Phylogenetic Study on Mineralization during Tooth Development. I.Initial Mineralization in Teeth of Elasmobranchs.

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Mineralization during odontogenesis in elasmobranchs was investigated using electron microscopy and enzyme histochemistry.Abundant fine tubular vesicles, 20-30nm in diameter and limited by a unit membrane, occupy the matrix at the stage of enameloid matrix formation. The tubular vesicles often continue into the parts of odontoblasts, suggesting that the tubular vesicles are derived from odontoblasts. Fine enameloid crystals initially appear at the tubular vesicles and a unit membrane surrounds the crystals. Hence, it is assumed that the initial crystals are formed in the tubular vesicles and odontoblasts are deeply involved in the initial mineralization.At the stage of enameloid mineralization, many mell-defined crystals are scattered through the enameloid layr. No accumulation of such crystals along the collagen fibrils is found in elasmobranch enameloid. At the stage of enameloid maturation, at which mineralization in dentine begins, many matrix vesicles, which often contained slender crystals, are found in the predentine. The matrix vesicles are probably involved in the mineralization at least in the part of dentine. Otherwise, columnar inner dental epithelial (IDE) cells exhibit well-developed sER,many mitochondria, vesicles, vacuoles and electron-dense bodies in the cytoplasm. Large numbers of round or elongated vesicles are found near the distal end. Alkarine phosphatase and calcium-magnesium dependent adenosin triphosphatase activities are localized at the cell membrane of the IDE cells. Acid phosphatase activity is observed in the cytoplasm of the outer dental epithelial (ODE) cells in some specimens. It is postulated that the IDE and ODE cells play an important role in the removal of the organic matrix from the enameloid and in the calcium transport to the enameloid at the stages of enameloid mineralization and maturation.