[Microscopy and electron microscopy of the Hertwig sheath in the mouse]

The structure and ultrastructure of Hertwig's sheath have been studied in the mice on molar tooth germs collected on day 16. On a morphological basis, two parts, a diaphragm portion and a sheath part could be described. The diaphragm portion consisted of two layers, one internal and one external with in between occasionally cells disposed in quincunx. Mitotic activity was noted in the external layer. This fact raised the possibility of the existence of a cellular flow from this layer towards the internal counterpulpal layer. The permanent existence of the sheath during root elaboration could thus be explained. In the internal layer structural modifications and cell axis variations seemed to be related to the odontoblast differentiation. In the "sheath" part electron microscopy revealed the existence of a fine layer of unmineralized collagen fibrils which remained under the internal epithelial layer. The observations related to the structure of the periradicular sheath cells questioned the hypothesis whether these cells assume secretory potentials.