Urothelium

Abstract

The wall of the urinary passages, including the surfaces of the renal pelvis, the ureters, the urinary bladder, and proximal parts of the urethra, is covered by a unique specialized epithelium, the transitional epithelium or urothelium. The urothelium is stratified and composed of three types of cells: basal precursor cells, intermediate cells, and large superficial umbrella cells. The latter line the lumina of the organs and are responsible for the main specific urothelial functions. Results indicate that in the adult bladder after injury, intermediate cells function as progenitors for superficial cells. The urothelium enables the retention of urine and forms a barrier that makes it impermeable to water and movement of ions and metabolites. The various types of superficial cells in the distinctive parts of the urinary tract epithelium differ considerably in the expression of uroplakins, cytokeratins, and in the structure of the apical cell surface. In the bladder, the urothelium adapts to the cyclical changes of luminal contents and must maintain the permeability barrier under variations in pressure during filling and voiding. The permeability barrier (“blood-urine barrier”) between tissue fluids and urine depends on the high-resistance tight junctions between the superficial cells, and on their unique apical plasma membrane visible under the electron microscope.