A light and electron microscopic study of the early stages of root surface formation in molar teeth in the rat

The forming root surfaces of first molar teeth of twelve-day old rats were examined by light and transmission electron microscopy. A thin layer of the outermost dentinal matrix, approx. 1.0–1.5 μm wide, did not mineralize initially. The layer was lined externally by epithelial cells containing organelles suggestive of secretory activity. Electron-dense granular material resembling that produced by ameloblasts was observed in the epithelial intercellular spaces and in the unmineralized layer. The so-called initial layer of acellular cementum in the rats is therefore not cementum but a dentinal matrix to which epithelial secretory products are added.