The majority of epithelial tissue in our body makes up surface epithelia, otherwise known as covering and lining epithelium. The epithelial cells are going to create sheets of tissue that are going to line surfaces of the body and can cover organs within body cavities. One of the important characteristics of all covering and lining epithelium is that the basal surface of the tissue will always be bound to connective tissue.
And this combination of an epithelial tissue bound to connective tissue will create a structure now known as an epithelial membrane. So we're defining epithelial membranes as that combination of epithelial tissue bound to connective tissue, right? So there's our epithelial tissue, there's our connective tissue, and the connection, if you recall, between these two different types of tissue will occur at a structure known as the basement membrane. Now the basement membrane is not made of cells.
but is instead a meshwork, a mixture of proteins and other substances that are being produced by cells. And there's going to be two distinctive layers or laminae of the basement membrane. The basal lamina is the portion of the basement membrane that's created by the epithelial cells.
And the reticular laminar or reticular layer is that portion of the basement membrane that is created by the connective tissue cells. One of the functions of the basement membrane is to actually adhere or connect those two distinctive tissues to each other. But in addition to that, the basement membrane is important. because this mesh-like junction is semi-permeable. Certain substances are going to be able to diffuse across that basement membrane.
And if you recall, epithelial tissue is avascular, but yet these cells are still going to require things like oxygen and nutrients. and for the most part they're going to rely on getting their oxygen and nutrients from the underlying connective tissue that's really well vascularized so we can have diffusion of oxygen and nutrients going across from the connective tissue to the epithelial tissue and then we can also have diffusion of wastes carbon dioxide that will diffuse from the epithelium to the underlying connective tissue. Now there are three types of epithelial membranes or three major types of epithelial membranes that we have within our body. We have mucous membranes that are sometimes just referred to as mucosa.
We have serous membranes or serosa and we have our cutaneous membrane which is otherwise known as our skin. Mucus membranes are those epithelial membranes that are going to line passageways in our body that have an opening to the outside environment. So somewhere along the way, right, we'll have a tube-like passageway that will open up to the outside environment, and that passageway will be lined with a mucous membrane or mucosa.
So think digestive tract. or respiratory tract, right? There's openings to that outside environment.
And in a mucous membrane, the epithelial cells, or at least some of the epithelial cells within that layer, will produce mucus, right? So maybe here we're kind of seeing a goblet cell associated with the mucous membrane, and we'll have mucus being secreted. to the surface of the tissue in order to moisten and protect that surface.
So again, we're going to find mucous membranes lining our digestive tract, our respiratory tract, urinary, and reproductive passageways. Now, serous membranes are going to be epithelial membranes that are found in body cavities that are not open to the outside environment. And our serous membranes...
are going to be a double layered membrane that are going to line the inside of the cavity wall and cover organs within the cavity. So the cavities that we're talking about here now, if you recall, are going to be our thoracic cavity and our abdominal pelvic cavity and We have many organs within these cavities, but we also have these double-layered serosa or serous membranes. And if you recall, we will have a parietal layer, which is going to be lining the inside of the cavity wall. And then we'll have a visceral layer on the surface of the organ.
Now, We've got three specific named serous membranes in our body. The pleura was the membrane that surrounded and was associated with the lungs. The pericardium is the membrane associated with the heart. And the peritoneum is that membrane associated with our abdominal pelvic cavity.
If we look a little bit closer at this relationship between that parietal and visceral layer, the superficial layer is the parietal layer. The deeper layer is the visceral layer. And in between those two layers, we are going to have...
A little bit of slippery fluid that's going to be secreted, and it's going to be secreted by the epithelial cells. Because as these layers are created, right, so remember we're going to have epithelial tissue, and then a little bit of connective tissue that it's going to be bound to. All the epithelial surfaces are going to face each other. And Between those epithelial surfaces now, this slippery fluid called serous fluid is going to serve to lubricate the surface of the membranes and it's going to reduce friction within these body cavities so that the organs that are so jam-packed in these body cavities, they can sort of move easily within the cavities. So those are our serous membranes.
The last of the epithelial membranes is what's known as our cutaneous membrane, otherwise known as our skin. And this is the epithelial membrane that's going to cover and protect the basically the entire surface of our body from the external environment. So just like any epithelial membrane, we're going to have two parts to it, right? We're going to have our epithelial tissue.
And it's going to be bound to connective tissue. And we're going to have a basement membrane connecting those two areas there. But here now we're simply going to refer to the epithelial portion of the cutaneous membrane as our epidermis.
That's literally the name of it. And then the connective tissue portion. is called the dermis.
So we'll come back and talk a little bit more about the cutaneous membrane when we do the integumentary system, which is going to be our next chapter that we'll cover. Now, if we can just focus in on the epithelial part of an epithelial membrane. So we know that we're going to always have an underlying connective tissue.
And when we cover connective tissues, we'll talk about exactly what kind of connective tissue it is that's always going to be attached to epithelia. But for right now, let's just focus on the epithelial portion. of our membranes. And we actually have many different types of epithelial tissue within our body. And each of those different types is going to have a different structure, which will ultimately affect its function.
So in order to characterize the different epithelial in our body, we take... two, basically two major things into consideration. We're going to look at how many layers, how many layers of cells we have associated with the epithelia. And then we're also going to consider, right, the shape of our cells. So when it comes to the number of layers of cells within epithelial tissue, we may simply have one layer of cells bound to connective tissue.
And if we have one layer of cells bound to connective tissue, we're going to call that type of epithelium a simple epithelium. So the word simple relates to one layer of cells. Now the other possibility is we could have more than one layer of cells. And anytime we have more than one layer of cells, we're going to call that a stratified epithelium.
Now it doesn't matter if there are two layers of cells or 200 layers of cells, it's still going to be considered stratified. Now lastly, we're going to see with a particular type of epithelial tissue, it's a little tricky because it's a type of epithelial tissue where it's going to look like there is more than one layer of cells, but in reality, all of the cells are going to touch that basement membrane. So really, truly, it's a single layer of cells that looks multi-layered. And because of the tricky nature of this particular type of epithelial tissue, it is given the name pseudo-stratified.
So pseudo means false. So falsely stratified. And we'll take a look at that and what it actually looks like.
We'll take a look at all of this underneath the microscope. So one of the things that we're going to consider when we start to look at epithelial tissues in the body is we're going to start to pay attention to how many layers of cells are there in this tissue. Is there one layer? Is it a simple epithelia? Is it more than one layer of cells?
We'll call it a stratified epithelia. Or is it a tricky one that makes you think it's more than one layer, but really truly all of the cells contact that basement membrane? That's pseudostratified.
Then we'll also pay attention to cell shape. So there are three basic shapes that we tend to see with epithelial cells. Some epithelial cells are relatively flat.
So when we're looking at them from the side view, they tend to be these really thin, flattened cells. We call those cells squamous cells. So the word squamous means flat.
So these are flat cells. Now, if we're looking at them from the side, they're easy to see that they're flat. Sometimes it's a little bit tricky because if we're looking at them from above, from sort of a top view, if you want to call it that, they'll tend to look like floor tiles that are all interconnected together. So again, when we start to look at these tissues under the microscope, we'll look.
at squamous cells from what we'll call, we'll call them a top view, and then we'll look at them from a side view. We'll learn to identify them both ways. But basically, we're looking at these flat cells, these squamous cells. Now, other epithelial cells might be about as tall as they are wide. And we're going to call those cells cuboidal cells.
They're sort of cube-shaped. Sometimes they're really more kind of roundish, to be honest with you, because there are no right angles. in the body.
So if we see a cell that's about as tall as it is wide, with the nucleus basically resting in the center of the cell, that's a cuboidal cell. And then lastly, we can see cells that are relatively taller than they are wide. The nucleus tends to hang out towards the base of the cell, and those are our columnar cells.
So epithelial cells basically, as far as their shape goes, might be flat, might be cuboidal, might be columnar. So squamous, cuboidal, columnar cells. And we will see as we move forward that the functions of epithelial tissue are going to be reliant upon looking at The combination of how many layers of cells we have combined with the shape of the cells.
But in addition to those two things, there's one more thing that I want you to pay attention to when we start getting in and looking at the different types of epithelial tissue under the microscope. We want to look for the possibility of some modifications. that we might find in columnar cells, or sometimes we'll see them in cuboidal cells as well in the body. And these modifications are going to be found at the apical surface of the tissue.
So at the apical surface of some columnar cells, usually. maybe cuboidal cells, we might see these short little folds in the plasma membrane that are going to be called microvilli. So microvilli are important to certain epithelial cells when we need to have an increase in the functional surface area, right, at that exposed surface of the cell.
So microvilli are these small folds, and they're permanent folds. They're actually supported by proteins within the cell itself. And that maintains the folding there and that gives us an increased surface area.
So if you see cells that have microvilli at their surface, and I'll point them out to you when we get into the lab portion and we start looking at stuff under the microscope. Think to yourself, hey, these cells have increased surface area and most likely they are going to be involved in the process of absorption, which is bringing stuff. into the cell. So the more surface area, the more effective absorption can happen. And so we're going to see, for example, cells with microvilli lining the surface of our small intestine where we do our nutrient absorption.
Now, another modification that we want to pay attention to is the presence of different kind of folds known as cilia. So Cilia are longer folds in the plasma membrane that are supported by a different type of protein than what supports microvilli. Cilia are supported by a type of protein called microtubules.
And the cilia... that we see here, actually look like and act like the tentacles of sea anemones. So if you've ever seen a little sea anemone, think back to Finding Nemo, right?
Sea anemones have these little tentacles that wave within or that move within the watery environment and they create a fluid current. When it comes to the anemones, the fluid current is designed to get little plankton, right, to come closer to the digestive structure of the anemone. So in our body, We have cells that have cilia at the surface, and those cilia move and create almost like a beating pattern, a moving pattern. And this movement of cilia will allow substances to move across the surface.
the cell. So remember the surface of the cell, right, most likely we're going to see ciliated cells. There's going to be mucus at the surface. I'm going to see them found in mucus, certain mucus membranes within our body. So as the cilia move like the tentacles of the sea anemone, it creates a little movement current and we can move substances across the surface of the tissue.
And this is going to be really important, for example, in our respiratory system. It's our way of moving dust and gunk up, you know, moving glucose particles that are going to trap the dust and gunk and moving that up and out of our respiratory passageways. We also have ciliated cells that line the fallopian tubes, the uterine tubes.
Then that will help. propel an egg cell across the surface of that epithelium. So when you see microvilli associated with the tissue, think increased surface area for absorption.
When you see cilia associated with the tissue, think movement of substances across the surface of the cell. Those are going to really modify the functions of the tissues. And when it comes to the actual now specific types of surface or covering and lining epithelium that we have in our body, you can see that as we start to put together now the number of layers with the shape of cells, we've got a variety of different types. So simple squamous epithelium, right? We have one layer of these flat cells.
bound to an underlying connective tissue. Simple cuboidal epithelium, right? One layer of these sort of cube-shaped cells about as tall as they are wide, again, bound to an underlying connective tissue. Simple columnar epithelium, one layer of our tall column-like cells. Now, columnar epithelium, remember, can have some modifications to it.
We don't usually include the microvilli. We don't add it to the name of the tissue if it's there, but it is important to just recognize that it's there. But if simple columnar epithelium is ciliated, if it has cilia at its surface, it's not going to be a problem.
We want to include that in the name. So we would have simple columnar epithelium that's non-ciliated, may or may not have microvilli, and we can have simple columnar ciliated epithelium. So those are our simple varieties.
For our multilayered types of tissue or stratified tissue, when we consider our classification scheme, right, we're talking about a tissue that's going to have multiple layers of cells, more than one, right, attached to that basement membrane, which is attached to the underlying connective tissue. And when we're classifying stratified tissue, the only shape of cells that we're going to pay attention to are the cells that are at the most apical surface. So it doesn't matter what the shape of the deeper cells are. We just look at the apical layer, the most superficial layer of cells. And if they are flat, we're going to call that tissue stratified squamous epithelium.
Now we're going to learn that we've actually got two kinds of stratified squamous epithelium. keratinized variety and a non-keratinized variety. So stratified squamous keratinized epithelium, which is sometimes referred to as stratified squamous dry.
Stratified squamous non-keratinized epithelium, which is moist or wet. We can have a stratified cuboidal epithelium. So again, now we've got more than one layer of cells and the surface cells are about as tall as they are wide. That would be called stratified cuboidal.
And then if we have more than one layer of cells, but the surface cells are a little taller than they are wide, those are going to be called, or that tissue will be called stratified columnar epithelium. Now, if you notice down here, right, the last two, we have a variety of epithelium. That's a tricky one.
That's going to be a pseudostratified epithelium. Now the only shape of cells that will allow us to have that pseudostratified appearance will be columnar cells. And how it works is basically we're going to have tall columns next to some shorter ones.
And when you look at this under the microscope, here's a microscope. it will trick you into thinking that it's a stratified tissue. But in fact, all of the cells are contacting that basement membrane. So those columnar cells are pseudostratified in their appearance.
And the particular pseudostratified tissue that we are going to look at, which is the most common type we have, has cilia at its surface. So if the cells are tall enough to hit the surface, we'll see these little hair-like or fringe-like structures that are cilia. So when we put this all together, we're going to call this particular tissue pseudostratified ciliated columnar epithelium, which occasionally you will see me abbreviate. PSCC epithelium for short.
Now, I am allowed to abbreviate it, but you at this point in your education are not. So you will have to learn how to write out pseudostratified ciliated columnar epithelium. And if that's what we see, that is what you'll have to write down, for example, for your exam.
So make sure that you're clear on practicing, right? how to write that down because that is a long name for tissue. And then the last tissue that we have doesn't go by a typical naming strategy.
It's simply called transitional epithelium. Transitional epithelium is actually a stratified tissue, but you'll never see it called stratified transitional epithelium. It's simply just referred to as transitional epithelium. It's only found within our urinary system.
So lining the inside of our ureters, we'll talk about the tubes that connect the kidneys to the urinary bladder and lining the inside of our urinary bladder. We have transitional epithelium. So we'll talk about why is that called transitional epithelium and what is so unique about it. So there you have it.
During class, we will go through in a little bit more detail these specific types of surface epithelium. You'll work on trying to identify them underneath the microscope. We'll consider that their structure, their anatomy, will determine their function.
And that function will determine where we find that particular type of epithelium within the body.