Cell kinetics of mouse urinary bladder epithelium. II. Changes in proliferation and nuclear DNA content during necrosis regeneration, and hyperplasia caused by a single dose of cyclophosphamide.
The effects of cyclophosphamide (CPA) on the mouse (hr/hr strain) urinary bladder epithelium are described with regard to histology, proliferation (mitotic activity, LI, and cell number), and changes in ploidy. An initial toxic effect on the epithelium was evident after injection of CPA, demonstrated by a focal cell loss of approximately 50% during the period from 10 to 24 h after the injection of CPA. The toxicity is partly direct and partly mediated through DNA replication, and leads to cell death and cell loss. The necrotic phase was followed by a rapid regeneration with normalization of the diploid and tetraploid cell populations in 2-4 days. From 36 to 48 h the octoploid cell number increased by 3-4 times the normal number, and remained high for the rest of the observation period, with a maximum on the 5th day and a slight decrease during the following 2 weeks. The labeling index (LI) was significantly decreased 2-4 h after administration of the drug and evidence of partial synchronization was seen. The LI started to increase at 12 h and reached a maximum at 36 h, after which there was a gradual decrease. With microflow fluorometry the increase in DNA synthesis was seen to appear initially in the diploid population, starting at 10 h, and then in the tetraploid population, at 24 h. The tetraploid DNA synthesis reached a maximal value of almost 7 times the normal one at 36 h. An increased LI was followed by an increase in mitotic activity with a peak at 48 h and a gradual decrease during the first week. Diploid, tetraploid, and octoploid cells all underwent mitotic division. The pattern of DNA synthesis in the different cell populations strongly suggests that the development of higher ploidies occurs mainly by repeated synthesis of DNA in the same cells, and not by fusion. Cells of higher ploidy develop by repeated cycling of DNA along the vertical axis, and new cells within each ploidy population are formed by mitotic division and migration along the horizontal axis.