Ultrastructure of the epithelial-mesenchymal interface in the mouse tooth germ.

The structure of the epithelial-mesenchymal interface of a developing mouse embryonic tooth germ was examined at the time of cell differentiation by using transmission and scanning electron microscopy. During odontoblast differentiation, the basal lamina was continuous beneath the inner enamel epithelium, and the fibrils at the mesenchymal aspect increased in density and length. Numerous cell processes of the preodontoblasts were seen in close contact with the basal lamina. This supports the idea that the basement membrane is involved in the differentiation of mesenchymal cells into odontoblasts, probably by providing special attachment sites for the aligning mesenchymal cells. The rearrangement of intracellular organelles in the cells of enamel epithelium and a change of epithelial morphology were already seen at the time of the initial predentin secretion by odontoblasts. At this time, the basal lamina was still present. The penetration of epithelial microvilli into the epithelial-mesenchymal interface through breaks in the basal lamina related to the onset of predentin mineralization. This suggests that the determination of epithelial cells into ameloblasts already occurs before the disappearance of the basal lamina.