The ureters are fibromuscular tubes that transport urine from the renal pelvis to the urinary bladder, where it’s stored until it’s emptied through the urethra during urination.
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.
Although the basement membrane is often hard to identify when using light microscopy.
In this cross-section of the ureter, the lumen is on the far left.