AbstractThe ultrastructure of the cells of the enamel organ related to enamel formation was studied using the lower incisors of adult male rats. In the region of enamel deposition, stratum intermedium cells are stabilized by a system of intercellular bridges and intracellular fibrils. The mitochondria in these cells are positioned toward the extracellular channels through which any direct intercellular exchange between the capillaries and ameloblasts must occur. Tentatively, the mitochondrial arrangement is considered to be related to the movements of electrolytes and water across the capillary‐ameloblast interval.In the region of transition, enamel deposition ceases and the ergastoplasm of the ameloblasts is removed, apparently by cytosegresomes, with an accompanying reduction in the height of the ameloblasts. Here, vesicles containing stippled material are infrequent compared to their occurrence in ameloblasts concerned with enamel deposition. Other vesicles, characteristically found in ameloblasts related to maturing enamel, first appear in the transition region and seem to originate from the cell membrane abutting on the enamel.In the region of maturation, cytosomes are common in the Golgi region whereas vesicles and mitochondria predominate in the distal ends of the ameloblasts. The papillary cells contain an unusually large number of mitochondria, elaborate microvilli and vesicles, which suggests that these cells are extremely active, presumably in the movement of materials related to enamel maturation.The changes in structure of the papillary cells, which occur concomitantly with those of the ameloblasts during enamel formation, are indicative of interrelated functional changes and strongly support the concept of ameloblasts and papillary cells acting together as a functional unit.