The dentino-enamel junction: a structural and microanalytical study of early mineralization.

The spatial localization of enamel and dentin apatite crystals of the rat tooth has been studied by electron microscopic methods--bright field, selected-area dark field, and electron spectroscopic imaging. The sequential events of dentin calcification followed by the formation and growth of enamel crystals were determined and compared to previous studies. In dentin, initial sites of mineral deposition occur in areas subjacent to the dentino-enamel junction (DEJ). The subsequent expansion of these deposits progresses towards the DEJ to the terminal ends of dentin collagen fibrils. Concomitantly, an electron-dense enamel matrix is released by ameloblasts; with the presence of this matrix, the growth of enamel crystals occurs from the underlying calcified dentin. Enamel crystal growth continues to within close proximity of the plasma membrane of ameloblasts. A close spatial relationship between enamel and the crystals of calcified dentin collagen fibrils was observed by selected-area dark field imaging. Such areas of crystal intimacy show a co-localization of calcium and phosphorus extending from calcified collagen fibrils to enamel sheaths which encase enamel crystals. A working model of the spatial relationship between crystals of dentin and enamel is presented and discussed in light of mechanisms by which calcified dentin may promote the formation of enamel crystals.