| Abstract: | Secretory ameloblasts in lower second molars of 1-week-old kittens were studied with the elctron microscope after perfusion-fixation with and without decalcification. Ameloblast height varied from 40–65 μm. Tomes' process presented a stepped profile with two type-1 faces (presumably enamel secreting) and one type-2 face (thought to be non-secreting). The type-1 face was associated with extensive membrane infoldings. The enamel facing the type-1 face was lined by a dense border, and stippled material was present between cell membrane and enamel. The type-2 face was characterized by minute membrane invaginations and, sometimes, stippled material between enamel and cell membrane. In cross section, the distal part of Tomes' process showed bilateral symmetry, with one dorsal, one ventral and two lateral aspects. It was suggested that enamel formation occurs in three stages. First, interrod enamel on the lateral aspects of Tomes' processes is formed by two adjacent amelblasts. Next, the interrod enamel on the dorso-ventral aspects of Tomes' processes is formed mainly by the dorsal extension of one Tomes' process. Finally, the enamel rod is formed by one ameloblast. Stippled material was consistently present in the extracellular spaces between Tomes' processes and in irregularly shaped vesicles in the ameloblast apex, and occasionally in large masses between the ameloblast cell bodies. Spherical vesicles within the ameloblast, thought to be lysosomes, also could contain material resembling stippled material. The possibility was considered that stippled material is partly derived from enamel, and finally phagocytosed by ameloblasts. There was no clear morphologic evidence for merocrine secretion playing a role in the formation of either enamel, stippled material or the rod sheath. |
