urotheliale

Urothelium, or urothelial epithelium, is a specialized stratified lining that covers most of the urinary tract, from the renal pelvis and ureters to the bladder and the proximal urethra. It is a form of transitional epithelium that can stretch significantly as the bladder fills and empties, enabling urine storage and passage without tissue damage.

The tissue typically consists of three cell layers: basal cells at the base, intermediate cells in the middle, and umbrella (superficial) cells on the surface. Umbrella cells form a tight, flexible barrier and display apical membrane plaques enriched with uroplakins, which help protect underlying tissues from urine while allowing expansion. Tight junctions between cells contribute to the barrier function, limiting paracellular leakage and maintaining impermeability to many solutes.

Functionally, the urothelium serves as a dynamic barrier and a reservoir for urine. It can rapidly adapt its surface area through changes in umbrella cell size and number, maintaining integrity under varying volumes and pressures.

Developmentally, the urothelium originates from endoderm and renews primarily by proliferation of basal cells, with turnover altered in response to injury or inflammation.

Clinical relevance includes urothelial carcinoma, the most common cancer of the urinary tract, historically termed transitional cell carcinoma. Risk factors include smoking and exposure to certain industrial chemicals. Diagnosis combines cystoscopy, urine cytology, and biopsy, with treatment ranging from transurethral resection and intravesical therapies to more extensive surgery for invasive disease. Non-malignant conditions affecting the urothelium include cystitis and metaplastic changes.