Characterization of putative secretory sites on ameloblasts of the rat incisor.

The distribution and structure of the putative sites where enamel matrix is secreted from the ameloblast were studied by correlating the external topography with the distribution of organelles in Tomes' process cut in various planes of section. Both the interrod and rod secretion sites are associated with deep membrane infoldings. It was found that the interrod secretion site completely surrounds each ameloblast, and the marked interdigitation of adjacent cells results in a cooperative growth front for interrod enamel. In contrast, the rod secretion site is present on only one surface of the interdigitating portion of Tomes' process. Numerous granules were observed adjacent to the membrane infoldings associated with both sites, and granules were seen fused to membrane infoldings suggesting that the matrix of enamel is a merocrine secretion product.