Differentiation of the mammalian urothelium reaches its peak in the superficial cell layer, which consists of large umbrella cells. Umbrella cells are unique for their luminal plasma membrane, as 70–90% of its area is covered by urothelial plaques. The plaques are asymmetrically thickened membrane domains with diameters of 600–1500 nm, separated by narrow rims of nonthickened membranes, called hinge regions. The organization of uroplakins in plaques defines their rigidity and is of major importance for the proper formation and maintenance of the urinary bladder’s permeability barrier. Urothelial plaques are also present in fusiform vesicles, each containing two plaques. FVs function as transporting compartments for the delivery of urothelial plaques to the apical plasma membrane. During bladder stretch, mechanoreceptors activate exocytosis of FVs by purinergic signalling, modulated by cAMP, Ca2, extracellular ATP, adenosine, the epidermal growth factor receptors and the actin cytoskeleton. By that means, 25–55% of cytoplasmic FVs are incorporated into the apical plasma membrane, which makes its size increase, and urinary bladder can accommodate filling with urine. It has been shown that Rab 27b, Rab11a, syntaxin-1, SNAP-23 and synaptobrevin play important roles in apical targeting of FVs. FVs may also be formed by endocytosis during contractions of the urinary bladder. Upon voiding, the redundancy of the apical plasma membrane is internalized and designated for degradation in lysosomes. Two hypotheses have been put forward to explain a biosynthetic origin of urothelial plaques. According to the first one, it has been assumed that thickened membranes assemble in Golgi cisternae from the cis- to the trans- side, and eventually SAR131675 transcisternae mature into FV. Finally, the FV detaches from the Golgi stack. According to the second hypotheses, urothelial plaque assembly begins with the synthesis of four major uroplakins, followed by the formation of UPIa/UPII and UPIb/UPIII dimers in the endoplasmic reticulum. In the Golgi apparatus, two N-glycosylation sites on the UPII pro-sequence are converted into complex glycans, which results in the tethering of dimers into the heterotetramers. It is assumed that by cleavage of the UPII pro-sequence into the trans-Golgi network, heterotetramers assemble into a 16-nm uroplakin particle and particles are arranged into urothelial plaques in the post-Golgi compartments. The maturation stages of urothelial plaques, following the exit of the heterotetramers out of the Golgi apparatus, have not been shown in umbrella cells.