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Ultrastructural and cytochemical observations on the alternating morphologic changes of the ameloblasts at the stage of enamel maturation.

The functional role of the alternating morphologic changes of the rat and hamster incisor ameloblasts at the stage of enamel maturation was investigated. Special attention was paid to the distribution of the intravenously injected horseradish peroxidase (HRP) throughout the ameloblastic layer. The ameloblasts at the stage of enamel maturation were divided into two groups with respect to their distal cell borders: ruffle-ended (RA) and smooth-ended ameloblasts (SA). In the ameloblastic layer, SA were distributed as several band-like structures (SA-bands) which ran transversely or obliquely along the distal surface of the ameloblastic layer. Intravenously injected HRP permeated the intercellular spaces of SA and reached the surface of the enamel, however it did not penetrate the distal junctions of RA. Five min after the injection, HRP was incorporated into the cytoplasmic vesicles of RA, while no incorporation was shown in SA. After 1 hr, however, HRP was incorporated into the incisal one-third of the ameloblasts in each SA-band. Acid phosphatase activity (p-nitrophenylphosphatase) was shown in the cytoplasmic vesicles, both in RA and SA. The most intense alkaline phosphatase activity was located on the ruffled border of RA, while no activity was detected on the unmodified apices of SA. Contact microradiography revealed a gradual increase of the radio opacity of enamel towards the incisal direction independent of the alternating changes of overlaying RA and SA. However, the fluorescence of injected tetracycline showed intense labelling at the portions of enamel which corresponded to RA. The portions of enamel being overlayed by SA were located in between such labelled areas and showed only faint fluorescence of tetracycline. These results suggest that, RA actively degrade, resorb and then digest the organic matrices of the enamel and also transport minerals into the enamel. It is also suggested that SA are formed from the RA which have become inactive metabolically and here the exhausted cytoplasmic organelles seem to be renewed and reactivated.