urothelium

The urothelium, also known as transitional epithelium, is a specialized stratified epithelium that lines much of the urinary tract. It covers the renal pelvis, ureters, urinary bladder, and part of the proximal urethra. The urothelium is adapted to stretch as the bladder fills while maintaining a tight barrier against urine.

Structurally, the urothelium consists of basal, intermediate, and superficial umbrella cells. Basal cells rest on the basement membrane, while intermediate cells serve as a proliferative reserve. The luminal surface is lined by umbrella cells, which are large and dome-shaped and can be binucleated in some species. These cells form urothelial plaques on their apical membranes that contain uroplakins, contributing to both impermeability and mechanical resilience. The epithelium is joined by tight and adherens junctions, creating a robust barrier.

Functionally, the urothelium acts as a highly selective barrier that prevents urinary solutes and toxins from diffusing into underlying tissues. It also accommodates substantial stretching without compromising integrity. When distended, umbrella cells flatten to preserve a continuous surface, and the urothelium can participate in signaling and regenerative responses following injury or inflammation.

Development and clinical relevance: In mammals, the urothelium derives from the endoderm of the urogenital sinus during embryogenesis. Pathologically, it is the origin of urothelial (transitional) carcinomas, most commonly in the urinary bladder. Other conditions, such as infections and inflammatory diseases, can disrupt its barrier function and integrity.