Transitional epithelium is a stratified tissue made of multiple cell layers, where the cells constituting the tissue can change shape depending on the distention in the organ. When the organ is filled with fluid, cells on the topmost layer of this epithelium can stretch and appear flattened. Alternately, they can also appear cuboidal with a rounded shape when the fluid pressure is low. This epithelium is found lining the urinary bladder, ureters and urethra, as well as in the ducts of the prostrate gland. Due to its location in the excretory system, especially in the ureters and urinary bladder, one of the primary functions of this tissue is to be an extremely effective permeability barrier, impenetrable to water and most small molecules. The second important function of these cells is to allow the organ to stretch and increase its volume depending on fluid pressure. Source: https://biologydictionary.net/transitional-epithelium/