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Recommended video: Stratified epithelium [13:06]
Structures and types of stratified epithelia.

Urothelium, also called transitional epithelium, is a type of stratified epithelial tissue that lines most of the urinary tract, from the calyces in kidneys to the proximal part of the urethra.

As its name suggests, this epithelium in the non-distended state, resembles stratified cuboidal epithelium, but undergoes a transition in their appearance to stratified squamous epithelium when stretched, while maintaining the actual number of layers.

It consists of an apical layer of large, dome-shaped cells called umbrella cells, that are sometimes bi- or multinucleated. These cells have specialized membranes that are impermeable and able to withstand the hypertonic and potential cytotoxic effects of urine, thus, offering protection to underlying tissues. Additionally, these cells have a unique characteristic of accommodating a great degree of stretch with bladder filling and distention. 

Below the apical layer is an intermediate region composed of few to several
layers of columnar cells. Beneath this intermediate region sits a single layer of compact, cuboidal basal cells that rest on a thin basement membrane.

Terminology English: Urothelium
Synonym: Transitional epithelium

Synonym: Epithelium transitionale
Definition Transitional epithelium or urothelium, found only in the lining of
the urinary system, is stratified, with large rounded surface cells pro-
tective against urine
Location Renal calyces, renal pelvis, ureters, urinary bladder, urethra
Function Protection barrier for underlying cells, distensibility

Learn more about the urothelium in the following study unit:

Urothelium: want to learn more about it?

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