Transitional epithelium

Transitional epithelia is a type of tissue consisting of multiple layers of epithelial cells which can contract and expand. These cells, part of the epithelium, are usually found in the urinary tract, especially around the urinary bladder. They are also found in the ureters and superior urethra.

Appearance
These cells appear to be cuboidal when the organ or the tube is not stretched (in reality, squeezed like when your bladder fills up, the tissue actually "compress" hence, "stretched"). When these cells are stretched ("compress") they look flat and irregular, squamous.

Transitional epithelium is also known as urothelium .

Functions
Some functions of the transitional epithelium cells: accommodates fluctuation of volume of the liquid in an organ or tube (i.e. urinary bladder), and protection against the caustic effects of urine.

Surfaces
The free surface of the tissue (the part that faces outward into the organ or tube) is called the apical surface or free surface.

The part that lines the bottom of the tissue is called the basement membrane.