The fine structure of tomes' process of rat incisor ameloblasts and its relationship to the elaboration of enamel

Abstract

The shape of Tomes' process and its relationship to enamel precursors, the growing enamel, and the apical terminal bars were studied with light and electron microscopes in the enamel organs of the lower incisors of adult rats. It was found that the proximal part of Tomes' process has a complex cross section which is described here as consisting of head, body, and large and small foot processes. The shape of the cross section is related to the direction of tooth eruption and the direction of the presumed sideways motion of the ameloblast. At the level of the apical terminal bars the ameloblast cross section is rectangular, with the long axis in the direction of the axis of the tooth. The major terminal bars have a large number of filaments showing little recognizable order, and an almost uninterrupted zonula adherens. The minor terminal bars have fewer filaments which often form well-defined bundles, and their zonula adherens is more frequently interrupted by membrane invaginations, maculae occludentes, and distensions of the extracellular space. Stippled material was seen first in irregularly shaped vesicles in the ameloblast apex just proximal to the terminal bar level. It appears to be secreted into the extracellular space at the level of the terminal bars and distal to them. It is incorporated into the interrod enamel in an erratic fashion and remains uncalcified for a period of time. The anomaly of its distribution may indicate that it is not an essential enamel component in the rat incisor. No new stippled material was seen to be secreted in rod enamel formation. The first enamel to be formed is a broad band of interrod enamel between Tomes' processes of the same row. It is formed in close proximity to, and thus presumably to a large extent by, Tomes' processes of the neighboring (in apical direction) row. A thin band of interrod enamel between rows is then deposited by ameloblasts of the adjacent rows, while the enamel rod is formed by one ameloblast. At the enamel growth fronts, the three enamel components (crystallites, pericrystal membrane, background matrix) appear almost simultaneously in the enamel space. Secretion granules seem to participate in rod enamel formation by a type of exocytosis.