Biology: Animal Tissues: Epithelial Tissue

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I want you to think still one more time about this whole idea of the fact that the whole evolutionary of life shows the very first fossil organisms were not organisms at all, but were one-celled. And then I want you to think about the fact that one cell, through millions and millions of years, gave rise to colonial organisms. But they weren't even organisms yet, because they were just colonies of single cells. But then, somewhere along the way, multi-cellularity began, and with that came a whole bunch of problems, not the least of which is homeostasis. I want you to think about this. If you are that multi-cellular organism with layer upon layer upon layer upon layer upon layer of cell and you're living in the sea--well, your outside cells are really pretty happy, but what about those cells on the inside? They may be doing great jobs for you, but how are you going to keep them happy? How are you going to provide them with oxygen? How are you going to provide them with nutrients? Who's going to take on the job of doing these things? Who's going to specialize? That's a key word I want to use today. I want to use the whole idea of specialization.
You see, it's a very basic dogma in biology, an organizational dogma that we have, that when you have a single cell in a multi-cellular organism, that cells form together to form something called "tissues." You learned that back in probably second grade. Tissues, we know, form organs. Organs, you know, form systems. Systems, you know, form you and I--an organism. Well, to really, really, really, really understand homeostasis, we have to understand it not only at the level of what the cell does, but we have to really understand it at the level of this thing called the tissue, because it is the tissues that take on the responsibilities of talking to the other tissues, because you know, you may have heard this before--in an organism the structure of an organ follows its function. What an organ does is controlled by what it is, or vice versa, what an organ is is going to determine what it does--structure follows function. Well, guess what organs are made out of? Tissues. Well, that suggested, tissues must follow the same rule which you already know cells do.
We're going to jump to the next level here. We want to look at tissues. We're going to spend a lot of time talking about tissues, because we could spend seven CDs just talking about tissues, but we're not going to. What we're going to talk about for a little bit is we're going to talk about tissues, and we're going to talk about this whole idea of structure following function in this science, which you and I are going to delve into called "histology," the study of tissues.
Well, what do you think of when you think of a tissue? Well, you know, some people, when they think of tissue, think of really kind of gross--well, it depends on you--some people think of what they consider gross stuff. This is organ tissue. And for those of you who were wondering, that was leftovers from my breakfast. That's liver. Maybe some of you think of this when you think of a tissue. This has some pretty cool tissues on it. We'll talk about those in a minute, too. But you know, no matter what you think about, there's a lot of different types of tissue. Did I say a lot? What's a lot? You're saying, "Man, every organ in your body we're going to be talking about the tissues?" Well, you know what, tissues are only organized into four major groups. Let's talk about those four major groups.
The first group is called "epithelial tissues." See that? There's a little blood there. This is pretty cool. So let's talk about epithelial tissues. The first kind of tissue is epithelial, and that's the one we're really going to concentrate on right now, but I've got to introduce the other ones, and then we'll come back to epithelial tissues. Second one is a tissue that you've probably heard of called "connective tissue." Let's go back to my friend the chicken here. Epithelial tissue--you've probably heard of your own epidermis, your skin. Well, that's a form of epithelial tissue. But connective tissues--do you ever wonder why it's hard to pull bones apart or why muscles are attached to bones? Well, that's connective tissue. Oh, and I just gave away my third type of tissue. Look at that. That's muscle. That stuff that's attached to the bone, that's muscle tissue, so that's number three. And four--brain tissue, or nervous tissue. That was a brain made of nervous tissue.
Now you're probably saying, "Wait a minute. What about these guys? What about plants. What kind of tissue is that?" Well, this is an animal system discussion. I don't want to talk about plants today. Plants are another lecture.
We're here today to talk about epithelial tissue. What is epithelial tissue? It's really kind of a complex group of tissues that have a lot of different jobs. First of all, let's talk a little bit about epithelial tissue. I just happen to have some artwork of a nice epithelial tissue right here. Epithelial tissue, if we take a look at this, what you see is this particular type of epithelial tissue, the first thing you get when you look at this thing is it's all cells. Now, you might say, "It's a tissue. Isn't tissue made out of cells?" Yeah, but notice there's nothing in here but cells. So the first thing I want you to know about epithelial tissue is it's mostly cellular. So epithelial tissue is almost exclusively cellular. You're going to see some other tissues later that are not mostly cellular.
You'll notice also about this is that the tissue has at least one surface exposed to the environment. Now remember, did I say a little while ago structure follows function? Are you thinking one surface exposed to the environment? Could there be some exchange thing happening here? Stay tuned. So two, it's exposed to the environment, at least one surface. Environmental exposure.
A third thing about epithelial tissue--do you see all the blood vessels in here? No, you don't. Why? Because there are none. Epithelial tissue is what we call "avascular." It lacks blood vessels--avascular.
And four, epithelial tissue, because of where we find these things--and I've got to tell you, somebody said to me, "What kind of cell would you like to be?" I'm not so sure I'd pick an epithelial tissue. They have a tough life. It's tough to be an epithelial tissue. And quite often, as we'll see later, a layer at the bottom of these things called the germinative layer, because these things get worn out, and so the point is that these things are highly mitotic. They really, really, really do a lot of mitosis, which I'm sure you all remember is cell division. Why? Because they're always getting beat up.
Okay, so mostly cellular, environmental exposure, one surface, at least, they're avascular, and they're highly mitotic. Okay, how are they organized? Well, this is one cell layer, but you'll also notice here that there's what looks like some kind of membrane at the bottom. So organizationally they have what's called a "basement membrane." All epithelial tissues share this. They're connected at the bottom to a basement membrane, and they're connected to each other by intercellular connections.
Now, if you don't know about intercellular connections, you'd better look back, because we've talked a lot about intercellular connections before. Intercellular connections are the way cells talk to each other. The point is that when we have intercellular connection, they can be used for talking to each other, but they can be used for other things, too. For example, here's three different types of what we called "cell junctions." We have these up here, these tight junctions, which literally are waterproof connections that things can't squeeze through. You see they're like rivets and they go right through that thing, almost like the cells are sewn together.
We also have gap junctions. Now, gap junctions are junctions where there's a larger gap and where materials can go back and forth between cells. And we have what are called desmosomes. Desmosomes are where cells are connected and can't be ripped apart no matter how hard you try and that's why some tissue is hard to rip. Junctions--cool stuff and very important.
So intercellular connects and basement membranes. Now, let's think about functions. And you know what? The rest of this should be very easy for you. If you understand the why of epithelial cells, you'll understand the what. What do they do? Why do they do it? Well, let's think about those things. I want you to think about what a surface cell like that would do, an epithelial cell. Well, let's think about it. It can be used for protection because it covers surfaces. If it's on the surface it can control permeability. So if you have a lot of tight junctions in there, guess what? Things are not going to get through. If it's near skin or near a sensory thing, it may be what is called innervated, which means it may have connections for sensing, because where do you want your senses to be? Well, near where the environment is going to stimulate them. And last, but not least, epithelial cells may secrete. They may be secretory.
Okay, now let's take a look at some of the epithelial cells. We have like three major groups of these things. The first kind means flat and it's called "squamous." You're going to tell me, by the way, what these things do when you look at these things. So we'll talk about squamous epithelial right now. Squamous epithelial is--oh, I just happen to have some right here. Now, this is a simple layer of squamous epithelial. Notice the irregular shape of these things. This particular squamous epithelium is one cell layer thick. Think of that. How thin is that? That's one cell layer thick. So what do you think is going to happen through simple squamous epithelium? In simple squamous epithelium you're going to get diffusion. Now, can you think of any place you'd like diffusion to happen? Think about it. What diffuses? Things like gases, things like nutrients. In fact, if you know anything at all about your lungs, you know that indeed, diffusion of gases has to occur at these places called the alveoli, and the alveoli are one cell layer thick, and guess what kind of cell they have--you've got it--simple squamous epithelium. So there's two kinds of squamous epithelium--simple, and another kind called "stratified."
Now, stratified epithelium is just like what I just showed you except here we're going to have layers of these things. And you know what these act like? If you've ever seen the shingles on the roof of a house. Think of how protective they are. And that's what stratified squamous epithelium does. It's stratifications like in your skin. Keep your skin protected. So that's squamous epithelium. See, structure follows function. This is just great stuff. Here's another one.
Now we're going to make the cells bigger. We're going to make them cubic. We're going to call them "cuboidal." Cuboidal epithelium is another type. These are boxlike, and now what do you think about this? Well, we're thinking maybe bigger cells with perhaps more volume, like this one I showed you before. And if you have more volume, what can you do with that more volume? You can do more protein synthesis. And if you can do more protein synthesis, you can do some secretion. This is simple because watch this. Guess what? There's two types. Simple, where there's one cell layer, and stratified, where there's two. Now, once again, where are we going to find these? We're going to find these in glands--salivary glands, the thyroid, and all your glands which we will call the etcetera glands. On the other hand, this one is stratified cuboidal, it's kind of rare, but we'll find sections of this in sweat glands, mammary glands, those milk-producing glands. Again, what do these have in common? Secretion. Why? Cell volume.
Now, let's get really big. Let's get columnar, our last type of epithelium. Columnar epithelium is bigger yet. Now, we're not only talking about the ability to produce huge amounts of juice, but maybe even some absorption. Let's talk about a juice producer first. First of all, guess what the first kind of these are called? You guessed it. Simple. One layer. Don't forget that basement layer. Don't forget there may be a germinative layer, too, but we're talking about the epithelium itself. And what are these going to do? Well, a good place where we might find these is the stomach. Guess what it makes? Stomach juice. Gastrointestinal material. The enzymes that help you digest your food in your stomach, the hydrochloric acid, all columnar epithelium.
And if you want to add a wrinkle to this columnar epithelium and go to something called a "pseudo stratified"--oh, there's a new one for you. We haven't talked about that yet. Why? Take a look at this. Here's why it's called pseudostratified. It looks stratified, but it's really not. These cells are all different lengths. The big long one looks like it's stratified, but really they're all coming down to the basement membrane. In pseudo stratified there's something I really want to show you here, but first we want to write down pseudostratified. Pseudostratified, fake stratified, has along the top of it extensions, microvilli, cilia. What happens here is these are specialized for absorption. So, for example, a pseudostratified epithelium, with its cilia, are going to line your nasal passages, your respiratory membrane, your trachea, all those places where you need extra surface area--so pseudostratified respiratory surfaces.
And last, but not least, the least common of all the columnar, stratified. It's rare, but we need to know that it's there. The stratified is much more protective. You'll find this in the urethra, the mouth, the epiglottis, the pharynx. This is protective as well as productive.
You never thought it was going to be this complicated, did you? But think about it. If you are going to have organs, you're going to have to have tissues that start everything out. What better tissue to have than an epithelium tissue to act as either a thin layer or a protective layer. We'll take a look at some of those other tissues in another lecture.
Animal Systems and Homeostasis
Introduction to Animal Systems and Homeostasis
Animal Tissues: Epithelial Tissue Page [3 of 3]