Fate of HERS during Molar Root Development in Mice

Tooth root development begins after the completion of crown formation in mammals. Previous studies have shown that Hertwig's epithelial root sheath (HERS) plays an important role in root development. The fate of HERS has remained unknown, with at least six possibilities: separation into the epithelial rest of Malassez, apoptosis, incorporation into the advancing cementum front, epithelial-mesenchymal transformation, migration, and differentiation into cementoblasts.

Objective: To investigate the morphological fate and analyze the dynamic movement of HERS cells in vivo. Methods: We generated K14-Cre; R26R mice and observed the HERS cells at Postnatal day 0.5, 3.5, 5.5, 7.5, 10.5, 14.5, 21.5, 30.5, and 60.5 by X-gal staining. Result: The molar epithelial cells are allƒn £]-galactosidase-positive before root formation. HERS cells are detectable on the surface of the root throughout the beginning to completion of root formation and do not disappear. Most of the HERS cells are attached to the surface of the cementum, and others separate to become the epithelial rest of Malasez. HERS cells secrete extracelluar matrix components onto the surface of the dentin, before dental follicle cells penetrate the HERS bilayer to contact dentin. Dental follicle cells also contact dentin and secret protein matrix during cementogenesis, after dissociation from the HERS. HERS cells participate in the cellular cementum and can become cementocytes. HERS cells are active in the furcation of the tooth and may be the primary producer of cementum. The formation of cellular cementum in furcation is different from that of the apical region of the root. During root development, the HERS is not interrupted, and the HERS cells may continue to communicate with each other through the network structure. Conclusion: HERS cells remain on the surface of the root throughout root formation and participate in cementogenesis. The network of HERS cells is important for tooth root development.