Localization of actin during differentiation of the ameloblast, its related epithelial cells and odontoblasts in the rat incisor using NBD-phallacidin

Abstract

Using NBD-phallacidin, which specifically binds to F-actin, we investigated changes in the localization of actin during the differentiation of ameloblasts, related epithelial cells and odontoblasts in rat incisors. In cryosections treated with NBD-phallacidin, intense fluorescence was observed in undifferentiated epithelial cells in the apical loop and at the proximal extremity of undifferentiated inner enamel epithelial cells. During differentiation, the distal extremity began to exhibit strong fluorescence. In cross-sections of secretory ameloblasts, the fluorescence took the form of polygons of uniform intensity at the proximal end, and of rectangles of non-uniform intensity at the distal end. At the distal end, the fluorescence was more intense at right angles to the long axis of the incisor. At the distal end, this pattern was established just before the appearance of the enamel layer. These patterns were maintained during the secretory stage of ameloblasts. The location, pattern and time of appearance of these sites were identical to those of the terminal webs in ameloblasts. NBD-phallacidin weakly labelled the peripheral cytoplasm of the cell body of ameloblasts, and also labelled Tomes’ process. The cells forming the stratum intermedium were mainly labelled at their periphery (i.e. forming larger polygons), while the overlying epithelial cells exhibited labelling throughout their cytoplasm. Except for the terminal webs, the cell bodies of odontoblasts were weakly labelled throughout the period of differentiation. Young odontoblasts secreting pre-dentin were first labelled on the terminal web, with the fluorescence becoming gradually more intense as the thickness of the dentin increased. Thread-like fluorescence probably indicating actin localization in the odontoblast process and its branch - was seen only in the predentin in undecalcified sections, and in both the predentin and dentin in decalcified sections. The observed changes in the distribution of F-actin during the differentiation of ameloblasts and odontoblasts might be involved in the morphogenesis of incisor teeth as well as in the formation of enamel and dentin. The localization of actin in the dentin of decalcified sections supports the view that the odontoblast process extends almost as far as the enamel-dentin junction.