alt=transitional epithelium|thumb|Transitional epithelium animation, highlighting the epithelial layer, then underlying connective tissue. Contrast the messy appearance of the epithelial surface to other epithelial tissues.

Transitional epithelium is a type of stratified epithelium. Transitional epithelium is a type of tissue that changes shape in response to stretching (stretchable epithelium). The transitional epithelium usually appears cuboidal when relaxed and squamous when stretched. 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 (such as 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.

Cell layers

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 mitochondria; 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. Because of its importance in acting as an osmotic barrier between the contents of the urinary tract and the surrounding organs and tissues, transitional epithelium is relatively impermeable to water and salts. This impermeability is due to a highly keratinized cellular membrane synthesized in the Golgi apparatus. The membrane is made up of a hexagonal lattice put together in the Golgi apparatus and implanted into the surface of the cell by reverse pinocytosis, a type of exocytosis. The cells in the superficial layer of the transitional epithelium are highly differentiated, allowing for maintenance of this barrier membrane.

Function

The transitional epithelium cells stretch readily in order to accommodate fluctuation of volume of the liquid in an organ (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 a 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 to the cellular membranes of neighboring cells. This barrier prevents re-absorption of toxic wastes and pathogens by the bloodstream.

Clinical significance

Urothelium is susceptible to carcinoma. Because the bladder is in contact with urine for extended periods, chemicals that become concentrated in the urine can cause bladder cancer. For example, cigarette smoking leads to the concentration of carcinogens in the urine and is a leading cause of bladder cancer. Aristolochic acid, a compound found in plants of the family Aristolochiaceae, also causes DNA mutations and is a cause of liver, urothelial and bladder cancers. Occupational exposure to certain chemicals is also a risk factor for bladder cancer. This can include aromatic amines (aniline dye), polycyclic aromatic hydrocarbons, and diesel engine exhaust.

Carcinoma

Carcinoma is a type of cancer that occurs in epithelial cells. Transitional cell carcinoma is the leading type of bladder cancer, occurring in 9 out of 10 cases. It is also the leading cause of cancer of the ureter, urethra, and urachus, and the second leading cause of cancer of the kidney. Transitional cell carcinoma can develop in two different ways. Should the transitional cell carcinoma grow toward the inner surface of the bladder via finger-like projections, it is known as papillary carcinoma. Otherwise, it is known as flat carcinoma.

Transitional cell carcinoma patients have a variety of treatment options. These include nephroureterectomy, or the removal of kidney, ureter, and bladder cuff, and segmental resection of the ureter. This is an option only when the cancer is superficial and infects only the bottom third of the ureter. The procedure entails removing the segment of cancerous ureter and reattaching the end. The exact causes of IC/BPS are unknown, but there is evidence of an association between increased permeability of the urothelium and IC. Since the purpose of the urothelium is to act as a highly resistant barrier, the loss of this function has serious clinical implications. Many patients with IC have exhibited a loss of umbrella cells.

Urothelial lesions

  • Papillary urothelial lesions
  • Papillary urothelial hyperplasia
  • Urothelial papilloma
  • Papillary urothelial neoplasm of low malignant potential (PUNLMP)
  • Low-grade papillary urothelial carcinoma
  • High-grade papillary urothelial carcinoma
  • Invasive urothelial carcinoma
  • Flat urothelial lesions
  • Reactive urothelial atypia
  • Urothelial inverted papilloma
  • Urothelial atypia of unknown significance
  • Urothelial dysplasia
  • Urothelial carcinoma in situ
  • Invasive urothelial carcinoma
  • Invasive urothelial carcinoma (NOS)
  • Urothelial carcinoma with inverted growth pattern
  • Urothelial carcinoma with squamous differentiation
  • Urothelial carcinoma with villoglandular differentiation
  • Urothelial carcinoma, micropapillary variant
  • Urothelial carcinoma, lymphoepithelioma-like variant
  • Urothelial carcinoma, clear cell (glycogen-rich) variant
  • Urothelial carcinoma, lipoid cell variant
  • Urothelial carcinoma with syncitiotrophoblastic giant cells
  • Urothelial carcinoma with rhabdoid differentiation
  • Urothelial carcinoma similar to giant cell tumor of bone

<gallery>

File:Illu epithelium.jpg|Types of epithelium

File:Illu_transitional_epithelium.svg|Schematic view of transitional epithelium

File:Gray1141.png|Vertical section of bladder wall.

File:Gray1134.png|Transverse section of ureter.

</gallery>

See also

  • List of distinct cell types in the adult human body
  • List of human cell types derived from the germ layers

References

Bibliography

  • Histology at utmb.edu
  • - "ureter"
  • - "urinary bladder"
  • - "Transitional Epithelium", Ureter
  • "ureter"
  • www.urothelium.com is an online resource for information about Human Urothelium and the "Biomimetic Urothelium"
  • Histology at qmul.ac.uk
  • Diagram at umich.edu
  • Histology at wisc.edu