Epithelium is a type of tissue that forms continuous sheets of cells, covering all internal and external surfaces of the body. This tissue acts as a selective barrier, providing protection, absorption, and secretion. Transitional epithelium, also known as urothelium, is a highly specialized variant. Its unique structure and function allow it to manage the extreme volume changes required by the organs it lines, which often leads to confusion regarding its classification.
Understanding Epithelial Layering
Transitional epithelium is classified as a stratified tissue, meaning it is composed of multiple layers of cells. This is in contrast to simple epithelia, which consists of only a single layer of cells where every cell touches the basement membrane. The multiple layers provide structural reinforcement and protection against the harsh environment it faces.
This stratified tissue is organized into three distinct cell layers. The deepest layer is the basal layer, which consists of cuboidal or columnar cells connected to the underlying basement membrane. These basal cells are responsible for generating new cells to constantly renew the epithelial sheet.
Above the basal cells lies the intermediate layer, which is highly proliferative and provides for rapid cell regeneration. The most superficial layer is composed of large, specialized cells known as umbrella cells or facet cells. The presence of these multiple stacked layers confirms the tissue’s classification as stratified, regardless of the changing shape of its top cells.
Where Transitional Epithelium Is Found
The distribution of transitional epithelium is restricted, found almost exclusively within the organs of the urinary system. Its presence begins in the renal pelvis, where urine first collects after being formed by the kidney. The tissue then extends through the ureters, which transport urine down to the bladder.
The urinary bladder itself is lined with this specialized tissue to accommodate its primary function as a storage reservoir. Transitional epithelium continues into the proximal section of the urethra, the tube through which urine exits the body. Because of this strict localization, the tissue is often referred to as urothelium, reflecting its role lining the urinary tract.
This specific location demands the unique properties of the transitional epithelium to prevent the contents of the lumen from contacting the underlying body tissues. The environment it lines contains urine, a fluid concentrated with waste products and salts. The tissue must withstand the mechanical stresses of expansion and contraction while maintaining an effective chemical barrier.
The Role of Stretching and Barrier Protection
The name “transitional” refers to the tissue’s ability to change its appearance and cell shape in response to fluid volume changes. When the bladder is empty or relaxed, the umbrella cells on the surface appear large and rounded, giving them a dome-like shape. As the bladder fills with urine and becomes distended, the entire tissue layer stretches and thins out dramatically.
During this distension, the superficial umbrella cells flatten considerably, becoming thin and squamous-like. This change allows the tissue to accommodate a large volume of liquid without rupturing or compromising the barrier integrity. The tissue can transition between a thicker, multi-layered state when empty and a thinner, more stretched state when full.
A primary function of the urothelium is to act as an impermeable barrier. This is necessary to prevent the reabsorption of water, salts, and potentially toxic waste products from the urine back into the bloodstream. This impermeability is established by specialized structures within the tissue, particularly the tight junctions that seal the spaces between the umbrella cells.
The apical surface of the umbrella cells also features a modified cell membrane composed of a rigid hexagonal lattice. This crystalline plaque structure provides a keratinized surface that is resistant to the chemical properties of urine. This dual mechanism of mechanical stretch and chemical impermeability ensures the bladder can safely store urine for extended periods.