01.01 Epithelial (Skin) Tissues

In this lesson we’re going to talk about Epithelial tissues and their structure and function within the body.

The big thing you need to know is that epithelial tissues make up the majority of the structural tissues within the human body including our skin, mucous membranes, and parts of glands. So it’s quite prevalent throughout the body.

Let’s look at a few general characteristics of epithelial tissues. First thing to know is that the cells are densely packed. So that means they are right up next to each other with very little space in between them, if any. Those cells are usually in layers and then rest on top of a basement membrane. That basement membrane connects the epithelial tissue to a connective tissue. There’s a separate lesson on connective tissues you can check out as well. So the big thing to note here is that there aren’t actually any blood vessels inside the epithelial cells. Instead, the blood vessels are out here in the connective tissue. So nutrients will diffuse across the basement membrane into the epithelial tissue, and wastes will come out and into the blood vessels. Epithelial tissue also usually doesn’t contain any nerve endings - the one exception to that is the skin. Epithelial tissue is also highly regenerative, which just means it uses mitosis to replace any dead cells - think about how often and easily our skin regenerates, right?

A couple more quick terms to know here with epithelial tissues is that they have a basal surface - that’s what rests on the basement membrane. Then, they usually have a free surface. A free surface is one that doesn’t come into contact with any other tissue - so think about the outside of our skin, or the inside of our mouth. There’s also something called a potential free surface - this is when it is in contact with another tissue, but that tissue could go away and it could become an actual free surface. The best example of this is the inside of blood vessels or the heart - if blood is PRESENT, it’s a potential free surface, but if not - it’s an actual free surface. Blood itself is considered a tissue, in this case.

So let’s talk quickly about the main functions of epithelial tissue - there are four: protection, absorption, filtration, and secretion. I’m gonna give you a couple examples of each function. First is protection - we see this in the skin which protects us from bacterial infection or against damaging UV rays. Or, inside the respiratory tract. The epithelium in there has cilia, which are like tiny hairs, and it also secretes mucus. This helps us to trap particles before they get all the way into our lungs so we can protect from infection.

Next is absorption. Epithelial tissue can have what’s called microvilli. Basically it’s finger-like projections off the outside of the cell. What this does is it actually increases the surface area. Instead of just having this straight length of cell membrane, we actually have all of this. This allows us to more efficiently absorb nutrients through the cell into our bloodstream. Two examples of this are in our small intestine or in our kidneys.

Filtration is the process of only allowing certain molecules or particles to pass through. This happens in epithelial tissue that is only 1 cell layer thick like in the capillaries or alveoli. It allows for small molecules to pass and leaves the larger ones behind - it’s like a coffee filter. The water can get through, but not the coffee grounds.

Last is secretion. Like I said before, epithelial tissue makes up the secretory portion of most of the glands in our body. The tissue will actually synthesize or create whatever the secretion is for that specific gland and then release it into the gland to be secreted. We’ll look at glands in more detail in a separate lesson.

Last thing I want to talk about is classification - there are two things we look at to classify epithelial tissues - the number of cell layers and the shape of the cell in the TOP layer. If there is only 1 layer, we call it simple, if there are 2 or more layers, it’s called Stratified. If the top layer of cells is flat, it’s called squamous. If it’s cube-like, it’s called cuboidal. And, if it’s column-like, it’s called Columnar. So - if I have a single layer of flat cells, it’s called Simple Squamous. If I have multiple laters and the top layer is cube-like, it would be called stratified cuboidal. In the lesson on types of epithelial tissue, we’re going to dive into each one of these combinations and their functions.

So, let’s recap really quick. Epithelial tissue makes up most of the major structures in the body. It rests on a basement membrane which connects to an underlying connective tissue - this is important because it usually doesn’t have blood vessels and needs to get those nutrients from the vessels in the connective tissue. And remember it’s highly regenerative. The four main functions of epithelial tissue are protection, absorption, filtration, or secretion. And we classify it by the number of layers and the shape of the cells in the top layer.

So that’s it for the basic structure and function of epithelial tissue. Make sure you check out the lesson on types of epithelial tissue and the rest of the anatomy and physiology course. Now, go out and be your best self today. And, as always, happy nursing!