Turnover of asymmetric unit membranes in the transitional epithelial superficial cells of the rat urinary bladder.

Asymmetric thick unit membranes were observed on the luminal surface, fusiform vesicles, and multivesicular bodies of superficial cells of rat transitional epithelium. When HRP-labeled Ricinus communis lectin (RCA-I) was injected into the rat urinary bladder, RCA-I was deposited along the luminal cell membrane and in some multivesicular bodies, but not in the fusiform vesicles either before or after contraction. When the bladder was sliced by Vibratome and stained with HRP-labeled RCA-I after fixation, RCA-I was observed in many cell organelles, including fusiform vesicles and multivesicular bodies as well as the luminal surface. When small pieces of tissue were stained en-bloc with HRP-labeled RCA-I, RCA-I was found along the luminal cell surface but not in the fusiform vesicles nor the multivesicular bodies. When HRP alone was injected into the bladder, HRP was observed in some multivesicular bodies after contraction but not in the fusiform vesicles. Various lysosomes were observed by electron microscopy. Some were wrapping multivesicular bodies in ringlike fashion, and some contained asymmetric unit membranes. These findings suggest that the asymmetric unit membranes are carried to the luminal cell membrane via the fusiform vesicles and that old luminal cell membranes are removed via the multivesicular bodies to be degraded by lysosomes.