The ureters, bladder, and the initial portion of the urethra are all lined with transitional epithelium, which is also called urothelium.
This specialized epithelium is only found in the urinary system and it allows the conducting passageways and bladder to expand a lot while staying impermeable to water and ions.
Let’s first take a look at a cross-section of the ureter at low magnification, which shows the four main concentric layers of the ureter: the transitional epithelium that lines the mucosa, the lamina propria, the muscularis externa, and the adventitia.
The transitional epithelium is also further divided into 3 layers.
At high magnification, we can see that the most superficial layer consists of large ovoid cells that are called dome or umbrella cells because of their curved apical surface and because they often cover other epithelial cells beneath them.
When the ureter is distended, there are fewer folds in the epithelium, but the epithelium is also able to expand further by flattening and decreasing the amount of overlap of the umbrella cells.
The second layer that comprises the transitional epithelium is an intermediate layer of cuboidal and low columnar cells.
When umbrella cells are damaged, cells from the intermediate layer are able to quickly differentiate in order to replace the damaged umbrella cells.
The third layer of the transitional epithelium is the basal layer, which is a single layer of cuboidal basal cells that rest on the basement membrane.
In this cross-section of the ureter, the lumen is on the far left.
So the first layer starting from the left is the transitional epithelium, which is purple when stained with hematoxylin and eosin.
The next main layer beneath the transitional epithelium is a supporting layer called the lamina propria, which is a thick layer of dense collagenous and elastic connective tissue.
Surrounding the lamina propria, is the muscularis externa, which is made of 2-3 layers of smooth muscle.
The proximal portion of the ureter has an inner longitudinal layer and an outer circular layer.
The direction of these muscle layers are actually the opposite of how they’re arranged in the GI tract.
The distal portion of the ureter has an additional outer longitudinal layer of smooth muscle.
Keep in mind that this is a cross section of the ureter, so the individual muscle fibers that run longitudinally will look round in this image, and the middle circular layer of smooth muscle will appear to have long fibers, since they are arranged parallel or along the same plane as this section of tissue.
Finally, the outermost layer of the ureter is the adventitia, which is composed of loose connective tissue, blood vessels, adipocytes, and nerves.
Alright, let’s move onto the bladder, which has a similar overall structure as the distal ureter.
So it also has the same four main concentric layers, which includes the transitional epithelium with umbrella cells, the lamina propria, the muscularis externa, and the outer adventitia.