Uroepithelial

Uroepithelium, also known as urothelium, is a specialized stratified epithelium that lines the urinary tract, including the renal pelvis, calyces, ureters, bladder, and the proximal portion of the urethra. It is adapted to repeated stretching as the bladder fills and empties while forming a tight barrier against urine.

Histologically, the uroepithelium consists of three cell layers: basal cells resting on the basement membrane, intermediate cells, and large superficial umbrella cells at the luminal surface. Umbrella cells may be binucleate and possess a highly specialized apical membrane with urothelial plaques formed by uroplakins. These plaques, along with tight junctions and a supportive cytokeratin network, create a robust barrier that protects underlying tissues from urine’s chemical constituents and fluctuations in volume.

The primary functions of the uroepithelium are to provide a selectively permeable barrier and to accommodate distension. The epithelium can transition from a thick, protective lining when the bladder is empty to a flattened, stretched form when distended, while maintaining barrier integrity. It also participates in sensory signaling related to bladder fullness and urine flow.

Developmentally, the uroepithelium is derived from the endoderm of the urogenital sinus during embryogenesis. Its distinctive structure and barrier properties are essential for urinary storage and transport, and they have clinical relevance in diseases such as urothelial (transitional cell) carcinoma and in practices using immunohistochemical markers to identify urothelial origin.