Current studies on tooth enamel development in lower vertebrates.

The tooth enamel is a unique tissue in the human body on many characteristic features, in both morphological and developmental aspects. This tissue is widely distributed in the vertebrate world. This hard tissue is divided into two groups, mainly from the developmental feature. One is the enamel in the upper classes of the adult amphibian, called true or ectodermal enamel. The other mainly distributes in fish (both of osteichytyes and chondroichtyes), called enameloid or mesodermal enamel. Enameloid seen in fish has many characteristics compared with enamel, although enameloid after eruption is analog tissue to enamel. Many studies on the enameloid with a long history show its character. (1) The enameloid matrix is mainly composed of collagen fibrils to make thick bundles, most of which are interwoven in a complicated manner. (2) Mineralization of enameloid is initiated by the matrix vesicles similar to bone and dentine. (3) Growth of crystals does not stop the retention of their thin needle shape as with bone and dentine during mineralization, but finally begins to show a large hexagonal shape, as seen in the mature enamel of mammals. (4) Enameloid contains much fluoride and the fluoride exists as a fluoroapatite in enameloid. Many of the recent studies on this subject applied various histochemical methods, especially immunohistochemical studies, which show some evidence that enameloid and enamel might not be continuous in enamel evolution. It is now necessary to reconsider the enameloid from the phylogenetic aspect.