Cytochemical studies of ameloblasts and the surface layer of enamel of the rat incisor at the maturation stage.

In order to elucidate the cytochemical properties of the membranous structure between enamel and ameloblasts of the rat incisor at the maturation stage, chromic phosphotungstic acid (Cr-PTA) and periodic acid-silver methenamine (PA-silver) techniques for electron microscopy were employed in combination with a digestion test with hyaluronidase, neuraminidase, collagenase or trypsin. Also, acid phosphatase activity of ameloblasts at the maturation stage was examined with a modified GOMORI's metal salt method. An intensely Cr-PTA reactive band approximately 0.1 micron thick appeared along the surface layer of enamel at the transitional stage, and at the very beginning of the maturation stage another intensely Cr-PTA reactive band which was seen by uran-lead stain to be a delicate electron-dense membranous structure appeared as well between enamel and ameloblasts. A lot of cytoplasmic small vesicles or tubular structures, both intensely reactive to Cr-PTA, were observed near the apical membranes of the overlying ameloblasts indicating that those organelles must have been responsible for the secretion of the latter band. Acid phosphatase activity was clearly demonstrated at Cr-PTA reactive large vesicles in the cytoplasm of those cells. The PA-silver staining technique manifested a band heavily deposited with silver grains along the surface layer of enamel, i.e., where the former band existed, but showed no particular reaction at the latter, the band-like layer between enamel and ameloblasts. Hyaluronidase or neuraminidase treatment remarkably decreased the Cr-PTA reaction of the latter band. Trypsin or collagenase treatment, on the other hand, not only eliminated the Cr-PTA reaction but digested the band itself. These results suggest that the membranous structure between enamel and ameloblasts of a rat incisor is not so-called enamel cuticle but a basal lamina produced by overlying ameloblasts and that the basal lamina contains collagenous components even though it lies on enamel.