[Architectural ultrastructure of the urinary bladder epithelium. I. The influence of the contracted and distended state].

The luminal surface and three dimensional architecture during contraction and distension of the normal rat and human urinary bladder mucosa were observed by scanning electron microscopy (SEM) after cryofracture of specimen and by transmission electron microscopy (TEM). The bladder epithelium consists of three distinct types of cells, superficial, intermediate and basal cells. SEM and TEM shows that the epithelium is stratified in various states. Some of the intermediate cells reach the basal lamina by fine cytoplasmic processes, but most of them and superficial cells never directly reach the basal lamina. In the rat bladder, the contracted epithelium consists of three to five cell layers and becomes three cell layers in the distended state. Since neighbouring cells are joined by desmosomes, the contact relations between each cell do not change in contracted and distended states. SEM shows an irregular luminal surface of the superficial cells in contracted state. Under TEM this luminal surface showed a scalloped contour, and most of the cell membranes intruding into the cytoplasm gave the appearance of discoidal vesicles. Between superficial cells there are numerous interdigitations in the distended state which show an accordion-like appearance. Between neighbouring cell layers the interdigitations are deep in the contracted state and shallow in the distended state. In the human bladder, the contracted epithelium consists of six to eight cell layers and becomes three to six cell layers on the distended state. Ultrastructural features of the human bladder epithelium during contraction and distension are basically similar to those of the rat bladder epithelium. In contrast, during contraction and distension mainly cellular shape changes from protruded to flattened, but the microvilli-like membrane junctions between intermediate and basal cell layers demonstrate little change.