Formation of intermediate cementum. I: early mineralization of aprismatic enamel and intermediate cementum in monkey.

Intermediate cementum and aprismatic enamel outlines the dentinal surface in the crown and root. Since the formation of these tissues is incompletely known and there have been diverging opinions whether they are of epithelial or mesenchymal origin, the present study was undertaken to study the pattern of early mineralization of these hard tissue layers by means of scanning electron microscopy and tetracycline labeling in monkey teeth. The most apical portion of the forming roots were covered by smooth spherulites, distinctly different from "microspherites" in cementum and calcospherites in dentin. These spherulites were labeled by tetracycline in a way that showed that they were formed and mineralized in a phase separate from the adjacent dentin. Apical to the honeycomb pattern of prismatic enamel formation, in the crown, a zone of smooth spherulites in the aprismatic enamel was found. These spherulites were similar to the ones at the forming root tip. Tetracycline was incorporated into the aprismatic enamel, which was in contrast to the prismatic enamel where no fluorescence was found. The mineralization pattern of aprismatic enamel and intermediate cementum were similar as seen by both tetracycline labeling and scanning electron microscopy. Thus, it is suggested that they are of similar origin and that the epithelial root sheath participates in the formation of the intermediate cementum but not of the proper dental cementum.