Molecular ultrastructure of the urothelial surface: insights from a combination of various microscopic techniques.

The urothelium forms the blood-urine barrier, which depends on the complex organization of transmembrane proteins, uroplakins, in the apical plasma membrane of umbrella cells. Uroplakins compose 16 nm intramembrane particles, which are assembled into urothelial plaques. Here we present an integrated survey on the molecular ultrastructure of urothelial plaques in normal umbrella cells with advanced microscopic techniques. We analyzed the ultrastructure and performed measurements of urothelial plaques in the normal mouse urothelium. We used field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) on immunolabeled ultrathin sections (immuno-TEM), and freeze-fracture replicas (FRIL). We performed immunolabeling of uroplakins for scanning electron microscopy (immuno-FESEM). All microscopic techniques revealed a variability of urothelial plaque diameters ranging from 332 to 1179 nm. All immunolabeling techniques confirmed the presence of uroplakins in urothelial plaques. FRIL showed the association of uroplakins with 16 nm intramembrane particles and their organization into plaques. Using different microscopic techniques and applied qualitative and quantitative evaluation, new insights into the urothelial apical surface molecular ultrastructure have emerged and may hopefully provide a timely impulse for many ongoing studies. The combination of various microscopic techniques used in this study shows how these techniques complement one another. The described advantages and disadvantages of each technique should be considered for future studies of molecular and structural membrane specializations in other cells and tissues.

Freeze-fracture replica immunolabelling reveals urothelial plaques in cultured urothelial cells.

The primary function of the urothelium is to provide the tightest and most impermeable barrier in the body, i.e. the blood-urine barrier. Urothelial plaques are formed and inserted into the apical plasma membrane during advanced stages of urothelial cell differentiation. Currently, it is supposed that differentiation with the final formation of urothelial plaques is hindered in cultured urothelial cells. With the aid of the high-resolution imaging technique of freeze-fracture replica immunolabelling, we here provide evidence that urothelial cells in vitro form uroplakin-positive urothelial plaques, localized in fusiform-shaped vesicles and apical plasma membranes. With the establishment of such an in vitro model of urothelial cells with fully developed urothelial plaques and functional properties equivalent to normal bladder urothelium, new perspectives have emerged which challenge prevailing concepts of apical plasma membrane biogenesis and blood-urine barrier development. This may hopefully provide a timely impulse for many ongoing studies and open up new questions for future research.

Urothelial plaque formation in post-Golgi compartments.

Urothelial plaques are specialized membrane domains in urothelial superficial (umbrella) cells, composed of highly ordered uroplakin particles. We investigated membrane compartments involved in the formation of urothelial plaques in mouse umbrella cells. The Golgi apparatus did not contain uroplakins organized into plaques. In the post-Golgi region, three distinct membrane compartments containing uroplakins were characterized: i) Small rounded vesicles, located close to the Golgi apparatus, were labelled weakly with anti-uroplakin antibodies and they possessed no plaques; we termed them "uroplakin-positive transporting vesicles" (UPTVs). ii) Spherical-to-flattened vesicles, termed "immature fusiform vesicles" (iFVs), were uroplakin-positive in their central regions and contained small urothelial plaques. iii) Flattened "mature fusiform vesicles" (mFVs) contained large plaques, which were densely labelled with anti-uroplakin antibodies. Endoytotic marker horseradish peroxidase was not found in these post-Golgi compartments. We propose a detailed model of de novo urothelial plaque formation in post-Golgi compartments: UPTVs carrying individual 16-nm particles detach from the Golgi apparatus and subsequently fuse into iFV. Concentration of 16-nm particles into plaques and removal of uroplakin-negative membranes takes place in iFVs. With additional fusions and buddings, iFVs mature into mFVs, each carrying two urothelial plaques toward the apical surface of the umbrella cell.