Figure 1923

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 19.24. x5(000.

their walls. In the tubular portions (ureters and urethra), usually two layers of smooth muscle lie beneath the lamina propria:

• The inner layer is arranged in a loose spiral described as a longitudinal layer.

• The outer layer is arranged in a tight spiral described as a circular layer.

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 muscular sheets. Peristaltic contractions of the

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