Renewal of normal urothelium in adult mice

Normal unstimulated urothelium of adult mice appears to be essentially quiescent, as demonstrated by an 3HTdR-pulse labelling index of 0.11%. There is no evidence that this labelling index may be artificially low. Results from continuous labelling experiments, where 3HTdR was supplied in the drinking water of mice for varying periods, render it possible that the urothelium satisfies its small renewal requirements via a low growth fraction of cycling cells. The remainder may consist of quiescent cells, which may be capable of resuming cell cycle progression when required, and of post-mitotic cells, which are incapable of further proliferation. There are three possible levels of adult urothelial proliferation with cell cycle times of about a) one year (if the urothelium is homogeneous and the growth fraction unity), b) three weeks (if the growth fraction is about 5%), and c) 15 h (when the urothelial cells are rapidly regenerating).