Ultrastructure of the rat urothelium in en face section.

We examined en face the fine structure of rat urothelium and describe here some unique subcellular features distinguishable in the various tissue layers by this technique. The most striking finding is that large, compact fibrous bundles lie in the cytoplasm at both sides of the superficial cell tight junction for the entire length around it apical surface. Basal to these bundles, an intricate series of desmosomes alternates with other specialized elaborations of the plasma membrane. These desmosomes anchor the extensive filamentous network that interconnects the nucleus, flattened superficial vesicles, and plaques of the lumenal membrane. En face sections of the urothelium show that large numbers of vesicles form inthe intermediate cell Golgi complexes and are then stored just under the apical membrane. These intermediate vesicles are smaller, more round, and less variable in size than superficial vesicles. After intermediate cells reach the superficial layer, the vesicles mature, undergoing modifications that apparently increase their tendency to fuse.