Development of the distal end and Tomes' processes of ameloblasts observed by freeze-fracture and ultrathin section electron microscopy.

A freeze-fracture examination of distal portions of differentiating and secretory ameloblasts was carried out using upper molar tooth germs from one to four days old rats. Ultrathin sections were prepared for comparison. Freeze-fracture provides a unique opportunity to observe plasma membrane events in distal portions of ameloblasts during differentiation and development. Our results suggest that the distal plasma membrane and Tomes' processes of differentiating and secretory ameloblasts are highly active in endocytosis. In differentiating ameloblasts, endocytic activity is likely to be associated with removal of the basal lamina. During matrix secretion and maturation, ameloblasts are probably removing material from the maturing matrix; in particular amelogenins and water. The present study provides morphological evidence that an endosomal system is present in differentiating and secretory ameloblasts during periods of endocytic activity. This would provide a system whereby ligands, fluid components, membrane receptors and plasma membrane, which enter the cells during endocytosis, may be separated from each other and sorted out for despatch to different destinations.