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Transitional

Transitional epithelium, also known as urothelium, is a specialized type of epithelial tissue. It is found primarily in the urinary system. This includes the urinary bladder, ureters, and part of the urethra. It’s designed to accommodate changes in the volume of the organs it lines, particularly in response to stretching.

Histological slide showing transitional epithelium in longitudinal section, stained purple (H&E stain). Epithelial cells exhibit a layered appearance with rounded nuclei in the top layer. Underlying connective tissue is less densely stained, containing scattered fibroblasts and small blood vessels. The epithelium appears folded orrugated. High magnification.
Figure 1: Transitional epithelium tissue, stained purple under a microscope.

Cell Shape

The surface cells of transitional epithelium often have a rounded or domed appearance on their apical surface. When the organ is distended, this shape flattens out when the bladder, ureters, or urethra are stretched.  This change in shape allows the epithelium of the bladder to stretch. It achieves this even though it does not contain any elastic fibers.

Often, cells in transitional epithelium appear columnar or cuboidal toward the basal surface on a collapsed bladder.  The apical cells usually appear domed or deceptively cuboidal.  As the bladder stretches, so do these cells, elongating their width.  The columnar shapes become cuboidal and the cuboidal shapes become squamous.

Like other types of epithelial tissue, transitional epithelium is held together by tight junctions. The cells, however, are relatively impermeable to water and solutes.  This impermeability prevents leakage of urine waste products. It stops them from entering the tissues around the bladder, ureters, and urethra.

Layering

Transitional epithelium is stratified, meaning it consists of multiple layers of cells. The number of layers varies depending on the degree of distension of the organ. When the organ is empty, it appears as if only a few layers of cells are present. As it stretches, additional layers are revealed.

Function

The primary function of transitional epithelium is to accommodate changes in volume.  It is important for the bladder to be able to stretch to accommodate urine.  Equally important is the ability of the bladder to return to a flaccid, empty shape.  Transitional epithelium is able to accomplish both these functions with its ability to compress layers and squish cells.

Locations

Transitional epithelium is uniquely suited to the urinary tract. It can undergo significant stretching and distension without tearing. This is due to its ability to change shape and the flexibility of its cell layers.

Specializations

Like other types of epithelial tissue, transitional epithelium is held together by tight junctions. These junctions help to maintain the integrity of the epithelial barrier.  Transitional epithelium is relatively impermeable to water and solutes, which helps to prevent the leakage of urine into surrounding tissues.


Diagram illustrating transitional epithelium lining the lumen of the bladder with dome-shaped cells on the apical surface and labeled lumen.
Diagram illustrating transitional epithelium lining the lumen of the bladder with dome-shaped cells on the apical surface and labeled lumen.
Microscopic image of transitional epithelium, stretched. The layered tissue section shows a stratified cellular structure, with visible cell nuclei stained purple-blue and cytoplasm in pink-red hues with a light background space. Cell morphology in this image demonstrates the transitional epithelium's unique ability to stretch
Figure 2: Stretched transitional epithelium histology showing cell layers
Microscopic view of transitional epithelium, stained pink. The image focuses on the tissue's edge, showcasing the stratified cell structure. The top layer shows rounded, dome-shaped cells. At the bottom, the connective tissue is visible as brighter pink, less defined material. The white background provides contrast.
Figure 3: Microscopic view of transitional epithelium, a type of tissue found in the urinary tract, showing cell layers and structure.
High-magnification microscopic image of transitional epithelium, stained pink. The tissue appears as a curving structure against a lighter background. The upper layers show densely packed, rounded cells contrasting with irregular shapes near the base. Darkly stained nuclei are visible within each cell. Some other red or blood substances can also be seen.
Figure 4: Microscopic view of transitional epithelium tissue, showing pink-stained cells.
Histological slide of transitional epithelium tissue stained pink. The view shows the characteristic multilayered structure, with columnar basal cells transitioning to cuboidal and then large, rounded apical cells. Nuclei are visible within the cells. Illumination is bright, highlighting cellular details. The composition emphasizes the tissue's transitional nature.
Figure 5: Transitional epithelium tissue showing multiple layers of cells with varying shapes. Histological specimen.

Figure 1: Bladder non-distended, H&E, 40X Slide DU 98-1
Figure 2: Ureter, over distended, H&E, 20X Slide 091-1
Figure 3: Bladder, human, H&E, 40X Slide 212
Figure 4: Ureter, undistended, H&E, 40X Slide 019-2
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.

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General Information
Simple Squamous
Stratified Squamous
Simple Cuboidal
Stratified Cuboidal
Simple Columnar
Stratified Columnar
Transitional
Pseudostratified Ciliated Columnar

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