Transitional epithelium (also known as urothelium) is a type of tissue consisting of multiple layers of epithelial cells which can contract and expand. These cells, part of the epithelium, are found in the urinary bladder, in the ureters, and in the superior urethra and gland ducts of the prostate.
These cells appear to be cuboidal with a domed apex when the organ or the tube in which they reside is not stretched. When the bladder fills, the tissue actually compresses and the cells become stretched, and they appear to be flat, irregular, and squamous.
The transitional epithelium cells accommodate fluctuation of volume of the liquid in an organ (Note that the distal part of the urethra becomes non-keratinized stratified squamous epithelium in females, The part that lines the bottom of the tissue is called the basement membrane.