Fine structural changes and lysosomal phosphatase cytochemistry of ameloblasts associated with the transitional stage of enamel formation in the rat incisor.

Trimetaphosphatase (TMPase) and cytidine-5'-monophosphatase (CMPase) were used as lysosomal markers in the transitional ameloblasts (TA) to investigate the distribution of lysosomal structures and to correlate the cytochemical findings with the ultrastructural features of these cells. Of particular interest were the cytochemical and morphological changes which occur as the ameloblasts approach the maturation stage of enamel formation. The sequence of changes observed provides a basis for designation of three regions of the transitional zone (early and late TA and modulating ameloblasts). In the early TA region, the cells decreased in height and contained phagic vacuoles as well as numerous TMPase and CMPase reactive structures. Late transitional ameloblasts had invaginations at their distal ends as well as membrane-bound structures, both filled with fine granular material. Dense bodies, phagic vacuoles, and other elements of the lysosomal system were enzyme reactive. Modulating ameloblasts lacked the phagic vacuoles but exhibited large numbers of multivesicular bodies, vesicles, and secretory granules. Their distal ends were morphologically altered indicating a change towards ruffle- or smooth-ended varieties of maturation ameloblast. In the former, increased granular material was observed within cell membrane invaginations and associated membrane-bound structures. In the latter, intercellular spaces widened and were filled with granular material. The present cytochemical findings of an extensive lyosomal system in transitional ameloblasts confirm the function of those cells in reducing the secretory ameloblast population and in the selective elimination of their protein-synthesizing organelles. Furthermore, this extensive lysosmal system and the present morphological findings are consistent with a potential role for transitional ameloblasts in contributing to the marked loss of enamel protein known to occur during maturation.