Localization and Function
The urothelium historically received the name transitional epithelium because its appearance depends on the physiological state of the organ. This tissue exclusively lines organs of the urinary system that are subject to significant and regular volume changes—primarily the urinary bladder and urinary pathways.
When the organ fills with fluid, its walls experience intense tension. To prevent tearing and preserve the barrier function, transitional epithelium cells flatten out. As a result, the entire epithelial layer becomes visually thinner, yet its initial stratified structure is fully preserved.
Histoarchitecture: Stratified Structure
Like any stratified epithelium, the urothelium consists of multiple cell tiers, each possessing distinct morphological characteristics:
- Basal layer. Located directly on the basement membrane. It consists of small cells with characteristic oval nuclei.
- Intermediate layer. Formed by polygonal (many-sided) cells that make up the bulk of the layer in the unstretched state.
- Superficial layer. Composed of very large cells. A distinctive feature of this layer is that many of its cells may be binucleated.
Morphology of Superficial Cells and Stretching Mechanism
Superficial layer cells play a key role in tissue adaptation to pressure. In a relaxed (unstretched) organ, they have a convex, dome-like shape.
When the organ fills and its wall stretches, these cells dramatically change configuration, becoming flat. The secret to this elasticity lies in a specialized plasma membrane reserve. In the apical (upper) portion of superficial cells, this membrane reserve is stored as:
- Deep folds (invaginations).
- Specialized intracellular vesicles.
During stretching, these structures unfold, quickly and safely increasing the total surface area of the cell.
Diagnostic Features on Microscopic Slides
When studying histological slides, students often encounter difficulties identifying the urothelium. The reason is that during slide preparation, cell shape is frequently distorted by mechanical and chemical processing, and the classic dome shape of superficial cells is not always clearly visible.
The most reliable diagnostic criterion: pay attention to the nuclei of the superficial layer cells. In transitional epithelium, they consistently retain a rounded shape. This reliably distinguishes the urothelium from stratified squamous epithelium, whose surface cells contain flattened, squamous nuclei.