Calcified Tissues: Chemistry and Biochemistry

Over 90% of the calcium in the human body is deposited as calcium phosphate (hydroxyapatite) within the “hard tissues” of the skeleton and the teeth. Hydroxyapatite is also frequently found pathologically in metastatic and dystrophic deposits (Dieppe and Calvert 1983). The extracellular matrices of the physiologically calcified tissues (calcified cartilage, ligamentous and tendinous insertions, the mineralized matrix of bone itself, as well as the dentine, cementum, pulp and enamel of teeth) contain from 20% to 90% calcium phosphate. The presence of the mineral within these tissues provides them with strength and rigidity, allowing them to function properly. The mineral content is generally expressed either as the proportion of mineral found per unit weight of dry tissue (ash content) or as the proportion of mineral in the intact organ. These parameters may be quite different; for example, in the osteoporotic bone the mineral content is markedly reduced due to the increased porosity (decreased density) of the bone, while the distribution of mineral in the tissue of that bone (ash content) may not be altered. In general, the extent of mineralization depends upon tissue function, site within the tissue, age and species. For example, the ash content of embryonic chick bones increases during development from 13% to 51% (Pellegrino and Biltz 1972). In general, the ash content of most bone in mature animals is between 50% and 70%. However, the bones of the ear are 80%–90% mineral (Bonar and Lees 1984). The hydroxyapatite content of the extracellular matrices of some representative mammalian hard tissues is summarized in Table 7.1.