Basic Bone Biology

Om Shree Ganapati namah

Basics of bone biology / Role of inflammation / Relevance to orthodonticsDr. Sangamesh B. M.D.S., MOrth RCS(Edinburgh)

Assistant Professor

Contents

Gross bone anatomy

Bone cells

Periodontal ligament

Functional histology

Bone turnover

Bone modeling and remodeling

BoneHighly specialized support tissue characterized by

Rigidity

Hardness

The bones in the skeleton are not all solid

Strength to act as levers for muscles

Give form to soft tissues

Provide protective cavities for the vital organs

Outer cortical or compact bone

Inner trabecular or spongy bone

Periosteum

Endosteum

Structure of the bone

Types of bone

Primary bone or the woven bone

Secondary bone or the lamellar bone

Trabecular / cancellous / spongy

Cortical / compact / dense

Woven (immature, fracture)

Large, rounded osteocytes

Osteocytes irregularly spaced

Randomly oriented collagen fibres

Variable collagen fibre diameter

Rapid matrix mineralisation

Forms rapidly

Rapid turnover

Lamellar (mature, adult)

Smaller, flattened osteocytes

Osteocytes regularly spaced

Collagen fibers show regular,

plywood orientation confers strength

Regular collagen fiber diameter

Delayed matrix mineralisation (few days)

Forms slowly

Slow turnover

Types of bone formation

Endochondral ossificationformation of long bones from cartilage model

Alberts et al Molecular Molecular Biology of the Cell

Growing knee joint (cat)growth plate

Endochondral bone formation

is by the replacement o f t h e h y a l i n e cartilage model with bone tissue.

Intramambranous bone formationis the replacement of the connective tissue membrane sheets and results in the f o r m a t i o n o f fl a t bones.

Intramembranous ossification CalvariumPeriosteum

Outer Cortical bone is solid with few small canals

Inner trabecular bone is like scaffolding or a honey-comb

Spaces between the bone are filled with fluid bone marrow cells and some fat cells

Alveolar Bone

Tooth eruption (cat)ultra-low power section of developing jaw

1 mm

Supporting the teeth

Alveolar bone

Periodontal ligament

Alveolar bone proper / Bundle bone

Central spongiosa

Outer cortical plates

Alveolar bone proper / Bundle bone

Because alveolar process is regularly penetrated by collagen fiber bundles, it is also called bundle bone

It appears more radiodense than surrounding supporting bone in X-rays called lamina dura

Low-power scanning electron microscope image of normal bone

architecture in the 3rd lumbar vertebra of a 30 year old womanmarrow and other cells removed to reveal thick, interconnected plates of boneTrabecular

bone

Relevance of Architecture and Geometry

Normal Loss of Loss ofQuantity and Quantity Architecture Architecture

Trabcular bone element perforated by osteoclast action

Low-power scanning electron microscope image of osteoporotic

bone architecture in the 3rd lumbar vertebra of a 71 year old womanmarrow and other cells removed to reveal eroded, fragile rods of bone

Trabcular bone element eroded by osteoclasts

Normal Moderate Osteoporosis

Severe Osteoporosis

Courtesy Dr. A. Boyde

Compact bone

Mineral deposition

Composition of the boneInorganic bone - 67%

65-70% inorganic mineral (hydroxyapatite)

Crystalline complex of Calcium and phosphate (hydroxyapatite) Ca5(PO4)3(OH)2

Organic bone - 33%

Collagen - 28%

Cells - 5%

Osteocalcin

Sialoprotein

Phosphoprotein

OsteonectinBone specific protein

Water 45 - 50% Ash 30 - 35% Protein 10 - 15% Fat 5 - 10%

Composition of Ash:

Calcium 36% Phosphorus 17%

Magnesium 0.8%

Structural and Metabolic Bone

Fractions

Cortical outer half provides strength

Inner half provides metabolic Ca++

Trabecular High turnover rate

Major source of metabolic Ca++

Calcium Electrical- carries current during an action potential across membranes, and can result in changes in intracellular free Ca2+ Cofactor for extracellular enzymes and regulatory proteins - stability or maximal activityIntracellular regulator - as a result of change in [Ca2+] inside cellsStructural (bones, tissues)

Vitamin D 1,25-dihydroxyvitamin D is thought to be the biologically active form which upregulatores calcium binding proteins to enhance calcium absorption conserves calcium at the kidney and increases bone resorption

Phosphorus Structural in bone, phospholipids Buffer and regulator of acid-base balanceEnergy currency of cells

Magnesium

Magnesium is thought to enhance bone quality by influencing hydroxyapatite crystal growth.

CollagenFiber

OrientationAlternate

Parallel

Twisted Plywood

Cells of the bone

Osteoblasts are derived from the mesenchymal stem cells

Transcription Factors & Differentiation

of Mesenchymal Progenitors

T Katagiri & N Takahashi. Oral Diseases. 2002

Primitive

progenitor Preosteoblast Osteoblast

Osteocyte

Regulated self-renewal,

Choice of cell fate

Commitment,

differentiation

Extracellular matrix

synthesis & mineralization

C3HT101/2

MC3T3.E1

Metaphyseal bone cells

Diaphyseal bone marrow stromal cells

BONE

Osteoblast T issue Matrix

Cell adhesion molecules

Cell membrane

Cytoskeleton

Nuclear Matrix nuclear pore

nucleolus nucleoskeleton

Integrins

ECM

The Osteoblast Tissue Matrix

Osteocytes

osteoblast

osteocyte

Osteocytes

3Inhibitors of Osteoclast FormationOPGsRANK-FcGM-CSF IFN!IFN"IL-18IL-12OCILTSA-1LegumainsFRP-1IL-4IL-13IL-10

- binds RANKL- binds RANKL- direct - direct - direct (feedback loop?)- indirect via T-cells- indirect via T-cells- direct- direct- direct- ?

Th2 cytokines}Dr. Jack Martin

Osteoclast Differentiation

OPG

RANKLRANK

ActivatedOsteoclast

HematopoieticStem Cells

MononuclearOsteoclastOsteoclast

Progenitor

Inactive Osteoclast

1,25(OH)2D3

Osteoblasts

Bone

Characteristics of Osteoclasts

! Multinucleated Cells-contain 4-20 nuclei in vivo

! Tartrate-resistant acid phosphatase positive

! Calcitonin receptor positive

! Vitronectin receptor positive

! Positive for cathepsin K

! Resorb bone

Ultrastructural Features

! Highly vacuolated foamy cells

! Polarized cells have ruffled border and

sealing zone

Ultrastructural Characteristics of

the Osteoclast

BONE

Clear Zone

Resorption Lacuna/ Pit

Vitronectin

Receptors (VNR, !v"3)

C a lcitonin Receptors (C TR)

Ruffled Border H+

H+

H+Cl-

Cl- Cathepsin K

TRAPLysosomal enzymesProton pump

V-ATPase

VNR and collagen receptors (!2b1)

Carbonic

Anhydrase II

H+HCO3-

Drs. Quinn/Martin 2002

Transmission Electron Micrograph of a

Human Osteoclast

Stenbeck, Seminars Cell Developmental Biol 2002

1

The Osteoclast

! Multinucleated giant cell found in bone

! Found in contact with calcified bone

surface

! Function is bone resorption

! Life span in vivo is up to 2 weeks with a

half-life around 6-10 days

Osteoclasts on Bone

Dr. Otts Web Site 2002

Osteoclast on Bone

Dr. Otts Web Site 2002

In vitro Generated Murine

Osteoclast

Galvin et al BBRC

Location of Osteoclasts

! Attached to or at the bone surface

! BMU-basic multicellular unit

! Howships lacunae- generally 2

osteoclasts/lacunae, but can be up to 5

Bone Remodeling

Osteoblast

Reversal/ FormationReversal/ Formation

New Bone

Osteoclast

ResorptionResorption

3Inhibitors of Osteoclast FormationOPGsRANK-FcGM-CSF IFN!IFN"IL-18IL-12OCILTSA-1LegumainsFRP-1IL-4IL-13IL-10

- binds RANKL- binds RANKL- direct - direct - direct (feedback loop?)- indirect via T-cells- indirect via T-cells- direct- direct- direct- ?

Th2 cytokines}Dr. Jack Martin

Osteoclast Differentiation

OPG

RANKLRANK

ActivatedOsteoclast

HematopoieticStem Cells

MononuclearOsteoclastOsteoclast

Progenitor

Inactive Osteoclast

1,25(OH)2D3

Osteoblasts

Bone

Characteristics of Osteoclasts

! Multinucleated Cells-contain 4-20 nuclei in vivo

! Tartrate-resistant acid phosphatase positive

! Calcitonin receptor positive

! Vitronectin receptor positive

! Positive for cathepsin K

! Resorb bone

Ultrastructural Features

! Highly vacuolated foamy cells

! Polarized cells have ruffled border and

sealing zone

Ultrastructural Characteristics of

the Osteoclast

BONE

Clear Zone

Resorption Lacuna/ Pit

Vitronectin

Receptors (VNR, !v"3)

C a lcitonin Receptors (C TR)

Ruffled Border H+

H+

H+Cl-

Cl- Cathepsin K

TRAPLysosomal enzymesProton pump

V-ATPase

VNR and collagen receptors (!2b1)

Carbonic

Anhydrase II

H+HCO3-

Drs. Quinn/Martin 2002

Transmission Electron Micrograph of a

Human Osteoclast

Stenbeck, Seminars Cell Developmental Biol 2002

Osteoclast differentiationOsteoclast migrationOsteoclast polarizationRuffled border formationOsteoclast actin ring formationDissolution of boneOsteoclast bone resorptionOsteoclast apopotsis

Functions of boneProvide structural support to the body

Provide protection of vital organs

Provide an environment for marrow (Blood

forming and fat storage)

Act as mineral reservoir for calcium

homeostasis in the body

Periodontal Ligament

Periodontal LigamentPDL is the soft specialized connective tissue situated betweencementum and alveolar bone proper

Ranges in thickness between 0.15 and 0.38 mm and is thinnest in the middle portion of the root

The width decreases with age

Tissue with high turnover rate

Contains fibers, cells and intercellular substance

Embryogenesis

The PDL forms from the dental follicle shortly after root development begins

Cells

OsteoblastsOsteoclasts (critical for periodontal disease and tooth movement)Fibroblasts (Most abundant)Epithelial cells (remnants of Hertwigs epithelial root sheath-epithelial cell rests of Malassez)Macrophages (important defense cells)Undifferentiated cells (perivascular location)CementoblastsCementoclasts (only in pathologic conditions)

Ground SubstanceAmorphous background material that binds tissues and fluids - major constituent of the PDL

Similar to most connective tissue ground substanceDermatan sulfate is the major & glycosaminoglycan

70% water; critical for withstanding forces

When function is increased PDL is increased in size and fiber thickens, bone trabeculae also increase in number and thicker

However, in reduction of function, PDL narrows and fiber bundles decreases in number and thickness (this reduction in PDL is primarily due to increased cementum deposition)

PDL fibers

Collagen fibers: I, III and XII. Groups of fibers that are continually remodeled. (Principal fiber bundles of the PDL). The average diameter of individual fibers are smaller than other areas of the body, due to the shorter half-life of PDL fibers (so they have less time for fibrillar assembly)

Oxytalan fibers: variant of elastic fibers, perpendicular to teeth, adjacent to capillaries

Eluanin: variant of elastic fibers

Dentoalveolar groupAlveolar crest group (ACG): below CE junction, downward, outwardHorizontal group: apical to ACG, right angle to the root surfaceOblique group: most numerous, oblique direction and attaches coronally to boneApical group: around the apex, base of socketInterradicular group: multirooted teeth. Runs from cementum and bone , forming the crest of the interradicular septum

At each end, fibers embedded in boneand cementum: Sharpeys fiber

Principal FibersRun between tooth and bone.

Can be classified as dentoalveolar and gingival group

Gingival ligament fibers The principal fibers in the gingival area are referred to as gingival fibers. Not strictly related to periodontium. Present in the lamina propria of the gingiva.

Dentogingival: most numerous; cervical cementum to f/a gingivaAlveologingival: bone of the alveolar crest to f/a gingivaCircular: around neck of teeth, free gingivaDentoperiosteal: runs apically from the cementum over the outer cortical plate to alv. process or vestibule (muscle) or floor of mouthTransseptal: cementum between adjacent teeth, over the alveolar crest

The PDL gets its blood supply from perforating arteries (from the cribriform plate of the bundle bone).

The small capillaries derive from the superior & inferior alveolar arteries.

The blood supply is rich because the PDL has a very high turnover as a tissue.

The posterior supply is more prominent than the anterior.

The mandibular is more prominent than the maxillary

Interstitial SpacePresent between each bundle of ligament fibersContains blood vessels and nervesDesigned to withstand the impact of masticatory forces

Nerve supply

The nerve supply originates from the inferior or the superior alveolar nerves.

The fibers enter from the apical region and lateral socket walls.

The apical region contains more nerve endings (except Upper Incisors)

Tooth support

Shock absorber: Withstanding the forces of mastication

Sensory receptor necessary for proper positioning of the jaw

Nutritive: blood vessels provide the essential nutrients to the vitality of the PDL

FUNCTIONS OF PERIODONTIUM

Bone turnoverBone modeling

Technique for quantifying the remodeling process

How much (static)How long (dynamic)Cell activity

. . . the origin or causation of the phenomenon would seem to lie partly in the tendency of growth to be accelerated under strain. . . . accounting therefore for the rearrangement of . . . the trabeculae within the bone.

DArcy Thompson, 1917

Rules for Bone Adaptation

Bone responds only to dynamic loads

The loading period can be short

Rate related phenomena are critical to

response

The Mechanostat: Essential Principles

Threshold-driven

Modeling and Remodeling are antagonistic

Operate within different strain ranges

Architecturally antagonistic

Bone envelopes are controlled by local conditions

Signal Transduction External signals Odorants Chemicals that reflect metabolic status Ions Hormones Growth factors Neurotransmitters Light Mechanical forces

Signal Transduction Steps

Recognition

Ionic bonds

Van der Waals interactions

Hydorphobic interaction

Transduction

Transmission

Modulation of the Effectors

Response

Termination

Bone modeling

Modeling Activation Resorption (A-R)Activation Formation (A-F)

Drift (Cortical and Trabecular)Occurs through Modeling Processes

Old bone New bone Osteoid

3. Resorption

4. Reversal

5. Formation

6. Quiescence

1. Quiescence

2. Activation

LC

POC

OB

LC

OCHL

?

CL

CL

BSUCL

The QuantumConcept of

Bone Remodeling

Gene 1

Environment 2

Phenotype

Gene 2 Gene 3 Gene 4

Environment 1

Gene x Environment Interactions Underlying Complex Disease

Role of inflammationDr Anand K. Patil

Documents

Basic Bone Biology