An electron microscopic study of ectomesenchymal contacts in rat incisors

In order to clarify connections between epithelium and mesenchymal tissue in the early stage of odontogenesis, the formation of ectomesenchymal contacts and the presence of coarse-textured material at the interface between ameloblasts (AMs) and odontoblasts (ODs) on growing rat incisors were studied by electron microscopy. The presecretory zone was classified into five regions according to the ultrastructure of each preameloblast (PA). The region where collagen fibrils could be observed was designated PZ-2, the region where predentin existed PZ-3, the region where the basal lamina began to disappear PZ-4, and the region where the basal lamina had disappeared PZ-5. In PZ-2, the cytoplasmic processes of PAs penetrated the basal lamina and reached the dental papilla cells. In some locations, ectomesenchymal contacts were also observed, in which the cytoplasmic processes of the PAs were in contact with those of the preodontoblasts (POs). In PZ-3, the distal cytoplasm contained large amounts of type-1 vesicles, and secretory granules. In the distal membrane of the PA, membrane invaginations containing fibrillar structures were visible. Also in some areas, the cytoplasmic processes at the distal ends of the PAs invaded the predentin. In the predentin, a large amount of coarse-textured material, considered to be the precursor of the enamel matrix, was observed. In PZ-3, a large number of cytoplasmic processes extended into the predentin, while in PZ-4, microvillus-like processes extended from the distal ends of the PAs, showing a high frequency of ectomesenchymal contact. It was suggested that secretory activity of the PAs was induced after ectomesenchymal contact had been accomplished, but that the PAs did not undergo morphological change into secretory ameloblasts.