urotheliums

The urothelium is a specialized type of stratified epithelium that lines the urinary tract, from the renal calyces and pelvis to the ureters, bladder, and the proximal part of the urethra. It is also known as transitional epithelium because it was initially thought to be a transitional form between simple and stratified squamous epithelium. However, its unique structure and function are distinct from both.

The urothelium is characterized by its ability to stretch and contract, accommodating changes in the volume of urine. Its surface cells, called umbrella cells or dome cells, are large and dome-shaped. These cells possess a thick apical membrane containing specialized proteins like uroplakins, which are crucial for forming a barrier against the highly concentrated and potentially toxic urine. This barrier prevents water and solutes from diffusing back into the body and protects the underlying tissues from damage.

The structure of the urothelium varies depending on the degree of distension of the organ it lines. When the bladder is empty, the urothelium appears thicker, with several layers of cells. As the bladder fills with urine, the urothelium thins and stretches, reducing the number of cell layers visible. This remarkable plasticity is essential for the function of the urinary organs.

The urothelium also plays a role in immune surveillance and defense within the urinary tract. It can secrete antimicrobial peptides and cytokines, contributing to the protection of the urinary system from infection. Furthermore, alterations in the urothelium are often indicative of underlying urinary tract pathology, making it a subject of study in urological diseases such as bladder cancer and interstitial cystitis.