Electron microscopic study of early formation of the tooth enameloid of a fish (Hoplognathus fasciatus). I. Odontoblasts and matrix fibers.

An electron microscope study was made on the tooth germs of Hoplognathus fasciatus in early developmental stage. Special attention was given to the odontoblasts, enameloid matrix fibers, calcification of enameloid and the hitherto controversial origin of the enameloid. 1. The ameloblasts and the odontoblasts are demarcated by a single layer of basement membrane which persists until immediately before the calcification of the enameloid matrix. 2. This histogenesis of the enameloid matrix begins with the formation of non-striated fibers 140-180 A thick. They are arranged in a direction vertical to the basement membrane and parallel to the sides of odontoblasts. Fibers with regular cross striations of 640 A periodicity later appear and the entire enameloid matrix is formed by these fibers. 3. Along with the formation of the enameloid, the odontoblasts assume a high columnar form, with a marked increase in cell organelles which show marked polarity suggesting active protein synthesis. Numerous odontoblastic processes are noted in the enameloid matrix. Granules, representing precurosors of enamleoid matrix fibers occur in the odontoblasts. Based on these findings, the enameloid matrix fibers must be of mesodermal origin. 4. Deposition of crystals of small size, needle and tube in shape, occurs in the circumference of the fiber bundles. As calcification progresses, crystals appear in the central portion of the bundles. Later, large crystals of rod and platelet shapes become intermingled. In addition to this, small crystals are fused, forming aggregates.