Which epithelium lines the urinary tract and is capable of stretching?

• A. Simple columnar epithelium
• B. Transitional epithelium (urothelium)
• C. Stratified squamous epithelium
• D. Pseudostratified ciliated columnar epithelium

Answer: (B)

Stretchable lining of the urinary tract is essential because these organs fill and empty repeatedly, so the tissue must expand without tearing or leaking. Transitional epithelium, also called urothelium, is built for this task. It lines the renal pelvis, ureters, bladder, and proximal urethra, and its surface cells can shift shape from tall, dome-shaped umbrella cells when the bladder is relaxed to flatter cells as it stretches. Tight junctions between cells and a specialized apical membrane with uroplakin plaques keep urine from penetrating the tissue, maintaining a robust barrier even during expansion. This combination of location and distensibility is unique to transitional epithelium, making it the best choice. Other epithelia don’t provide the same balance of lining the urinary tract and accommodating large distension: simple columnar lines many ducts and the gut but isn’t stretch-ready for urine storage; stratified squamous resists abrasion but isn’t designed to distend; pseudostratified ciliated columnar lines the respiratory tract, not the urinary system.