Transitional Epithelium: Function, Structure, and Location

Transitional epithelium, also known as urothelium, is a specialized, stratified tissue that lines organs of the urinary system. Consisting of multiple layers of cells, its structure allows it to change shape and adapt to the function of the organ it lines. This adaptability makes it a distinct component of the body’s urinary structures.

The Unique Structure of Transitional Epithelium

Transitional epithelium is composed of three to four layers of cells. The bottom, or basal layer, is in contact with the basement membrane, a thin, supportive layer of tissue. The cells of this basal layer are cuboidal or columnar in shape and are the least differentiated, serving to support the other layers. They are attached to the underlying connective tissue, the lamina propria.

Above the basal layer are the intermediate layers, which consist of proliferative cells that can multiply to replace those lost due to abrasion or infection. These cells are involved in processing and packaging proteins and lipids. The cells in the intermediate layers are polyhedral or pear-shaped.

The most superficial layer, facing the organ’s lumen, is made of large, dome-shaped cells called umbrella cells, which can be multinucleated. A feature of these umbrella cells is the presence of specialized proteins called uroplakins on their surface. These proteins form rigid plaques that contribute to the tissue’s structural integrity. The epithelium’s appearance changes with the organ’s state; when relaxed, the cells are cuboidal, but when stretched, they flatten and appear more scale-like.

Function and Location of Transitional Epithelium

Transitional epithelium has two primary functions: distensibility and protection. Distensibility is the tissue’s ability to stretch significantly without damage, which is evident in the urinary bladder. The epithelium transforms from about five to seven layers thick in a relaxed state to just two or three layers when the bladder is full. This change is facilitated by the umbrella cells, which flatten to accommodate the increased volume.

The second function is providing a protective barrier. The uroplakin plaques on the umbrella cells create a highly impermeable barrier between urine and the underlying tissues, preventing toxic substances from leaking into the bloodstream. The tight connections between epithelial cells also contribute to this impermeability, limiting the passage of water and salts.

Due to these functions, transitional epithelium is found almost exclusively in the urinary system. It lines the following organs, which need to expand and contract while containing urine:

• The renal pelvis of the kidneys
• The ureters
• The urinary bladder
• The upper part of the urethra

Associated Medical Conditions

A significant medical condition associated with this tissue is urothelial carcinoma, also known as transitional cell carcinoma. This is the most common type of bladder cancer, accounting for about 90-95% of cases, and it originates in the transitional epithelium cells. It can be non-invasive, growing only within the urothelium, or invasive, spreading into the bladder wall’s deeper layers. Symptoms include blood in the urine, and diagnosis involves urine analysis and cystoscopy.

Defects in the transitional epithelium’s barrier function can contribute to other conditions. In chronic urinary tract infections (UTIs), damage to the urothelium may allow bacteria to penetrate the tissue, leading to persistent inflammation. This can compromise the protective glycosaminoglycan (GAG) layer covering the epithelium.

Another condition linked to urothelial dysfunction is painful bladder syndrome, or interstitial cystitis. One theory suggests that a defect in the epithelium allows irritating substances in urine to penetrate the bladder wall, causing pain and inflammation. This can lead to symptoms like chronic pelvic pain, urinary frequency, and urgency, even without an infection.