Abstract
The mechanism of the perforation in the epithelial root sheath of the lower first molar tooth germ of the mouse was studied by light and electron microscopy. The findings obtained were as follows : (1) By frontal serial sections and reconstructed models of the epithelial root sheath, the first perforation area appeared at the buccal and slightly mesial region of the root bifurcation. (2) In the prospective perforation area, the epithelial cells of the root sheath and undifferentiated follicular cells had many small cytoplasmic processes which were in intimate contact with or penetrated through the basal lamina. (3) In the perforating region, the basal lamina was absent, and the epithelial intercellular space was enlarged, then a close approximation or the direct cell-to-cell contact between the mesenchymal cells of the dental sac and the outer epithelial layer of the root sheath was observed. (4) The cytoplasmic process of the mesenchymal cells penetrated through the intercellular space of the root sheath and became attached to the dentine. By the contact with the dentinal matrix, the mesenchymal cells differentiated into the highly active functional cells. These results suggest that during the early root formation, the epitheliomesen-chymal interaction between the epithelial root sheath and the dental sac may exist and play an inductive role in the perforation of the epithelial root sheath.
