No abstract

Under scanning electron microscopy (SEM) the surface cells of the guinea pig bladder have pentagonal or hexagonal outlines. Their borders are clearly defined since they are elevated. They possess a large reserve of surface membrane which is markedly folded and wrinkled when the bladder is empty. The folds disappear and the cells become flat during distension. The luminal surface is characterized by numerous reticular ridges which are a remarkably constant feature and persist even under acute artificial distension. A small proportion of the surface cells are small and have less than five sides. Since they show only sparse microvilli as a surface feature, they have a smooth appearance. These are believed to be young surface cells which have just emerged from the intermediate layer, and have not yet acquired the ridged pattern of mature cells.

Following artificial ulceration of the guinea pig bladder epithelium, study by light microscopy showed that mitotic activity occurs within 24 hours in the basal layer of the remaining epithelium; which leads to thickening, disorganization and cell shedding. At the ulcer margin, the epithelium forms a rolled edge, from the extremity of which a two-layered sheet of flat cells grows over the edematous ulcer bed, and the ulcer site is protected during dilation of the bladder by localized muscle spasm. Healing is effected within about one week, as the mitotic activity declines. Organization of the new epithelium commences at the periphery and the cells acquire glycogen and alkaline phosphatase. Study by SEM, which included normal bladders, confirmed that new epithelium spreads from a rolled epithelial edge, and also showed that cells undergoing rejection have globular profiles with surface microvilli. The immature epithelial cells are markedly distorted from mutual pressure during migration and their surfaces show only short microvilli. Ulcers heal after four to seven days, depending on size, and the new cells rapidly assume the pentagonal and hexagonal outlines and the reticular pattern of surface ridges, characteristic of mature surface cells. The ridges apparently develop by fusion of rows of microvilli. By the tenth day it is difficult to identify the original ulcer site.