The majority of epithelial tissues make up linings and coverings. Example: The outer layer of skin that covers your entire body is made up of stratified squamous epithelium and so is the lining of your mouth. Epithelial tissues also make up most exocrine glands. Examples: sweat glands and salivary glands.
Epithelial tissues have a bottom side “basal surface” and a top side “apical surface.” The basal surface meets up with an underlying connective tissue while the apical surface is the free surface– facing air or the inside of a tubed organ.
Naming epithelial tissues: Most epithelial tissues have three words in their name.
- The first word has to do with the number of cell layers present.
- just one cell layer = simple
- more than one cell layer = stratified
- The second word has to do with the shape of the cells at the apical surface.
- cells are relatively flat —> squamous
- cells relatively box or cube shaped —> cuboidal
- cells are column shaped (taller than they are wide) —> columnar
- The last word in the name is always “epithelium” or “epithelial”
Example: That top layer of skin mentioned earlier is made up of multiple layers of cells that are flattened at the apical surface, so the proper name for that epithelium is “stratified squamous epithelium.”
There are two exceptions to the three word rule of naming epithelial tissues. Those two exceptions are pseudostratified columnar epithelium and transitional epithelium.
- Pseudostratified columnar epithelium is made up of a single layer of cells, but some of these cells are shorter than others giving the appearance of multiple layers.
- Transitional epithelium contains multiple layers of cells that do not have a clear shape and that are vary rounded at the surface (at least in well prepared slides). Because of a lack of clear shape we call it transitional.
Below are example images of epithelial tissues (click the image for a larger version). I’ve tried to include as many examples of each, the objective is to see past the differences in appearance and look for the major features (in this case those major features are the shape of the cells and the number of layers).
Pseudostratified columnar epithelium (450 X magnification). This section comes from a trachea. Note that there appears to be at least two layers of cells in the membrane, but there is only one… also note the cilia at the apical surface (the cilia’s job is to move mucus up the trachea).
Transitional epithelium (450 X magnification). Note that the cells have a seemingly random arrangement (no clear shapes in the layers) and the cells at the apical surface tend to be large and rounded. This section comes from the wall of a urinary bladder.
Transitional epithelium (450 X magnification). This is the epithelium that lines the ureters (tubed organs that transport urine from the kidneys to the bladder). This looks a bit different, but we still have multiple layers of cells with no clear single shape.
Simple squamous epithelium lining one of the tiny air sacs (alveoli) in a lung (450X magnification).
Simple squamous endothelium (450 X magnification). It’s called endothelium instead of epithelium because this epithelium lines the inside of the vascular system– even the inside of the heart). In the above image (from a spinal cord slide) you can see an oblique section of a blood vessel (cropped and labeled below).
The above image is the same as the previous image but I’ve labeled some of the simple squamous endothelial cells (just for clarity)
Simple squamous endothelium (450 X magnification). The above two images are of two very small blood vessels in a spinal cord.
Squamous cells (450 X magnification) The above image shows squamous cells that have been isolated from the tissue. This should give you an idea of how these cells look. They are flat at their side, but if we see them from above or below they cover a wide area… they are kind of like an egg– if you were to crack the egg and drop it out into a frying pan… the egg white flattens out and the yolk makes a lump in the middle (nucleus).
Stratified squamous epithelium (450X magnification). This slide is from an esophagus. Notice that the cells towards the apical surface are flattened.
Stratified squamous epithelium (450X magnification). This happens to be from thin skin… again, notice that the cells at the apical surface are very flat… in this case they are flat and dry, so dry that they flake off. This epithelium is also keratinized (the cells fill with the tough protein keratin).
Simple cuboidal epithelium (450X magnification). The above image is from the kidney. Each of the circles above is a kidney tubule, and most of them are lined with simple cuboidal epithelium.
Stratified cuboidal epithelium (450 X magnification). The above image is from a sweat gland found in human scalp. The inside of the gland is lined with stratified cuboidal epithelium.
Simple columnar epithelium (450 X magnification). In the above image the pointer is pointed at a line of simple columnar epithelial cells. The reason you see so many lines of cells here is that this is from small intestine that has a lot of villi (small projections specialized to absorb nutrients). Notice that each line of simple columnar epithelium as white space on one side (the apical surface) or dark staining tissue on the other side (basal surface). We can also see goblet cells in this image, they appear as kind of chubby white cells. Goblet cells make mucus.
Simple columnar epithelium (450 X magnification). The line of cells at the pointer is a good example of simple columnar epithelium. This comes from a kidney and this epithelium lines papillary ducts.
Stratified columnar epithelium (450 X magnification). Note that the epithelium at the pointer has two layers of cells and that the cells that, at the surface, are longer than they are wide… this is from a vas deferens or ductus deferens (tubes that carry sperm from the testicles).
Stratified columnar epithelium (450 X magnification). Again we see two layers of cells and the topmost layer are longer than wide. This section comes from the duct of a salivary gland.
Glandular epithelium (100 X magnification). The pointer is on part of a salivary gland. this is a classic example of “glandular appearance.”
Glandular epithelium (450 X magnification). The above image is from a pancreas. Most of the cells that you see are glandular epithelial cells (there is also a blood vessel and some connective tissue in view).
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