Renal Pelvis
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.
Ureter
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.
Bladder
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.
