TEM observations on the ameloblast/enamel interface in the rat incisor

The structure of rat incisor enamel is established at the topographically complex interface between secretory ameloblasts and forming enamel. The aim of this study was to gain additional information on this interface by sectioning parallel with the rows and the long axis of Tomes' processes and prisms. Rats were sacrificed and fixed by glutaraldehyde/paraformaldehyde perfusion. After dissection, demineralization and embedding transverse jaw/incisor segments were cut, reembedded, and reoriented. Sections were prepared for and observed in the transmission electron microscopy (TEM). The intraenamel part of Tomes' process was about 18 microns long. The forming prism occupied a longitudinally grooved invagination on its apical aspect. The parts of Tomes' process forming the side walls of the groove were attenuated and showed variation in extent and outline. Prism growth occurred over the whole grooved area. An estimation of Tomes' process secretory area in rat compared with data from humans suggests that there may be a relationship between secretory area and rate of prism formation. Prism crystals were oriented obliquely or parallel to the secretory surface of Tomes' process. At interprism growth sites matrix deposition was irregular and required some redistribution to conform to the pattern of interprism sheets.