Localization and Functional Purpose
The urothelium was historically named transitional epithelium because its appearance depends on the state of the organ. This tissue exclusively lines the organs of the urinary system that are subject to significant and regular volume changes—primarily the urinary bladder and urinary tract.
When the organ fills with fluid, its walls experience strong tension. To prevent ruptures and preserve the barrier function, the cells of the transitional epithelium flatten out. As a result, the entire epithelial layer becomes visually thinner, yet its initial stratified structure is fully preserved.
Histoarchitecture: Layered Structure
Like any stratified epithelium, the urothelium consists of several cell tiers, each with its own 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 main bulk of the layer in the unstretched state.
- Superficial layer. Represented by very large cells. A distinctive feature of this layer is that many of its cells can be binucleated.
Morphology of Superficial Cells and the Stretching Mechanism
The cells of the superficial layer 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 drastically change their configuration, becoming flat. The secret to such elasticity lies in the presence of a specific plasmalemma reserve (cell membrane). In the apical (upper) part of the superficial cells, this membrane reserve is stored in the form of:
- Deep folds (invaginations).
- Special intracellular vesicles.
During stretching, these structures straighten out, quickly and safely increasing the total surface area of the cell.
Diagnostic Features on a Slide
When studying histological slides, students often face difficulties in identifying the urothelium. The fact is that during slide preparation, cell shape is often distorted due to mechanical and chemical influences, and the classic dome-like 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 always retain a rounded shape. This makes it possible to unmistakably distinguish the urothelium from stratified squamous epithelium, whose surface cells contain flattened, flat nuclei.
Briefly: answers to topical questions
What are the specific transmembrane proteins that form plaques on the apical surface of urothelial cells called?
Urothelial-specific proteins are referred to in sources as uroplakins.
Information that they are transmembrane and form plaques on the apical surface of urothelial cells is not provided in the supplied sources.
To distinguish the urothelium from stratified squamous epithelium, look at the nuclei of the topmost layer: in squamous epithelium, the nuclei are flat (like pancakes), while in transitional epithelium, they are round (like balls), even if the cell itself has flattened.
Class and exam questions
Why is this epithelium called transitional?
The name reflects its ability to visually "transition" from one state to another: the thickness of the layer and cell shape change from tall dome-like (in an empty organ) to flat (in a stretched one).
Does the epithelium lose its stratification when the urinary bladder stretches?
No, the mechanism of adaptation consists in the flattening of cells, not in a change in their number. The layer becomes thinner, but the stratified histoarchitecture is fully preserved.
How do superficial cells manage to increase their surface area so significantly?
In the apical part of these cells, there is a membrane reserve (in the form of invaginations and vesicles) that unfolds upon tension, preventing cell rupture.
What else to study on this topic
- Structural features of the urothelial basement membrane
- Cytological mechanisms of binucleated cell formation
- Histological processing artifacts distorting cell shape
- Interaction of folds and membrane vesicles during stretching
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Transitional Epithelium" and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the section "Introduction to Histology. Epithelial Tissues"
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.