UNUSUAL NATURE OF ROOT FORMATION IN MOLAR TEETH OF LABORATORY RAT

The cellular cementum phase of root formation in molar teeth of Sprague-Dawley rats was studied in thin sections for transmission electron microscopy. Once cellular cementum formation is established, cells of Hertwig's epithelial root sheath are embedded en masse between cementum and dentine at the advancing root edge. This developmental event is coincident with an alteration in the rates of formation of cementum and dentine; cellular cementum coming both to precede and to be of greater bulk than dentine at the advancing root edge. This mode of root formation does not seem to have been described for other mammals and should be taken into account in studies utilizing rat molar periodontium. Despite the unusual fate of Hertwig's epithelial root sheath, there is evidence of morphological continuity between epithelial sheath and dental sac compartments prior to cementogenesis. The possible roles of epithelial root sheath and of occlusal function in cellular cementum formation are discussed. © 1969 Academic Press, Inc.