The urinary bladder receives the urine from the two ureters and stores it until neural stimulation causes it to contract and expel the urine via the urethra. It, too, is lined with transitional epithelium. Beneath the epithelium and its underlying connective tissue, the wall of the urinary bladder contains smooth muscle that is usually described as being arranged as an inner longitudinal layer, a middle circular layer, and an outer longitudinal layer. As in most distensible hollow viscera that empty their contents through a narrow aperture, the smooth muscle in the wall of the urinary bladder is less regularly arranged than the description indicates, allowing contraction to reduce the volume relatively evenly throughout the bladder.
This orientation micrograph of the urinary bladder reveals the full thickness of the bladder wall. The luminal surface epithelium is at the top of the micrograph. One of the ureters can be seen as it passes through the bladder wall to empty its contents into the bladder lumen. Most of the tissue to the sides and below the ureteral profile is smooth muscle.
Figure 1, urinary bladder, human, H&E x60.
This micrograph shows most of the entire thickness of the urinary bladder. An unusual feature is the presence of one of the ureters (U) as it is passing through the bladder wall to empty its contents into the bladder lumen. The transitional epithelium (Ep) lining the bladder is seen on the right. Beneath the epithelium is a relatively thick layer of connective tissue (CT) containing blood vessels (BV) of various sizes. Note that the connective tissue stains some what denser than the smooth muscle of the underlying mus-cularis (M). The epithelium and connective tissue constitute the mucosa of the bladder. The muscularis consists of smooth muscle arranged in three indistinct layers. It should be noted that as the ureter passes through the bladder wall, it carries with it a layer of longitudinally oriented smooth muscle (SM(L)). Medium-size arteries (A) and veins (V) are occasionally seen in the muscularis.
Figure 2, urinary bladder, human, H&E x250.
This higher magnification of the left rectangle of Figure l shows the transitional epithelium (Ep) and the underlying connective tissue (CT) that represent the mucosa of the ureter. Adjacent to the mucosa are bundles of longitudinally
Figure 3, urinary bladder, human, H&E x250.
This higher magnification of the right rectangle of Figure l shows the bladder epithelium (Ep) and the underlying connective tissue (CT) of the bladder wall. The transitional epithelium is often characterized by the presence of surface cells that exhibit a "dome" shape. In addition, many of these cells are binucleate (arrows). The thickness of transitional epithelium is variable. When the bladder is fully distended, as few as three cell layers are seen. Here, in the con-
sectioned smooth muscle (SM(Lj) that belong to the ureter. A small lymphatic vessel (Lym) is present in the connective tissue adjacent to the smooth muscle. Note the lymphocytes, identified by their small round densely stained nuclei, within the lumen of the vessel.
tracted bladder, it appears that there are as many as ten cell layers, a result of the cells folding over one another as the smooth muscle contracts and the lining surface is reduced. The connective tissue consists of bundles of collagen fibers interspersed with varying numbers of lymphocytes identified by their densely stained round nuclei. A vein (V) filled with red blood cells is also evident in the mucosal connective tissue.
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