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Bone I. Structure Review A. Compact bone vs. spongy bone 1. Compact- dense matrix, on surface of bone, resists pressure from one general direction. Lacunae, canaliculi & lamellae (see below, under “bone cells”) arranged in concentric circles, forming units called osteons. Blood vessels travel through a central canal in the center of each osteon. 2. Spongy- matrix bundled into struts called trabeculae, inner part of bone, resists pressure from many directions, not organized into osteons. Forms in branches, called trabeculae. Blood vessels extend between trabeculae. Red marrow common in spongy bone. B. Coverings 1. Periosteum- contains a fibrous, dense irregular connective layer and a cellular layer composed of bone cells. The dense irregular connective layer is an extension of the deep fascia. The cellular layer contacts the bone. Collagen fibers, "Sharpey's fibers," tie the periosteum to the bone. 2. Endosteum- a layer of bone cells that lines the inner part of bones: marrow cavities & trabeculae. C. The matrix: collagen (strong & flexible) laid down by calcium salts (strong and brittle) creates a strong, non-brittle structure. Calcium salts: Calcium phosphate, calcium hydroxide, calcium carbonate. Calcium phosphate & calcium hydroxide combine to form the complex salt, hydroxyapatite. Other minerals (ex, magnesium) are also part of the matrix crystals. II. Histology Review A. Bone cells- what they do and where they live 1. Osteoprogenitor cells- give rise to osteoblasts. Important in repair, growth & restructuring of bone. Located in periosteum & endosteum. 2. Osteoblasts- produce & secrete the organic matrix (collagen). Collect and deposit calcium salts upon collagen. Located in periosteum & endosteum. Give rise to osteocytes. 3. Osteocytes- mature bone cells. In a growing bone, as osteoblasts produce matrix, the matrix grows around them and they become encapsulated. Now they are considered osteocytes. Osteocytes live in cavities of the matrix called lacunae. Osteocytes send extensions through tunnels in the matrix called canaliculi. Neighboring osteocytes communicate with each other through gap junctions at the ends of their cytoplasmic extensions. Layers of calcified matrix between canaliculi are called lamallae. Osteocytes maintain the matrix locally, breaking it down and building it up. 4. Osteoclasts- dissolve the bone matrix. Not related in origin to other bone cells. Located in periosteum & endosteum. B. Remember that bones are constantly being remodeled; that dynamic nature is because of the activity of the above listed bone cells. When the activity of osteoblasts outpaces that of osteoclasts, bones grow, and vice-versa. III. Bone remodeling and repair- bones are constantly being remodeled: built up & broken down. A. Repair- when bones are fractured, a blood clot forms around the injured area to reduce blood loss. Fibroblasts and Osteo -cytes, -blasts, and -progenitors undergo mitosis and daughter cells migrate to injured area. Some fibroblasts differentiate to chondrocytes and begin building cartilage around the injured area. Dead cells, pathogens and debris are cleaned up by macrophages. Over time, the cartilage is replaced by bone. Remodeling of the affected area may continue for months to years. B. Factors affecting bone remodeling: 1. Hormones [Hormones that regulate blood levels of calcium] a. Calcitriol (active V-D)- required for adequate dietary absorption of Calcium b. Parathyroid Hormone (PTH)- stimulates osteoclasts to dissolve bone & release calcium to blood c. Calcitonin- inhibits osteoclast activity [Hormones that are age related and may affect bones indirectly, for example, by affecting general metabolic rates of cells] d. Growth Hormone and Thyroxine (thyroid hormone)- stimulate osteoblast activity, importance in this respect declines after adolescence. e. Sex hormones (estrogen, testosterone)- support bone maintenance in adults. Women's estrogen levels decline more abruptly (after menopause) than men's testosterone, which may account for the higher incidence of osteoporosis in women (it does affect men). 2. Nutrients (these are among the most critically relevant to bone, but certainly many other nutrients can affect bone either directly or indirectly)a. Minerals- calcium, phosphorus, magnesium b. Vitamins- C (required for collagen production), K (importance in bone health only recently recognized), D (active vitamin D is calcitriol) c. Protein 3. Physical stress to bones (weight-bearing exercise)- both compression on bones and pulling from muscles. Stimulates osteoblast activity.