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Urinary Tract Histology

  • The Urinary Tract includes the ureters, bladder, and urethra and conducts urine from the kidneys to the outside world. The urinary tract is lined by a highly stretchy yet impermeable epithelium known as the urothelium, specialized for the task of urine conduction. Importantly, the urothelium also lines the minor and major calyces as well as the renal pelvis which for all intensive purposes can be thought of as part of the urinary tract.
  • The urothelium (AKA Transitional Epithelium) is a stratified epithelium with about 5-6 layers of urothelial cells. Urothelial Cells in the lower layers are polygonal to cuboidal, whereas the most superficial layer of cells are known as Umbrella Cells because they can stretch significantly. The key function of the urothelium is its capacity to stretch significantly without losing its impermeability to the movement of water and solutes. This is a key function that prevents water and electrolytes from diffusing back and forth between the urine and the ECF even when the epithelium is stretched tremendously as occurs in a full bladder.
  • The ureter is a muscular tubule which conducts urine from the renal pelvis to the bladder. The inner-most layer of cells is a urothelium which sits on top of a loose collagenous lamina propria. The outer portion of the Ureter is composed of two muscular layers which help conduct the urine in bolus form to the bladder.
  • The bladder is a muscular vesicle which stores urine in between periods of urination. The inner layer of the bladder is composed of urothelium which sits on top of a loose collagenous lamina propria. The outermost section of the bladder is defined by the Detrusor Muscle which is composed of several layers of smooth muscle cells that contract during voiding of the bladder.