Skeletal System Part I Biology 2121
Skeletal System Functions Axial vs. Appendicular Number of Bones 1. Support 2. Mineral and Lipid storage 3. Hematopoiesis 4. Protection 5. Leverage
Introduction Bone Shapes Structural Types 1. Long 2. Short 3. Flat 4. Irregular 5. Sutural 6. Sesamoid Structural Types 1. Compact 2. Spongy
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
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
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
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
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
Intramembranous Ossification Osteoblasts differentiate in mesenchymal fibrous connective tissue ‘Dermal ossification’ Dermal bones formed Examples: flat bones of skull; mandible; clavicle
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
Endochondrial Ossification
Abnormal Bone Growth Gigantism Dwarfism GH overproduced in puberity Acromegaly Post-puberity (bones thicken) Dwarfism Pituitary growth failure
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
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
Calcium Homeostasis
Consequences of Calcium Imbalances Osteopenia is a natural process Reduction in bone mass – osteoporosis Risk Factors Sex-hormones Smoking Other
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