Types of Epithelium
Annnddddd we're back with another revision post, which realistically wasn't needed but I really enjoyed doing the last one!
Today we've got histology work, and one of the first 'self study' modules we've been asked to complete covers epithelium cells, one of the most common cell types in the body (in fact, it's actually all over you!) So, with that said, let's discuss...
Tissues are made up of sheets of cells known as epithelium, with the simplest of these sheets being just a single cell thick. Below is a lovely video I made earlier (ha) of some of the more detailed aspects of what these cells look like. 'Tight' junctions hold the cell membranes together (in this image that's the gap between the two lines at the edges of the rectangle) and 'adhering' junctions hold the lateral sides of the cells together.
You'll also notice the thick, black line at the bottom of the diagram. This is what's known as the 'basement membrane', and it's an extracellular matrix (fluid outside the cell) secreted by the body's connective tissue. If that was a lot to follow, don't panic: all you really need to know from this is that the basal (bottom) surface of the epithelium cells are anchored to the connective tissue by the basement membrane.
(If you want to get technical, this is achieved by proteins from the cell membrane cross linking with proteins in the basement membrane, which in turn are linked to the extracellular matric of the connective tissue!)
Take note of my use of apical, lateral, and basal surfaces: these are important because the proteins that are embedded within them are different, which causes apicobasal polarity (essentially just meaning the surfaces are different.) There's also no blood supply to the epithelium, so these cells rely on diffusion from the capillary beds underneath to obtain the oxygen and nutrients they need to survive - this also makes them perfect for acting as a partially permeable membrane!
Now, there's also different types of epithelium, and these are categorised based on their shape and layers. 'Simple' epithelium types have just a single layer of cells, with all of them directly in contact with the basement membrane. However, stratified cells have two or more layers, and only the bottom layer is in contact with the membrane.
Let's start with simple epithelium.
The most basic is simple squamous epithelium, shown in the diagrams below. These cells have flat, thin, central nuclei, and are usually found lining blood vessels and diffusion pathways such as in the alveoli. It is a single cell thick and has one layer.
Indeed, simple cuboidal and simple columnar are very similar in appearance, although columnar cells are slightly more elongated and tend to have their nuclei positioned towards the bottom. These cells are found lining gland ducts involved in excretion and absorption processes (cuboidal) and tissues involved with secretion (columnar). In fact, you're most likely to find columnar cells at the same time as you find pseudostratified epithelium.
Pseudostratified epithelium is so named because on first look it can appear that it has two layers. However, all cells are connected to the basement membrane, which ultimately means that it is considered 'simple' epithelium. Pseudostratified cells can often be ciliated, which if you remember back to A-Level Biology, you'll know help to waft mucus up the trachea to be coughed up. At the same time, goblet cells (columnar epithelium) are secreting said mucus to trap pathogens! Isn't biology wonderful?
(How sarcastic can you say that? Prize for the winner.)
Now we move on to stratified epithelium.
This stuff is interesting, because it's essentially layers of cells. The most common type of epithelium in your body (I'm guessing, don't quote me on that) is stratified squamous epithelium, because it's what makes up your skin. Also found in the vagina and oesophogus, its built to withstand abrasion and therefore functions as protection for vulnerable areas of the body. It does this because the top cells are highly differentiatied, and when they are scraped off or damaged, the cells from the middle of the structure push up and continue to differentiate, replacing the lost top layer. The middle layer is constantly differentiating, and will push the top layer of skin cells off when they have matured, whereas the basal (bottom) layer remains undifferentiated.
Stratified cuboidal epithelium is largely the same as simple cuboidal epithelium, however this time it is most commonly found lining gland ducts and tends to be two or more layers thick.
Moving on, stratified columnar cells are found in the eye conjunctiva as well as the anus, and aren't really much more interesting than their simple partners. Soz and all that.
What is interesting and doesn't want to make me fall asleep however, is transitional epithelium. This stuff is really cool, and actually can change shape! When relaxed, it tends to be for or five layers thick, however when it's stretched out it 'shrinks' to only two or three layers! This makes it perfect for lining the bladder, as it can also withstand the toxicity of urine as well. The umbrella cells on the top layer make in pockets in the membrane when they join together, thus increasing the surface area when the epithelium is stretched out.