Organic Matrices and Mineral Crystallites in Vertebrate Scales, Teeth and Skeletons

The earliest fossil vertebrates, the ostracoderms, more than 500,000,000 years ago had skeletons which included the hard tissues calcified cartilage, bone, dentine and enamel. It seems plausible that the modes of mineralization pioneered in these primitive tissues have persisted throughout vertebrate radiation without major changes. All four types utilize the calcium phosphate mineral apatite to form inorganic crystallites which develop in close association with matrix proteins. In the collagen-associated tissues—calcified cartilage, bone and dentine—the short, needle-or lath-like crystallites develop along collagen fibrils. Non-collagenous components of the matrix in these tissues include proteoglycans, glycoproteins and matrix vesicles. Special calcium-binding or phosphatebinding constituents may promote crystal nucleation and growth. In enamel (enameloid) large hexagonal crystallites develop in association with the glycoproteins amelogenin and enamelin secreted by cells of the inner dental epithelium (ameloblasts). Enamel (enameloid) of the shark tooth resembles mammalian enamel in size and shape of crystallites. Biochemical and immunohistochemical techniques are being used in several laboratories to determine how these proteins control the mineralization of enamel, and whether they are homologous in all vertebrate classes.