Hertwigs sheath in the rat incisor

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Abstract
The histodynamic nature of Hertwig's sheath of the mandibular rat incisor was studied utilizing tritiated thymidine. The epithelial sheath is composed of three distinct cell layers: (1) inner dental epithelium, (2) stellate reticulum, and (3) outer dental epithelium. Incisal to the basal loop area, the outer layers eventually diminish and disappear leaving only the inner dental epithelium. This layer ultimately becomes perforated and is replaced by connective tissue elements. Epithelial rest cells are not identifiable in the periodontal ligament. Autoradiographic study shows labeling to be more frequent in the inner dental epithelium than the other two epithelial layers. At the one hour period, 97 per cent of the labeled cells in the inner dental epithelium are located in the basal two‐thirds of the area between the basal loop and the site of predentin deposition. A study of cell kinetics indicates that the inner dental epithelium is made up of a renewing cell population. The cells have a total generation time of 25.3 hours; a synthesis period of nine hours; a G2 period of two to six hours; a mitotic period of approximately one‐half hour, and a G1 period ranging from 9.8 to 13.8 hours. The rate of cell migration is of the same order as the rate of tooth eruption (358 μ per day).