These two types of epithelia are difficult to classify using the shape of the cells in the surface layer and the number of the cell layers as criteria.
Transitional epithelium
Transitional epithelium is found exclusively in the excretory urinary passages (the renal calyces and pelvis, the ureter, the urinary bladder, and part of the urethra).
The shape of the cells in the surface layer of a transitional epithelium varies with the degree of distension of the organs whose lumen is lined by this type of epithelium. In the 'relaxed' state of the epithelium, it seems to be formed by many cell layers. The most basal cells have a cuboidal or columnar shape. There are several layers of polyhedral cells, and, finally, a layer of superficial cells, which have a convex, dome-shaped luminal surface. In the distended state of the epithelium only one or two layers of cuboidal cells are followed by a superficial layer of large, low cuboidal or squamous cells. In the distended state the epithelium will resemble a stratified squamous epithelium.
Pseudostratified columnar epithelium
All cells of this type of epithelium are in contact with the basement membrane, but not all of them reach the surface of the epithelium. Nuclei of the epithelial cells are typically located in the widest part of the cell. Consequently, the nuclei of cells which do or do not reach the surface of the epithelium are often located at different heights within the epithelium and give the epithelium a stratified appearance. The epithelium will look stratified but it is not - hence its name "pseudostratified". Pseudostratified columnar epithelia are found in the excretory ducts of many glands.
Suitable Slides
transitional epithelium: sections of ureter or bladder - H&E
pseudostratified epithelium: sections of the trachea - H&E
pseudostratified epithelium: sections of the trachea - H&E
| Bladder, Monkey - H&E At a first glance a transitional epithelium looks like a stratified cuboidal epithelium. Several rows of nuclei appear to be topped by a layer of dome-shaped cells which bulge into the lumen of the ureter. The shape of the surface cells and the number of rows change if the bladder is distended. The number of rows decreases. This decrease should tell us that many of the nuclei located in different layers of the epithelium belong to cells which are all in contact with the basement membrane. With distension, the shape of the cells in the surface layer will become squamous. Draw the epithelium and label the features you can see. Add a simple schematic drawing of how you expect the epithelium to look like if the ureter is distended. It has not yet been resolved if all the epithelial cells are in contact with the basement membrane. Some texts consider transitional epithelium as a specialised stratified epithelium while others group it with pseudostratified epithelia. Maybe it is best to also consider it 'transitional' in this regard. |
| Trachea, Human - H&E At least two, sometimes three rows of nuclei are seen in the pseudostratified columnar epithelium lining the trachea. The nuclei belong to cells which are all in contact with the basement membrane. The epithelial lining of the trachea is also one of the few examples of a basement membrane clearly visible in H&E stained sections. Epithelial cells can be ciliated or they can be goblet cells (unicellular exocrine glands). Basal cell regenerate other cell types of the epithelium. Capillaries and small vessels are visible in the connective tissue beneath the epithelium. A ciliated pseudostratified columnar epithelium with goblet cells is a characteristic feature of parts of the respiratory system, where it is call respiratory epithelium. It contains several cell types in addition to ciliated, goblet and basal cells. Draw the epithelium at high magnification and label your drawing. |
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