Transitional epithelium
| Transitional epithelium | |
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| Code | TH H2.00.02.0.02033 |
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Anatomical terminology
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Transitional epithelium is a type of tissue consisting of multiple layers of epithelial cells which can contract and expand. It is so named because of this function in the transition of degree of distension. This tissue structure type is found in urothelium, including that of the renal pelvis, urinary bladder, the ureters, the superior urethra, and the prostatic and ejaculatory ducts of the prostate.
The appearance of transitional epithelium depends on the layers in which it resides. Cells of the basal layer are cuboidal, or cube-shaped, and columnar, or column-shaped, while the cells of the superficial layer vary in appearance depending on the degree of distension. These cells appear to be cuboidal with a domed apex when the organ or the tube in which they reside is not stretched. When the organ or tube is stretched (e.g. when the bladder is filled with urine), the tissue compresses and the cells become stretched. When this happens, the cells flatten, and they appear to be squamous and irregular.
The transitional epithelium cells stretch readily in order to accommodate fluctuation of volume of the liquid in an organ. (Note that the distal part of the urethra becomes non-keratinized stratified squamous epithelium in females; the part that lines the bottom of the tissue is called the basement membrane.) Transitional epithelium also functions as barrier between the lumen, or inside hollow space, of the tract that it lines and the bloodstream. To help achieve this, the cells of transitional epithelium are connected by tight junctions, or virtually impenetrable junctions that seal together the cellular membranes of neighboring cells. This barrier prevents reabsorption of toxic wastes and pathogens by the bloodstream.
Transitional epithelium is made up of three types of cell layers: basal, intermediate, and superficial. The basal layer fosters the epithelial stem cells in order to provide constant renewal of the epithelium. These cells' cytoplasm is rich in tonofilaments and ; however, they contain few rough endoplasmic reticulum. The tonofilaments play a role in the attachment of the basal layer to the basement membrane via desmosomes. The intermediate cell layer is highly proliferative and, therefore, provides for rapid cell regeneration in response to injury or infection of the organ or tube in which it resides. These cells contain a prominent Golgi apparatus and an array of membrane-bound vesicles. These function in the packaging and transport of proteins, such as keratin, to the superficial cell layer. The superficial cell layer, that which lines the lumen, is the only fully differentiated layer of the epithelium. It provides an impenetrable barrier between the lumen and the bloodstream, so as not to allow the bloodstream to reabsorb harmful wastes or pathogens. All transitional epithelial cells are covered in microvilli and a fibrillar mucous coat.
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