Morphology and cytidine-5'-monophosphatase cytochemistry of odontogenic epithelium at the enamel-free areas of mouse molars.

This epithelium in developing first and second molars of 1-15 days old balb/c mice was examined by light and electron microscopy. The lysosomal content of these epithelial cells was assessed using CMPase as a marker. At the early developmental stages (4-7 days), the epithelial cells at the enamel-free areas had morphological features of secretory cells. A distinct afibrillar layer was identified covering the enamel-free dentine surfaces. This layer was morphologically different from both the underlying dentine and the enamel at its periphery. At a later stage (9 days), the epithelial cells had extensive infoldings of their distal membranes. Also, the secretory organelles were markedly reduced and mostly replaced by abundant mitochondria concentrated in the distal cytoplasm. Numerous CMPase-reactive lysosomal structures were demonstrated in the 9- and 11-day-old specimens. These cytochemical and morphological features are consistent with a resorptive activity for the enamel-free cells during this developmental stage (9-11 days). Subsequently, at 15 days, these cells had regressed into a flattened squamous epithelium that remained in contact with the tooth surface until eruption.