Transitional epithelium of the urinary bladder
Transitional epithelium is the layered lining of the urinary bladder, whose cells slide across one another so the bladder can stretch as it fills with urine.
Remember
Transitional epithelium lets the bladder lining stretch: its cells change shape and slide across one another as the organ fills.
Memory aid
transitional epithelium is the one in transition
The name reminds you that its surface cells move between cuboidal and flat shapes as the bladder fills.
Epithelia (sheets of cells that line surfaces) are classified by cell shape and by how many layers they form. A squamous cell is flat, a cuboidal cell is box-like and a columnar cell is tall. A simple epithelium has one layer, and a stratified one has several. Transitional epithelium is a stratified type found in the ureters (the tubes that carry urine from the kidneys) and the urinary bladder.
Its name is a memory aid: its surface cells make a transition between shapes as the bladder fills. When the bladder is empty, the lining is folded and looks thick and multilayered, with cuboidal surface cells that have rounded, umbrella-shaped tops. As the bladder fills with urine, the folds flatten out, the surface cells become squamous, and the lining looks thinner and less layered.
The key property is that the cells can slide across one another. The lining stretches as the bladder fills. Once the bladder empties, the cells return to their earlier shape. The large umbrella cells (named for their domed tops) stop urine leaking through the wall, so the barrier stays intact as the bladder expands.
Other epithelia are built for different jobs. Stratified squamous epithelium is layered too, but it protects the skin and the lining of the mouth against wear and tear. Pseudostratified columnar epithelium (literally false layers) only looks layered, because every cell touches the basement membrane (the thin sheet of protein beneath an epithelium), and it lines the respiratory tract (the airways). Simple cuboidal epithelium is a single layer of cells that secretes and absorbs, with no stacked layers to slide over one another.
Key facts
- Site
- found only in the urinary system, such as in the ureters and the bladder.
- Key property
- the cells slide across one another, so the lining can stretch.
- Empty bladder
- a thicker, folded, multilayered lining whose surface cells are cuboidal and rounded.
- Full bladder
- a thinner, less layered lining, because the folds flatten and the surface cells become squamous.
- Naming
- epithelia are named by cell shape and layering; transitional is listed with the layering categories (simple, stratified and pseudostratified), but it is a stratified type.
- Also called
- urothelium.
Exam traps
- Stratified squamous epithelium is multilayered, but its job is protection against wear and tear, as in the skin and the lining of the mouth, rather than repeated stretching.
- Pseudostratified columnar epithelium only looks layered, because every cell touches the basement membrane, so it is really a single layer and belongs to the respiratory tract rather than the bladder.
- Flat surface cells in a full bladder do not make its lining simple squamous epithelium, because a single thin layer is built for the exchange of substances that must pass quickly, not for changing thickness.
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