Scanning electron microscopy of developing roots of molar teeth of the laboratory rat

Abstract

Molar teeth of young rats were rendered anorganic and their developing roots were examined with the scanning electron microscope. It was possible to compare the morphology and ultrastructure of advancing root edges in various stages of development. It was found that the roots of any one tooth develop asynchronously and that the structure and composition of the tissues at the advancing edge of any one root alter in a characteristic way as development proceeds. The latter is due, in part, to the formation of a deep cleft which separates dentine and cellular cementum, and, in part, to the gradual overtaking of dentine by an increasing bulk of cellular cementum. The extensive, primary cleft is ultimately replaced by numerous, small, irregular clefts during closure of the root “apex”. The findings add the necessary third dimension to present understanding of root formation in rat molars and emphasize the atypical nature of this process. Raised developmental “seams” were found on all molars examined, extending over the undersurface of the crown and the contiguous (interradicular) surface of the roots. These seams would not be resolvable in their entirety by any other method presently available. The role played by Hertwig's epithelial root sheath in cleft formation, and its likely role in seam formation is discussed.