A topographical study of the circadian rhythm in labelling index of mouse gingival and floor-of-mouth epithelium, including changes in labelling activity with individual cell position on the epithelial ridges

Abstract

A continuous strip of epithelium from the mandibular teeth to the ventral surface of the tongue of B6D2F-1 mice was examined autoradiographically after tritiated thymidine flash-labelling. Five areas were defined: area 1, the gingival sulcus epithelium adjacent to tooth enamel; area 2, the free gingival margin epithelium; area 3, the attached gingiva; area 4, the floor of mouth with undulating basement membrane; area 5, the floor of mouth with flat basement membrane. Data for the circadian variation in the proportion of DNA synthetic cells were recorded into a microcomputer, which enabled a large number of cells to be scored. The topographical position of each basal cell along the rete ridges and the incidence of labelling were noted. In each of the five areas a statistically significant circadian variation in labelling index (LI) was demonstrated, with a peak at 04.00–06.00 h and a trough at 20.00 h, although area 1 was slightly out of phase with the rest. The 24-h average LI values were almost double those obtained from a single flash-labelling at 10.00 h. The peak to trough ratio in LI was greatest in area 5 and fell towards area 1. Within the attached gingiva, cells deepest in the epithelial ridges had a larger peak to trough ratio than more superficial basal cells. For a group of mice labelled at 10.00 h the mean LI of the basal epithelial cells in areas 1–5 was 7.5 ± (3.0)% (SD). Various aspects of the distribution of DNA synthesis in relation to topography were examined. Cell labelling was highest adjacent to the teeth and lowest in the floor of mouth. In areas 1, 2 and 3 labelling was greatest in the anterior (first molar) region of the mandible. Where rete ridges were present, labelling was greatest in the most deeply positioned basal cells. These findings indicate heterogeneity in proliferation indices even within specific regions of gingival epithelium, consistent with the concept of a proliferative hierarchy within the basal layer of the epithelium.