The skeletal system or bones protect the internal organs, provide rigid kinematic links and muscle attachment sites, facilitate muscle action to provide body movement and act as a storage reservoir for certain minerals (Ca, P). Bone structure is complex consisting of cells surrounded by an extracellular matrix. The matrix is biphasic containing both inorganic material of mineral salts that make bone hard and rigid combined with organic material that provides flexibility and resilience. This combination of strong, brittle material embedded in the weaker but flexible matrix results in a material that is stronger per unit weight than either material on its own. The mineral portion of bone is primarily calcium and phosphorus in the form of hydroxyapatite, Caio(P04)6(OH)2, and accounts for 65-70% of the dry weight. About 95% of the extracellular matrix is comprised of type I collagen fibers that are tough but pliable and resist tension with little deformation. Proteoglycans, protein polysaccharides or glycosaminoglycans (GAG) and a smaller amount of non-collagenous proteins such as bone morphogenetic protein (BMP) surround the collagen fibrils.
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