Ultrastructure of the ameloblast-stratum intermedium border during ameloblast differentiation.

The molar tooth germs of newborn mice were isolated and the zone of ameloblast differentiation studied with the electron microscope. Special attention was paid to the changes that occur in the contacts such as desmosomes, tight junctions and close attachments associated with cell interdigitations between the inner enamel epithelium (IEE) and the stratum intermedium (SI) cells from the beginning of differentiation to the onset of enamel secretion. These changes are accompanied by variations in the width and configuration of the intercellular space separating the two cell layers. The terminal web persists from the beginning of cell elongation to the appearance of Tomes's process. Through its attachment to the zonulae adherentes, a terminal web-lateral junction system is created which stabilizes the differentiating ameloblasts. Also the SI cell layer is interpreted as a stabilizer for the IEE layer, particularly during the differentiation phase. This interpretation does not conflict with information that the SI cells transport materials required for enamel secretion.