MORPHOLOGY AND DISTRIBUTION OF MITOCHONDRIA IN AMELOBLASTS WITH SPECIAL REFERENCE TO A HELIX-CONTAINING TYPE

In the enamel organ of perfusion fixed, demineralized rat incisors, helical inclusions were found in mitochondria of ameloblasts at specific stages of amelogenesis. In the matrix formation stage, elongated mitochondria are clustered in the proximal end of the ameloblasts (adjacent to stratum intermedium). In general features, these mitochondria resemble the common types found in other mammalian epithelial protein secreting cells. However, dilated intercristal spaces filled with a granular content are fairly often observed. The redifferentiation and rearrangement of the ameloblasts into the transporting epithelium of the maturation stage is accompanied by the appearance of a second population of mitochondria. In the distal end of the ameloblasts a group of elongated mitochondria is associated with the infoldings of the distal plasma membrane. In the proximal end of the ameloblasts, however, rounded mitochondria with elongated intracristal dilatations containing up to 20 helices, are observed. The helices have a diameter and a pitch of about 140 Å and are formed of filaments approximately 40 Å in width. The two populations of mitochondria persist during the subsequent pigmentation stage of the ameloblasts. © 1968 Academic Press Inc.