 There are two characteristics that we're going to use to classify epithelial tissues. And they are the number of layers in the tissue and the shape of the apical or the cells lining the space. So I'm going to tell you if we have one layer of cells, the tissue is called simple. If there are more than one layer of cells, if there's more than one layer of cells, then it's called stratified. So epithelium can be simple or stratified. We combine that with the shape of the apical cells and we have three options for our shapes. We can have super flat cells. They almost look like little pancake cells or little like fried egg cells. If they're flat and even the nucleus gets kind of flat, although sometimes the cell is super flat and the nucleus is just like this little bump in the cell, that's called a squamous shaped cell. Some of our cells are box shaped. The nuclei in the box shaped cells are really often really round. That looks a little bit more overly. And I'm telling you all these characteristics and the fact is that we'll see lots of variation. If the cells on the apical edge of the tissue are square, then it's called cuboidal tissue. And again, you can have one layer of cuboidal shaped cells and you're going to have a simple cuboidal epithelium. Super straightforward. Sometimes the cells are tall and if you get a tall guy, you can have actually columnar because it looks like a column standing there tall and proud. You can have simple columnar epithelium. You can have stratified columnar epithelium. You always look, even if I had, let's look at something like how many things are going through my brain right now. Okay, I'm going to draw you a tissue and tell me what kind of tissue it is. Pictionary. Okay, I can't help it. I can't draw for forever. Ooh, I'm going to draw something else for you because it's fun and because it's fun. And I'm not done. I'm not done, doggies. What do you notice? What's happening to these cells? Oh my gosh. I will not tell you what hour it is in the evening and how tired I am. I feel like I'm falling asleep right here playing Pictionary with you. Okay, what do you think? What kind of tissue is this? Is it simple or stratified? Please tell me you know it's stratified because look at how many layers of cells I drew. I took the time to draw all those crazy layers. Definitely stratified. Now, what is the shape? Squamous cuboidal or columnar. Is it columnar? Definitely not. You're at all. Cuboidal? There's some cuboidal cells. Squamous? There's some squamous cells. How do you know which one it is? It's stratified something, something. Where's my apical edge, my friends? So what kind of tissue am I talking about? Stratified squamous epithelium. It's that straightforward. I would like to... What I'd like you to do is check out your lab handout. That will tell you the six kinds of epithelium that we're going to know. Stratified epithelium, we're only going to look at stratified squamous epithelium. Stratified cuboidal and stratified columnar do exist. They're just not that common in the human body, but we will see simple squamous, simple cuboidal and simple columnar. Of course, there are some exceptions. And transitional epithelium is one of them. Transitional epithelium looks stratified. It looks like stratified cuboidal. If this is my basement membrane, it looks like, I mean, you've got this whole thing going on here, you've got these layers and all the cells in the layers actually look cuboidal. But then at the top, the luminal or apical edge, the cells actually get this like rounded sort of balloon-like look to them. And transitional epithelium, I'm going to tell you where it's found. I'm going to give you an example. It's found in the bladder. And I told you it looks stratified. The fact is that transitional epithelium, if it's relaxed, it looks stratified and it has those bubbly luminal cells in the bladder. If it's not relaxed and it's filled instead with fun things like urine, it actually stretches out. This is a stretchy tissue and it stretches out and it actually looks simple. So it's a bizarre tissue depending on where you, if you get it relaxed or you get it not relaxed, then you might be confused. But I will always show it to you in relaxed form with these really distinctive bubbly luminal cells. The only thing you can confuse it with is stratified cuboidal epithelium and we're not going to be studying that in this class. So if you see something that looks like stratified cuboidal, follow these rules, call it transitional because that's what it's going to be. The other tissue that's unique is one that's going to look like this. It's actually simple. Okay, but it's called pseudostratified. Pseudostratified columnar, columnar epithelium. And it has a basement membrane just like you would expect. It's columnar, so sure thing. It's got a basement membrane and it actually does have an essentially columnar shape. In a traditional or normal columnar tissue, the nuclei are actually lined up sort of evenly. In pseudostratified columnar epithelium, it looks like it is stratified tissue and that's because the nuclei are all over the place. And they're all over the place because, well, I actually have absolutely no idea why they're all over the place. But do you see how every single cell does reach the basement membrane and the lumen? That's the requirement for a simple tissue. Every single cell indicates it touches both the basement membrane and the lumen. So we just have one layer of cells, but because the nuclei are all over the place for whatever reason, it looks like it's stratified. You have to be kind of careful when you're identifying this tissue. Good lord. What's happening? Sometimes pseudostratified columnar epithelium is ciliated and that can also be a really nice clue that we actually don't have a stratified tissue here. It's a pseudostratified. Okay, I have no idea what just happened and why it moved all around like that. Good lord. Let's stop. And the next thing is to move on to connective tissues.