Figure 1922

Photomicrograph of transitional epithelium (urothelium). This H&E-stained specimen shows the 4- to 5-cell-layer 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. x450.

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" cells because of the curvature of the apical surface (see Fig. 19.22). 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. 19.23). 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. 19.24). Each cell appears to fold inward upon itself. As a result 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. 19.25).

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

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