FIGURE 20.25 • Photomicrograph of transitional epithelium (urothelium). This H&E–stained specimen shows the 4to 5-celllayer thickness of the epithelium in the relaxed ureter. The surface cells exhibit a rounded or dome-shaped profile. The connective tissue below the epithelium (Ep) is relatively cellular and contains a number of lymphocytes. Blood vessels (BV) are also abundant in this area. 450.
when the bladder is distended, as few as three layers are seen. This change reflects the ability of the cells to accommodate to distension. The cells in the distended bladder, particularly the large surface cells and those in the layers below, flatten and unfold to accommodate the increasing surface area. As the individual cells unfold and flatten, the resulting appearance is the “true” three layers.
In routine histologic sections obtained from the empty bladder, the surface epithelial cells are usually cuboidal and bulge into the lumen. They are frequently described as “dome shaped” or “umbrella” cells because of the curvature of the apical surface (see Fig. 20.25). When examined with the TEM, the plasma membrane exhibits an unusual feature (e.g., modified areas of the plasma membrane called plaques) are seen (Fig. 20.26). These plaques appear to be more rigid and thicker (up to 12 nm) than the rest of the apical plasma membrane. Actin filaments are observed stretching from the inner surface of the plaques into the cytoplasm. In the undistended urinary bladder, the plaques give the luminal surface of cells an irregular scalloped contour (Fig. 20.27). Each cell appears to fold inward on itself. Because of this folding, the plaques appear as a series of fusiform vesicles. Their lumina, however, are in continuity with the cell’s exterior. As the bladder distends, the fusiform vesicles unfold and become part of the surface as the cell stretches and flattens (Fig. 20.28).
FIGURE 20.26 • Transmission electron micrograph of urinary bladder epithelium. The mucous membrane of the urinary bladder consists of transitional epithelium (Ep) with an underlying lamina propria (LP). The epithelial cells contain unique fusiform vesicles, which are evident here at this relatively low magnification. These are seen at higher magnification in Figure 20.24. 5,000.
Smooth muscle of the urinary passages is arranged in bundles.
A dense collagenous lamina propria underlies the urothelium throughout the excretory passages. Neither a muscularis mucosae nor a submucosal layer is present in their walls. In the tubular portions (ureters and urethra), usually two layers of smooth muscle lie beneath the lamina propria:  Longitudinal layer, the inner layer that is arranged in a loose spiral pattern Circular layer, the outer layer that is arranged in a tight
Note that this arrangement of the smooth muscle is opposite that of the muscularis externa of the intestinal tract. The smooth muscle of the urinary passages is mixed with connective tissue, so that it forms parallel bundles rather than pure
FIGURE 20.27 • Transmission electron micrograph of the apical portion of a transitional epithelial cell. a. The cytoplasm displays small vesicles, filaments, and mitochondria, but the most distinctive feature of the cell is its fusiform vesicles (FV). 27,000.
b. The higher magnification shows that the membrane forming the vesicles appears to be similar to the plasma membrane of the cell surface (arrows). Both membranes are thickened and give the impression of possessing a degree of rigidity greater than that of plasma membrane in other locations. The thickened plasma membrane represents a sectioned view of a surface plaque. The fusiform vesicles are formed by the infolding of the plaques in the cells of the relaxed urinary bladder. 60,000.