunnel fills in, &

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a new osteon is created.As a osteon is forming, osteoblasts under the periosteum

deposit new outer circumferential lamellae, further increasing the thickness of the

bone .as the additional periosteal blood vessel becomes enclosed further growth

process continues. As a new bone tissue is being deposited on the outer surface of

bone, the bone tissue lining the medullary cavity is destroyed by the osteoclasts in the

endosteum .In this way the medullary cavity enlarges as the bone increases in

diameter

FACTORS AFFECTING BONE GROWTH:

The regulation of postnatal somatic growth is complex. Genetic, nutritional

factors and hormones exert regulatory functions

Calcium-Regulating Hormones

Three calcium-regulating hormones play an important role in producing healthy

bone: 1) parathyroid hormone or PTH, which maintains the level of calcium and

stimulates both resorption and formation of bone; 2) calcitriol, the hormone derived

from vitamin D, which stimulates the intestines to absorb enough calcium and

phosphorus and also affects bone directly; and 3) calcitonin, which inhibits bone

breakdown and mayprotect against excessively high levels of calcium in the blood.

Parathyroid hormone or PTH

PTH is produced by four small glands adjacent to the thyroid gland. These

glands precisely control the level of calcium in the blood. They are sensitive to small

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changes in calcium concentration so that when calcium concentration decreases even

slightly the secretion of PTH increases. PTH acts on the kidney to conserve calcium

and to stimulate calcitriol production, which increases intestinal absorption of

calcium. PTH also acts on the bone to increase movement of calcium from bone to

blood. Excessive production of PTH, usually due to a small tumor of the parathyroid

glands, is called hyperparathyroidism and can lead to bone loss. PTH stimulates bone

formation as well as resorption. In recent years a second hormone related to PTH was

identified called parathyroid hormone related protein (PTHrP). This hormone

normally regulates cartilage and bone development in the fetus, but it can be over-

produced by individuals who have certain types of cancer. PTHrP then acts like PTH,

causing excessive bone breakdown and abnormally high blood calcium levels, called

hypercalcemia of malignancy.

Calcitriol

Calcitriol is the hormone produced from vitamin D. Calcitriol, also called 1,25

dihydroxy vitamin D, is formed from vitamin D by enzymes in the liver and kidney.

Calcitriol acts on many different tissues, but its most important action is to increase

intestinal absorption of calcium and phosphorus, thus supplying minerals for the

skeleton. Vitamin D should not technically be called a vitamin, since it is not an

essential food element and can be made in the skin through the action of ultra violet

light from the sun on cholesterol. Vitamin D deficiency leads to a disease of defective

mineralization, called rickets in children and osteomalacia in adults. These conditions

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can result in bone pain, bowing and deformities of the legs, and fractures. Treatment

with vitamin D can restore calcium supplies and reduce bone loss.

Calcitonin

Calcitonin is a third calcium-regulating hormone produced by cells of the

thyroid gland, although by different cells than those that produce thyroid hormones.

Calcitonin can block bone breakdown by inactivating osteoclasts, but this effect may

be relatively transient in adult humans. Calcitonin may be more important for

maintaining bone development and normal blood calcium levels in early life. Excesses

or deficiencies of calcitonin in adults do not cause problems in maintaining blood

calcium concentration or the strength of the bone. However, calcitonin can be used as

a drug for treating bone disease.

Sex Hormones

Along with calcium-regulating hormones, sex hormones are also extremely

important in regulating the growth of the skeleton and maintaining the mass and

strength of bone. The female hormone estrogen and the male hormone testosterone

both have effects on bone in men and women. The estrogen produced in children and

early in puberty can increase bone growth. The high concentration that occurs at the

end of puberty has a special effect—that is, to stop further growth in height by closing

the cartilage plates at the ends of long bone that previously had allowed the bones to

grow in length. Estrogen acts on both osteoclasts and osteoblasts to inhibit bone

breakdown at all stages in life. Estrogen may also stimulate bone formation. The

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marked decrease in estrogen at menopause is associated with rapid bone loss.

Testosterone is important for skeletal growth both because of its direct effects on bone

and its ability to stimulate muscle growth, which puts greater stress on the bone and

thus increases bone formation. Testosterone is also a source of estrogen in the body; it

is converted into estrogen in fat cells. This estrogen is important for the bones of men

as well as women. In fact, older men have higher levels of circulating estrogen than do

postmenopausal women.

Growth hormone

Is an important regulator of skeletal growth. It acts by stimulating the

production of another hormone called insulin-like growth factor-1 (IGF-1), which is

produced in large amounts in the liver and released into circulation. IGF-1 is also

produced locally in other tissues, particularly in bone, also under the control of growth

hormone. The growth hormone may also directly affect the bone—that is, not through

IGF-1. Growth hormone is essential for growth and it accelerates skeletal growth at

puberty. Decreased production of growth hormone and IGF- 1 with age may be

responsible for the inability of older individuals to form bone rapidly or to replace

bone lost by resorption. The growth hormone/IGF-1 system stimulates both the bone-

resorbing and bone-forming cells, but the dominant effect is on bone formation, thus

resulting in an increase in bone mass.

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Thyroid hormones

Increase the energy production of all body cells, including bone cells. They

increase the rates of both bone formation and resorption. Deficiency of thyroid

hormone can impair growth in children, while excessive amounts of thyroid hormone

can cause too much bone breakdown and weaken the skeleton . The pituitary hormone

that controls the thyroid gland, thyrotropin or TSH, may also have direct effects on

bone.

Cortisol

Cortisol, the major hormone of the adrenal gland, is a critical regulator of

metabolism and is important to the body’s ability to respond to stress and injury. It has

complex effects on the skeleton .Small amounts are necessary for normal bone

development, but large amounts block bone growth. They can cause bone loss due

both to decreased bone formation and to increased bone breakdown, both of which

lead to a high risk of fracture.There are other circulating hormones that affect the

skeleton as well. Insulin is important for bone growth, and the response to other

factors that stimulate bone growth is impaired in individuals with insulin deficiency

.A recently discovered hormone from fat cells, leptin, has also been shown to have

effects on bone.

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Local factors in regulation of growth1,18

Local factors are necessary for intercellular communication and include

cytokines and growth factors. A cytokine can be defined as a soluble low molecular

weight cell product that affects the activity of other local cells in a paracrine manner;

they may act on their cell of origin by an autocrine mechanism, or via release into the

circulation may affect cells at a distant site, behaving as classic endocrine agents. In

hard tissues another mechanism of control exists, where locally produced growth

factors, or those in the circulation, are incorporated into mineralized matrix and are

released during matrix dissolution by osteoclasts or chondroclasts. The term cytokine

is now generally used to include molecules that were originally defined as growth

factors, e.g., the insulin-like growth factors (IGFs), the transforming growth factors

(TGF alpha and TGF beta), platelet-derived growth factor (PDGF), and fibroblast

growth factors (FGFs)

Local mediators in skeletal tissues

Factor Expression of mRNA or protein in bone

and cartilage cells

Growth factors

Insulin-like growth factors (IGF-I & II) Osteoblasts (OB) & chondrocytes (C)

Transforming growth factors (TGFbs 1-3) OB & C

Fibroblast growth factors acidic and basic

(aFGF & bFGF)

OB & C

Platelet derived growth factor (PDGF) OB

Bone morphogenetic proteins BMPs 1-7 OB

Interleukins (IL)

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IL-1 b OB & C

IL-3 (Multi CSF) OB

IL-4

IL-6 OB & C

IL-8 OB & C

Tumour necrosis factors

TNFa OB

TNFb

Interferons

IFNg

Colony stimulating factors

GM-CSF OB & C

M-CSF OB & C

Others

Prostaglandins OB & C

PTH-RP OB & C

CGRP

Insulin-like growth factors (IGF-I & IGF-II)

Of the growth factors, those with the most potent effects on growing skeletal

tissue are the IGFs, previously known as somatomedins. IGFs are synthesized in the

liver and circulate bound to carrier proteins .The major factors regulating IGF

concentrations in serum are growth hormone, nutritional intake and thyroid hormones,

the latter being necessary for growth hormone secretion. The traditional view was that

growth hormone acted indirectly on the growth plate via IGF-I, a potent mitogen for

growth plate chondrocytes. However, there is increasing evidence that growth

hormone has direct effects on the growth plate .In addition to having effects on the

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growth plate chondrocytes, locally synthesized and circulating IGFs retained in bone

matrix are important in the regulation of bone remodelling. Osteoblasts synthesize

IGFs; with human bone cells producing more IGF-II relative to IGF-I, and in human

bone matrix IGF-II is present in 10-15-fold greater concentrations than IGF-I. Both

IGF-I and IGF-II stimulate osteoblast and chondrocyte proliferation, induce

differentiation in osteoblasts and maintain the chondrocyte phenotype .Some of the

anabolic effects of PTH and oestrogen on bone may be effected by alterations in the

local synthesis of IGFs. Local concentrations of IGFs will also be regulated by

osteoblastic synthesis of binding proteins (IGFBPs), IGFBPs synthesis itself being

altered by growth hormone and oestradiol

Transforming growth factors (TGFs)

TGFs have diverse effects on growth and differentiation in normal and

neoplastic cell types. Most important in skeletal tissue are members of the TGF-β

gene family which includes the activins, inhibins, mullerian inhibiting substance, bone

morphogenetic proteins (BMPs), the drosophila decapentaplegic gene complex

product (dpp), and products of the mammalian Vgr gene. At least three isoforms of

TGF- β have been isolated in mammalian tissues (TGF-β). There is considerable

sequence identity and shared biological effects between these isoforms. TGF- β is

produced by several cell types, with bone matrix one of the most abundant sources of

both TGF- β1 and TGF- β2. Regulation of TGF- β, like that of many cytokines, occurs

not only at a transcriptional or translational level; it is secreted and stored in a latent

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form that requires activation to become functional. Considerable evidence exists

supporting a role for TGF- β in morphogenesis, in the regulation of endochondral

ossification and in bone remodelling .High levels of TGF- β messenger RNA are

expressed in the growth plate of fetal human long bones. . TGF-β regulates the

synthesis of collagen by growth plate chondrocytes; increasing the synthesis of type I

relative to type II collagen,it may therefore control mineralisation by regulation of

hypertrophic chondrocyte differentiation. The effects of TGF- β on endochondral

ossification may be to stimulate growth in the undifferentiated cell, with different

effects on the terminally differentiated chondrocyte. TGF- β has a role to play in

regulation of bone remodelling, having effects on the proliferation and differentiation

of osteoblastic cells. TGF- β inhibits interleukin-1 and 1,25(OH)2D3 induced bone

resorption and the formation of multinucleated osteoclast-like cells in a human

marrow culture system. These diverse effects of TGF- β on bone cells have led to the

hypothesis that TGF- β may have a role in the coupling of bone formation to bone

resorption. One proposed mechanism is that during bone resorption latent TBF- β is

released from bone matrix and activated (possibly by the low pH and/or proteases), to

act locally on bone cells.

Bone morphogenetic proteins (BMPs)

This large family of proteins has aroused considerable interest in the bone cell

field, since the discovery that the implantation of demineralised matrix at

subcutaneous or intramuscular sites leads to bone formation. The factors in bone

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matrix responsible for this induction of bone formation were named the bone

morphogenetic proteins (BMPs). There are now known to be 7 members of this family

(BMPs 1-7); all except BMP1 are members of the TGF- β family. BMP1 has been

classed as a novel regulatory protein. Chromosome mapping has shown that the

BMP2A and BMP3 genes map to conserved regions between mouse and human,

while the BMP1 gene does not.BMPs are the only molecules so far discovered capable

of independently inducing endochondral ossification in vivo. TGF- β1 and TGF- β2

enhance the osteoinductive properties of BMPs; however, injection of TGF- βs on

their own leads to extensive fibrous tissue formation only Recombinant forms of

BMP2 and BMP4 induce ectopic bone formation, and BMP2 will heal cortical bone

defects by an endochondral process BMP2 stimulates the growth and differentiation

of growth plate chondrocytes in vitro, and results in the development of the osteoblast

phenotype in a rat pluripotential cell line Osteoblasts have been shown to have high

affinity binding proteins for BMP on the cell surface). Indirect lines of evidence

demonstrate that BMPs have a critical role in bone development. Firstly, the protein

encoded by the decapentaplegic locus (dpp) in Drosophila is a member of the TGF- β

family member with 75% sequence homology to BMP2, suggesting a common

ancestral gene. Developmental anomalies produced by mutations of the dpp gene are

similar to patterns of disease expression in fibrodysplasia ossificans progressive, a

developmental disorder characterised by deformations of the hands and feet and

heterotopic chondrogenesis. In addition, the chromosomal locations of the BMP genes

overlap with the loci for several disorders of cartilage and bone formation. More direct

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evidence is provided by a recent study which demonstrated that BMP2, together with

fibroblast growth factor-4, is important in regulating limb growth in the mouse

embryo.

Fibroblast growth factor (FGF)

FGF is a heparin binding peptide that exists in two forms, acidic and basic, with

55% sequence homology between the two.FGFs are potent mitogens for osteoblasts,

chondrocytes and endothelial cells, and stimulate proliferation of mesenchymal cells

in the developing limb that leads to limb outgrowth. FGF receptors are expressed in

limb mesenchyme as is mRNA for FGF-4.There is increasing evidence that basic FGF

(bFGF) is also important at later stages of bone growth, bFGF interacts with two

classes of binding sites on bovine growth plate chondrocytes: a high-affinity bFGF

receptor and a low-affinity heparin-like binding site FGFs are not secreted proteins

since a leader sequence is lacking, so they may only be released from their cell of

origin after membrane disruption. In this way FGF released from the degenerating

chondrocyte may act as a mitogen for metaphyseal vessels (since FGF is a potent

angiogenic factor) and cells of the osteoclast lineage.). During bone remodelling, FGF

synthesised by osteoblasts and stored in bone matrix may be released following

osteoclastic bone resorption. Activated FGF may then be important in stimulating

bone formation by increasing the number of osteoblastic precursor cells. bFGF has no

effect on osteoblast differentiation.

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Platelet-derived growth factor (PDGF)

PDGF, a dimeric 30kDa peptide, was initially isolated from human platelets and

is known to exist in both homo- and heterodimeric forms . PDGF has been found in

bone matrix extracts and is secreted by human osteosarcoma cells and untransformed

rat osteoblasts. However, its synthesis by normal human osteoblasts or chondrocytes

has not been reported. The PDGF located in bone matrix may be sequestered from the

systemic circulation. PDGF is mitogenic for osteoblasts, fibroblasts and periosteal

cells, although it is possible some of these effects may be mediated by IGF-I, since

PDGF increases IGF-I synthesis in mesenchymal cells.PDGF may play a role in bone

development and growth. Both homodimeric forms of PDGF bind and increase DNA

synthesis in growth plate chondrocytes, having an additive effect with IGF-1. In the

stunted child, where disease may be a significant contributing factor, cytokine effects

on the bone growth plate may be of particular importance. For example, in post-

menopausal osteoporosis, cytokine production by circulating cells may be altered, and

this mechanism is believed to be important in the uncoupling of bone formation from

resorption characteristic of this disease.

Tumor necrosis factors (TNF)

Alpha and beta forms of TNF exist and, although there is only 28% sequence

identity, they share the same receptors, and their range of biological activities

overlaps, with many similar functions to IL-1. A second form of the TNF receptor

exists that binds to circulating TNF, and is shed after cleavage of the extracellular

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TNF cell surface receptor. TNFa is produced by most cell types, including osteoblasts,

in response to a range of non-specific signals .TNFb is only induced by specific

antigens and has only been shown to be synthesized by activated T cells. In skeletal

tissues, TNFs stimulate bone and cartilage resorption and cell division . Since TNFa

induces neo-vascularisation in vivo, it may work with other local factors, including

FGF and TGFa to stimulate vascular invasion of the growth plate.

Interleukin 1 (IL-1)

IL-1 exists in two 17 kDa forms, alpha and beta, that have a similar spectrum of

biological activity but little sequence homology. IL-1 was originally isolated from

cells of the monocyte series but has subsequently been shown to be expressed by most

cell types, including human osteoblasts. The range of biological effects of IL-1 is

extensive, with activities previously attributed to leucocyte endogenous mediator

(LEM), mononuclear cell factor (MCF), osteoclast activating factor (OAF) and

catabolin now known to be those of IL-1 .The first cell surface receptor to be

identified for IL-1 was found to be a member of the immunoglobulin superfamily.

There is evidence that there may be a soluble form of IL-1 receptor. The most potent

inducer of IL-1 synthesis is endotoxin, but it is also induced by a number of other

cytokines and in an autocrine manner by IL-1. IL-1b stimulates bone, and increases

the proliferation of osteoblast cells and the production of other cytokines by

osteoblasts. IL-1 mRNA has been localised in the calcified cartilage zone of growth

plate, and together with BMP enhances ectopic bone formation , and cartilage

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formation .Since IL-1 suppresses cell proliferation and proteoglycan synthesis in

chondrocytes, and decreases types II and IX collagen synthesis, it may suppress the

cartilage phenotype in the hypertrophic zone that precedes the onset of mineralisation

. Local synthesis of IL-1 and TNFa may also be important in the remodelling of

matrix in the metaphysis via stimulation of the synthesis of proteinase enzymes by

bone and cartilage cells.

Interleukin 6 (IL-6)

IL-6 is a 23-28 kDa protein produced by many cell types including fibroblasts,

bone and cartilage cells as well as monocytes. Synthesis in osteoblastic cells is

stimulated by a range of factors including IL-1 and PTH. The considerable overlap in

the biological activities of IL-6 and IL-1 has led to the suggestion that IL-6 mediates

some of the actions of IL-1.. Direct effects have been demonstrated in osteosarcoma

cells, although it has not been shown to affect cell growth or differentiation in primary

cultures of human osteoblast.IL-6 may mediate some of the effects of oestrogen on

bone.

Interleukin 8 (IL-8)

IL-8, or neutrophil activating factor (NAF), is an inflammatory mediator

produced by a wide variety of cell types. IL-8 is a potent attractant for neutrophils and

may have an important role to play in diseases such as rheumatoid arthritis and

osteoarthritis. Other members of the IL-8 supergene family may also have effects

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within connective tissues, including the monocyte chemotactic and activating factor

(MCAF), macrophage inflammatory protein (MIP-2) and platelet factor-4.

Interferons (IFN)

This is a family of molecules that are potent inhibitors of malignant and normal

cell proliferation. There are three types, alpha, beta and gamma, and of these only

IFNg has significant osteotropic effects. Its principal role is believed to be that of an

antagonist to IL-1 and TNFa induced bone resorption. IFNg inhibits bone resorption,

in part, by inhibiting osteoclast formation from precursors.

Colony stimulating factors (CSFs)

These molecules are of importance in hematopoietic differentiation and those

studied most in relation to bone are the monocyte./macrophage CSF (M-CSF),

granulocyte-macrophage (GM-CSF) and multi-CSF (IL-3), because of the assumption

that osteoclasts and monocytes share a common ancestor. Both GM-CSF and M-CSF

are produced by marrow stromal cells, and M-CSF is known to be required for normal

osteoclast development

Parathyroid hormone related peptide (PTHrP)

PTHrP is a peptide closely related to PTH that is produced by normal tissues,

with similar effects to PTH on bone. It has been established as having an important

role in regulating the hypercalcaemia that is associated with some malignancies.

PTHrP has also been identified as a fetal hormone which may regulate placental

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calcium (Ca2+

) flux. This peptide may also have an important role in skeletal

development, having been localised in embryonic bone.

Calcitonin gene related peptide (CGRP)

This peptide is a separate product of the calcitonin gene. There is increasing

evidence to suggest it is important in the local regulation of skeletal tissues. CGRP

inhibits bone resorption, it has effects on osteoblast cells, and may regulate cytokine

synthesis by osteoblast precursors. Its location in nerve cells is evidence for a potential

role as a neurogenic modulator of bone cells

THEORIES OF BONE GROWTH14,15

FUNCTIONAL MATRIX THEORY

Moss formulated the functional matrix theory and stated as follows.

“No direct genetic influence on the size, shape or position or skeletal tissues,

only the initiation of ossification.

According to Moss, head is designed to carry out functions like neural

integration, respiration, digestion, hearing, olfaction and speech each accomplished by

certain (tissues and spaces) in head. These tissues and spaces are called functional

cranial component. Thus the component handling speech consist of lips, teeth, tongue,

oral cavity, nasal cavity etc. This is further divided into functional matrix consisting of

soft tissue and space that performs a functions and a skeletal unit that protect/support

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its functional matrix.Skeletal unit is subordinate to and supportive of functional

matrix; the bone tissue enables the matrix the function. As a consequence cells of

bone need not have genetic information for morphologic orientation; the functional

matrix will provide the direction.

To understand how the matrix influence the size, shape of bone it is better to

consider in term of two types of functional matrix and two types of skeletal units.The

first type of functional matrix is named periosteal. This relates the matrix that

influence the bone directly through periosteum (e.g.. Muscles) The periosteal matrix

affects a micro skeletal unit. A tooth is responsible for the alveolar bone that supports

it; extracts the tooth (periosteal functional matrix) and the micro skeletal unit (the

immediate alveolar process) disappears The second type is termed capsular the masses

and space surrounded by capsular are included in this. For eg. : neural mass is

contained within a capsule of scalp, duramater (etc.) These capsule tend to influence

macro skeletal units which means portions of several bones are simultaneously

affected. eg inner surface of calvarium.The neural mass with in its capsule elicits a

reaction on surfaces of the calvarium that transcends a localized area. As a result,

apposition on the occipital, parietal temporalis and frontal occurs as if all of them

were but one bone. This sharing of reaction by several adjacent bones constitute a

macroskeletal unit.

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VAN LIMBORG’S VIEW OF CRANIOFACIAL GROWTH

According to van Limborg embryologic origin of components of skull

determines the kind of growth that occurs there.The cartilaginous base, the nasal

capsule and otic capsules are sites of endochondral ossification and become known as

chondrocranium. All the bones rising from these cartilage precursors have for varying

degrees of time, the capability for interstitial expansion while they are growing. Direct

deposition of bone, intramembranous ossification forms the calvarium, middle face

and mandible an aggregation called the desmocranium.

SICHER

Sicher stated that the destiny of skull tissue is controlled largely by its own

intrinsic genetic information.In Sichers view bone forming elements (Cartilage,

sutures and periosteum) are growth centers. For eg. sutures attaching maxillary

complex to cranium can both drive the mid face down by cellular proliferation and

determine the extent of activity through their genetic composition. This theory fails

because the independence of the skull growth cannot be consistently demonstrated.

Studies have demonstrated that orbits exist only to house the eye. Manipulation of

primordia of the eye in the embryo can create an animal with one, two or even three

orbits. Postnatally eye continues to affect the surrounding bone. If the eye is

enucleated and not replaced by a prosthesis the orbit will cease to expand.This and

other data are not consistent with an all intrinsic genetic theory

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SCOTT

His theory depicts cartilage and periosteum as growth centers and classified

sutures as passive and secondary. He also implicates that periosteal tissue is

dominated by intrinsic genetic factors.Scott correlated sutural growth with

synchondrosis activity and with other tissue growth such as brain. Some local factors

could also possibly modify the process. Van Limborg feels that periosteum should fall

into the same category as sutures, since there can’t that much difference in their

cellular profile

MODELING AND RE MODELING 1,4

The process of building the skeleton and continuously reshaping it to respond to

internal and external signals is carried out by specialized cells that can be activated to

form or break down bone. Both modeling and remodeling involve the cells that form

bone called osteoblasts and the cells that break down bone, called osteoclasts In

remodeling there is an important local interaction between osteoblasts or their

precursors (the cells that will develop into osteoblasts by acquiring more specialized

functions—a process called differentiation) and osteoclasts or their precursors. Since

remodeling is the main way that bone changes in adults and abnormalities in

remodeling are the primary cause of bone disease, it is critically important to

understand this process. Osteoblasts are derived from precursor cells that can also be

stimulated to become muscle, fat or cartilage; however, under the right conditions

these cells change (or differentiate) to form new bone, producing the collagen that

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forms the scaffolding or bone matrix. This calcium- and phosphate-rich mineral is

added to the matrix to form the hard, yet resilient, tissue that is healthy bone.

Osteoblasts lay down bone in orderly layers that add strength to the matrix. Some of

the osteoblasts are buried in the matrix as it is being produced and these are now

called osteocytes. Others remain as thin cells that cover the surface and are called

lining cells. Osteocytes are the most numerous cells in bone and are extensively

connected to each other and to the surface of osteoblasts by a network of small thin

extensions. This network is critical for the ability of bone to respond to mechanical

forces and injury.

When the skeleton is subjected to impact there is fluid movement around the

osteocytes and the long-cell extensions that provides signals to the bone cells on the

surface to alter their activity, either in terms of changes in bone resorption or

formation. Failure of the osteoblasts to make a normal matrix occurs in a congenital

disorder of the collagen molecule called osteogenesis imperfecta. Inadequate bone

matrix formation also occurs in osteoporosis, particularly in the form of osteoporosis

produced by an excess of the adrenal hormones called glucocorticoid- induced

osteoporosis. This form of osteoporosis differs from primary osteoporosis and most

other forms of secondary osteoporosis because with glucocorticoid-induced

osteoporosis inhibition of bone formation is the dominant mechanism for weakening

of the skeleton. The osteoclasts remove bone by dissolving the mineral and breaking

down the matrix in a process that is called bone resorption. The osteoclasts come from

the same precursor cells in the bone marrow that produce white blood cells. These

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precursor cells can also circulate in the blood and be available at different sites in need

of bone breakdown. Osteoclasts are formed by fusion of small precursor cells into

large, highly active cells with many nuclei. These large cells can fasten onto the bone,

seal off an area on the surface, and develop a region of intense activity in which the

cell surface is highly irregular, called a ruffled border. This ruffled border contains

transport molecules that transfer hydrogen ions from the cells to the bone surface

where they can dissolve the mineral. In addition, packets of enzymes are secreted

from the ruffled border that can break down the matrix. Excessive bone breakdown by

osteoclasts is an important cause of bone fragility not only in osteoporosis, but also in

other bone diseases such as hyperparathyroidism, Paget’s disease, and fibrous

dysplasia Inhibitors of osteoclastic bone breakdown have been developed to treat

these disorders

Removal and replacement of bone in the remodeling cycle occurs in a carefully

orchestrated sequence that involves communication between cells of the osteoblast

and osteoclast lineages. It is controlled by local and systemic factors that regulate

bone remodeling to fulfill both its structural and metabolic functions. The activation

of this process involves an interaction between cells of the osteoblastic lineage and the

precursors that will become osteoclasts. What stops this process is not known, but the

osteoclasts machinery clearly slows down and the osteoclasts die by a process that is

called programmed cell death. Thus the amount of bone removed can be controlled by

altering the rate of production of new osteoclasts, blocking their activity, or altering

their life span. Most current treatments for osteoporosis work by slowing down

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osteoclastic bone breakdown through use of antiresorptive agents. The activation and

resorption phases are followed by a brief reversal phase. During the reversal phase the

resorbed surface is prepared for the subsequent formation phase, in part by producing

a thin layer of protein, rich in sugars, which is called the cement line and helps form a

strong bond between the old bone and the newly formed bone. These three phases are

relatively rapid, probably lasting only 2 to 3 weeks in humans. The final phase of bone

formation takes much longer, lasting up to 3 or 4 months. Thus active remodeling at

many sites can weaken the bone for a considerable period of time (even if formation

catches up eventually), as many defects form in the bony structure that have not yet

been filled. Formation is carried out by large active osteoblasts that lay down

successive layers of matrix in an orderly manner that provides added strength. The

addition of minerals to the collagenous matrix completes the process of making strong

bone. Any error in this complex process can lead to bone disease. Since remodeling

serves both the structural and metabolic functions of the skeleton, it can be stimulated

both by the hormones that regulate mineral metabolism and by mechanical loads and

local damage acting through local factors. Repair of local damage is an important

function of remodeling. Over time repeated small stresses on the skeleton can produce

areas of defective bone, termed micro-damage. Replacement ofthat damaged bone by

remodeling restores bone strength. Signals for these responses are probably developed

by the network of osteocytes and osteoblasts, which, through their multiple

connections, can detect changes in the stress placed upon bone and in the health of the

small areas of micro-damage. Factors that affect the formation, activity, and life span

97

of osteoclasts and osteoblasts as they develop from precursor cells can affect the

remodeling cycle. Drugs have been developed that act in these ways, with the goal of

reducing bone loss or increasing bone formation and maintaining skeletal health.

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FUNCTIONS OF BONE;1,2,7,8,9

Support -- Bone provides a framework for the body by supporting soft tissues and

providing points of attachment for the tendons of most of the skeletal muscles.

Protection -- Bones protect many internal organs from injury very well, such as

cranial bones protect brain, vertebrae protect spinal cord and rib cage protects the

heart and lungs.

Movement : Most skeletal muscles attach to bones. When the muscles contract, they

pull on bones to activate lever systems, and movement is produced. Hence known as

musculoskeletal system

Mineral homeostasis: Bone tissue stores a number of minerals, particularly calcium

and phosphorus., which contribute to the strength of bone. Under control of the

endocrine system, bone releases the minerals into the blood or stores the minerals in

bone matrix to maintain critical mineral balances.eg In pregnancy the demands of the

fetus for calcium require a suitable diet and after menopause hormonal control of

calcium levels may be impaired: calcium leaches out leaving brittle osteoporotic

bones.

Blood cell production: Within certain parts of bones aconnective tissue called red

bone marrow produces red blood cells, white blood cells, and platelets bya process

called hemopoiesis.Red bone marrow, one of the two types of bone marrow , consists

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of developing blood cells within a network of reticular fibers. Also present are

adipocytes, macrophages and fibroblasts.

Triglyceride storage: In the newborn, all the bone marrow is red and is involved in

hemopoiesis. However, with increasing age, blood cell production decreases, and most

of the bone marrow changes from red to yellow. Yellow bone marrow consists

primarily of adipocytes abd few scattered blood cells.

CALCIUM AND PHOSPHATE METABOLISM1,2,20,21

Calcium and phosphate metabolism is based on a balance between intestinal

absorption, bone mineralization and demineralization, and urinary filtration and

reabsorption. The major direct regulators of this balance (besides dietary intake) are

parathyroid hormone (PTH) and 1,25 (OH)2-cholecalciferol (aka calcitriol), with a

minor contribution from calcitonin.

CALCIUM METABOLISM

The extracellular fluid (ECF) ionised calcium (1 mmol/L) concentration is 104

times the concentration of the intracellular fluid (ICF) ionised calcium with the latter

varying during normal function by up to 10-fold (e.g. from 10-4

to 10-3

mmol/L).

Nonionised calcium is predominantly found in bone providing an important structural

function to the human body, whereas the ionised calcium is responsible for a variety

of physiol-ogical effects that are characteristic of the cell type (e.g. secretion,

neuromuscular impulse formation, contractile functions, clotting). Normal plasma

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calcium, which consists of protein bound, ionised and complexed calcium, ranges

from 2.10 - 2.55 mmol/L. Normal plasma ionised calcium ranges from 1.15 - 1.30

mmol/L.

DIETARY REQUIREMENTS

Normal adult males require about 0.8 gm/day. Children, pregnant and lactating

mothers requires more. If the dietary calcium intake is low, the absorption of calcium

by the intestine becomes very avid, whereas the dietary calcium intake is high the

intestinal absorption of calcium becomes poor.thus the intestine regulates the calcium

metabolism by adjusting the absorption of calcium. This adjustment of absorption is

made possible by changing the availability of 1,25 dihydroxycholecalciferol (vit D

derivative)

Two main sources of calcium in the diet are (i) milk and (ii) green vegetables.

However, the green vegetables contain ‘phytic acid,’which retards calcium

metabolism but many green vegetables also contain an enzyme phytase which splits

and nullifies phytic acid.

ABSORPTION :CALCIUM BALANCE

Calcium is also secreted in the various gastrointestinal juices.this ia added to the

dietary calcium. A part of this combined (dietary + from GI juice) and the amount of

fecal calcium is excreted via the feces.In dietary intakes of about 0.5 gm/day, the

daily fecal calcium excretion is about 400mgm. The rest of the calcium is absorbed. In

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healthy adults, urinary calcium excretion is about 100 mgm/day, and thus there

is,neither accumulation nor loss of calcium in the body . In such a state, the individual

is, said to be in calcium equilibrium.

In a growing child, there is positive calcium balance, that is, the combined

value of fecal calcium and urinary calcium is less than the dietary calcium intake.

Positive calcium balance occurs when the bones are depositing calcium, that is when

they grow.Conversely, in some diseases of the bone, there is net loss of calcium from

the body and the man is said to be in negative calcium balance.

Two mechanisms are responsible for the calcium absorption(i) Active transport,

where, calcium absorption occurs against a concentration gradient and is dependent on

1,25 dihydroxycholecalciferol. Active transport occurs in the duodenum (ii) Passive

transport occurs lowers down in thesmall intestine and the amount absorbed by this

process is very small (nearly 15%)

All conditions which lower the availability of the 1,25

dihydroxycholecalciferol, reduce calcium absorption.Thus , rickets and advanced

renal danmage are important causes of reduced calcium absorption.

SERUM CALCIUM

Normal serum calcium value is 10mgms/100ml , the range being 9-11

mgms/100ml. About half of this occurs in the ionized form, this is the active form. Of

the rest 50%, most that is about 4mgm/100ml occurs, bound with serum protein.The

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rest occurs in complexex with citrate and phosphate. The combined amount of

complexed (citrate and phosphate) calcium and ionizable calcium is the ultrafiltrable

calcium of serum(diffusible fraction)

Therefore, in conditions of severe protein malnutrition, the protein bound

calcium fraction becomes very low but the ionized fraction remains normal. The total

serum calcium therefore falls.The protein bound fraction of calcium is physiologically

inactive whereas the ionized fraction is active.Therefore, although the total serum

calcium value is very low in severe protein deficiency, the patient does not suffer from

calcium deficiency syndrome.

The protein bound fraction of calcium increases at the cost of ionized fraction,

when the blood pH becomes high.Therefore in alkalosia, the patient may in the ECF

show signs of calcium deficiency (tetany) although the total calcium remains within

normal limits.

FUNCTIONS OF CALCIUM

� Calcium is the chief mineral found in the bone and gives bone its hardness.

Bone is aliving tissue and when there is deficiency of Ca++

in the ECF, calcium

resorption occurs from the bone to maintain the serum calcium level.

Conversly, whwre the serum calcium is high, the Ca++

from the serum is

deposited in the bone Bone thus acts act as a reservoir of calcium.Teeth also

contain a high proportion of calcium.

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� Ca++

is essential for muscular contractions, for activities of the enzymes of

blood coagulation, for activation of digestive enzymes like trypsin and amylase.

� Ca++

acts as the second messenger in some hormonal actions.

� Some neurotransmitters are stored normally within the vesicles of the nerve

terminals.Their discharge require the presence of calcium.

� An optimal Ca++

concentration in ECF is necessary for the correct functioning

of the neuromuscular system.If the Ca++

is low, the neuromuscular system

becomes hyperirritable and as a result, tetany is likely to develop. Conversely,

excess Ca++ i

in the ECF makes the neuromuscular System Sedate(Hypoirritate)

REGULATION OF IONISED CALCIUM IN THE EXTRACELLULAR FLUID

In health, the plasma ionised calcium does not vary by more than 5% and is

maintained largely by the actions of PTH and vitamin D. Calcitonin does not normally

regulate plasma calcium levels and is only secreted when hypercalcaemia exists.

Parathyroid hormone

The main factor controlling the secretion of PTH is plasma ionised calcium,

stimulating the calcium-sensing receptor on the cell membrane of the parathyroid

chief cell to inhibit secretion of PTH with hypercalcaemia and promote secretion of

PTH with hypocalcaemia.

The production of PTH is inhibited by 1,25 (OH)2D3.

The

main sites of PTH action are in bone and kidney. Magnesium is also required for

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normal PTH function as hypomagnesaemia can cause hypocalcaemia due to impaired

synthesis and/or release of PTH and impaired peripheral action of PTH.

Bone: PTH acts on an osteoblast cell membrane receptor, activating adenylate

cyclase and increasing intracellular cAMP, which increases the cell permeab-ility to

calcium. The increase in cytosolic calcium activates a pump that drives calcium from

the bone to the ECF. The pump is enhanced by 1,25 (OH)2D3

An increase in the

activity of the pump is associated with an increase in plasma alkaline phosphatase.

Kidney: PTH acts on a renal tubule membrane receptor, activating adenylate

cyclase and increasing intracellular (and urine) cAMP which, in turn, decreases

proximal renal tubule phosphate (as well as HCO3

-

) reabsorption. PTH also increases

distal nephron calcium reabsorption and stimulates the 1α -hydroxylase conve-rsion of

25 hydroxycholecalciferol (25 (OH)D3) to 1,25 (OH)2D3, thereby acting indirectly on

the gastrointestinal tract by increasing absorption of calcium.

Vitamin D

The major action of 1,25 (OH)2D3 is to increase the ECF calcium and phosphate

by directly increasing calcium and phosphate absorption from the intestine. It does

this by binding to a steroid receptor to alter mRNA transcription, with the mRNAs

produced controlling formation of intracellular calbindin-D proteins (members of the

troponin-C superfamily of calcium binding proteins that also includes calmo-dulin13

).

In the intestine, increases in calbindin-D9k and calbindin-D28k levels are associated

105

with an increase in calcium transport, although the exact mechanism as to how they

facilitate calcium transport is unknown. The calbindin-D proteins can also increase

intestinal absorption of magnesium, zinc, cobalt and strontium. 1,25 (OH)2D3 also

facilitates normal osteoid mineralisation by providing sufficient concentrations of

calcium and phosphate to the calcifying centres. It also demineralises osteoid by

augmenting PTH action (although it also decreases PTH production by altering PTH

gene transcription) and may increase distal nephron calcium reabsorption

by

regulating the distal nephron intracellular calbindin-D28k levels.

AT-10

This synthetic vitamin D (a photochemical derivative of ergosterol) seems to

have a position intermediate between the actions of the natural D vitamins and

parathyroid hormone. Its action to increase calcium absorption from the gut is greater

than the parathyroid hormone effect (which is almost nil), but less than that of D

vitamins. Its action to cause an increased urinary phosphate excretion is considerably

greater than that of D vitamins but it is not quite as effective as parathyroid hormone.

Apart from its great expense and the need for caution in its administration (to prevent

a marked hypercalcaemia), it may be used as a substitute for parathyroid hormone.

Calcitonin

It is secreted by the C cells of the thyroid gland, predominantly when the

plasma calcium is greater than 2.45 mmol/L (i.e. a plasma ionised calcium of 1.15

mmol/L). Therefore its major role appears to be the control of hypercalcaemia.

106

Gastrin, glucagon, and beta-adrenergic agonists also stimulate calcitonin secret-ion,

and may play a part in stimulating calcitonin and reducing the plasma calcium in acute

illness. Calcitonin acts by almost completely inhibiting osteoclastic bone reabsorption,

thereby reducing plasma calcium and phosphate levels without altering plasma

magnesium levels.

ACTION OF OTHER HORMONES

Estrogens either decrease bone reabsorption as well as stimulate new bone

formation,or that they may have a stronger tendency to convert stored protein into

osteoid than androgens.This does not rule out a possible specific direct action of

oestrogens, or for that matter of androgens, on calcium metabolism in its retention by

renal tubular cells.

107

The adrenal steroids (especially the glucocorticoids), with their anti-anabolic

and/or catabolicaction on proteins, tend to decrease bone matrix formation and

perhaps even increase bone resorption, the net result being a strong tendency toward a

negative nitrogen and calcium balance.

Thyroxin, because of rapid protein turnover, produces an increased urinary

calcium and phosphorus excretion and tends to shift the patient into negative balance.

Its action may be as with glucocorticoids, and/or a direct renal tubular effect.

PHOSPHATE METABOLISM

DISTRIBUTION AND THE PHYSIOLOGICAL IMPORTANCE OF

PHOSPHORUS

Phosphorus has many functions and it is widely distributed in the body. Its

concentration in the plasma is highest in the summer and lowest in the winter. It seems

to vary with the concentration of ultraviolet light. This is probably explained by the

fact that ultraviolet rays increase absorption of calcium, allowing more free soluble

phosphorus to be absorbed.The urine and stools per 24 hours each contain about 1

gram of phosphorus. The inorganic form exists as an electrolyte component of

intracellular fluid and urine. It exists as the ions of monobasic and dibasic salts of

orthophosphoric acid. The actual proportions of these two salts vary with the pH of

the internal milieu and play an important role in the body's acid-base balance. The

inorganic form is present in the complex apatite mineral structure of bone. Inorganic

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phosphorus takes part in the formation of complex molecules which constitute the

chief negative ions of the intracellular space. It is vital in the process of

phosphorylation in the various phases of carbohydrate metabolism to form high

energy bonds, and the subsequent release of this energy when these bonds are split.

Organic phosphorus is found in the phospholipids as lecithins and cephalins,

and in nucleic acid, nucleotides, nucleoproteins and in phosphoproteins as casein. It is

present in enzyme systems containing adenosine triphosphate and diphosphate, which

with phosphocreatine pro vide the main sources of fast-acting readily available

energy. The blood inorganic phosphorus exists almost entirely in the plasma fraction;

in infancy the concentration is about 5-6.5 mg. % and it gradually diminishes to the

adult level of 3-4.5 mg. % (1.3-1.6 mM/litre, with a valence of 1; or 2.5-3.1 mEq./l.

expressed as HPO4=/HPO4-, the average valence being 1.8 at a pH of 7.4).

ABSORPTION OF PHOSPHORUS

In contrast to calcium, phosphorus is absorbed later and lower down the

gastrointestinal tract. This is reasonable since the organic phosphate esters must be

split by the action of the pancreatic and intestinal juices. It is probable that the sodium,

potassium, and calciuim acid salts are absorbed as such by a process of diffusion. The

role of phosphorylation in the process of phosphorus absorption has not been fully

elucidated. Vitamin D favours the absorption of calcium and there will be less of this

element left for precipitation with phosphorus. As a result, vitamin D may be said to

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indirectly enhance absorption in the gastrointestinal tract. Phosphorus absorption is

decreased by the ingestion of a diet with a high Ca/P ratio, the relative excess of

calcium tending to precipitate phosphorus. The administration of cations such as

beryllium, strontium, magnesium, barium, thallium and aluminium, causes formation

of insoluble unabsorbable salts with phosphorus. A deficient vitamin D intake will

indirectly lower phosphorus absorption.

EXCRETION OF PHOSPHORUS

Phosphorus is eliminated via the bowels and the kidneys. If the diet is a

balanced one, theurinary phosphorus is about 55% of the total excretion. If calcium

intake drops and if phosphorus intake is constant, the amount of phosphorus absorbed

will rise and the proportion of the intake excreted in the urine will also increase.

Therefore if the phosphorus intake is low, 70% of it is excreted via the urine, and if a

low calcium/high phosphorus diet is given, 80%//o of the phosphorus appears in the

urine. The proportion excreted in the fcces will increase when a diet contains much

calcium or has factors which tend to inhibit phosphorus absorption. The urinary

phosphorus (about 1 gram/24 hours) is derived from the plasma inorganic phosphorus

and from the splitting off of this element from its organic esters by the enzyme

alkaline phosphatase. The renal threshold of this element is 2-3 mg. per 100 c.c. of

plasma, excretion falling to a minimum at concentrations below this level. With renal

functional impairment, as in glomerulonephritis, the urinary phosphorus decreases and

the frecal phosphorus correspondingly rises. The urinary phosphorus increases with

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the administration of parathyroid hormone, 47 thyroxin, glucocorticoids, acids or

acidforming foods, AT-10, and with large doses of vitamin D. Actually, evidence

suggests-'8 that while vitamin D acts directly on the renal tubules to decrease the

reabsorption of phosphorus, it may act indirectly by inhibiting the parathyroid glands,

and cause an increased urinary reabsorption. One effect in a given individual may

predominate over the other, or they may balance each other.

DIETARY SOURCES OF PHOSPHORUS

Phosphorus is present in all natural foods, chiefly in milk and its products,

meat, liver, egg yolk, cereals, nuts and leguminous vegetables. Diets high in meat and

milk will provide an adequate phosphorus intake.

111

BONE MINERALISATION1,8,19

Calcium can be deposited both inside cells & extracellular compartment. Within

the cells, the calcific deposits are initially in the form of amorphous granules in the

mitochondria. These calcium phosphate packets may then be translocated from the

mitochondrial sites to the extracellular mineralization zone.In the extracellular matrix,

precipitation is first in association with matrix vesicles. It appears to begin on the

membrane & extend to extravesicular clusters which then coalesce into continuous

extracellular mass of heavily calcified tissue within & around the collagen fibrils.

Where precipitation first begins of the collagen fibrils, specific sites corresponding to

the hole regions are initially involved.as mineralization progresses, axial periodicity of

the collagen is first accentuated. Mineralization begins several days after matrix

formation. It proceeds rapidly, with about 66% of the final amount of mineral being

deposited within few hours. This primary mineralization process reflects increasing

number of crystals. The subsequent secondary mineralization phase occurs over

longer periods of months. The entire mineralization phase is accompanied by water &

non-collagenous protein loss. Heterogeneous nucleation occurs on the surface of

foreign particles. The best nucleating agents are the crystals of the precipated

compound itself. These include new crystal formation by secondary nucleation. After

nucleation, the crystals grow until a critical size is reached.

112

THEORIES

Basically, therories regarding the phenomenon of mineralization can be

classified along two lines.First is the physiochemical theory, which involves an

interaction of the components of the organic matrix.Second is the cellular theory,

which involves the activites of cells,organelles (mitochondria), and extracellular

substances (matrix vesicles).

The cellular theory of calcification has its origin in the work of Robinson

(1923) who identified enzyme alkaline phosphatase in bone.Robinson hypothesized

that this enzyme could produce elevated localized conenetration of phosphate when

provided with asuitable substrate, i.e.,phosphate esters.the release of inoraganic

phosphate would result in the exceeding of the solubility product of calcium

phosphate salts, resulting in the precipitation of these salts.However such substrates

were never found.this enzyme is invariably present at the sites of calcification,but, it

can also be identified at sites that do not calcify.Alkaline phosphatase may remove

esterified phosphate inhibitors, such as nucleotide phosphates and pyrophosphates,

both known inhibitors of the initiation and crystal growth processes.

A wide variety of intracellular organelles has been shown to accumulate and

concentrate calcium like mitochondria.Based on the numerous observations that

demonstrate the ability of this organelle to sequester calcium, it has been suggested

that mitochondria normally function as a means of initiating crystal formation in bone

and cartilage. In this way, osteoblasts could store calcium and subsequently release

113

micropackets of amorphous calcium phosphate,which would diffuse to calcifying

sites.This amorphous calcium phosphate would eventually cryatallize as

hydroxyapatite at the calcification front. While in case of mitochondrial theory, the

following can be argued upon:

� When mitochondria are isolated and provided with an energy source (ATP),

calcium ions, and a permeable anion, the organelles will gorge themselves with

calcium.If inorganic phosphate is present, mitochondria become filled with an

electron dense precipitate identified as amorphous calcium phosphate and

occasionally, demomstrate some apatite crystals.

� In pathologic conditions that render the plasma membrane permeable, the

mitochondria would eventually mineralize. This mineralization is the basis for

calcification mechanism in the epiphyseal growth plate, where chondrocytes

undergo progressive degeneration (disruption) characterized by the uptake and

subsequent release of mitochondrial calcium granules in the zone of

extracellular calcification.

Other cytoplasmic organelles have also been demonstrated to concentrate calcium

ions: the plasma membrane- endocytotic vesicles,and the specialized smmoth

endoplasmic reticulum the sarcoplasmic reticulum.Both of these structures have been

shown to possess an inwardly directed calcium ATPase for the translocation of the

calcium ions.

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Matrix vesicles :.Bonucci proposed a roel for the osteoblast , considering initial

nucleation sites to be cellular’buds’ or extrusions.Secondary nucleation, by the

formation of critical nuclei which break away from the thermodynamically unstable

surfaces of already formed crystals, leads to further aggregation of crystals around this

initial locus, thus to the formation of spherulitic mineralization nodules:these coalesce

to give seams of mineralized bone, the association with collagen fibers being

secondary. Such cellular ‘seeds’ derived from osteoblasts would be absent in the

general connective tissue.

The vesicles are membrane- bound spheres about 0.1-0.2μm in diameter. They

typically have an electron dense core.The bone salts crystals within the matrix vesicles

are often seen first in association with the inner surface of the vesicle membrane and

subsequently accumulate in the vesicles.

Matrix vesicels appear to be the loci of earliest crystal formation in newly forming

bone, and indeed in the initial mineralization of all mineralized tissue throughout

vertebrates. They occur in calcifying cartilage, woven bone, dentin, and subperiostesl

bone.Thus matrix vesicels with similar properties can be produced by chondroblasts,

odntoblasts and osteoblasts and their formation is not uniform around the cell

membrane.In each case, it is thought that they are derived by polarized budding of the

relevant cell.Being membrane-bound, vesicles separate internal and external

environment,and the outer surface of the vesicle is the same as that of the cell.they

show alkaline phosphatase, adenosine 5’ phosphate (ATP)ase, inorganic

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pyrophosphatase, and nucleoside triophopshate pyrophatase activity and contain

acidic phospholipids.it is possible that matrix vesicels provide the enzymes and

environment to concentrate calcium and phosphate sufficiently to initiate

crystallization., primarily along the inner leaflet of the membrane,which then spreads

outside the vesicle.

Physiochemically, it has been thought for many years that collagen might initiate

crystallization by a process of epitaxy, i.e., the deposition of calcium phosphate on (or

in ) collagen.In this way , collagen serves as a host material for the deposition of the

apatite crystals.Early microscopic studies of collagen with 600 to 700 A axial

periodicity described the presence of 6 A ‘hole zones’ in the nonoverlap region of the

collagen fibril and showed the presence of the apatite crystals within the hole zones of

developing embryonic bone.Experiments have demostrated that purified collagen

preparations will initiate hydroxyapatite formation from metastable solutions with

calcium phosphate ions products well below those needed for spontanoues

precipatation.Only type I collagen is effective in this regard.this type of collagen is not

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only found in bone but in collagenous tissue that ordinarily do not mineralize(skin).

Because of this fact, it has been hypothesized that bone collagen possess some

molecular adjunct that allows this form to undergo mineralization.These special

‘helper molecules’ may be polypeptides, polysaccharides, phosphoproteins

etc.Alternatively, specificity may reside in the presence of phosphorylated sites on the

collagen molecule. This specificity might include the presence of a high proportion of

phosphoserine residues, implying a covalent bonding of calcium to phosphorus during

calcification.the colavent bonding would not involve epitaxy.In addition, asmall

molecular weight protein containing gamma-carboxyglutamic acid(osteocalcin) has

been identified in bone.This molecule is absent in soft tissues.It has a high affinity for

calcium so it may function in calcification.

The major problem hindering the universal recgnition of collagen as a nucleating

system is the fact that not all collagens, in different locations, undergo spontanous

nucleation and subsequent calcification.Also, certain collagens are characterized by

the presence of specific nucleation inhibiting molecules.Finally,continued

fibrillogenesis and crossliniking,i.e.,extracellular polymerization , may block the

appropriate nucleation sites as well as retard the inward diffusion of th enecessary

ions.

In addition to collagen, many other preparations have been hypothesized to

posseses the nucleating ability: tendon ,phospholipids,proteolipids, phospholipid-

calcium-phosphate complexes, lysozymes,noncollagenous proteins and elastin.

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Factors influencing mineralization

Local Factors

Collagen – Collagen has holes and pores in which nucleation, crystal growth,

secondary nucleation and multiplication of the solid phase can occur.

Non Collagenous Molecules

Name Composition Possible Function

Osteopontin Phosphoprotein inhibits crystal growth

Osteonectin Phosphoprotein inhibits crystal growth

Bone Sialo Protein Phosphorylated Glycoprotien Nucleator for

mineralization

GLA Protein Protein & y-carboxy

glutamic acid

Regulator of crystal

growth

Biglycan and Decorin Chondroitan Sulfate

Proteoglycans

Phospholipids

Pyrophosphate

Removed at

mineralization front to

permit mineralization

calcium binding at

mineralization front.

inhibitor of calcification

Growth Factors

FGF :Increase osteoblastic precursor population and also increase collagen

synthesis.

IGF :Increase bone cell proliferation and total protein synthesis.

TGF, PGDF :increase proliferation of osteo-progenitor and total protein synthesis.

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Interleukin 1 :At low doses, it stimulates collagen synthesis but is inhibitor in higher

concentrations.

Tumor necrosis factor: stimulate proliferation and collagen synthesis in pre-

Osteoblasts

Systemic Factors

PTH, 1,25 - Dihydroxy Vitamin D3, estrogen

Role of alkaline phosphates

� Hydrolyzes phosphate ions from organic radical at an alkaline pH

� Marker of osteoblast activity.

Incremental lines

Due to variations in the degree of mineralization at the boundaries between

periods of activity and rest.

BLOOD SUPPLY AND NUTRITION OF BONE1,2

NUTRIENT ARTERY

Forms the main supply during the period of growth. It enters the shaft through

the nutrient foramen. The tortuosity of the artery before entrance into the foramen

prevents it form being damaged and form change in blood pressure during active

muscle contractions.

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PERIOSTEAL VESSELS:-

These are numerous and carried along the muscular attachments. They form a

plexus in the inner layer of the periosteum. They pierce the outer part of compact bone

and supply it they communicate vessels in the harvesian canals by passing through the

Volkmann’s canal.

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METAPHYSEAL( JUXTA-EPIPHYSEAL) VESSELS

They are numerous small vessels derived from arterial anastomosis around the

joint; they supply the metaphyseal region, the epiphysis and articular capsule.

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EPIPHYSEAL VESSELS

Derived from arterial anastomosis around the joints

Present in only those bones where the capsular ligament is attached to the

epiphysis instead of metaphysis

Large irregular bones, short bones, and flat bones

These bones receive a superficial blood supply from the periosteum and

frequently from large nutrient arteries that penetrate directly into the medullary bone.

The two systems anastomose freely.

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APPLIED ANATOMY

Like any other tissue in the body necrosis of the bone can occur due to interruption

of its blood supply resulting from physical injury (trauma) or form non-traumatic

occlusion of blood vessels (Caisson’s Disease). In such aseptic necrosis of bone, there

could be two situations

1. Bone with some residual supply: Involved bone is invaded by vascular fibrous

stroma and the necrotic bone is gradually resorbed and replaced by new bone.

Part of bone with blood supply undergoes atrophy with decalcification.

2. Bone with completely no blood supply: Involved bone shows increased density

because there is no blood supply to remove the mineral salts. Sequestrum is an

example of the effect of loss of blood supply to bone.

Thereby increased density of bone may represent

� Zones of hypertrophic living bone

� Necrotic bone with reactive bone about it

� Simply necrotic bone

In situations where there is increased circulation, bone responds by hypertrophy and

increase in length. Increase in length of lower limb with an arterio-venous aneurysm is

an example.

Circulatory disturbances in specific regions of the long bones:

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1. Legg–Calve–Perthes Disease: Circulatory disturbance to the capital femoral

epiphysis plays an important role in the pathogenesis of the disease.

2. Physeal Trauma: Epiphyseal vessels are responsible for nourishing the

reproductive cells of the physis. Interruption of these vessels results in

irreparable damage to the growth plate. Metaphyseal vessels supply calcium

and Vitamin D via serum and phosphates via erythrocytes, aiding in the

calcification of the matrix, removal of degenerate cells, and deposition of

lamellar bone. The metaphyseal vessels are of no nutritional significance to

proliferating chondrocytes of the physis.

Metaphysis:

1. Haematogenous Osteomyelitis: In children infective foci are known to localize

in the metaphysis within two hours following intravenous inoculation. This is

explained by the nature of blood supply to this region. The last ramifications of

the intramedullary nutrient artery to the metaphysic turn down in sharp loops

and empty into a system of large sinusoidal veins where the rate of blood flow

is decreased. This creates an ideal medium for the bacteria to settle down and

proliferate. In the adult there is a free anastamosis between the metaphyseal and

epiphyseal arteries and thereby osteomyelitis, when it does occur may appear

anywhere. In infants a few vessels do cross the physis. At eight months the

physeal cartilage becomes a barrier that becomes definitively established by 18

months. Therefore in infants osteomyelitis can lead to septic arthritis.

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2. Metastasis: Metastatic deposits are more common in the axial skeleton than the

appendicular skeleton. However when they occur they are known to deposit in

the region of the metaphysis. This is attributed to the fact that the metaphyseal

circulation is richer compared to epiphyseal or diaphyseal circulation.

Diaphysis

1. Intramedullary Reaming: Cortical reaming and nail insertion injures the

medullary vascular system resulting in avascularity of significant portions of

the cortex. Medullary reaming disturbs the intracortical transport mechanism by

altering the high intramedullary pressure required for venous drainage into the

periosteal veins and the pulsating intramedullary pressure necessary for

nourishment of osteocytes. Research has shown that there is more rapid

revascularization with nails placed without preparatory reaming compared with

nails placed after reaming. Hence there is a shift of interest to interlocking

nailing systems that do not require reaming.

2. Fracture Healing: Periosteal circulation plays a very important role in healing of

fractures. Soft tissue stripping thereby, while performing an internal fixation of

a fractured bone must be kept to a minimum to encourage the participation of

periosteum and its circulation in fracture healing.

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VENOUS AND LYMPHATIC DRAINAGE OF BONE1,2

Blood is drained from bone through veins that accompany the arteries and

frequently leave through foramina near the articular ends of the bones. Lymph vessels

are abundant in the periosteum.

NERVE SUPPLY OF BONE1,2

Nerves are most rich in the articular extremities of the long bones, vertebrae,

and larger flat bones. Many nerve fibers accompany the blood vessels to the interior of

the bones and to the perivascular spaces of the Haversian canals. The periosteal nerves

are sensory nerves, some of which are pain fibers. Therefore, the periosteum is

especially sensitive to tearing or tension. Accompanying the arteries inside the bones

are vasomotor nerves, which control vascular constriction and dilation.

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DEVELOPMENT OF FACIAL BONES22

The facial bones develop intramembranously from ossification centers in the

neural crest mesenchyme of the embryonic facial precursor. An interaction between

the ectomesenchyme of the facial ‘processes’ and the overlying epithelium is believed

to be prerequisite to the differentiation of the facial bones. In the frontonasal process,

intramembranous single ossification centers appear in the 3rd

month for each of the

nasal and lacrimal bones in the membrane covering the cartilaginous nasal capsule.

The first ossification centers to appear early in the 8th week in are those for the medial

pterygoid plates of the sphenoid bone and for the vomer. The ossification centre for

the medial pterygoid plate first appears in a nodule of secondary cartilage that forms

the pterygoid hamulus, but subsequent ossification of the pterygoid plate in

intramembranous Further intramembranous ossification centers develop for the greater

wing of sphenoid (in addition to its endocrinal sphenoidal center), and for the lateral

pterygoid plate. Bony fusion of the medial and lateral pterygoid plates takes place in

the 5th

month IU. Single ossification centers appear for each of the palatine bones, and

two bilaterally for the vomer in the maxillary mesenchyme surrounding the

cartilaginous nasal septum in the 8th week IU. A primary intramembranous

ossification centre appears for each maxilla early in the 8th week at the termination of

the infraorbital nerve first above the canine tooth dental lamina.

Secondary cartilages appear at the end of the 8th

week in the region of the

zygomatic and alveolar processes that rapidly ossify and fuse with the primary

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intramembranous center. Two further intramembranous center. Two further intra

membranous premaxillary centers appear anteriorly on each side in the 8th

week and

rapidly fuse with the primary maxillary centre. Single ossification centers appear for

each of the zygomatic bones and the squamous portion of the temporal bone in the 8th

week IU. In the lower 3rd

of the face, the mandibular process develop bilaterally

single intramembranous centers for the mandible and 4 minute centers for the

tympanic ring of the temporal bone.

DEVELOPMENT OF PALATE23,24

The human palate in its embryological development passes through stages

representing divisions of the oronasal chamber found in primitive crossopterygian

fish, reptiles and early mammals

As a result of medial growth of maxillary prominences, the two medial nasal

prominences merge not only at the surface but also at a deeper level. The structure

formed by the two merged prominences is the intermaxillary segment It is composed

by

� Labial component – forms philtrum of upper lip.

� An upper jaw component which carries 4 incisors.

� Palatal component – which forms the triangular primary palate.

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Secondary palate :

Primary palate is derived from intermaxillary segment. Main part of definitive

palate is formed by two shelf like overgrowth from the maxillary prominences. These

outgrowths, the palatine shelves, appear in 6th week of development and are directed

obliquely downwards on each side of the tongue. In the 7th week, palatine shelves

ascend to attain a horizontal position above the tongue and fuse, forming secondary

palate. Anteriorly shelves fuse with triangularprimary palate, the incisive foramen is

the midline landmark between primary and secondary palate. At the same time as

palatine shelves fuse, the nasal septum grows down and joins with the cephalic aspect

of newly formed palate. Late in the 7th week 10 between the 47

th and 54

th days a

remarkable transformation in position of the lateral shelves takes place, when they

alter from vertical to horizontal, as a prelude to their fusion and partitioning the

oconasal chamber.

Several mechanism have been proposed for the rapid elevation of the palatal

shelves. This movement has been variously ascribed to biochemical transformation in

the physical consistency of the connective tissue matrix of the palatal shelves ; to

variations in vasculature and blood flow to these structures, to rapid differential

mitotic growth; to an intrinsic shelf force and to muscular movements The

withdrawal of the embryo’s face from against the heart prominence by uprighting of

the head facilitates jaw opening mouth opening reflexes have been implicated in the

withdrawal of the tongue from between the vertical shelves, and pressure differences

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between the nasal and oral regions due to tongue muscle contraction may account for

palatal shelf elevation. The epithelium overlying the edges of the palatal shelves is

especially thickens, and thin fusion upon mutual contact is crucial to contact palatal

development. Fusion also occur between the dorsal surface of the fusing palatal

processes and the lower edge of the midline nasal septum. A combination of

degenerating surface cells and epithelial surface coat accumulation of polyaminonic

substances such as glycoprotein may facilitate epithelial adhesion between contacting

palatal processes. It appears that programmed cell death of the fused epithelium is an

essential prerequisite for mesenchymal coalescence of the shelves to occur. The

epithelium at the leading edges of the palatal shelves may contribute to failure of

fusion by not breaking down of the shelf approximation leading to epithelial pearl

formation or by not maintaining an adhesiveness beyond a critical time - The fusion

of the 3 palatal processes initially produces a flat, unarched roof to the mouth. The

fusing lateral palatal shelves overlap the anterior primary palate as indicated by the

sloping pathways of the junctional incisive neurovascular canals that carry the

previously formed incisive nerves blood – vessels The site of junction of the 3 palatal

components is marked by the incisive papilla overlying the incisive canal. The line of

the fusion of the lateral palatal shelves is traced in the adult by the mid palatal suture,

and on the surface by the midline saphe of the hard palate the fusion 1 seam is

minimized in the soft palate by extra territorial mesenchymal invasion.

Ossification of palate proceeds during the 8th

week IV from the spread of bone

the mesenchyme of the fused lateral palatal shelves and from trabecular appearing in

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the primary palate as premaxillary centers all derived from the single primary

ossification centers of the maxillae posteriorly, the hard palate is ossified from the

trabecular spreading from the single primary ossification centers of each of the

palative bows. mid palatal sutures is first evident at 10½ weeks when an upper layer

of fiber bundles develop across the midline. The palatine bone elements of the palate

remain separated from the maxillary elements by the palatomaxillary sutures into

adulthood. In the most posterior part of the palate ossification doesn’t occur, group

rise to the region of the soft palate. Branchiomeric mesenchymal tissue of the 1st and

4th

branchial arches migrates into this faucial region, supplying the musculatures of

the soft palate and fauces. The tensor veli palatine is derived from the first arch and

the levator veli palatine muscle, and by the vagus nerve for all the other muscles.

Growth of the hard palate takes place in length, breath and height, converting in into

an arched roof for the mouth. The fetal palate initially increases in length more rapidly

than in width between 7 – 18 weeks 10 subsequently, growth in width overtakes that

in length. In earily prenatal life the palate is relatively long, but from the 4th

month

onwards, it becomes wider as a result of midpalatal along the lateral alveolar margins

At birth, the length and breath of the hard palate are mostly equal the later postnatal

increase of palatal length is due to appositional growth in the maxillary tuberosity

region, and to some extent at the transverse palatine suture. Growth at the midpalatal

suture cases between 1 and 2 yrs of age although no synostosis growth in width of the

midpalatal suture is layer in its posterior than in the anterior part, obliteration of the

midpalatal suture may starts in adolescence, but complete fusion is rarely found

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between 30 yrs of age. Great variability exists in the timing and degree of fusion of

this suture. Appositional lateral growth occurs in the palate up until seven years of

age, at which age the palate achieves its ultimate anterior width. Appositional growth

continues posteriorly after lateral growth has ceased, accounting for a lengthens of the

palate over its width during late childhood during infancy and childhood, bone

apposition also occurs on the entire inferior surface of the palate, a companied by

concomitant resorption from is superior (nasal surface) descent of the palate, and

enlargement of the nasal cavity. Increase of nasal capacity must keep pace with

general body growth thatdetermine increasing respiratory requirements. A

fundamental drive in facial growth is provision of an adequate nasal capacity, that if

not met is diverted to the mouth for maintains of respiration. The appositional growth

of the alveolar processes contributes to a deepening as well as a widening of the vault

of the bony palate, at the same time adding to the height and breath of the maxillae.

The lateral alveolar processes help to form an anterioposterior palatal fallow, which,

together with a concave floor produce by a tongue curled from side to side, results in a

palatal tunnel ideally scuted to receive a nipple. A variable number of transverse

palatal rugae develop in the mucosa covering the hard palate. This rugae are most

prominent in the infant and are useful for holding the nipple while it is being milked

by the tongue. The anterior palatal furrow is well marked during the first year of life,

concomitant with the active suckling period, and normally flattens out into the palatal

arch after three to four yrs of age when suckling has been discontinued persistence of

thumb or finger sucking habits may retain the palatal furrow into childhood.

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133

MAXILLA 1,14,23,24

ANATOMY:

The maxillae form by their union, the whole of the upper jaw. Each assists in

forming the boundaries of three cavities, viz., the roof of the mouth, the floor and

lateral wall of the nose and the floor of the orbit; it also enters into the formation of

two fossae, the infratemporal and pterygopalatine, and two fissures, the inferior orbital

and pterygomaxillary. Each bone consists of a body and four processes—zygomatic,

frontal, alveolar, and palatine.

The Body (corpus maxillae).—The body is pyramidal in shape, and contains a

large cavity, the maxillary sinus (antrum of Highmore). It has four surfaces—an

anterior, a posterior or infratemporal, a superior or orbital, and a medial or nasal.

Surfaces.—The anterior surface is directed forward and lateral. It presents at

its lower part a series of eminences corresponding to the positions of the roots of the

teeth. Just above those of the incisor teeth is a depression, the incisive fossa, which

gives origin to the Depressor alae nasi; to the alveolar border below the fossa is

attached a slip of the Orbicularis oris; above and a little lateral to it, the Nasalis arises.

Lateral to the incisive fossa is another depression, the canine fossa; it is larger and

deeper than the incisive fossa, and is separated from it by a vertical ridge, the canine

eminence, corresponding to the socket of the canine tooth; the canine fossa gives

origin to the Caninus. Above the fossa is the infraorbital foramen, the end of the

infraorbital canal; it transmits the infraorbital vessels and nerve. Above the foramen is

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the margin of the orbit, which affords attachment to part of the Quadratus labii

superioris. Medially, the anterior surface is limited by a deep concavity, the nasal

notch, the margin of which gives attachment to the Dilatator naris posterior and ends

below in a pointed process, which with its fellow of the opposite side forms the

anterior nasal spine.

The infratemporal surface is convex, directed backward and lateralward, and

forms part of the infratemporal fossa. It is separated from the anterior surface by the

zygomatic process and by a strong ridge, extending upward from the socket of the first

molar tooth. It is pierced about its center by the apertures of the alveolar canals, which

transmit the posterior superior alveolar vessels and nerves. At the lower part of this

surface is a rounded eminence, the maxillary tuberosity, especially prominent after the

growth of the wisdom tooth; it is rough on its lateral side for articulation with the

pyramidal process of the palatine bone and in some cases articulates with the lateral

pterygoid plate of the sphenoid. It gives origin to a few fibers of the Pterygoideus

internus. Immediately above this is a smooth surface, which forms the anterior

boundary of the pterygopalatine fossa, and presents a groove, for the maxillary nerve;

this groove is directed lateralward and slightly upward, and is continuous with the

infraorbital groove on the orbital surface.

The orbital surface is smooth and triangular, and forms the greater part of the

floor of the orbit. It is bounded medially by an irregular margin which in front

presents a notch, the lacrimal notch; behind this notch the margin articulates with the

lacrimal, the lamina papyracea of the ethmoid and the orbital process of the palatine. It

135

is bounded behind by a smooth rounded edge which forms the anterior margin of the

inferior orbital fissure, and sometimes articulates at its lateral extremity with the

orbital surface of the great wing of the sphenoid.

It is limited in front by part of the circumference of the orbit, which is

continuous medially with the frontal process, and laterally with the zyogmatic process.

Near the middle of the posterior part of the orbital surface is the infraorbital groove,

for the passage of the infraorbital vessels and nerve. The groove begins at the middle

of the posterior border, where it is continuous with that near the upper edge of the

infratemporal surface, and, passing forward, ends in a canal, which subdivides into

two branches. One of the canals, the infraorbital canal, opens just below the margin of

the orbit; the other, which is smaller, runs downward in the substance of the anterior

wall of the maxillary sinus, and transmits the anterior superior alveolar vessels and

nerve to the front teeth of the maxilla. From the back part of the infraorbital canal, a

second small canal is sometimes given off; it runs downward in the lateral wall of the

sinus, and conveys the middle alveolar nerve to the premolar teeth. At the medial and

forepart of the orbital surface just lateral to the lacrimal groove, is a depression, which

gives origin to the Obliquus oculi inferior.

136

The nasal surface presents a large, irregular opening leading into the maxillary

sinus. At the upper border of this aperture are some broken air cells, which, in the

articulated skull, are closed in by the ethmoid and lacrimal bones. Below the aperture

is a smooth concavity which forms part of the inferior meatus of the nasal cavity, and

behind it is a rough surface for articulation with the perpendicular part of the palatine

bone; this surface is traversed by a groove, commencing near the middle of the

posterior border and running obliquely downward and forward; the groove is

converted into a canal, the pterygopalatine canal, by the palatine bone. In front of the

opening of the sinus is a deep groove, the lacrimal groove, which is converted into the

nasolacrimal canal, by the lacrimal bone and inferior nasal concha; this canal opens

into the inferior meatus of the nose and transmits the nasolacrimal duct. More

anteriorly is an oblique ridge, the conchal crest, for articulation with the inferior nasal

concha. The shallow concavity above this ridge forms part of the atrium of the middle

meatus of the nose, and that below it, part of the inferior meatus.

137

The Zygomatic Process (processus zygomaticus; malar process).—The

zygomatic process is a rough triangular eminence, situated at the angle of separation

of the anterior, zygomatic, and orbital surfaces. In front it forms part of the anterior

surface; behind, it is concave, and forms part of the infratemporal fossa; above, it is

rough and serrated for articulation with the zygomatic bone; while below, it presents

the prominent arched border which marks the division between the anterior and

infratemporal surfaces.

The Frontal Process (processus frontalis; nasal process).—The frontal

process is a strong plate, which projects upward, medialward, and backward, by the

side of the nose, forming part of its lateral boundary. Its lateral surface is smooth,

continuous with the anterior surface of the body, and gives attachment to the

Quadratus labii superioris, the Orbicularis oculi, and the medial palpebral ligament. Its

medial surface forms part of the lateral wall of the nasal cavity; at its upper part is a

138

rough, uneven area, which articulates with the ethmoid, closing in the anterior

ethmoidal cells; below this is an oblique ridge, the ethmoidal crest, the posterior end

of which articulates with the middle nasal concha, while the anterior part is termed the

agger nasi; the crest forms the upper limit of the atrium of the middle meatus. The

upper border articulates with the frontal bone and the anterior with the nasal; the

posterior border is thick, and hollowed into a groove, which is continuous below with

the lacrimal groove on the nasal surface of the body: by the articulation of the medial

margin of the groove with the anterior border of the lacrimal a corresponding groove

on the lacrimal is brought into continuity, and together they form the lacrimal fossa

for the lodgement of the lacrimal sac. The lateral margin of the groove is named the

anterior lacrimal crest, and is continuous below with the orbital margin; at its junction

with the orbital surface is a small tubercle, the lacrimal tubercle, which serves as a

guide to the position of the lacrimal sac.

The Alveolar Process (processus alveolaris).—The alveolar process is the

thickest and most spongy part of the bone. It is broader behind than in front, and

excavated into deep cavities for the reception of the teeth. These cavities are eight in

number, and vary in size and depth according to the teeth they contain. That for the

canine tooth is the deepest; those for the molars are the widest, and are subdivided into

minor cavities by septa; those for the incisors are single, but deep and narrow. The

Buccinator arises from the outer surface of this process, as far forward as the first

molar tooth. When the maxillæ are articulated with each other, their alveolar processes

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together form the alveolar arch; the center of the anterior margin of this arch is named

the alveolar point.

The Palatine Process (processus palatinus; palatal process).—The palatine

process, thick and strong, is horizontal and projects medialward from the nasal surface

of the bone. It forms a considerable part of the floor of the nose and the roof of the

mouth and is much thicker in front than behind. Its inferior surface is concave, rough

and uneven, and forms, with the palatine process of the opposite bone, the anterior

three-fourths of the hard plate. It is perforated by numerous foramina for the passage

of the nutrient vessels; is channelled at the back part of its lateral border by a groove,

sometimes a canal, for the transmission of the descending palatine vessels and the

anterior palatine nerve from the spheno-palatine ganglion; and presents little

depressions for the lodgement of the palatine glands. When the two maxillæ are

articulated, a funnel-shaped opening, the incisive foramen, is seen in the middle line,

immediately behind the incisor teeth. In this opening the orifices of two lateral canals

are visible; they are named the incisive canals or foramina of Stenson; through each of

them passes the terminal branch of the descending palatine artery and the nasopalatine

nerve. Occasionally two additional canals are present in the middle line; they are

termed the foramina of Scarpa, and when present transmit the nasopalatine nerves, the

left passing through the anterior, and the right through the posterior canal. On the

under surface of the palatine process, a delicate linear suture, well seen in young

skulls, may sometimes be noticed extending lateralward and forward on either side

from the incisive foramen to the interval between the lateral incisor and the canine

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tooth. The small part in front of this suture constitutes the premaxilla (os incisivum),

which in most vertebrates forms an independent bone; it includes the whole thickness

of the alveolus, the corresponding part of the floor of the nose and the anterior nasal

spine, and contains the sockets of the incisor teeth. The upper surface of the palatine

process is concave from side to side, smooth, and forms the greater part of the floor of

the nasal cavity. It presents, close to its medial margin, the upper orifice of the incisive

canal. The lateral border of the process is incorporated with the rest of the bone. The

medial border is thicker in front than behind, and is raised above into a ridge, the

nasal crest, which, with the corresponding ridge of the opposite bone, forms a groove

for the reception of the vomer. The front part of this ridge rises to a considerable

height, and is named the incisor crest; it is prolonged forward into a sharp process,

which forms, together with a similar process of the opposite bone, the anterior nasal

spine. The posterior border is serrated for articulation with the horizontal part of the

palatine bone.

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Ossification.—The maxilla is ossified in membrane. It is ossified from two centers

only, one for the maxilla proper and one for the premaxilla.These centers appear

during the sixth week of fetal life and unite in the beginning of the third month, but

the suture between the two portions persists on the palate until nearly middle life. The

frontal process is developed from both centers.

The maxillary sinus appears as a shallow groove on the nasal surface of the

bone about the fourth month of fetal life, but does not reach its full size until after the

second dentition.

The maxilla was formerly described as ossifying from six centers, viz.,

� one, the orbitonasal, forms that portion of the body of the bone which lies

medial to the infraorbital canal, including the medial part of the floor of the

orbit and the lateral wall of the nasal cavity;

� a second, the zygomatic, gives origin to the portion which lies lateral to the

infraorbital canal, including the zygomatic process;

� from a third, the palatine, is developed the palatine process posterior to the

incisive canal together with the adjoining part of the nasal wall;

� a fourth, the premaxillary, forms the incisive bone which carries the incisor

teeth and corresponds to the premaxilla of the lower vertebrates;

� a fifth, the nasal, gives rise to the frontal process and the portion above the

canine tooth;

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� and a sixth, the infravomerine, lies between the palatine and premaxillary

centers and beneath the vomer; this center, together with the corresponding

center of the opposite bone, separates the incisive canals from each other.

DEVELOPMENT:

PRENATAL DEVELOPMENT:

At fourth week of intra uterine life prominent bulge appears at ventral aspect

corresponding to developing brain. Below this bulge a shallow depression

corresponding to stomadeum Floor of stomadeum is formed by buccopharyngeal

membrane .Mesoderm of developing forebrain proliferates downwards and forms a

projection overlapping stomadeum. This projection is called FRONTONASAL

PROCESS The stomadeum is thus overlapped superiorly by frontonasal process and

mandibular process form lateral walls. Mandibular arch now gives a bud on dorsal end

called maxillary process, which grows ventro-medio-caudally to mandibular process

Ectoderm overlying frontonasal process shows bilateral thickening called NASAL

PLACODE .Nasal placode become sunken to form NASAL PITS There by dividing

frontonasal process into medial nasal process & lateral nasal processes As maxillary

process grow frontonasal process become narrower Eventually maxillary process fuse

with frontonasal process A primary intramembranous ossification centre appears for

each maxilla early in the 8th

week at the termination of the infraorbital nerve first

above the canine tooth dental lamina Secondary cartilages appear at the end of the 8th

week in the region of the zygomatic and alveolar processes that rapidly ossify and fuse

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with the primary intramembranous center. Two further intra membranous

‘premaxillary centers appear anteriorly on each side in the 8th week and rapidly fuse

with the primary maxillary centre. Single ossification centers appear for each of the

zygomatic bones and the squamous portion of the temporal bone in the 8th

week IU.

Growth of the maxilla is dependent upon a number of functional matrices acting

upon different areas of the bone that theoretically allow its subdivision into ‘skeletal

units’The ‘basal body’ develops beneath the infraorbital nerve, later surrounding it to

form the infraorbital canal The ‘orbital unit’ responds to the growth of the eyeball.

The ‘nasal unit’ is dependent upon the septal cartilage for its growth, while the teeth

provide the functional matrix for the ‘alveolar unit’ The ‘pneumatic unit’ reflects

maxillarysinus expansion which is more a responder than determiner of this skeletal

unit.

POSTNATAL DEVELOPMENT:

Postnatally maxilla grows by :

� Displacement

� Growth at suture

� Surface remodeling

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DISPLACEMENT:

Two types:

Primary displacement – By maxilla’s own growth in downward and forward

direction This results in the whole of maxilla being carried anteriorly The amount of

this forward displacement equals the amount of posterior lengthening

Secondary displacement –By growth of cranial base pushing maxilla in

downward and forward direction The nasomaxillary complex is simply moved

anteriorly as the middle cranial fossa grows in that direction.It is important

mechanism of growth during primary dentition years but becomes less important as

the growth of cranial base slow

GROWTH AT SUTURES

Sutural attachments of maxilla are oblique and almost parallel to etch other thus

growth at suture results in downward and foreword movement of maxilla. This leads

to opening up of space at the sutural attachments New bones are formed on either

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side of the suture leading to increase in overall size Sutural attachment of maxilla are

frontonasal suture, frontomaxillary suture, zygomatico maxillary suture,

pterygopalatine suture

SURFACE REMOLDING

Bone resorption and deposition leading to increase in size of maxilla in all

direction and flattening of palate. Bone deposition occurs along the posterior margin

of the maxillary tuberosity. This causes lengthening of the dental arch & enlargement

of the antero-posterior dimension of the entire maxillary body. This helps to

accommodate the erupting molars. Bone resorption is seen on the floor of the nasal

cavity . To compensate there is deposition on the palatal side . Thus a net downward

shift occurs leading to increase in maxillary height.

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MANDIBLE1,14,23,24

ANATOMY

The mandible, the largest and strongest bone of the face, serves for the

reception of the lower teeth. It consists of a curved, horizontal portion, the body, and

two perpendicular portions, the rami, which unite with the ends of the body nearly at

right angles.

The Body (corpus mandibula).”The body is curved somewhat like a

horseshoe and has two surfaces and two borders.

Surfaces.The external surface is marked in the median line by a faint ridge,

indicating the symphysis or line of junction of the two pieces of which the bone is

composed at an early period of life. This ridge divides below and encloses a triangular

eminence, the mental protuberance, the base of which is depressed in the center but

raised on either side to form the mental tubercle. On either side of the symphysis, just

below the incisor teeth, is a depression, the incisive fossa, which gives origin to the

Mentalis and a small portion of the Orbicularis oris. Below the second premolar tooth,

on either side, midway between the upper and lower borders of the body, is the mental

foramen, for the passage of the mental vessels and nerve. Running backward and

upward from each mental tubercle is a faint ridge, the oblique line, which is

continuous with the anterior border of the ramus; it affords attachment to the

Quadratus labii inferioris and Triangularis; the Platysma is attached below it.

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The internal surface is concave from side to side. Near the lower part of the

symphysis is a pair of laterally placed spines, termed the mental spines, which give

origin to the Genioglossi. Immediately below these is a second pair of spines, or more

frequently a median ridge or impression, for the origin of the Geniohyoid. In some

cases the mental spines are fused to form a single eminence, in others they are absent

and their position is indicated merely by an irregularity of the surface. Above the

mental spines a median foramen and furrow are sometimes seen; they mark the line of

union of the halves of the bone. Below the mental spines, on either side of the middle

line, is an oval depression for the attachment of the anterior belly of the Digastric.

Extending upward and backward on either side from the lower part of the symphysis

is the mylohyoid line, which gives origin to the Mylohyoid; the posterior part of this

line, near the alveolar margin, gives attachment to a small part of the Constrictor

pharyngis superior, and to the pterygomandibular raphe. Above the anterior part of

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this line is a smooth triangular area against which the sublingual gland rests, and

below the hinder part, an oval fossa for the submaxillary gland.

Borders The superior or alveolar border, wider behind than in front, is

hollowed into cavities, for the reception of the teeth; these cavities are sixteen in

number, and vary in depth and size according to the teeth which they contain. To the

outer lip of the superior border, on either side, the Buccinator is attached as far

forward as the first molar tooth.

The inferior border is rounded, longer than the superior, and thicker in front

than behind; at the point where it joins the lower border of the ramus a shallow

groove; for the external maxillary artery, may be present.

The Ramus The ramus is quadrilateral in shape, and has two surfaces, four

borders, and two processes.

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Surfaces.The lateral surface is flat and marked by oblique ridges at its lower

part; it gives attachment throughout nearly the whole of its extent to the Masseter. The

medial surface presents about its center the oblique mandibular foramen, for the

entrance of the inferior alveolar vessels and nerve. The margin of this opening is

irregular; it presents in front a prominent ridge, surmounted by a sharp spine, the

lingula mandibula, which gives attachment to the sphenomandibular ligament; at its

lower and back part is a notch from which the mylohyoid groove runs obliquely

downward and forward, and lodges the mylohyoid vessels and nerve. Behind this

groove is a rough surface, for the insertion of the Pterygoideus internus. The

mandibular canal runs obliquely downward and forward in the ramus, and then

horizontally forward in the body, where it is placed under the alveoli and

communicates with them by small openings. On arriving at the incisor teeth, it turns

back to communicate with the mental foramen, giving off two small canals which run

to the cavities containing the incisor teeth. In the posterior two-thirds of the bone the

canal is situated nearer the internal surface of the mandible; and in the anterior third,

nearer its external surface. It contains the inferior alveolar vessels and nerve, from

which branches are distributed to the teeth. The lower border of the ramus is thick,

straight, and continuous with the inferior border of the body of the bone. At its

junction with the posterior border is the angle of the mandible, which may be either

inverted or everted and is marked by rough, oblique ridges on each side, for the

attachment of the Masseter laterally, and the Pterygoideus internus medially; the

stylomandibular ligament is attached to the angle between these muscles. The

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anterior border is thin above, thicker below, and continuous with the oblique line.

The posterior border is thick, smooth, rounded, and covered by the parotid gland.

The upper border is thin, and is surmounted by two processes, the coronoid in front

and the condyloid behind, separated by a deep concavity, the mandibular notch

The Coronoid Process (processus coronoideus) is a thin, triangular eminence,

which is flattened from side to side and varies in shape and size. Its anterior border is

convex and is continuous below with the anterior border of the ramus; its posterior

border is concave and forms the anterior boundary of the mandibular notch. Its lateral

surface is smooth, and affords insertion to the Temporalis and Masseter. Its medial

surface gives insertion to the Temporalis, and presents a ridge which begins near the

apex of the process and runs downward and forward to the inner side of the last molar

tooth. Between this ridge and the anterior border is a grooved triangular area, the

upper part of which gives attachment to the Temporalis, the lower part to some fibers

of the Buccinator.

The Condyloid Process (processus condyloideus) is thicker than the coronoid,

and consists of two portions: the condyle, and the constricted portion which supports

it, the neck. The condyle presents an articular surface for articulation with the articular

disk of the temporomandibular joint; it is convex from before backward and from side

to side, and extends farther on the posterior than on the anterior surface. Its long axis

is directed medialward and slightly backward, and if prolonged to the middle line will

meet that of the opposite condyle near the anterior margin of the foramen magnum. At

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the lateral extremity of the condyle is a small tubercle for the attachment of the

temporomandibular ligament. The neck is flattened from before backward, and

strengthened by ridges which descend from the forepart and sides of the condyle. Its

posterior surface is convex; its anterior presents a depression for the attachment of the

Pterygoideus externus.

The mandibular notch, separating the two processes, is a deep semilunar

depression, and is crossed by the masseteric vessels and nerve.

DEVELOPMENT

PRENATAL DEVELOPMENT:

At fourth week of intra uterine life developing forebrain and heart two

prominences on ventral surface of mandible. These prominences are separated by

shallow depression corresponding to stomadeum – primitive mouth Floor of

stomadeum is formed by buccopharyngeal membrane The mandibular arch forms the

lateral wall of stomadeum. Mandibular arch now gives a bud which grows ventro-

medio- cranial to the main arch and is called “Mandibular process” The mandibular

process grows from both sides and fuse in midline to form mandible. The prior

presence of the nerve has been postulates as being necessary to induce osteogenesis by

the production of neurotrophic factors. The mandible is derived from ossification of

an osteogenic membrane formed from ectomesenchymal condensation at 36-38 days

of development. The mandibular ectomesenchyme must initially interact with the

epithelium of the mandibular arch before primary ossification occur.

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MECKLE’S CARTILAGE :

The ossifying membrane is locate lateral to Meckel’s cartilage and it’s

accompanying neurovascular bundle ossification spreads from the primary center

below and around the inferior alveolar nerve and its incisive branch and upwards to

form a trough for the developing teeth. Spread of the intramembranous ossification

dorsally and ventrally forms the body and ramus of the mandible. Meckel’s cartilage

becomes surrounded and invaded by four ossification stops dorsally at the scar that

will later become the mandibular lingula, from where Meckel’s cartilage continues

into the middle ear. The prior presence of the neurovascular bundle ensures the

formation of the mandibular foramen and canal, and the mental foramen. The first

bronchial arch core of Meckel’s cartilage almost meets its fellow of the opposite side

ventrally. It diverges dorsally to end in the tympanic cavity pharyngeal pouch, and is

surrounded by the forming petrous portion of the temporal bow the dorsal end of

meckel’s cartilage ossicle, the stapes, is deceived primary from the cartilage of the

second branch. Arch from the cartilage of second branch. Arch (Reichert’s cartilage)

The major portion of Meckel’s cartilage disappears. Parts of the cartilage transform

into the sphenomandibular and anterior malleolar ligaments. A portion of Meckel’s

cartilage contributes to the formation of the scar of the sphenoid bow. A further small

part of its ventral end, from the mental foramen ventrally to the symphysis, forms

accessory endochondral ossicles that are incorporated to into the chin region of the

mandible. Merkel’s cartilage dorsal to the mental foramen undergoes resorption on its

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lateral surface at the same time as intra membranous bony trabeculae are forming

immediately lateral to the resorbing cartilage.

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CONDYLE :

Around 5th week intrauterine life an area of mesenchymal condensation

develops superior to developing mandible. By 10th week it transforms in to cone

shaped cartilage By 4th month fuses with developing mandible.

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CORONOID PROCESS

Around 10th – 14th month a accessory cartilage appears in response to

developing Temporalis muscle in the region of coronoid process. Becomes

incorporated in developing mandible.

POSTNATAL GROWTH OF MANDIBLE

RAMUS:

It moves progressively posterior by combination of deposition & resorption.

Resorption occurs at the anterior part of the ramus while bone deposition occurs at the

posterior region

CORPUS OR BODY OF THE MANIDIBLE:

Displacement of ramus results in the conversion of former ramal bone into the

posterior part of the body of the mandible. Additional space Is made by the means of

resorption of the anterior border of the ramus

ANGLE OF THE MANDIBLE:

On the lingual side of the angle of the mandible , resorption takes place on the

posterio-inferior aspect while deposition occurs on the anterosuperior aspect On the

buccal side, resorption occurs on the antero -superior part while deposition takes

place on the postero-superior part resulting in the flaring of the angle of the mandible

as age advances.

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ALVEOLAR PROCESS:

Develops in response to tooth buds. As the tooth erupts the alveolar process

develops & increases in height & thickness of the body of the mandible

CHIN:

The mental protuberance forms by bone deposition during childhood. Its

performance is accentuated by bone resorption that occurs in the alveolar region

above it creating a concavity.

CONDYLE:

The growth of the soft tissues including the muscles & connective tissue carries

the mandible forward away from the cranial base. Bone growth follows secondarily at

the condyle to maintain constant with the cranial base

CORONOID PROCESS:

Follows ‘V’ principle. Bone deposition occurs on the lingual (medial) surfaces

of the right & left coronoid process The deposition on the lingual of the coronoid

process brings about a posterior growth movement in the ‘V’ pattern

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ALVEOLAR BONE25,26,27,28

Alveolar bone is a specialized part of the mandibular and maxillary bones that

forms the primary support structure for teeth. Although fundamentally comparable to

other bone tissues in the body, alveolar bone is subjected to continual and rapid

remodeling associated with tooth eruption and subsequently the functional demands of

mastication. The ability of alveolar bone to undergo rapid remodeling is also

important for positional adaptation of the teeth but may be detrimental to the

progression of periodontal disease.

The alveolar bone, which makes up the alveolar process and alveolar bulbs of

the upper and lower jaw, is that part of the facial skeleton, which forms the alveoli and

crypts of the developing teeth and the sockets of erupted teeth, giving protection to the

former, and providing a means of attachment for the latter. The morphology of the

alveolar bone depends on the size, shape and position of the teeth.There is no distinct

boundary between alveolar bone and supporting basal bone. It is continuous with and

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indistinguishable structurally from the basal maxilla and mandible. Most authors use

the term "alveolar bone" to describe only the compact cortical bone lining the tooth

socket and reserve the term "supporting alveolar bone" or "sustentacular bone" for the

remainder of the tooth bearing area or alveolar process. The alveolar process is,

therefore, composed of both alveolar bone and supporting alveolar bone. Some

authors use the term alveolar bone in a broader sense to encompass those parts of the

jaw that bear teeth as well as the alveoli and crypts of developing teeth. Also known

as Processes Alveolaris in case of maxilla, Pars Alveolaris in mandible.

DEVELOPMENT

The alveolar bone develops around the tooth germs.Towards the end of the 2nd

month of the intrauterine life, both maxilla and mandible are in the form of a groove

which opens towards the oral cavity. The tooth germs and the alveolar nerve and

vessels lie in this groove. Septa develop between adjacent tooth germs. Much later, a

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bony plate grows to separate the tooth germs from inferior alveolar nerve and vessels.

It only grows in association with active eruption.

As periodontal ligament forms, osteoblasts differentiate from the cells in the

outer part of the dental follicle and deposit new bone around the developing

periodontal ligament fiber bundle against the wall of the crypt. This leads to a gradual

reduction of this space between the crypt wall and tooth, until the normal dimension

of the periodontal ligament is reached. Alveolar bone appears first labial to the tooth

germs, then lingual and basal, while the tooth germ is in bell stage. In deciduous tooth

germs, the alveolar bone forms the roof of their bony crypt, while in the permanent

tooth germs the alveolar bone lacks at the upper region of the tooth germ, probably

because of persistence of epithelial remnants (Gubernaculum Dentis). Therefore,

pores called "foramen gubernaculare" or canal are found in infants lingual to the

erupted deciduous teeth. The succedaneous tooth germs at first lie within the bony

compartment of their corresponding deciduous tooth, and acquire their own

compartment only after the deciduous tooth begins to erupt. The alveolar bone proper,

and possibly, a part of the alveolar process, are to be considered as those parts of the

jaw, initially induced and formed from the dental sac proper.

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STRUCTURE

As a result of functional adaptation, two parts of the alveolar process may be

distinguished:

� Alveolar bone proper

� Supporting alveolar bone

Alveolar bone proper

The alveolar bone proper, forming the alveolar wall, is 0.1 to 0.4 mm thick, and

has the character of a fine holed sieve and attached to the trabeculae of the spongiosa.

Foramina in this osseous plate are especially numerous in the coronal and apical

region of the alveoli. Openings correspond to the Volkmann's canals. Also called

Cribriform plate or Lamina Cribriformis.

161

In X-ray, it appears as radiopaque-Lamina dura- because the mesial and distal

plate of the alveolar bone proper lie parallel to the path of X-ray beam, thus produce

an overlapping effect. It has same degree of mineralization as surrounding bone.

Absence of lamina dura may be due to the technical factors like over exposure, change

in beam angulation, long cone technique as well as pathological alteration.

Alveolar bone proper contains osteon and interstitial lamellae, but is

distinguished by the presence of Bundle bone. The term bundle bone was coined by

Stein and Weinman(1925), who stated that the entire alveolar bone proper or at least

the surface adjacent to the PDL, may consist of multiple layers of bone parallel to the

surface of the alveolar wall, which are penetrated by bundles of Sharpey's fibers,

embedded nearly right angled to the surface. Bundle bone is especially abundant in the

distal alveolar area of the pre-molar and molars of older, fully dentate individuals.

Alveolar bone proper consists of partly lamellated and partly bundle bone. The

bundle bone is characterized by the scarcity of the fibrils in the intercellular

substances. These fibrils, moreover, are arranged at right angles to the Sharpey's

fibers. Bundle bone contains fewer fibrils than does lamellated bone and appears dark

in routine H/E stain and much lighter in silver staining, than lamellated bone.

Supporting alveolar bone

� Cortical plate

� Spongy bone.

162

Cortical bone is continuous with the compact layer of maxilla and mandible. It is

covered by periosteum. Cortical bone is thinner in maxilla and there is more spongy

bone. It is thicker in the mandible and therefore spongy bone is less in mandible

In the maxilla, the buccal cortical plate is perforated by many openings, whereas

in the mandible it is dense. The cortical plate is thin in the anterior region and thick in

the posterior region. It fuses with the alveolar bone proper in the anterior region, The

cortical plates meet with the alveolar bone proper at the orifice of the alveoli. This part

is called the Alveolar Crest. The alveolar crest more or less parallels the outline of the

cervical margin of enamel, 1 to 3 mm apical to it, with greatest distance seen in the

older individuals.

The shape of the alveolar crest, under normal conditions, depends on the contour of

enamel of adjacent teeth, relative position of adjacent teeth, degree of eruption and

bucco-lingual width of the teeth.

Interdental septa are bony partitions that separate adjacent alveoli. Coronally, the

septa are thinner and here the alveolar bone proper are fused and cancellous bone is

frequently missing. The form of the interdental septa follows the alignment of

adjacent cementoenamel junction. In the posterior part, septa are flat. When the teeth

are in close approximation, the interdental septa are extremely narrow and in some

cases it is absent. It is commonly observed between the distobuccal root of the

maxillary first molar and the mesiobuccal root of the second molar. Interradicular

septa are the bony partitions between roots of the multirooted tooth.Interdental and

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interradicular septa contain the penetrating canal of Zuckerkandl and Hirschfeld which

house interdental and interradicular vessels and nerves.

Spongy bone is seen between cortical plate and alveolar bone proper & between

alveolar bone proper of adjacent teeth and roots of multi-rooted teeth. It consists of a

network of delicate trabeculae between which are the marrow spaces, filled mostly

with fatty marrow. In the region of maxillary tuberosity and angle of the mandible and

condylar process, erythropoeitic red marrow can be found even in an adult. Alveolar

process of maxilla contains more spongy bone than mandible.

Radiographically, spongiosa can be divided into two :

� Type I :- Trabeculae are arranged regularly and horizontally in a ladder like

fashion. More common in mandible.

� Type II:- Shows irregular arrangement of numerous delicate trabeculae.

Common in maxilla.

Trabeculae vary in size and shape. Accordingly they can be classified into :- Coarse,

Medium and Fine trabeculae.

Trabeculae are arranged preferentially along trajectories. The trajectories

represent planes or lines of stress, along which the forces acting on the body of the

jaw are captured and conducted. Stresses applied to the teeth are passed into the

alveolar bone via periodontal ligament and form here to jaw bone, thence to the skull.

The forces are transmitted along three Buttress system, anterior, middle and posterior.

164

The cortical plate is thickened and cancellous tissue is well developed in these areas.

Apart from buttresses, cortical bone of maxilla never show thickness.

VASCULAR, LYMPHATICS AND NERVE SUPPLY

The alveolar bone proper is perforated by numerous channels containing blood

and lymph vessels and nerves, which link the periodontal ligament and cancellous

portion of alveolar bone.

The blood supply of the alveolar bone comes from branches of the alveolar

artery. The periosteal vessels run over the cortical plates of bone and contribute to the

circulation supplying gingiva and periodontal ligament. The major supply comes from

the alveolar vessels that pass up to the centre of the alveolar septa, sending branches

laterally from the marrow spaces, and by way of canals through the cribriform plate to

the periodontal ligament. The interdental vessels pass upward to supply the septum

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and the interdental papilla. In the periodontal ligament, the vessels generally take a

longitudinal course with ascending and descending branches.

The lymphatic drainage is mainly to the submandibular lymph nodes.The

branches from anterior, middle and posterior superior alveolar nerve innervate

alveolar bone of maxilla, where as in case of mandible it is innervated by the branches

from the inferior alveolar nerve.

Relationship of teeth and alveolar bone:

Evolution has complicated the relationship between teeth and jaws. In mammals

a complex tissue, the periodontal ligament, joins the two organs. This hasled to the

differentiation of specialized structures at the periodontal ligament interfaces with the

root (the cementum) and the jaw (the alveolar wall or alveolar bone proper) in which

the fiber bundles of the periodontal ligament are anchored. Once inserted, these fibers

are called Sharpey’s fibers.

166

After the eruption period, the relationship between the teeth and their

supporting structures remains dynamic, as the former migrate spontaneously within

the alveolar process. This implies adaptation mechanisms preserving the anchorage to

bone and the integrity of the periodontal ligament, which is a source of progenitor

cells renewing the tissues.

167

REFERENCES

1) Williams P, Bannister LH, Berry MM, Collins P, Dyson M, Dussek JE et al:

Gray’s Anatomy. 38th

edition. New York. Churchill Livingstone. 1995.

2) Tortora , Derrickson .Principles of Anatomy and Physiology.9th

ed .Wiley &

Sons.

3) Johnson DR. Introductory Anatomy: Faculty of Biological Sciences, University

of Leeds Available from :

URL:http://www.leeds.ac.uk/chh/lectures//anatomy3.html.

4) The basis of bone in health and disease Available from : URL:

http://www.surgeongeneral.gov/libraray/bonehealth/html.

5) Poddar S, Bhagat A. Handbook Of Osteology.11th

ed New Delhi:

Saunders;2002.

6) Yang YJ.Histology of Bone 2002 Availablefrom:URL:

http://www.emedicine.com/orthoped/topic403.html.

7) Ten Cate AR. Development, Structure and Function . 6th

ed .Mosby Inc ;2003.

8) Walter LD. Oral histology Cell Structure and Function. W B Saunders; August

1986.

9) Berkovitz BKB ,Holland GR, Moxham BJ Oral anatomy, histology and

embryology 3rd

edition.Mosby;2002.

10) Katagiri T .Regulatory mechanisms of osteoblast and osteoclast

differentiation.Oral Dis (2002) ;8:147–159.

11) Marks SC. The origin of osteoelasts: J Oral Pathol 1983;12: 226-256.

168

12) Blue Histology - Skeletal Tissues – Bone, School of Anatomy and Human

Biology. The University of Western Australia. Availablefrom:URL:

http://www.lab.anhb.uwa.edu.au/mbl40/corepages/bone/bone.html.

13) Mosekilde L. Bone Dynamics: School of Anatomy and Human Biology - The

University of Western Australia.Availa blefrom:

URL:http://www.lab.anhb.uwa.edu.au/mbl40/bonedynamics.html.

14) Bhalaji SI.Orthodontics – The Art and science 3rd

ed New Delhi:Arya (Medi)

Publishing House.

15) Thomas G, Vanarsdall R. Orthodontic :current principles and technique. 4th ed,

St Louis: Mosby; 2005.

16) Price JS, Oyajobi BO. The cell biology of bone growth. Available from: URL:

http://www.unu.edu./Unupress/food2/UID/06E/uid06e0u.html.

17) CarlTB.Epiphysealboneformation.Availablefrom:URL:http://cal.upenn.edu./pr

ojects/s aortho/chapter_02/02mast.html.

18) Price JS. Regulation of growth plate chondrocytes and bone cells. Available

from: URL: http://www.unu.edu./Unupress/food2/UID/06E/uid06e0u.html.

19) Christopher LB. Applied oral physiology. 2nd ed, John Wright; September

1988.

20) Baker C. The Essentials of Calcium, Magnesium and Phosphate Metabolism.

Crit Care Resus 2002; 4: 301-306.

21) Harvey Z. A review of normal calcium and phosphorus metabolism. Canad.

M. A. J 1956; 74(1):912-21.

169

22) Sadler TW. Langmans medical embryology. 9th edition. Lippincott Williams &

Wilkins; 2003.

23) SperberH. Craniofacial embryology.4th edition, London: Wright Sydney

Butterworths;1989.

24) Enlow. Essentials of Growth .2nd

ed .W B Saunders: 1996.

25) Bhaskar SN. Oral Histology and embryology 4th

ed .Mosby.

26) Avery JK. Essentials of Oral Histology.3rd

ed, St. Louis: Mosby; 2000.

27) Newmann MG, Takei H, Carranza FA. Glickmann's Clinical Periodontology.

9th

ed. Saunders; 2001.

28) Jaro S. Molecular and cellular biology of alveolar bone: Periodontol 2000

2000; 22: 299–126.