 we can classify epithelial tissues in two ways. We can classify them by shape, or we can classify them by the number of layers. So let's look at shape first. There are three possible shapes that our epithelial cells can take, and let's make sure that we're clear that this is the shape of the cell on the luminal edge, or the apical edge of the tissue. So that's important to keep in mind, and we'll see why in a second. You can have squished, flat pancake cells that look a little something like that. You can have square shaped cells that look a little something like that. And then you can have a tall boy that looks a little something like that. Now, this guy is called a squamous cell. Squamous, flat. Like squished, the squamous cell is squished. Okay, that's cool. What do you think? What would you call this one? I love it when it happens like that. It's cuboidal. Done. This would be an example of cuboidal epithelium. Here's squamous epithelium. What do you think this is? Rectangularoidal? No, it's columnar. Doesn't it look like a column? That's columnar epithelium. Now, before we can actually name our different structures of epithelial tissue or different types of epithelial tissue, we need to look... There's another piece of information. We need to know how many layers there are. You can have one layer, and then it's simple. Or you can have more than one. I want to put the little greater than or less than sign on there. Greater than one. But I can't remember which direction. And then that is called stratified. So, simple squamous epithelium. What's it going to look like, you guys? It's going to look like this. That's simple squamous epithelium. What is stratified squamous epithelium going to look like? It's way too fat to be in the squamous club. This is squamous. And, oh, we have another layer coming on, don't we? Oh, and another layer coming on. Look at the layers. Epithelial tissues can be like onions. They have layers. Do you notice about my stratified squamous epithelial tissue? What happened to the cells at the basal edge of the tissue? What shape do they have? Those are cuboidal all day long. But how do we decide the shape of the tissue? We have to look at the luminal edge. This is the lumen or the space. It's not a lumen when it's outside of your body. It's a space. So we look here and we go, oh, totally. This is stratified squamous epithelium. These are both epithelial tissues. So what do they both have? What is an anatomical structure that they both are going to have? Oops, I'm not keeping that. Pictionary. What am I drawing? If you say blood vessel, I will punch you. Just kidding, I don't even know you. I won't punch you. If you say blood vessel, I will give you this look. What would a blood vessel look like? It would look like this. And are there blood vessels in epithelia? No, it's avascular. So it's not a blood vessel. Look. The structure is present here as well. What is it, my friends? It's the basement membrane. Good job. I heard you yelling that. What else do you need to know? You need to know about the funky players. There are two funky players. And I'm going to go to a new page because I need to have lots of space to draw these guys. They're funky players because they just don't quite fit with anybody else. And the first one is... So maybe we should make a little note. Funky players. So the first one is pseudo stratified. Stratified. Pseudostratified columnar. Pseudostratified columnar epithelia. Pseudostratified columnar epithelial tissue. Why do I feel like there's something missing from that? Pseudostratified. Often pseudo stratified columnar epithelial tissue is ciliated. And so we add the word ciliated in here. But that's only if it is ciliated. So you want me to draw you a picture of it? I know you do. It's pseudo stratified. It looks stratified. But is it? No. Psych. It's pseudo. Here it comes. It really is a columnar cell. But the nucleus, the nuclei in this... I'm not doing a very good job of drawing them yet. The nuclei are all over the place. And it looks like there's multiple different kinds of... There's multiple layers is what it looks like. But if you look super close, you'll see that no, there's not multiple layers. Oh my, do you get what I'm drawing? It's a mess, but they're all over the place. And look, I want to draw one on this side because those look like they're lined up in exactly the same spot and they shouldn't be. And so I'm going to just draw another one over here like that. So you get the feeling that, you know, they're all mixed up. They're super stuck together, but they're all mixed up. Now if I were to add cilia to them, I could add cilia on the luminal side or the apical side. And then they become the ciliated. So where am I going to draw my basement membrane? That's right. There's my basement membrane. And you can find this. This actually lines your trachea. This is why these little guys are killed by tobacco smoke. Oh, and that is why you, they actually move mucus out of your trachea. And if you kill them by smoking, then that was my dog telling you to quit smoking. If you kill them by smoking, then you're going to have to like cop up the mucus instead of your cilia just taking care of it for you. So be nice to your cilia and quit smoking. That's your first message to quit smoking in my class. You ready for the next little tissue? This guy is called transitional. And here's the deal with transitional epithelium. It looks, depends. Sometimes it looks simple and sometimes it looks stratified. And when it looks stratified, this is what it looks like. What kind of tissue, what kind of cells do they look like? Cuboidal. Okay, that works. So it looks like it's cuboidal. There are multiple layers that all look cuboidal. So it's starting to look like if you didn't know that I was about to psych you out, what would you say that this looks like? You'd call it stratified cuboidal epithelium, right? Except they get kind of funky up here. They do this really distinctive like bubbling. Like they look rounded and bubbly on the luminal surface. Do you see that? Isn't that kind of cool? How they get this kind of bubbly look to them? This, that's your bladder. Your bladder is lined with transitional epithelium. And when your bladder is empty, it looks like this. When your bladder goes off and gets full and stretches and transitional epithelium will begin to look like, this whole thing spreads out and becomes, it looks simple. It looks like simple cuboidal. And maybe even gets a little squished and spread out even more. And it stretches and now it's simple. How weird. And then it relaxes and it looks stratified. So they call it transitional because it isn't, like what is that? It's simple sometimes and it's stratified other times. It's transitional. All right. Do you see? Here's what I want you to be thinking about. I want you to be thinking about how the structure of these different epithelial tissues can enable their function. Like how, why would we have a pseudo-stratified, ciliated, columnar epithelium lining our trachea? Why would we have transitional in our bladder? Why would we have simple squamous? Oh, where would you expect to see simple squamous? And where would you expect to see stratified squamous? And what about the cuboidals and the columnars? What are they actually doing? So once you think about those things, and I have a feeling that, I don't know if I ever tell you the answers to those. I kind of feel like you should know, I feel like I should say what they do. Ready? Simple squamous. Dude. That guy is all about diffusion. Stratified squamous, can I keep it here? Stratified squamous is all about protection. Diffusion of gases, of nutrients, they're super thin. So stuff goes in and out really easily. Cuboidal and columnar play a role in secretion. And columnar is excellent at absorption. Now as we go through our systems and you see these different kinds of epithelium, I want you to be thinking like, why is it like that? We are not going to see stratified cuboidal or stratified columnar tissues. So we're not even going to talk about where we might see them or what they might do. I love you. Now let's talk about glands.