1. Skeletal System Part I
Biology 2121

2. Skeletal System Functions Axial vs. Appendicular Number of Bones
1. Support
2. Mineral and Lipid storage
3. Hematopoiesis
4. Protection
5. Leverage

3. Introduction Bone Shapes Structural Types 1. Long 2. Short 3. Flat
4. Irregular
5. Sutural
6. Sesamoid
Structural Types
1. Compact
2. Spongy

4. Structural Characteristics of Bone
Structure of a Long Bone
Diaphysis – ”shaft”
medullary cavity
Epiphyses
distal and proximal
Metaphyses
Cartilage
Articulating surface
Hyaline cartilage
Periosteium
Covers compact bone
Endosteum
Medullary cavity membrane

5. Histology of Compact Bone Tissue
Osteons – Haversian Units
Central (Haversian canal)
Lamellae
Osteocytes in ‘lacunae’
Perforating Canals
Periosteal arteries
Canaliculi
Other Cell Types
Osteogenic
Osteoblasts
Osteoclast cells

6. Matrix of Bone Calcium salts interact to form hydroxyapaptite crystals
Calcium phosphate + calcium hydroxide
Calcium carbonate is added along with ions (sodium, magnesium, etc.)
Collagen fibers give bone flexibility

7. How Does Spongy Bone Histology differ from Compact Bone?
1. Do not contain osteons.
2. Lamellae – irregular arrangement
Trabeculae
3. Light
4. Support and protects red bone marrow
Location
Short, flat and irregular bone
Location of Red Marrow
Hips, ribs, breastbone, vertebrae, epiphysis of long bones

8. How do Bones Form?
Bones begin their formation during embryonic development
Ossification vs. calcification
Two methods of bone formation:
1. Intramembranous Ossification
2. Endochondral Ossification

9. Intramembranous Ossification
Osteoblasts differentiate in mesenchymal fibrous connective tissue
‘Dermal ossification’
Dermal bones formed
Examples: flat bones of skull; mandible; clavicle

10. Endochondrial Ossification
A cartilage model is replaced by bone.
Six-Eight weeks bones are made of cartilage
Cartilage continues to expand (interstitial growth) and new cartilage develops at outer surface (appositional growth)
Role of osteoclast cells
Ossification centers

11. Endochondrial Ossification

12. Abnormal Bone Growth Gigantism Dwarfism GH overproduced in puberity
Acromegaly
Post-puberity (bones thicken)
Dwarfism
Pituitary growth failure

13. What is needed to allow for normal bone growth and repair?
Diet
Calcium and phosphate
Homones
Calcitriol - from vitamin D
Growth Hormone
Estrogen – females
Androgens – males
Vitamins
D, A, K and B12

14. Homeostatic Control Two hormones must be balanced: Homeostatic Control
Parathyroid hormone (PTH)
Calcitonin
Homeostatic Control
Effects of PTH – counteracts low calcium levels
Stimulates osteoclast activity
Increases intestinal absorption of calcium ions
Decreases rate of calcium
Effects of calcitonin – counteracts high calcium levels
Inhibits osteoclast activity
Increases rate of excretion of calcium ions
Calcium Reserves

15. Calcium Homeostasis

16. Consequences of Calcium Imbalances
Osteopenia is a natural process
Reduction in bone mass – osteoporosis
Risk Factors
Sex-hormones
Smoking
Other

17. Bad Breaks, Repair and Remodeling
Fractures
Compound vs. Simple
Stress Fractures
Repair
Blood clot (hematoma) forms
Internal and external callus eventually replaced by bone.
Remodeling
Bone constantly removed and replaced
Young: 1/5th replaced every year
Spongy bone replaced faster, more often