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

Abstract

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 i.v. injected horseradish peroxidase (HRP) throughout the ameloblastic layer. The ameloblasts at the stage of enamel maturation were divided into 2 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. I.v. injected HRP permeated the intercellular spaces of SA and reached the surface of the enamel, it did not penetrate the distal junctions of RA. Five minutes after the injection. HRP was incorporated into the cytoplasmic vesicles of RA, while no incorporation was shown in SA. After 1 h HRP was incorporated into the incisal 1/3 of the ameloblasts in each SA.sbd.band. Acid phosphatase activity (p-nitrophenylphosphatase) was shown in the cytoplasmic vesicles, 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 radioopacity of enamel towards the incisal direction independent of the alternating changes of overlaying RA and SA. The fluorescence of injected tetracycline showed intense labeling at the portions of enamel which corresponded to RA. The portions of enamel being overlayed by SA were located in between such labeled areas and showed only faint fluorescence of tetracycline. RA apparently, actively degrades, resorbs and then digest the organic matrices of the enamel and also transports minerals into the enamel. SA are apparently formed from the RA which have become inactive metabolically. The exhausted cytoplasmic organelles apparently are renewed and reactivated.