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Lecture 6: Skeletal Tissue
I. Functions of Skeletal System
A. Support
B. Protection
C. Movement
D. Mineral Storage (Calcium + Phosphorus)
E. Hematopoiesis (blood cell formation in red marrow)
F. Energy Storage (lipids/fat stored in yellow marrow)
II. Histology of Skeletal Tissue (Osseous Tissue)
A. Different Cell Types
1. osteoprogenitor cells give rise to osteoblasts
a. found in periosteum, endosteum, and canals
2. osteoblasts secrete proteins, Ca, P
a. found on bone surface (where growth occurs)
3. osteocytes maintain bone integrity
a. in the bone tissue itself, matrix surrounds
4. osteoclasts degrade and absorb bone during growth
a. derived from white blood cells, on surface
B. Chemical Composition
1. 33% collagenous fibers as in connective tissue
2. 67% mineral salts - calcium phosphate + carbonate
3. hardening depends on correct amount of each
C. Classification of Bones
1. Long Bones – considerably longer than wide; shaft with 2
ends (most limb bones, finger bones, etc.)
2. Short Bones – roughly cube shaped (wrist bones)
i. Sesamoid bone – short bone within a tendon (patella)
3. Flat bones – Flattened, thin, and usually curved (most cranial
bones, ribs, sternum, and scapula)
4. Irregular bones – irregularly shaped (vertebrae, hip bones)
III. General Anatomy of a Long Bone (eg. humerus)
A. Diaphysis - main shaft of the bone
B. Epiphysis - large end of the bone
C. Metaphysis - where above meet during bone growth
D. Articular Cartilage - covers epiphysis, reduce friction
E. Periosteum - dense, white covering around the bone
a. fibrous layer - blood, lymph, nerves pass through
b. osteogenic layer - where bone cells originate
F. Medullary (marrow) Cavity - adults, yellow marrow
G. Endosteum - lines medullary cavity, houses bone cells
IV. Microanatomy of Compact (Dense) Bone
A. General Features
1. few empty spaces (dense)
2. thicker in diaphysis than epiphysis
3. concentric ring-like structure
B. Osteon (Haversian System) - Components of Compact Bone
1. central (Haversian) canal - vessels and nerves
2. osteocytes - mature bone cells (from osteoblasts)
3. lacunae - spaces where osteocytes reside
4. lamellae - rings around canal, house lacunae
5. canaliculi - projections from lacunae + osteocytes
C. Supporting Structures
1. perforating (Volkmann) canals - run perpendicular
2. interstitial lamellae - between osteons
V. Microanatomy of Spongy (Cancellous) Bone
A. General Features
1. many spaces filled with red marrow (not yellow)
2. most of epiphysis of long bones in adult
3. found in short, flat, irregular bones
B. Trabecular Lattice Structure
1. trabeculae - irregular, sponge-like network
2. lacunae - spaces within trabeculae for osteocytes
3. spaces - filled with red marrow (hematopoiesis)
VI. Ossification: The Formation of Bone During Development
A. Different Cells Involved
1. mesenchymal cells - migrate to location in embryo
2. chondroblasts - cartilage formation
3. osteoblasts - bone formation
B. Intramembranous Ossification (from fibrous membrane)
1. ossification center - where osteoblasts concentrate
2. osteoblasts secrete collagen fibers for matrix
3. calcification - Ca salts secreted to cement matrix
4. osteoblasts surrounded --> osteocytes
5. trabeculae - result when hardened bone forms
6. spongy bone now in place with red marrow
7. in time, spongy bone reconstructed -> compact bone
C. Endochondral Ossification (replacing hyaline cartilage)
1. cartilage "bone model" formed in the embryo
2. perichondrium - membrane around the cartilage
3. vessel penetrates cartilage, brings osteoblasts
4. cartilage converted into compact bone
5. perichondrium --> periosteum
6. chondrocytes gradually hypertrophy and die
7. vessels move into space and convert to bone
8. primary ossification center - in diaphysis
9. secondary ossification center - in epiphysis
10. epiphyseal plate - between the two, still cartilage
VII. Bone Growth and Remodeling
A. Four Zones of Epiphyseal Plate - Longitudinal Growth
1. zone of reserve cartilage
a. chondrocytes scattered throughout
b. anchor plate to epiphyseal bone
2. zone of proliferating cartilage
a. chondrocytes stacked together
b. replace dead cells at diaphyseal surface
3. zone of hypertrophic cartilage
a. larger chondrocytes, close to diaphysis
b. chondrocytes form mature cartilage
4. zone of calcified matrix
a. dead chondrocytes, calcified matrix around
b. absorbed by osteoclasts
c. osteoblasts lay down new bone on remains
d. metaphysis - between epiphysis and diaphysis
B. Bone Remodeling - Spongy Bone Converted to Compact Bone
1. osteoclasts - resorption of old bone tissue
a. lysosome release of digestive enzymes
b. cell "phagocytoses" (engulfs) particles
2. osteoblasts - lay down new bone in its place
3. bone constantly undergoes remodeling throughout life
a. Ca needed in muscle, nerve, blood clotting
b. fractures repaired immediately
4. Factors essential for proper bone growth
a. Ca and P in proper amount in diet
b. trace amounts of Boron and Manganese
c. Vitamin D - regulates Ca metabolism
d. Vitamin C - maintenance of bone matrix
e. Vitamin A - osteoclast/blast function
f. Vitamin B12 - osteoblast function
g. Human Growth Hormone (HGH) - pituitary
h. Calcitonin - thyroid, Ca absorption to bone
i. Parathormone - parathyroid, Ca release to blood
j. Sex Hormones - Testosterone + Estrogen