Transitional Epithelium

Transitional epithelium occupy the inner surface of all urinary system, in lower urinary system, include renal pelvis, ureters, bladder and urethra is called uroepithelium (urothelium). Urothelium is composed of at least three layers: a basal cell layer attached to basal membrane, an intermediate layer, and a superficial cells layer termed as umbrella cell. The shape of umbrella cells alters as a result of the fullness of organs. These cells have abundant cytoplasmic vesicles in their cytoplasm. It was reported that the vesicles are located apically close to the luminal surface of umbrella cells and covered by a thin membrane. These vesicles connect to each other with thin cytoplasmic processes. Apical membranes of umbrella cells are covered by raised ridges, also called as hinges or microplicae. Among the hinges, there are regions composed of asymmetric unit membranes structures called as plaque regions. It is detected that each plaque are composed of approximately 1000 sub-assemblies consisting of four types of proteins (UP1a, UP1b, UP2 and UP3) located at the inside of membrane termed as uroplakin (UP). Urothelium functions as a tight barrier for ion and liquid influx. Urothelium is also a sensitive structure that senses physiological, chemical and mechanical stimulations.