The entire urine collecting system is lined by a sheet of transitional epithelium known as urothelium. In the renal pelvis, the urothelial cells are two or three layers thick. The most superﬁcial cells are larger than the others and send projections down over the lateral surfaces of the cells beneath them, sometimes having an umbrella-like appearance. These “umbrella cells” have abundant eosinophilic cytoplasm and may be binucleate. Underneath the umbrella cells are smaller intermediate cells and basal cells.
From the minor calyces onward, these cells rest on top of a thin lamina propria, dual muscle layer, and adventitia. The outer of the two muscular layers consists of “typical” smooth muscle cells, which increase in number near the ureteropelvic junction and extend into the ureter. The inner layer, in contrast, contains “atypical” smooth muscle cells that terminate at the ureteropelvic junction. At present these atypical cells are thought to be the pacemaker cells responsible for the initiation of peristalsis. They are smaller than typical cells and their contractile ﬁlaments, instead of running parallel, appear randomly scattered, as in cardiac pace- maker cells. Another population of cells, which resembles interstitial cells of Cajal, has recently been identiﬁed in the renal calyces of some mammals, but its function is still being determined.
The ureter contains three to ﬁve layers of urothelial cells, which are thrown into folds with a characteristic stellate appearance. These cells sit on a well-developed, loose lamina propria that contains small vessels and nerves. There is no muscularis mucosae.
Outside of the lamina propria is the muscularis propria, which is continuous with the layer of “typical” smooth muscle cells seen in the renal pelvis. Its contractile ﬁbers are loosely arranged and interspersed with connective tissue. In the upper part of the ureter, there is a vague division into inner longitudinal ﬁbers and outer circumferential ﬁbers, although the distinction is often difﬁcult. In the lower half of the ureter, an additional outer ring of longitudinal ﬁbers tends to appear. Because of the urothelial folds and the well-developed longitudinal musculature, sizable calculi may pass through the ureter without injury to the mucosa. The outermost layer of the ureter contains a thick adventitia with longitudinally oriented small blood vessels.
The bladder contains ﬁve to eight irregularly folded layers of urothelial cells. An exception occurs at the trigone, where there are generally fewer layers of urothelial cells with a smooth, unfolded arrangement. As the bladder is distended, the urothelial cells ﬂatten out, with the most superﬁcial cells ﬂattening out to such an extent that they become barely visible. During this process, vesicles near the apical surfaces of the cells fuse with the plasma membrane to provide additional surface area.
Unlike in the ureter, the lamina propria occasionally contains a muscularis mucosae, which appears discontinuous and contains a haphazard arrangement of wispy, thin bundles of smooth muscle cells. In some instances, the ﬁbers can become hypertrophic and resemble those of the muscularis propria. Uncommonly, adipocytes can be found in the lamina propria.
The muscularis propria is known as the detrusor muscle; as in the lower ureter, it consists of inner and outer longitudinal ﬁbers with an intervening layer of circumferential ﬁbers. Except in the area of the bladder neck, these layers are typically indistinct, appearing as a meshwork of crisscrossed thick muscle bundles. Interspersed through the muscular layers are blood vessels, lymphatics, nerve ﬁbers, and even adipose tissue.