Transitional epithelium (urothelium) is unique in having 'umbrella cells' (facet cells) on its luminal surface. What is the structural basis for the ability of the bladder to distend without leakage?

• A The basement membrane is thick and elastic, providing the primary barrier function
• B Goblet cells interdigitate with umbrella cells to produce mucus as the permeability barrier
• C Gap junctions between all urothelial layers allow coordinated expansion
• D Umbrella cells have rigid asymmetric unit membranes (AUM/urothelial plaques) containing uroplakins, and fold/unfold on distension; tight junctions (zonula occludens) create an impermeable barrier ✓

Explanation

Transitional (urothelial) epithelium has several unique adaptations for distension and impermeability. The surface umbrella (facet) cells have a highly specialised apical membrane with crystalline arrays of hexagonal protein particles (uroplakins Ia, Ib, II, III) forming rigid 'urothelial plaques' (asymmetric unit membrane, AUM) that are biochemically distinct from standard lipid bilayers and are highly impermeable. When the bladder is distended, extra membrane from cytoplasmic vesicles (fusiform vesicles) is inserted into the surface, increasing surface area by up to 70%. Tight junctions (zonula occludens) between umbrella cells prevent paracellular leakage of urine. This combination creates the blood-urine barrier.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.