Tooth enamel and enameloid in actinopterygian fish

Abstract

The morphological features of tooth enamel and enameloid in actinopterygian fish are reviewed to provide basic data concerning the biomineralization of teeth in lower vertebrates. Enameloid, which covers the tooth surface, is a unique well-mineralized tissue and usually has the same functions as mammalian tooth enamel. However, the development of enameloid is different from that of the enamel produced by dental epithelial cells. Enameloid is made by a combination of odontoblasts and dental epithelial cells. An organic matrix that contains collagen is provided by odontoblasts, and then dental epithelial cells dissolve the degenerate matrix and supply inorganic ions during advanced crystal growth in enameloid. It is likely that enameloid is a good model for studying the growth of well-mineralized hard tissues in vertebrates. Some actinopterygian fish possess a collar enamel layer that is situated at the surface of the tooth shaft, indicating that the origin of tooth enamel is found in fish. Collar enamel is thought to be a precursor of mammalian enamel, although it is thin and not well mineralized in comparison with enameloid. In Lepisosteus and Polypterus, both of which are living actinopterygians, both enameloid and enamel are found in the same tooth. Therefore, they are suitable materials for examining the developmental processes of enameloid and enamel and the relationship among them.