Fine structure of the secretory and nonsecretory ameloblasts in the frog. I. Fine structure of the secretory ameloblasts.

Amelogenesis in the tooth germs of the frog Rana pipiens was examined by electron microscopy at different stages of tooth development. Cellular changes in secretory ameloblasts during this process showed many basic similarities to those in mammalian amelogenesis. Amelogenesis can be divided into three stages based on histological criteria such as thickness of enamel and the relative position of the tooth germ within the continuous succession of teeth. These stages are early, transitional and late. The fine structure of the enamel-secreting cells reflects the functional role of these ameloblasts as primarily secretory in the early stage, possibly transporting in the late stage and reorganizing between the two functions in the transitional stage. In early amelogenesis the cell exhibits well-developed granular endoplasmic reticulum, Golgi complex, microtubules, dense granules, smooth and coated vesicles, lysosome-like bodies in supranuclear and distal portions of the cell and mitochondria initially concentrated in the basal part of the cell. Numerous autophagic vacuoles are observed concomitant with the loss of some cell organelles at the transitional stage. During late amelogenesis the ameloblasts exhibit numerous vesicles, granules, convoluted cell membranes, junctional complexes and widely distributed mitochondria. Toward the end of amelogenesis, cells become oriented parallel to the enamel surface and the number of organelles is reduced. Amelogenesis in the frog is an extracellular process and mineralization seems to occur simultaneously with matrix formation.