Occurrence of amorphous and crystalline mineral deposits at the epithelial-mesenchymal interface of incisors in the calcium-loaded rat: implication of novel calcium binding domains.

Abstract

In order to clarify the regulatory factors that promote precipitation of enamel crystals in mammalian tooth germs, possible calcium binding domains were visualized at the epithelial-mesenchymal interface of rat incisor teeth by means of electron microscopy and X-ray microanalysis. Adult rats were loaded with calcium (30 mM Ca) by vascular perfusion and further loaded through fixation and dehydration in the presence of high doses of calcium. Electron microscopy of anhydrously prepared Epon sections of the calcium-loaded rat incisors revealed numerous electron-dense granular deposits, enriched with calcium and phosphorus, scattering in the fibrous mantle dentin matrix and the intercellular spaces of the inner enamel epithelium, but not in the pulp tissues including the odontoblastic cells layer. The electron-dense deposits were specific for the enamel-related portion and were never shown to occur in the cementum-related portion. Proceeding incisally, dense deposits in both the mantle dentin matrix and presecretory ameloblast layer gradually converted to fine needlelike figures resembling the early enamel crystallites. Mineral deposits in experimental rats disappeared concomitant with the onset of normal mineralization of mantle dentin. There was no spatial correlation between the dense deposits and either stippled material or matrix vesicles. These results indicate the presence of novel calcium-binding domains in the enamel-related portion of the epithelial-mesenchymal interface of rat incisors that form enamellike crystallites under calcium-loaded conditions. A contribution of these putative calcium-binding domains in the induction and spontaneous formation of enamel crystals is suggested.