Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 102

Bone is a material constructed of a flexible collagenous matrix that is intermingled with rigid mineral crystals. Thus, bone is harder and less pliable than tendons or cartilage. The extracellular matrix of bone is produced by osteoblasts and consists of inorganic and organic components. For example, osteoid is defined as the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue; it is composed primarily of collagen type I. In both unmineralized and mineralized bone, the organic component of bone (40% of bone’s dry weight) include collagen type I fibers (90% of the organic matrix and ~97% of the collagenous proteins) and a number of noncollagenous proteins, such as osteocalcin and bone sialoprotein, which may orchestrate mineralization (Hu, Peel, Ho, Sandor, & Clokie, 2009; Robey et al., 1993). The inorganic component of bone represents about 60% of the dry weight of bone matrix and is composed mainly of abundant calcium and phosphorus, as well as smaller amounts of bicarbonate, citrate, magnesium, potassium, and sodium. Calcium forms hydroxyapatite crystals with phosphorus, although it is also present in an amorphous form (Junqueira & Carneiro, 2005; Khurana, 2009). Calcium hydroxyapatite crystals are arranged parallel to collagen fibers. This orientation maximizes the collagen’s resistance to tensile (stretch) forces and calcium hydroxyapatite’s resistance to compressive forces (Gartner & Hiatt, 2007; Sela, Amir, Schwartz, & Weinberg, 1987). In life, bones also have a large amount of water (~25% of the total bone weight) (Ailavajhala, Oswald, Rajapakse, & Pleshko, 2019; Ailavajhala, Querido, Rajapakse, & Pleshko, 2020).