Transitional epithelium is a specialized, stratified tissue distinguished by its capacity to stretch. This tissue, also called urothelium, is composed of multiple cell layers that can change shape in response to tension. When the organ it lines is in a relaxed state, the cells appear cuboidal, but they flatten and become more squamous-like when stretched.
Unique Cellular Structure
The structure of transitional epithelium is organized into three distinct layers. At the bottom is the basal layer, which sits on a thin basement membrane and contains epithelial stem cells responsible for tissue renewal. The cytoplasm of these basal cells contains protein filaments that anchor this layer to the basement membrane.
Above the basal cells lies the intermediate cell layer, which consists of several rows of highly proliferative cells. This capacity for rapid cell division allows the tissue to quickly repair itself in response to injury or infection.
The topmost layer facing the organ’s lumen is the superficial cell layer, composed of large “umbrella cells.” These cells are the most differentiated and provide a barrier. When the tissue is relaxed, these cells have a domed, cuboidal appearance, but they flatten as the organ stretches. This structural change allows the tissue to expand without compromising its integrity and enables the organ to accommodate significant changes in volume.
Function and Protective Barrier
The design of transitional epithelium serves two main purposes: it allows for distension and provides a protective barrier. The ability of the cells to change shape enables organs like the urinary bladder to expand significantly as it fills with urine, increasing the organ’s volume while maintaining a continuous lining. This prevents the organ from rupturing under pressure.
This tissue also functions as an effective permeability barrier, separating the urinary tract’s contents from the rest of the body. The apical surfaces of the umbrella cells have a membrane that is impermeable to water and salts, preventing the passive movement of substances. This barrier is augmented by tight junctions, which are specialized connections that seal the membranes of adjacent cells together.
This seal is important for preventing harmful waste products, toxins, and pathogens in the urine from leaking into the underlying tissues and bloodstream. The cells are also highly resistant to the osmotic pressure exerted by urine, which protects them from damage and desiccation even when fully stretched.
Location in the Urinary System
Transitional epithelium is almost exclusively found lining the organs of the urinary system. Its presence in these locations is directly related to their function. Specific locations include:
- The renal pelvis, the central collecting part of the kidney
- The ureters that transport urine from the kidneys
- The urinary bladder where urine is stored
- The proximal portion of the urethra
The entire pathway that urine travels requires a lining that can handle fluctuations in fluid volume. For instance, the ureters must expand as pulses of urine pass through, and the bladder needs to accommodate a large volume. The properties of urothelium make it suited for these demands.
Clinical Relevance
The health of the transitional epithelium is linked to the proper functioning of the urinary tract. Damage to this tissue can lead to medical conditions like interstitial cystitis and overactive bladder. In urinary tract infections (UTIs), the protective barrier can be compromised, allowing bacteria to adhere to the lining and cause inflammation.
Another clinical issue associated with this tissue is the development of cancer. Transitional Cell Carcinoma (TCC), also known as Urothelial Carcinoma, is the most common type of bladder cancer and arises from the transitional epithelial cells. The degree to which cancerous cells penetrate the epithelium’s basement membrane is a factor in determining the disease’s prognosis.