SKELETAL TISSUE ANATOMY AND PHYSIOLOGY SUMMER, 2002

I BONE (OSSEOUS) - Functions include support, protection, movement, mineral storage, blood cell production and leverage

A. COMPOSITION: large matrix (Calcium Carbonate/Calcium Phosphate) with

collagenous fibers and osteocytes.

B. CELLS

1. OSTEOPROGENITOR CELLS - unspecialized; derived from mesenchyme; differentiate into
Osteoblasts; found in periosteum, endosteum and in perforating canals associated with blood
vessels; has mitotic capability

2. OSTEOBLASTS - associated with bone formation; secretes organic components with
mineral salts: found on bone surface: no mitotic ability

3. OSTEOCYTES - develop from osteoblasts entrapped by matrix: mature bone cells; maintain
daily cellular activities of bone tissue; no mitotic ability

4. OSTEOCLASTS - develop from circulating monocytes; found on bone surface; involved in
bone reabsorption

C. BONE CLASSIFICATION (based on shape)

1. Long - femur, tibia

2. Short - carpal, tarsal

3. Flat - skull roof, sternum

4. Irregular - vertebrae

5. Sesamoid - develop in tendons, tear drop shaped

6. Sutural ( Wormian) - small flat irregular shaped between flat bones of skull

D. LANDMARKS

1. DIAPHYSIS - shaft

2. EPIPHYSIS - end area

3. METAPHYSIS - junction of diaphysis and epiphysis

4. ARTICULAR CARTILAGE - hyaline cartilage cover of epiphysis

5. PERIOSTEUM - Fibrous (outer) contains connective tissue, blood vessels, mucus and
lymphatic vessels; Osteogenic (inner) contains elastic fibers, blood vessels, and cells capable
of becoming osteoblasts.

6. MEDULLARY (MARROW CAVITY)- within diaphysis: spaces filled with fatty yellow marrow

7. ENDOSTEUM - osteoblasts lining medullary cavity: has few osteoclasts

E. TYPES

1. SPONGY (CANCELLOUS)- many spaces filled with Red Marrow; short flat irregular

shaped + epiphysis of long bone; consists of trabeculae; located where bones are not heavily stressed or where stresses arrive from many directions

2. COMPACT- few spaces; deposited over spongy bone; >diaphysis than epiphysis; consists of Osteons (Haversian system); thickest where stresses arrive from a limited range of directions

E. OSSIFICATION PROCESSES (Osteogenesis) Preliminary Steps/Conditions:

• Use pre-existing connective tissue (fibrous membranes or cartilage)

• Migration of mesenchymal (embryonic CT) cells to bone forming areas. These Increase
cell # and size:

in areas without capillaries, become Chondroblasts;

in areas with capillaries, become Osteoblasts.

1. INTRAMEMBRANEOUS - directly onto fibrous membrane); forms dermal bones- ossification occurs deep in the dermis (e.g. roof skull bones, mandible and clavicle) Sequence:

a. Mesenchymal cells cluster, specialize into osteoprogenitor cells - form Center of
Ossification

b. Spicules develop (outward growth struts of developing bone) secreting matrix
(primarily collagenous fibers) along with calcification of matrix

c. Trabecula formation - once cluster of osteoblasts is completely
surrounded by calcified matrix

d. Adjacent trabecula fuse: form latticework

e. Trapped osteoblasts become osteocytes

f. Red marrow fills spaces between trabecula

g. Connective tissue surrounding this area becomes Periosteum
h. Surface area becomes reconstructed to compact bone

ENDOCHONDRAL - use cartilage template and its cover (Perichondrium); for most bones. Sequence:

a. Centrally located chondrocytes swell expand and eventually disintegrate leaving cavities

b. Blood vessels penetrate middle shaft area causing osteoprogenitor cells to enlarge -
form osteoblasts

c. Compact bone collar forms around middle area of diaphysis (Primary Ossification
Center): regionally perichondrium becomes periosteum

d. Cartilage cells swell and burst resulting in increased pH and increased salt depositing.

e. Calcification further restricts diffusing nutrients to cartilage so cartilage cells
die/degenerate leaving cavities

f. Blood vessels grow into cavities, enlarging them and cavities eventually join to form
marrow cavity

g. Collar area thickens and lengthens with the continual development of additional
osteoblasts

h. Blood vessels enter epiphysis (Secondary Ossification Center); initially form spongy bone

i. Replace spongy with compact except for articular cartilage area and epiphyseal plate Once secondary center is formed bone has replaced all cartilage except for:

1) Articular surfaces of epiphysis

2) Epiphyseal Plate - area between epiphysis and diaphysis

G. BONE VASCULARITY (for long bones)

1. Artery and vein which invade cartilage and form ossification center

2. Metaphyseal vessels - supply blood to inner surface of each epiphyseal plate

3. Periosteal vessels - vessels from periosteum are incorporated into developing bone surfaces

H. BONE GROWTH

1. INTERSTITIAL - primarily length growth; puberty into adulthood and continues as long as epiphyseal plate exists

a. Epiphyseal Plate Components
Epiphyseal Edge

• Zone of Reserve Cartilage - adjacent to epiphysis: irregularly scattered small
chondrocytes; anchors epiphyseal plate to bone

• Zone of Proliferating Cartilage - larger chondrocytes- stacked like coins, much
cell division; replaces dead diaphysis surface cells

• Zone of Hypertrophic Cartilage - larger chondrocytes; column shaped

• Calcified Matrix - few cells, primary dead, osteoclasts n matrix. Maintains
epiphyseal plate attachment to bone of diaphysis. Increase plate area by cell
proliferation and expansion

Diaphyseal Edge

b. Regulated by growth hormones (hGH) and sex hormones. Invasion of osteoblasts
and capillaries from diaphysis results in endochondral ossification. Initially form
spongy bone: reconstruct outer areas with compact bone

EPIPHYSEAL LINE - replacement of reserve cartilage area with bone indicates stopping of interstitial growth; not appositional

APPOSITIONAL - width growth only; throughout adulthood

a. Occurs simultaneously with length

b. Bone lining of marrow cavity destroyed (osteoclasts)

c. Periosteum osteoblasts add new surface osseous

H. BONE REMODELING

Bone constantly metabolically active; replaced continually throughout life span. Organic and mineral

components are continuously recycled and renewed

1. Mechanism: Balance of osteoblast / osteoclast activity

a. Osteoclasts form projections releasing Protyoletic enzymes - digest collagen; lactic/citric acid – dissolve bone salts

2. Regulated/Influenced by:

a. Exercise - shape and thickness reflects the stresses applied to bone.

b. Diet

• Ca, P, Mn and Zn

• Mg - deficiency inhibits osteoblastic activity

• Vit. D - increase Ca absorption

• Vit. C - deficiency decreases collagen production

c. Hormonal influence

• Growth Hormone and thyroxine - stimulate bone growth

• Calcitonin - decrease osteoclastic activity

• Parathyroid (PTH) Hormone -increases osteoclastic activity

• Sex Hormone - increase osteoblastic activity but degenerates cartilage at
epiphyseal plate

d. Miscellaneous

• Pulsating electromagnetic field - increase osteoblastic activity and ties up PTH

• Piezoelectric effect - mechanical stresses causes minerals to release low grade
current increasing osteoblastic formation