Uroepiteel

Uroepiteel, also known as urothelium, is the specialized epithelium lining most of the urinary tract, including the renal pelvis, ureters, bladder, and the proximal portion of the urethra. It is a type of stratified transitional epithelium that can undergo marked stretching as the bladder fills. The urothelium originates from the endoderm of the urogenital sinus and is adapted to provide both barrier and distensibility for urine storage and flow.

The epithelium consists of three cellular layers: basal cells attached to the basement membrane, intermediate cells, and large, highly specialized superficial umbrella cells. In a relaxed state, the tissue is about three to five cell layers thick, and it thickens with distension to accommodate urine. Superficial umbrella cells possess a highly specialized apical membrane with uroplakin-containing plaques, forming an asymmetric unit membrane that contributes to impermeability. Tight junctions between cells and a mucopolysaccharide-rich surface layer further reduce urine permeability. The urothelium rests on a supportive lamina propria and can renew itself from basal progenitor cells after injury.

Functions include forming a watertight barrier against urinary solutes and toxins, maintaining a stable luminal environment, and enabling large volume changes without leaking. It also participates in sensory signaling related to bladder fullness and repair processes following injury.

Clinical relevance: changes in the urothelium are central to urothelial carcinomas, cystitis, and other bladder disorders. Diagnostic and research markers include uroplakins and cytokeratins (such as CK20) expressed by umbrella cells.