Scanning electron microscopic examination of reversible hyperplasia of the rat urinary bladder

Urinary bladder damage caused by surgical incision, freeze-ulceration, or formalin instillation in male Fischer 344 rats was studied by light and scanning electron microscopy. The first two methods resulted in focal ulceration of the urinary bladder; the last induced diffuse mucosal damage. With each method, the damage was followed by regenerative hyperplasia and repair, the bladder mucosa returning to normal in 3-4 weeks. Epithelial cells in the hyperplastic areas had ropy microridges and uniform short microvilli on their luminal surfaces as observed by scanning electron microscopy. When the hyperplasia was marked, with nodular and papillary formation, occasional epithelial cells had pleomorphic microvilli on their surfaces. Rats treated either by surgical incision or freeze-ulceration had normal bladders after a 2-year observation period. Combined with results from previous experiments, pleomorphic microvilli are not a marker of neoplasia or irreversibility but appear with marked or prolonged mucosal proliferation even if reversible.