Mechanisms of Mineralization in the Enameloid of Elasmobranchs and Teleosts

Ultrastructural and cytochemical studies on the mineralization of enameloid were performed using Heterodontus japonicus, an elasmobranch, and Tilapia buttikoferi, a teleost as materials. The mineralization of the enameloid in the Heterodontus was divided into the following two steps: (1) initial crystallization in the tubular vesicles that originated from the odontoblasts, and (2) crystal growth that was accompanied by the degeneration and removal of the organic matrix around the crystals. In the Tilapia, the mineralization of the cap enameloid followed three steps: (1) initial crystallization at the matrix vesicles, (2) aggregation of fine slender crystals along collagen fibrils, and (3) crystal growth with the degeneration and removal of the organic matrix. The pattern of early mineralization and the composition of organic matrix in enameloid were considerably different between the two species examined, while in both species the odontoblasts were mainly involved in the formation of the organic matrix of enameloid and in the initial mineralization. In the next step, remarkable crystal growth associated with the degeneration and removal of the organic matrix occurred in both the elasmobranch and the teleost species. The absorptive functions of the dental epithelial cells in the later stages of enameloid formation is probably similar in the two types of enameloid, and is essential for the production of well-mineralized enameloid.