Cell cycle of normal bladder urothelium in developing and adult mice.

The present research has employed a novel, nonradioactive technique to quantitatively study normal urothelial proliferation in foetal, neonatal, juvenile and adult mouse bladder. Using whole mount histological preparations, the total number of urothelial nuclei per mouse bladder, and per given urothelial cell layer, have been assessed to provide data of the (unstimulated) kinetic behaviour of basal urothelial cells (the proliferative population), to analyse characteristics of the normal urothelial cell cycle. The urothelial cell cycle time increases from 30.6 h (foetal) to 40 weeks (adult), the duration of mitosis from 0.23 h (foetal) to 2.71 h (adult) and the duration of DNA synthesis from 2.52 h (neonatal) to 10.83 h (adult). These are average values for the urothelial cell cycle, which do not preclude the possible existence of proliferative units. The ratio of superficial nuclei to basal and intermediate nuclei, possibly indicative of a urothelial proliferative unit, declines to reach a plateau (1:40) in adult mice. These findings indicate that rapid urothelial proliferation during early murine development was likely to be a) biologically useful, since intrauterine foetal metabolic activity may require a functional bladder urothelium at an early stage, b) kinetically similar to acutely regenerating adult urothelial cells after cytotoxic insult. During murine life, the range of durations of mitosis and DNA synthesis is much less than the range of cell cycle times. Normal unstimulated urothelium of adult mice was confirmed to proliferate slowly.