Proliferation kinetics of the mouse bladder after irradiation

The proliferative response of the mouse bladder was investigated, using continuous labelling with tritiated thymidine, at various times after a single dose of electron radiation. Bladder epithelial and vascular cells were studied. The cell turnover rate in unirradiated epithelium and endothelium was found to be extremely slow (1 year). At 3 months after irradiation (25 Gy) there was little difference from the control proliferation rate of the epithelium but from 6 to 22 months after irradiation there was sustained rapid proliferation with the cell turnover time reduced to approximately 1 week. The increase in proliferative activity observed at 22 months was dose-dependent. Endothelial cells in the blood vessels of the submucosa also showed an increased turnover rate after irradiation and the timing of this response was similar to that of the epithelium. The onset of compensatory proliferation in both cell types coincided with marked histological and functional changes in the bladder. In this slowly proliferating tissue, the onset of rapid compensatory proliferation after irradiation is delayed and occurs at the time that functional impairment is observed. This supports the postulate that proliferation is unlikely to contribute much to the sparing effect of prolonged fractionated radiotherapy in slowly dividing tissues. (author)