What is transitional epithelium?

Transitional epithelium is a type of stratified epithelial tissue that is made up of multiple layers of specialized cells that have the ability to expand and contract depending on the distention in the organ. It is found lining the structures of the urinary system including the urinary bladder, ureters, and urethra. 

When the urinary bladder is empty and fluid pressure is low, the cells of the transitional epithelium are relaxed and appear cuboidal in shape. As the bladder fills with urine, the apical cells unfold and flatten, acquiring a more squamous or flattened appearance. The unfolding and flattening of the apical cells increases the surface area, allowing the bladder to expand to accommodate the urine. 

The primary function of transitional epithelium is to facilitate the expansion and contraction of the organs or structures that it covers or lines. Another, equally critical function of the transitional epithelium is that it acts as an effective permeability barrier preventing toxic wastes from entering or leaking into the bloodstream or the rest of the body.