Uroepithelium

Urothelium, also known as uroepithelium, is the specialized mucosal lining of the urinary tract. It lines the renal pelvis, calyces, ureters, urinary bladder, and the proximal urethra. As a stratified epithelium, it provides a watertight, highly impermeable barrier that can accommodate significant changes in luminal volume without leaking urine.

Histology and structure: The urothelium consists of basal cells attached to the basement membrane, several layers of intermediate cells, and a superficial layer of umbrella cells. Umbrella cells are large, often binucleated, and possess a distinct apical membrane enriched with uroplakin plaques. Tight junctions and a mucopolysaccharide-rich coat contribute to the surface barrier. The epithelium can rapidly remodel through proliferation of basal progenitor cells to replace damaged cells.

Function: The primary role is to act as a barrier against urine constituents and to prevent permeability to potentially irritating solutes and bacteria. It also exhibits marked plasticity, expanding and flattening umbrella cells as the bladder fills, thus preserving barrier integrity while accommodating stretch. The urothelium regenerates quickly after injury.

Development and clinical relevance: The urothelium originates from the endodermal lining of the urogenital sinus. It is innervated and supported by a vascularized lamina propria. Pathologically, urothelial cells give rise to urothelial carcinoma, and disorders of barrier function are implicated in conditions such as interstitial cystitis. Urothelial markers include uroplakins present in the umbrella cell plaques.