The Fine Structure of the Transitional Epithelium of Urinary Bladder and Its Functional Significance as Disclosed by Electron Microscopy

Abstract

The transitional epithelium and the subjacent lamina propria of the urinary bladders of horse-shoe-bats (Rhinolophus ferrum equinum Nippon) and rabbits (Lepus cuniculus) were observed with the electron microscope.
The epithelium consists of three layers, i.e., the superficial, middle anp basal layers. No microvilli are provided on the free surface of the superficial cells, but many incisures of various depth are observed. In stretched condition of the bladder wall, these incisures are markedly reduced in number and depth. A special layer of apical cytoplasm which contains many small dark granules probably of glycogen and a considerable number of vesicles are occasionally observed in the uppermost part of the superficial cell. This layer corresponds to the crust konwn by light microscopy, and many mitochondria are gathered in a zone just beneath this layer. Many vesicles and vacuoles are characteristically found in the superficial cells, the contents of these are either empty clear or filled with various granular or vesicular particles. According to an experiment that oil emulsion is infused into rabbit bladders, it was proved that vesicles or vacuoles of the superficial cell are capable to engulf the fatty substance contained in urine by a mechanism of the pinocytosis. Water and watersoluble electrolytes may also be absorbed into the epithelium by a similar mechanism. In the cytoplasmic matrix of the superficial cell many bundles of tonofilaments are observed.
In cells of the middle layer, pinocytotic vesicles and fat droplets are also observed, but there is a tendency that they are preferentially gathered at the region just beneath the superficial layer; and none of them are observed in the basal cells. Thus it seems likely that the fat and other ingested substance may be released into the narrow intercellular space at the level of the middle layer, and may pass through the basal layer by the way of intercellular channels. The basal cells are often dark in appearance and contain many small dark granules which are probably identical to the free RNA granules.
Periodically arranged desmosomes in short intervals are seen in the lateral cell boundaries of the superficial layer of the rabbit epithelium. These are thought to produce a useful resistance against stretching. A peculiar complicated folding of plasma membranes at the intercellular boundaries may be related to the remarkable change of cell outlines concerning the stretching and contraction of the bladder wall.
A thin basement membrane borders the epithelium from the underlying lamina propria. The wall of the subepithelial capillary consists of thinly flattened cytoplasm of the endothelium and the basement membrane outside it. Occasional pores are observed through the thin film of endothelial cytoplasm and may be probably related to the transfer of substances absorbed from the bladder lumen.