Transitional Epithelium Definition
Transitional Epithelium DefinitionTransitional epithelium is a layered tissue made up of several layers of cells, whereby the cells that make up the tissue can change their shape depending on the extent of the organ. When the organ is filled with fluid, the cells in the top layer of this epithelium can stretch and appear flattened. Alternatively, if the liquid pressure is low, it can also appear cube-shaped with a rounded shape.
This epithelium is found in the bladder, ureters, and urethra, as well as in the ducts of the prostate.
- The picture shows a cross-section of the bladder with an inset showing the histology of the epithelium, the underlying connective tissue (lamina propria), and the submucosa.
- Transitional epithelium is a type of layered epithelium that consists of several layers of cells, with the shape of the cell changing depending on the function of the organ. The epithelium is variable and appears cubic or round when relaxed, except for the apical layer, which appears to flatten when stretched. This epithelium is almost exclusively limited to the urinary tract, which is why it is also called “urothelium”.
The function of the transitional epithelium
Because of its location in the excretory system, particularly the ureters and bladder, one of the main functions of this tissue is to be a highly effective permeability barrier that is impermeable to water and most small molecules. The cells of this tissue are probably among the most resistant to osmotic pressure. Urine is hypertonic with a much higher concentration of many solutes than the cytoplasm of epithelial cells. However, these cells are protected from dehydration even when the epithelium is fully expanded. Toxic waste is also prevented from getting back into the bloodstream.
The second important function of these cells is to allow the organ to expand and increase in volume according to the pressure of the fluid. For example, when a large amount of water needs to be expelled from the body, a large amount of fluid will pass through the ureters, bladder, and urethra. The ability of the cells in the surface layer of this epithelium to change shape (change from a round cubic shape to a flattened squamous epithelial structure) allows these organs to stretch without exposing the underlying tissues to the toxic substances they contain.
Structure of the transitional epithelium
Transitional epithelia consist of 3-4 cell layers and the lowest or basal layer remains in contact with the basement membrane. The cells of this basal layer are connected to the lamina propria by tonofilaments and hemidesmosomes. These are among the least differentiated cells in this tissue and support the rest of the cells. Cells in the middle layers proliferate and can replace cells lost through abrasion or infection. They also have a large Golgi network that contains several membrane-bound vesicles. The surface layer of cells can change from a cubic appearance to a flattened appearance when the organ is stretched and contains a series of actin-based cytoplasmic protrusions called microvilli. A two-dimensional array of hexameric plaques covers the apical plasma membrane of these cells. Made up of a protein called uroplakin, these plaques are an important feature of this epithelium that helps create the urinary barrier to water, ammonia, urea, and many other solutes. They are also likely involved in these cells’ ability to change shape.
All cells of this epithelium are deeply connected by connecting complexes. Junction complexes are symmetrical connections between two cells, usually made up of three components: a band of tight junctions on the apical surface, followed by an intermediate series of adherent junctions and desmosomes on the base. These multiprotein complexes hold the epithelial cells together and form an uninterrupted surface in the lumen of the organ.
Location and Examples of transitional epithelium
Transitional epithelia are most commonly found in the male urinary tract and genital tract in men. These are areas where the volume and osmolarity of the organ can change rapidly. In the urinary system, the volume and concentration of solutes in the urine depend on several factors. Likewise, the prostate urethra in the male reproductive system is bounded by a continuous transitional epithelium with the epithelium of the bladder. It is the most stretchable part of the urethra that expands or contracts depending on the flow of urine or semen.
The bladder is an organ that holds much of the body’s toxic liquid waste before it is cleared from the body. When fully expanded, the bladder can hold almost 500 ml of urine, making it an organ that undergoes drastic changes in volume in a short period. Although three layers of muscle fibers contribute to the stretching and contraction of the organ, the transitional epithelium is also vital. Surface cell uroplakin-binding complexes and plaques protect the body from the effects of the storage of urea, ammonia, and other metabolites in the bladder. In addition, the plaques would help apical cells adjust the area of their plasma membrane. For example, when the bladder is stretched, the membrane area increases, possibly due to the fusion of vesicles in the Golgi network.
Transitional Epithelium Definition, Function, structure, location, and Example