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Histological examinations of erupted teeth and tooth germs in juvenile and adult dog salmon, Oncorhynchus keta, were carried out by light microscopy, scanning and transmission electron microscopy. As results of these observations, it was found that 1) cap enameloid may only be present in the teeth of juveniles and 2) the inner dental epithelial cells (i. d. e. cells) had great influence on the absorption of the majority of the cap enameloid matrix containing collagen fibers rather than on the formation of tooth matrix.In the teeth of juveniles, the tips of teeth were recognized as hypermineralized and collagen free portions by light microscopic, microradiographic, scanning and transmission electron microscopic observations. In the teeth of adults, however, no such features were seen in the tips of teeth. It seems that cap enameloid is absent in adult teeth.In the stage of matrix formation, the features of the inner dental epithelial cells (i. d. e. cells) indicated that the cells had begun their secretory activity. However, the morphological features of i. d. e. cells indicated that activity of the cells in matrix formation was at a low level and that a great quantity of epithelial secretions widely spread out in the tooth matrix was not obtained except around the basal lamina.On the contrary, active absorptive function at the distal margin of i. d. e. cells is expected and it is considered that absorption of organic substances from the cap enameloid area by i. d. e. cells had already begun in the late stage of matrix formation. Therefore, in the case of juvenile dog salmon, it is assumed that the i. d. e. cells play little part in the formation of tooth matrix and that the majority of the tooth matrix originates from the odontoblasts.In the stage of maturation, the taller and clear i. d. e. cells possessing a remarkable ruffled border at the distal margin were found in the portion surrounding the cap enameloid. It is assumed that such cells absorb the organic matrix containing collagen fibers from the cap enameloid area and make it a hypermineralized area. On the contrary, though the i. d. e. cells corresponding to collar enameloid became taller at this stage, the remarkable ruffled border was not found in these cells. The outermost layer of the collar enameloid area, about 0.5μm in thickness, became more electron-dense and the crystals, which appeared to be arranged perpendicular to the tooth surface, were gathered densely. Considerable difference in the formation mechanism may be present between cap and collar enameloid.

Formation of cap enameloid in the jaw teeth of dog salmon, Oncorhynchus keta.