Localization of F-actin in the secretory ameloblasts of the rat molar.

Abstract

Using rhodamine-phalloidin, which specifically binds to F-actin, we have investigated by means of light microscopy changes in the actin localization in ameloblasts in different regions of the rat molar. In the cervical side, a strong fluorescence was first seen at the proximal terminal web of the differentiation ameloblasts; then, after the appearance of the predentin, the distal terminal web began to exhibit a strong fluorescence. In cross sectioned secretory ameloblasts, the fluorescence took the form of polygons with a uniform intensity at the proximal end and rectangles with a polarized intensity at the distal end. Further, the secretory ameloblasts were aligned in rows with a horizontal orientation, whereas the Tomes' processes bent alternately, either to the right or to the left in each ameloblastic row, giving rise to a decussate pattern. In later stage secretory ameloblasts, an elongated polygonal fluorescence was seen at the distal end, and the Tomes' processes were aligned in a parallel fashion. These changes in the rat molar are similar to those seen in the rat incisor, from the inner enamel secretion to the outer secretion. These observations of the cervical side suggest that the morphology and modulation of the F-actin microfilament bundles in the terminal webs of the ameloblasts and the related arrangement of Tomes' processes in the molar are basically similar to those seen in the incisor. Therefore, in spite of the physical differences between the rat incisor and molar, the unique morphology and modulation revealed by the F-actin pattern and the Tomes' processes pattern that they share appear to be inherent ameloblastic characteristics. In the secretory ameloblasts in the fissure side of the molar, polygons in the distal terminal web and an irregular arrangement of Tomes' processes were seen. In cross sectioned secretory ameloblasts, the size of the fluorescent polygons in the terminal web varied in different regions. These results suggested that the fluorescent of the distal terminal web and Tomes' processes was related to the rod pattern and the cytoskeletal organization.