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Week 9 Study Guide0

Week 9 Study Guide0 - Week 9 Bone Growth and Calcium...

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Week 9 Bone Growth and Calcium Homeostasis Hormones Parathyroid Hormone Calcitriol Calcitonin (?) Prolactin (PRL) Sex hormones (estrogens and androgens) Growth Hormone (GH) and Insulin-Like Growth Factor-1 (IGF-1) Mechanisms of Bone Formation and Resorption Osteoblasts Osteoclasts Pathologies Hypocalcemia and Hypercalcemia Rickets and Osteomalacia Osteoporosis Bone Bone is composed of a calcium phosphate precipitate called hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 that is deposited upon a collagen matrix. There are two main layers of bone: the cortical or compact bone, which makes up about 80% of the bone mass and consists of a thick, dense structure, and the trabecular bone, which makes up the final 20% of the bone mass and consists of a spongy, porous matrix. Whereas the compact bone contributes a rigid characteristic to the bone structure, allowing the skeletal system to bear the weight of the body as well as external forces, the trabecular bone contributes an elastic characteristic, allowing bone to compress under longitudinal forces. Additionally, the porous structure of the trabecular layer allows for the vascularization of bone and development of bone marrow, in specific bones. Both components are crucial for bone structure in that if the skeletal system were composed solely of compact bone, it would easily shatter when subjected to pressure, like a ceramic mug. Further, because compact bone is quite heavy and dense, a skeletal system composed purely of compact bone would require high energy consumption. In contrast, if the bone were made purely out of the trabecular layer, then it would not be able to withstand the weight of the body or external forces. Thus, it is the combination of the two layers that gives bone its durable, yet flexible characteristic, like a sword. Bone is a dynamic tissue that is constantly undergoing remodeling due to the action of three main types of cells. Osteoblasts are monucleated bone forming cells that contribute to linear growth during the growing years as well as to bone sustainment in the adult. The osteoblasts secrete enzymes and osteoid, which is a mixture of collagen and other proteins that bind to calcium phosphate crystals, forming hydroxyapatite. The bone shaft is called the diaphysis and the two distal ends of bone are called the epiphyses. During the growing period, linear growth occurs at the epiphyseal plate, which is located at either end of the bone between the epiphysis and the diaphysis. The epiphyseal plate consists of chondrocytes, which are continually dividing cells that produce and lay down the collagen of cartilage. As the collagen layer thickens into cartilage, the chondrocytes degenerate and are replaced by osteoblasts, which cause the ossification of cartilage into bone by depositing a layer of osteoid, which then attracts
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and favors the deposition of calcium phosphate crystals, forming hydroxyapatite. Thus, bone consists of inner layer of cartilage that is covered with osteoid, which in turn is covered by hyrdoxyapatite. During the growing years, this process continues and is stimulated by the
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