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Study of Bones

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BonesThis study covers the structure and development of bone

tissue and its organization into bone• Location

– Forms the framework of the skeleton• Functions include

– (1) structural support– (2) a mineral storage site - especially for calcium and

phosphorus– (3) a site (red bone marrow) for the production of the formed

elements of blood– (4) protection - especially for the brain and organs of the chest– (5) Leverage and attachment sites for tissues and organs

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Classification of Bones• Four structural classifications are:

– (1) long, – (2) short, – (3) flat, – (4) irregular– (5) sesamoid

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Long BonesLong bones are longer than they are wide.

– Among the long bones are the bones of the limbs such as the humerus, ulna, radius, femur, tibia, and fibula.

Humerus Ulna Radius Femur Tibia Fibula

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Short BonesShort bones are boxy in shape. Among the

short bones are the bones of the wrist (carpals) and ankle (tarsals).

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Flat BonesFlat bones are thin and flattened. Among

the flat bones are the bones of the roof of the skull, the ribs, the sternum, and scapulae (shoulder blades).

Skull

Rib

Sternum Scapula

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Irregular BonesIrregular bones are irregular in shape, often

with curved, pointed, and ridged surfaces. Among the irregular bones are the vertebrae and some bones of the skull.

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Sesamoid Bones

• Generally small and flattened; shaped like a sesame seed

• Develop inside of tendons

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Structure of Bone• Bones are composed of both

compact and spongy bone tissue – Compact bone is found where great

strength is needed. • It makes up the external surfaces of all bones

and the shafts of long bones.

– Spongy bone composed of numerous interconnecting bony plates (trabeculae).

• The trabeculae are organized in an open framework that provides considerable strength with reduced weight. The open framework provides a site for red bone marrow.

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Features of a Long Bone1) Epiphyses

– Proximal– Distal

2) Diaphysis– Medullary cavity

3) Bone Membranes– Periosteum

• Fibrous layer• Osteogenic layer

– Endosteum

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– the expanded ends of the long bone.– Proximal epiphysis closer to point of

attachment– Distal epiphysis farther from point of

attachment– Houses red bone marrow within a

framework (trabeculae) of spongy bone tissue

– Articular surfaces lined with hyaline (articular) cartilage

Epiphyses

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Diaphysis (shaft)– Longitudinal axis of the bone– Consists mostly of strong, dense

bone called compact bone – Houses central cavity, marrow

(medullary) cavity, which mostly stores yellow (fatty) marrow

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Split Long Bone - FemurNumber the following correctly:• Articular (hyaline) cartilage • Compact bone• Diaphysis• Medullary cavity• Periosteum• Proximal epiphysis• Spongy bone

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Split Long Bone - FemurNumber the following correctly:• Articular cartilage• Compact bone• Diaphysis• Distal epiphysis• Epiphyseal line*• Medullary cavity• Periosteum• Spongy bone

*a line formed by trabeculae at the site where a cartilage growth area, the epiphyseal plate, was located.

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Structure of Flat Bone

• Bone has parallel surfaces of compact bone with spongy bone in between surfaces. Contains red marrow in the trabeculae (has no marrow cavity).

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Bone Histology• Matrix is material outside of cells. Matrix is hard and and

strong as it contains both calcium salts (hard) and collagen fibers (strong).

• Mature bone cells, the osteocytes, are surrounded by the matrix. Small cavities, lacunae, surround the cells.

• Small canals, canaliculi, branch between the lacunae and blood vessels.

• Bone is surrounded by periosteum (except at their joints). Periosteum has outer fibrous and inner osteogenic(cellular) layers.

• Bone has inner lining called endosteum. Lines the medullary cavity, plates (trabeculae) of spongy bone, and vascular channels in matrix.

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Bone Membranes (and structure)

• Periosteum –Fibrous vascular membrane that lines the outer surface of the

diaphysis– inner osteogenic layer

• osteoblasts and osteoclasts

– outer fibrous layer. • mostly dense irregular connective tissue provides attachment sites

for tendons, ligaments, and into the bone itself by fibers (Sharpey’sfibers)

• Endosteum –

a nonfibrous membrane that lines the medullary cavity and the trabeculae of the epiphyses.

• composed mostly of osteoblasts and osteoclasts

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Bone Membranes

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Structure of Bone– Bone matrix exhibits considerable

hardness and tensile strength• Hardness is due to the inorganic

component, the mineral salts. These consist mostly of calcium salts such as tri-calcium phosphate (hydroxyapatite) and account for about two-thirds of the matrix. Mineral salts make the matrix hard and non-compressible.

• Tensile strength is mostly due to theorganic components (mostly collagen fibers) and accounts for about one-thirdof the matrix.

Demineralized bone.The organic portion, mostly collagen fibers, remains after treatment with a weak acid which removes the mineral salts.

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Cells in Bone• Osteocytes

– Amitotic mature cells of bone found embedded in matrix. Found in small spaces called lacunae. Small canals, the canaliculi, interconnect the lacunae and central canals (blood vessels locations).

– Function in (1) monitoring and maintaining mineral content of surrounding bone tissue and (2) if bone is damaged may convert to osteoblast or osteoprogenitorcells to aid in bone repair.

• Osteoblasts– Produce new bone tissue by osteogenesis.

Osteoblasts produce and release organic components of bone matrix, the yet unmineralized components is called osteoid. Assist in the promotion of mineralization of osteoid into bone tissue. Osteoblastsentrapped in bone matrix develop into osteocytes

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• Osteoprogenitor– Stem cells that divide to produce osteoblasts for

normal growth and bone repair. Located in osteogenic(cellular) layer of periosteum and inner lining of bone, the endosteum.

• Osteoclasts– Osteoclasts breakdown existing bone matrix. Cells

are derived as precursor of white blood cell line and release acids and enzymes which destroy matrix, osteolysis. Cells are important in the maintenance of blood calcium levels and in bone remodeling.

Cells in Bone

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Bone Tissue COMPACT BONE– organized in units called osteons

(Haversian systems).• Osteon – each contains a

– Haversian (central) canal which contains blood vessels.

– Matrix consists of mineral salts (about 2/3 mostly tricalciumphosphate, or hydroxyapatite) and 1/3 collagen fibers

– Lamellae (concentric rings of matrix) surround each central canal

– Osteocytes in circular-rows separate the lamellae

– Canaliculi (small canals ) interconnect the osteocytes.

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Compact bone• Major function of compact bone is transfer

of weight (tension) along a limited range of directions.

• Since compact bone is thickest in shaft, shaft transfers weight along long axis of bone.

• Ostgeons run parallel with long axis and their laminated structure adds to strength of bone

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Spongy bone• Lamellae not organized as osteons• Organized into plates called trabeculae.• Located typically at ends of long bone and

occupies the interior of all other bone types.• Trabeculae are organized to supply the greatest

strength at stress points.• Trabeculae make the bones lighter and house red

marrow.

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Cartilages of the Skeletal System• Cartilages provide

– Strength, flexibility, and support– Avascular– Surrounded by layer of perichondrium

• dense irregular connective tissue and site of blood vessels and nerves

– Large amount of matrix (ground substance, fibers)– Chrondrocytes are cells of cartilage

• Hyaline and fibrocartilage– two most important cartilages of the skeletal

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Hyaline Cartilage• Primary cartilage of

skeletal system• Locations include

– Joints formed at the ends of long bones

– Joints formed between ribs and sternum (costal)

• Characteristics– Strong, flexible, supportive

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Fibrocartilage• Location:

– found in some joints (knee, pubic symphysis and intervertebral discs)

• Characteristics– Strong, compressible, and flexible

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Growth of Cartilage

• Appositional– Growth at the perichondrium

• Interstitial– Growth from within the cartilage

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Appositional Growth of Cartilage

• Appositional growth (growth from outside)– Occurs at the perichondrium

(rich in blood vessels)– Mitosis produces additional

cartilage cells– Secretion of matrix increases

amount of intercellular material

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Appositional Growth of Cartilage

• Interstitial growth (growth from within)– Chondrocytes within the

cartilage undergo mitosis– Secretion of matrix increases

amount of intercellular material

– Interstitial growth is limited due to avascular nature of cartilage

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Bone DevelopmentThe skeleton begins as an embryonic

framework composed of hyaline cartilage and fibrous membranes.

• Intramembranous ossification– Osteogenesis in membranes– Typically, produces flat bones

• Endochondral ossification– Osteogenesis in hyaline cartilage– Typically, produces long, short

and irregular bones.

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Intramembranous Ossification• Replaces the fibrous membranes to form flat

bones. – Osteoblasts (bone-producing cells) differentiate– Ossification sites develop as osteoblasts secrete

bone matrix. – Osteoblasts develop at the surface of the bone

matrix; some become trapped in the matrix and differentiate into osteocytes (bone cells)

– Trabeculae (small plates of bone) form, osteoblastactivity continues their enlargement

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Intramembranous Ossification– Trabecular (spongy) bone is formed as a

framework as the ossification sites continue to enlarge and replace the fibrous membrane.

– Periosteum forms as connective tissue at surface of the trabecular framework is replaced

– Osteoblast activity continues to increase the thickness of the bone

Eventually, bone is modified and modeled into compact bone that encloses the innertrabecular (spongy) bone.

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Fetal Skull Development• Ossification center is seen as a

cone-shaped region of thick bone • Fibrous membranes (fontanels),

are located between the developing bones

• The fontanels give “flexibility” to the head during birth and allow for continued growth of the cranium– Anterior (frontal) fontanel– Posterior (occipital) fontanel– Anterolateral (sphenoidal) fontanels– Posterolateral (mastoid) fontanels

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Endochondral Ossification

• Replaces the hyaline cartilage that is functioning as the embryonic skeleton. – Forms most bones of the body– The hyaline cartilage serves as the site and the

model for the formation of the bone.

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EndochondralOssification

Begins

• Periosteum is formed when perichondrium is invaded by osteoblasts– The periosteum is enriched with blood

vessels, and its inner layer develops osteo-blasts that begin to deposit bone matrix as a collar around the cartilage shaft.

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• The cartilage cells (chondro-cytes) in the center of the shaft begin hypertrophy (enlargement) and their surrounding matrix calcifies.– The mineralization results in the

inability of nutrients to diffuse to the chondrocytes. The chondrocytes die, and their surrounding matrix begins degeneration and cavities form.

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• Blood vessels from the periosteum invade the central area and form capillary networks.

• Osteoblasts arrive and begin the secretion of bone matrix on the remaining cartilage matrix to produce bony trabeculae.

Result:primary ossificationcenter

– This initial area of cartilage enlargement, degeneration, invasion by blood vessels and osteoblasts, and the deposition of bony trabeculae

Primary Ossification Center

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Formation of Diaphysis and Epiphysis• Formation of the diaphysis is

characterized by:– (1) the formation of periosteum– (2) the production of a bony collar

around the cartilage shaft– (3) ossification at the primary ossification

center, • Epiphyses, the two regions of hyaline

cartilage at the ends of the diaphysis. • Metaphysis is the region between the

diaphysis and an epiphysis where the diaphysis grows in length as the cartilage is replaced by newly formed bone.

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Growth of the Diaphysis• Hyaline Cartilage of Epiphysis

– Region of proliferation is region (away from metaphysis) of the hyaline cartilage which continually mitotically divides.

– Region of hypertrophy is region of older cells (toward the metaphysis) which undergo enlargement. They also undergo mineralization(calcification) of the matrix.

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Regions of Metaphysis• Epiphyseal surface of the metaphysis,

– the cartilage cells die, – most of the matrix degenerates, – cavities are formed.

• Diaphyseal surface of the metaphysis,– osteoblasts secrete bone matrix on remaining

cartilage spicules– network of bony trabeculae forms the

diaphysis.

• Growth in length – continues by forming bony trabeculae as the

cartilage is removed. The mitotic activity of the cartilage maintains its presence in the epiphysis. Later, the bone is remodeled by osteoclasts, and a marrow cavity is formed.

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Secondary Ossification Center

• Hyaline cartilage of the epiphysis(except the articular hyaline surface) is replaced by epiphyseal ossification. – 1)Periosteum is formed from

perichondrium.– 2)Bony collar produced around the

epiphysis– 3)Hypertrophy of chondrocytes in

the middle of the epiphysis. The matrix undergoes mineralization (calcification)

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Secondary Ossification Center

– 4) Chondrocytes die, • matrix begins to deteriorate, • cavities are formed. • Blood vessels enter the area

and form capillary networks.• Osteoblasts enter the area

and secrete bone matrix around the existing cartilage matrix and form numerous trabeculae.

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Secondary Ossification Center

– 5) Hyaline cartilage is replaced by trabecular(spongy) bone in an outward direction until two hyaline cartilage areas remain, the 1) articular cartilage and 2) the epiphyseal plate

• Articular (hyaline) cartilage remains as the permanent, outer cartilage surface of the joint.

• Epiphyseal plate remains as the inner cartilage plate, the where longitudinal growth of the bone continues to occur

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Longitudinal Growth• X-ray of adolescent shows

epiphyseal plate located between epiphysis and diaphysis

• Epiphyseal plates gradually narrow (due to hormonal regulation) as chondrocyteactivity decreases and osteoblastactivity increases.

X-ray of adolescent

Secondary ossification

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Epiphyseal line

• Epiphyseal plates are completely removed (eventually), and a line of bone, called the epiphysealline marks their prior location.

Split femur, adult

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Growth in Diameter(Appositional Growth)

• Occurs at osteogenic layer of the periosteum. – Osteoblasts secreting bone matrix onto

existing bone. – Periosteal blood vessels are encircled

with matrix, and osteoblast activity lays down layers of matrix (lamellae) to produce Haversian systems (osteons).

– Eventually, a layer of compact bone is formed. Overall, the thickness of layer of compact bone (thickness of diaphysis) is regulated by osteoblast and osteoclastactivity at the endosteum (lines the medullary cavity) and periosteum.

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Bone Remodeling

• Bone modifications include changes for strength, calcium maintenance, healing, etc.

• Haversian systems are initially laid down in compact bone in mostly parallel design.

• The initial Haversian systems areeroded and their “incomplete”replacement leaves asymmetrical fragments, the interstitial lamellae, of the original systems

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Dynamic Nature of Bone

• Bone remodeling is a life-long process• Functions to remodel the skeleton to

account for changes in growth, lifestyle, and for the maintenance of blood ionic calcium.

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Exercise and Bone

• Affect of stress is the generation of small electrical currents which stimulate osteoblasts to produce new bone material.

• Stressed bones become heavier and thicker with more prominent markings (for corresponding attachment to tendons and ligaments.

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Hormonal –Nutritional Effects

• Dietary maintenance of calcium and phosphate salts

• Calcitrol (hormone) synthesized in kidneys necessary for absorption of calcium and phosphates from intestine (made from cholecalciferol (vitamin D3) from diet and synthesized from skin.

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Hormonal –Nutritional Effects• Vitamin C necessary for synthesis of collagen

(gives bones strength).• Other vitamins such as A, K and B12 necessary in

protein synthesis.• Growth hormone (stimulates cell growtha and

protein synthesis ) and thyroxine (increases cell metabolism) are important in bone development.

• Sex hormones, androgens and estrogens, contribute to development and epiphyseal plate closure.

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Hormonal –Nutritional Effects

Parathyroid hormone (PTH)• Produced by parathyroid glands

• Produced when calcium levels are low• Promote activity of osteoclasts• Promotes production of calcitriol from kidneys• Promotes absorption of calcium and phosphate by intestines

Calcitonin• Produced by thyroid glands

• Produced when calcium levels are high• Promote activity of osteoblasts• Promotes kidney to excrete excessive calcium and since PTH

is low, intestinal absorption is decreased

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Bone Features• Include surface texture and structures of the

bone

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Bone FeaturesElevation and projections• Process - A projection or bump• Ramus – bone extension making an angle with the rest of

the structuresProcesses formed where tendons and ligaments attach• Crest - Ridge of bone, usually narrow• Trochanter - A very large, blunt process• Tuberosity - A large, rounded, roughened process• Tubercle - A small, rounded process• Spine - Narrow, sharp projection• Epicondyle - Process above a condyle for attachment to

tendons or ligaments

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Bone FeaturesProcesses formed for articultion with adjacent bones• Facet - Smooth, flat surface for articulation\• Head - Enlarged end of a bone, often rounded• Neck – narrow constriction between epiphysis and diaphysis• Condyle - Rounded projection with a smooth surface for

articulation to another bone• Trochlea – a smooth grooved process, pulley shaped processDepression• Fossa - Shallow depression• Sulcus – Shallow grooveOpenings

Foramen - A round or oval hole in a boneCanal - A passageway through the boneFissure- An cleft or deep furrowSinus – A chamber of cavity within the bone