Radiation response of mouse lip mucosal epithelium: a cell kinetic study

Changes in the cell proliferation kinetics of the epithelium of mouse lip mucosa have been assessed after local irradiation with a single dose of 18 Gy of 60Co gamma-rays. By the fifth day after irradiation, two distinct sub-populations of epithelial cells could be discerned. The larger of the two populations consisted of cells exhibiting varying degrees of radiation-induced damage, and the smaller population was composed of cells of normal size and appearance, intermingled between the radiation-damaged cells. There was a progressive decline in the epithelial cell density with time after irradiation, and focal denudation was seen after 11 days. Cell colonies were evident in the remnants of the epithelium by day 7. Degenerate cells could be identified in the basal layer of the mucosal epithelium, both before and after irradiation. The proportion of degenerate cells was increased 5 days after irradiation with the maximum number, approximately 3.6% being counted on days 7 and 8. In the first 2 days after irradiation, there was a reduction in the labelling index (LI) of basal cells in the epithelium. This was followed by recovery to control values on day 3. The LIs of both the radiation-damaged cells and those with a normal appearance remained relatively constant between days 5 and 11, at approximately 10 and approximately 60%, respectively. The LI of basal cells in the cell colonies was very high (approximately 70%). The estimated turnover time (TT) for the basal cell population with a normal appearance and for those in cell colonies (groups of normal cells with a cord length > or = 10 cells), was extremely short < 1 day. There was some fluctuation in TT values for basal cells exhibiting radiation-induced damage, with the shortest value (approximately 3 days) at 7 and 8 days after irradiation. It was concluded from the cell kinetic data that repopulation of the lip mucosal epithelium started between 3 and 5 days after irradiation and the overall response of the mucosa to irradiation was consistent with that predicted by a hierarchical model of cell proliferative organization.