The incisors of rodents (e.g., mice and rats) have an enamel-free lingual surface. It is not known how position specific differential evolution of labial versus lingual inner dental epithelium (IDE) is regulated. The patterns of mitotic activity of lingual and labial preodontoblasts and IDE were determined on sagittal sections of embryonic mouse incisors. No statistical differences were observed for the preodontoblasts. On the other hand the lingual IDE withdraw from the cell cycle before the corresponding labial cells. The incapacity of lingual IDE to differentiate into functional ameloblasts might be a consequence of a failure of cell kinetic dependent acquisition of competence to respond to specific epigenetic control mechanisms.