ANALYSIS OF MEMBRANE-STRUCTURE IN THE TRANSITIONAL EPITHELIUM OF RAT URINARY-BLADDER .2. DISCOIDAL VESICLES AND GOLGI-APPARATUS - THEIR ROLE IN LUMINAL MEMBRANE BIOGENESIS

The luminal plasma membrane of the rat urothelium is composed of plaques seen in freeze-fracture as hexagonal arrays of 12 nm-diameter subunits on the E-face. In the present freeze-fracture study, the structure of the discoidal vesicle and Golgi membranes of superficial urothelial cells was investigated and the relationship between discoidal vesicle and luminal membrane structure examined by comparative analysis of subunit number and density. The vesicle membranes exhibit a striking variation in subunit disposition and number, forming a complete spectrum from those appearing similar to luminal membrane to those with smooth E- and P-faces devoid of subunits. Golgi membranes also display smooth zones on both fracture faces, situated between regions bearing irregularly distributed intramembrane particles of the usual size and polarity. Subunits are not observed in Golgi membranes. The results are interpreted as sequential stages in assembly of luminal membrane and are discussed in relation to current concepts of membrane biogenesis. © 1979 Academic Press, Inc.