Normal bone is composed of organic matrix and inorganic matrix.
• The organic matrix includes cells, type I collagen (90% of bone protein), osteo-calcin, glycoproteins, and proteoglycans.
• The inorganic matrix includes calcium hydroxyapatite Ca10(PO4)6(OH)2, mag-nesium, potassium, chloride, sodium, and fluoride.
There are 3 cell types.
• Osteoblasts are responsible for the production of osteoid (unmineralized bone); they contain high amounts of alkaline phosphatase, have receptors for parathyroid hormone (PTH), and modulate osteoclast function.
• Osteocytes are responsible for bone maintenance; they are osteoblasts that have become incorporated in the matrix.
• Osteoclasts are responsible for bone resorption; they contain high amounts of acid phosphatase and collagenase, and resorb bone within Howship’s lacunae.
Bone remodeling occurs throughout life and is necessary to maintain healthy bones. Bone resorption by osteoclasts is tightly balanced with bone formation by osteo-blasts.
Important hormones involved in bone physiology include parathyroid hormone (PTH), calcitonin, vitamin D, estrogen, thyroid hormone, cortisol, and growth hor-mone.
Formation of bones is as follows:
• Intramembranous bone occurs as direct bone formation without a “cartilage model.” Intramembranous bones include flat bones such as the cranium, clavicle, vertebrae, wrist, and ankle bones. Intramembranous growth is also involved in appositional bone growth.
• Endochondral bone is indirect bone formation from a “cartilage model” at the epiphyseal growth plates; this type of bone formation occurs in long bones such as the femur, humerus, tibia, fibula, etc.
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