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Transitional epithelium is also called Uroepithelium or Urothelium (because it lines the urinary system), and it is a type of stratified epithelial tissue in which the surface cells change shape from being rounded to squamous in nature. Transitional epithelium is located in the urinary system, especially the urinary bladder. When the bladder is empty, the surface epithelial cells of the transitional epithelium are seen being rounded but these cells become flattened (squamous) as the bladder fills; therefore, the main function of Transitional epithelium is to enable the bladder to fill and stretch with-out tearing the lining this is the unique characteristic of urothelium (that ability to adapt to volume changes and respond to the tensile forces in the lumen of the organ in which the epithelium lines.
Transitional Epithelium Location
Transitional epithelium is found only in the urinary system. It lines the urinary passages and extends from the minor calyces of the kidneys to the ureters and down to the proximal urethra. Transitional epithelium covers the external surfaces of the renal papillae and reflects onto the internal surfaces of the surrounding minor calyces.
Description of the Structure of Transitional Epithelium
The transitional epithelium is a thick stratified epithelium that is made up of large rounded cells that vary considerably, depending on the degree of distension to which they are subjected. The superficial cells of a non-distended transitional epithelium such as that of the urinary bladder appear rounded or dome-shaped and often show a free convex border that bulges into the lumen. These superficial layer of large, dome-like cells are often called Umbrella cells; these Umbrella cells are especially well developed in the bladder where contact with urine is greatest. The umbrella cells measure up to 100 micrometer in diameter and have extensive intercellular junctional complexes that surround unique apical membranes. Most of the apical surface consists of asymmetric unit membranes in which regions of the outer lipid layer appear ultrastructurally to be twice as thick as the inner leaflet and are composed of lipid rafts containing mostly integral membrane proteins called Uroplakins that assemble into paracrystalline arrays of stiffened plaques measuring 16 nanometer in diameter. It is the presence of these membranous plaques that makes the epithelium impermeable to water and protects the cytoplasm and other underlying cells from the hyperosmotic effects of urine. In the distended organ, the superficial cells may vary in shape from squamous to cuboidal in shape.
Therefore, cells of the transitional epithelium are organized into three layers consisting of:
- a single layer of small basal cells resting on a very thin basement membrane that cannot be seen by light microscopy
- an intermediate layer that contains several layers of more columnar cells
- a superficial layer of very large, bulbous cells called umbrella cells that are occasionally bi-nucleated or multinucleated and are highly differentiated to protect underlying cells against the cytotoxic effects of concentrated (hypertonic) urine.
Histology of Transitional Epithelium
Urothelium (transitional epithelium) is stratified and lines much of the urinary tract system. The superficial cells are rounded or dome-shaped with specialized membrane features enabling them to withstand the hypertonic effects of urine and protect underlying cells from this toxic solution. The superficial cells sometimes contain large polyploid nuclei. In the major and minor calyces of the kidneys, the transitional epithelial layers are formed by two to three cells thickness, increasing in the ureter to four or five layers and to six, eight, or more layers in the bladder (Therefore, the calyces, renal pelvis, ureter, and bladder have similar histologic structure but their walls vary in thickness becoming gradually thicker as the epithelium moves closer to the urinary bladder). The mucosa of these organs is lined by the unique stratified transitional epithelium or.
Transitional epithelium lies on a very thin basement membrane that usually is not seen with the light microscope but the lamina propria consists of a compact layer of fibroelastic connective tissue with or without some diffuse lymphatic tissue.
Transitional Epithelium Functions
- The cells of the transitional epithelia are specialized to protect underlying tissues from the hypertonic and potentially cytotoxic effects of urine.
- The morphological features of the cells allow distension of transitional epithelium as the urinary bladder fills and stretches.
- Transitional epithelium forms a barrier to the diffusion of salts and water into and out of the urine
Medical importance of Transitional epithelium
Long term infection of the bladder known as Chronic cystitis can cause the transitional epithelium to become unstable leading to some benign changes in the transitional epithelium such as increased in number of cells (hyperplasia) or changes in their shapes (metaplasia); when these changes are not immediately addressed, then Bladder cancer known as transitional cell carcinoma arises from the unstable urothelium (transitional epithelium).