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Structures and types of simple epithelia.
Hello everyone! It's Megan from Kenhub here, and in today's tutorial, we're going to be looking at a type of epithelial tissue known as simple epithelium.
Before we dive in, let me first give you a quick overview of what we're going to talk about today, so we're going to work in a fashion that starts as very broad and general then we'll get more and more specific as the tutorial moves forward. So, first, we'll look at the tissues of the body then we'll move on to define epithelial tissue. After that, we'll focus specifically on simple epithelium – the topic of this tutorial – as well as exploring different types of simple epithelium and where it's found in the body. We'll then round off with some clinical notes that are relevant to the topic.
So first, I'll set the scene a little bit by talking to you about what epithelial tissue is and how it relates to other tissues of the body.
Now there are four basic tissue types that make up the organs and tissues found in the human body. There's epithelial tissue, connective tissue, muscle tissue and nerve tissue. In the image on the right, we can see all four tissue types. So, externally, we have the epidermis of the skin highlighted in green which is our epithelial tissue. Beneath that, we have the dermis in pink and the hypodermis in yellow which essentially consists of connective tissue, and then we have muscle tissue followed by nerve tissue in the form of nerves which I've circled for you in our image.
So now that we've covered the four basic tissue types of the human body, let's focus on the one that we're interested in which is, of course, the epithelial tissue.
So, epithelial tissue is a tissue made up of cells that cover surfaces both inside and outside of the body, and because of this, it's known as a covering tissue. Epithelial cells rest on a specialized form of extracellular matrix known as the basement membrane or basal lamina. We can see the basement membrane highlighted in green in our image and that it's situated between the epithelial cells whose basal surfaces are attached to it and the underlying connective tissue.
So when we're classifying epithelial tissues, several factors are considered. For example, the number of cell layers present. It can be classified as either simple when only one layer of cell is present or stratified when two or more layers of cells are present in the tissue. It can also be classified according to the shape of the epithelial cells – that is, as squamous, cuboidal or columnar. Here you can see I've made cartoon depictions of the cells. We have our fried egg-like squamous cell on the left, our cuboidal cell in the middle and our columnar cell on the right. It can also be categorized according to specializations that the epithelial cells may possess. For example, they may be ciliated, have villi or be keratinized. Finally, they may also exhibit some functional properties, for example, secretory, excretory or absorptive properties.
Now here we have a micrograph of the mesothelium of the visceral pleura. Mesothelium is a type of epithelial tissue that is comprised of simple squamous epithelium which we'll discuss later on in this tutorial. This type of epithelium lines several body cavities and the outer surface of most internal organs. For example, the pleurae, peritoneum and pericardium are all composed of mesothelium.
So now that we have an understanding of epithelial tissue in general, let's focus specifically on simple epithelium. So when we talk about simple epithelium, we are describing a type of epithelium based on the number of layers of cells present. Simple means that there is only one layer of epithelial cells present in this type of tissue. Here in this micrograph, we see a single layer of flattened epithelial cells that line the inner wall of an artery. These cells are known as endothelial cells.
In this next micrograph, we see another image of endothelial cells which are a type of simple epithelium highlighted in green but this time, they are lining the walls of a vein rather than an artery. So, there are four types of simple epithelium that we're going to discuss today. First, we have simple squamous epithelium which is a type of simple epithelium comprised of flattened epithelial cells like these seen here lining the wall of a vein. We then have simple cuboidal epithelium which is made up of cube-shaped epithelial cells as the name suggests followed by simple columnar epithelium where the epithelial cells are columnar in shape and their height is greater than their width. And, finally, we have pseudostratified columnar epithelium which is also comprised of only one layer of epithelial cells even though it appears to have multiple layers.
We'll now go into more detail about each of these four types of simple epithelium throughout the remainder of this tutorial. As we just saw, endothelial cells are epithelial cells that line the walls of blood vessels and here we see a micrograph again showing endothelial cells lining a vein. Endothelial cells are made up of simple squamous epithelium which consists of a single layer of flattened epithelial cells that rest on the basement membrane. It's a covering epithelium that lines blood vessels, lymphatic vessels and other fluid-filled cavities.
The simple squamous epithelium that lines blood and lymphatic vessels is known as endothelium while the simple squamous epithelium that lines other fluid-filled cavities is known as mesothelium. For example, in this micrograph, you can see the mesothelium lining the visceral peritoneum. Simple squamous epithelium is extremely thin so it allows for the exchange of nutrients, waste or respiratory gasses. As such, it can be found lining the alveoli and capillaries of the lungs, Bowman's capsule and the glomerulus in the kidneys and, as already mentioned, forms the lining of blood vessels, lymphatic vessels and several body cavities such as the pleurae of the lungs.
The second type of simple epithelium that we'll talk about is simple cuboidal epithelium. This micrograph shows the simple cuboidal epithelial cells found lining the proximal convoluted tubule of a nephron. As you can see, these cells are distinctly higher than the simple squamous cells we just saw, and this is because simple cuboidal epithelial cells are cuboidal in shape as the name implies. You can also see here that the nuclei of cuboidal cells or the little purple blobs you can see in the blue circle on your screen are more rounded and that the single layer of cells rests on the basement membrane. This type of epithelial cell can also have secretory, excretory or absorptive functions and they're found lining small ducts and tubules, for example, in the salivary glands such as the parotid gland or thyroid follicles in the thyroid.
The third type of simple epithelium that we'll see today is the simple columnar epithelium. As you can see in this micrograph, the simple columnar epithelium also comprises a single layer of cells resting on the basement membrane, however, these cells are columnar in shape which means they are even higher in appearance than the cuboidal cells we just saw. And although the cells are taller, their nuclei can be found situated closer to the basement membrane. We can see this in our micrograph where the nuclei or purple blobs are located towards the basement membrane. These cells can have either absorptive or secretory functions. For example, here we see that the simple columnar cells contain mucin-secreting goblet cells which are the pale pink circular structures you can see in the micrograph, and I've circled a few of them for you here on our image.
Simple columnar epithelium cells can withstand considerable wear and tear. As such, they're found lining the digestive tract and the cervix. If you think about the activity that goes on in the digestive tract and even in the cervix with sexual intercourse and childbirth, you can understand why it’s important that both these organs are lined with an epithelial tissue that can withstand wear and tear.
The final type of simple epithelium that we'll look at is called pseudostratified columnar epithelium. Although this type of epithelium is a simple epithelium, it doesn’t appear this way. It appears to have multiple layers because the positions of the nuclei of neighboring cells are found at different levels giving this type of epithelium a stratified appearance. This is not the case because it is, in fact, made up of a single layer of cells which all make contact with the basement membrane, and for this reason, it's called pseudostratified because it appears stratified even though it actually isn’t.
Pseudostratified columnar epithelium is made up of columnar cells and if you look at this cartoon I've drawn, you can see how the differing positions of the nuclei make it appear as though there is more than one layer of cells. These cells can have either absorptive or secretory functions and from this micrograph, we can see that these cells may also possess cilia. Ciliated pseudostratified columnar epithelium can be found lining the trachea and may contain mucus-producing goblet cells. The cilia are specializations that beat rhythmically propelling the mucus produced by the goblet cells along the surface of the cells. If you look at the micrograph, you'll see that the outer border of the cells appears blurry or fuzzy and that is, in fact, the cilia. I also want to mention again that this is a single layer of epithelium despite how things may look so don't let it fool you. Pseudostratified columnar epithelium is typically found lining most major airways such as the trachea and is also found lining the male urethra.
Now that we have an understanding of the different types of simple epithelium and where they're found, let's move on to some clinical notes relevant to the topic.
The epithelium that lines the esophagus is normally non-keratinized stratified squamous epithelium like what you see here in this micrograph. In some individuals who suffer from prolonged or a chronic gastroesophageal reflux disease also known as GERD, the squamous epithelial cells that line the esophagus are replaced with non-ciliated simple columnar epithelium and goblet cells as seen in this micrograph, similar to the epithelium found in the intestinal lining.
The change from one type of epithelial cell to another is known as metaplasia and the condition when it occurs in the lining of the esophagus as a result of GERD is known as Barrett's esophagus. This condition is a pre-malignant condition which can lead to adenocarcinoma. Note that this change in the epithelial cells of the esophagus occurs to protect it from reflux.
Before we finish our tutorial, let's first summarize what we've learned about today.
So, today, we've seen that simple epithelium is a type of tissue that is made up of epithelial cells that rest on a basement membrane. We have also seen that the term simple refers to the fact that we're talking about a single layer of epithelial cells. These cells can be described according to the number of cell layers, their shape, their specializations and their functional properties. In addition, we have also seen that simple epithelium can be classified into four subcategories.
First, we looked at simple squamous epithelium which is comprised of flattened epithelial cells and allows for the exchange of nutrients, waste or respiratory gasses. Then we had simple cuboidal epithelium whose cells are cuboidal in shape and can have secretory, excretory or absorptive functions. Next, we had simple columnar epithelium which as we saw is comprised of cells that are columnar in shape and can withstand considerable wear and tear. And, finally, we talked about pseudostratified columnar epithelium whose epithelial cells are columnar in shape and appears stratified even though there's only a single layer of cells. We also looked at some examples of where the different types of simple epithelium can be found in the body.
So that brings us to the end of our tutorial about simple epithelium. I hope you found it useful and thanks for watching.