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Transitional epithelium is a specialized form of stratified skin tissue. It is often called urothelial epithelium because the lining of the urinary tract is the most common place to find it. Transitional epithelium is very different from other types of epithelium because its cells are able to change shape to increase the amount of stretching the tissue can withstand. This ability to change shape is the reason it is referred to as transitional as the cells transition from one shape to another. Almost all urinary tract cancers begin in this type of tissue.
Epithelium can be either simple, having one layer, or stratified, i.e., multiple layers. Transitional epithelium has multiple layers but is a little more complicated in that the innermost layer appears to be several layers when the tissue is not stretched. Each cell in that layer is attached to the basement layer by an extension of the cell wall. When the tissue stretches, the piled up cells spread out to increase the surface area of the tissue. Transitional epithelium lines part of the urethra, the bladder, the ureters — i.e., tubes leading from the bladder to the kidneys, the renal calyces — i.e., the wider tube that connects the ureters to the kidneys, and the renal pelvis of each kidney.
Epithelial cells can be in three forms: squamous or thin and flat; cuboidal, i.e., roughly cube shaped; or columnar, i.e., elongated. Unlike ordinary epithelium, transitional epithelium cannot be classified as squamous, cuboidal, or columnar, however. Its cells appear to be cuboidal when the tissue is not stretched, although the cells in the topmost layer appear almost balloon-like because of an unusually high amount of cytoplasm. These same cells take on the appearance of squamous cells when the tissue stretches. This flattened shape also increases the surface area of the tissue, thereby allowing it to stretch farther.
While urinary cancer is rare, bladder cancer is found in the epithelium 90% of the time. Cancer can also strike the epithelium found in the renal pelvis of the kidney, the ureter, and the urethra. Some factors that increase the risk for transitional cell cancer are smoking, taking certain prescription medications like cyclophosphamide and phenacetin, long-term overuse of pain medication; and being around chemicals in the manufacture of rubber, plastic, or leather. Symptoms of this type of cancer can include painful urination sometimes with blood in the urine, recurring upper back pain, unexplained weight loss, and fatigue.
I understand why transitional epithelium is found in the bladder. This organ has to expand and contract all day and all night, so it has to be stretchy. If it weren’t, we would all be in trouble.
It is amazing how much urine it can hold when we sleep through the night. If I don’t wake up during the night some time, when I get up in the morning, I can feel how full my bladder is, and I can imagine how much it must have stretched to accommodate that amount of urine.