urothelial

Urothelial, referring to the urothelium, is the specialized epithelial lining of much of the urinary tract, including the renal pelvis, ureters, bladder, and the proximal part of the urethra. It forms a durable barrier between urine and the underlying tissues while allowing the urinary tract to accommodate varying volumes.

Histologically, the urothelium is stratified into basal, intermediate, and superficial umbrella (surface) cells. Umbrella cells are large and often binucleate, and their apical membranes contain uroplakin plaques. The superficial layer can become highly flattened as the bladder fills. Tight junctions and a mucopolysaccharide-rich protective layer contribute to impermeability, helping keep urine components from penetrating the tissue. During distension, vesicles called discoid fusiform vesicles fuse with the apical membrane to expand its surface area.

Functionally, the urothelium provides a robust barrier against urine’s potentially harmful constituents while remaining flexible to accommodate stretching and shortening of the urinary tract. It also participates in the controlled exchange of substances and maintains a relatively low permeability to water and ions.

Clinical note: the urothelium is the origin of urothelial carcinoma, the most common cancer of the bladder and a major form of urothelial cancers elsewhere in the urinary tract. Risk factors include cigarette smoking and occupational exposure to certain chemicals. Diagnosis typically involves cystoscopy with biopsy, urine cytology, and imaging studies.