Phase-contrast and electron microscopic study of the junction between reduced enamel epithelium and enamel in unerupted human teeth

Abstract

A phase-contrast and electron microscopic investigation was carried out on enucleated (unerupted) human teeth to study the area of the junction of reduced enamel epithelium with the enamel surface. Demineralized, as well as partially demineralized and undemineralized material was studied. The results indicated that reduced enamel epithelium is generally connected to the enamel surface by means of hemidesmosomes and a basement lamina approximately 400Å wide. This type of attachment corresponded to that observed in erupted human teeth. Vacuoles could be observed entirely within the cytoplasm of reduced ameloblasts or communicating with the zone of the basement lamina. These vacuoles could represent a secretory or absorptive activity by the reduced ameloblasts. Reduced ameloblasts were joined to each other and to other cells of the reduced enamel epithelium by desmosomes and zonulae occludentes. Neither type of cellular junction appeared to be restricted to any specific portion of the reduced enamel epithelium. Following stripping of the dental sac from the enamel surface, patches of a thin membranous structure remained attached to the enamel. These patches consisted of all the cells of the reduced enamel epithelium with some attached connective tissue. Although phase-contrast microscopy revealed a cuticular structure approximately 0.5–1.0μ wide on the enamel side of the ameloblasts, similar to Gottlieb's primary enamel cuticle, no such structure was observed with electron microscopy, suggesting that the primary enamel cuticle may be the result of an optical phenomenon. A cuticular structure was observed in continuity with the cementum layer, which was described as a type “A” cuticle. It was characterized by its position over enamel only, in close proximity to the cemento-enamel junction. It was granular and exhibited appositional lines. Although it appeared as a coronal extension of the cementum layer with phase-contrast microscopy, it differed from cementum at the ultrastructural level, primarily by being devoid of collagen fibrils. It may represent an end product of amelogenesis or more likely an afibrillar type of cementum. No evidence was found to support a recently published observation that a well defined cementum layer may be found on the enamel surface of unerupted human teeth.