 Let's get a good solid definition of epithelial tissue. What exactly is it? Like how would you define it on a quiz? Epithelial tissue is a sheet of cells that line a space. And I say line a space, and I put that in whatever quotation marks on purpose, and I want you to think about what the word line means. But I guess when we're talking about in reference to the space, the epithelial tissue is found lining the edges of the space. So think about this. My skin is lining a space. This is a space, do you agree? Guess what kind of tissue is found on the superficial edge of my skin? It's going to be epithelial tissue. Open your mouth. Your mouth is a space. What is a mouth? What is the space inside your mouth? It's your oral cavity. The oral cavity is lined by epithelial tissue. I can't help it. Your entire digestive system is a giant tube with a space in the middle of it. And that space, guess what, lines the inside of the tube. It's epithelial tissue. So no matter what, if you see a space and there is a tissue next to it, chances are excellent that that is an epithelial tissue. So characteristics of epithelium, you're always going to see it next to a space. Now, I say that for, let's see, baby histology students. That's you guys. I'm not a baby anymore. For the histology students, sometimes we look at a slice of something. And there's a giant space in it, like right in the middle. You're like, dude, what is that? I'm looking at a chunk of muscle and there's this huge space in the middle. Why? I wouldn't expect a space there. It's not a space. It's an accident. It's like an artifact of the process of making the slide in the first place. And that's something that you'll just get used to. You'll get used to seeing, oh, I would expect a space here. I wouldn't expect a space here. This is why histology is, I think, kind of challenging for students, because it takes time. And you just have to suck it up and look in the microscopes and get comfortable looking at them. So find your spaces. And then you'll know that you are finding your epithelium. Epithelial tissue is closely packed. Closely packed. The cells are closely packed together. I feel like I want to draw you a picture. So let me just do that. Look at how closely packed my cells are. What? These are all cells. Are they square? There are cells that are cuboidal in shape. They probably don't have these nice sharp edges that I've drawn. But look, this is a space. So what kind of tissue is this? It's epithelial tissue. And you know it because, whoa, here's a space. And I'm telling you that it's a real space and not some artifact in your slide. Look at how closely packed these puppies are. Are you kidding? That's awesome. They also, okay, because they are so closely packed, this is another characteristic. They're avascular. Do you know what that means? They're avascular, which means there's no blood. There's no blood supply. Think about your skin. We already decided here's a space. So we have epithelium lining the skin. So, oh my gosh, I can't even remember what I was going to say. There's no blood supply in the epithelial layer of your skin, but there are cells in there, right? And so what's the purpose of a blood supply? Thank you very much. I got there. Blood brings food and retrieves garbage. Do you think these guys need food and need their garbage taken away? They absolutely do. So guess what? They need a supply of blood, but it's just not in the epithelial tissue itself. They do need a blood supply. And look, how do the nutrients from the blood actually get to the epithelial tissue cells? They have to diffuse in. So the cells are like, we're starving. Send us some food. The blood supply is not embedded in these cells. The blood supply says, okay, I'm sending it over, and diffusion happens. And that's how the epithelial tissue gets its nutrients. So that's kind of an interesting characteristic. Let's try this one. They are epithelial tissues are highly regenerative. If they're highly regenerative, that means that they get scraped off and they'll just replace themselves, and they do it fast. They go through mitosis fast. Mitosis. I love mitosis. You remember that from Bio 1? So they make copies of themselves. They can replace themselves. They can regenerate themselves very quickly. In fact, there's some awesome fact about your intestines. Like your intestinal tract replaces all the cells like within 12 days or something crazy. Are you kidding? That's so fast. All right. What else do you need to know? Ready? They have the BM. What is this BM? You speak of a basement membrane. Epithelial cells, epithelial tissues, have a basement membrane. So I'm not going to write down basement membrane. I'm just going BM. But you, OK, I'll write it down. Basement membrane. And it's found on the... OK, are you ready for this? On the deep side of the tissue. And look, we need a word. We need a word to describe the space because the fact is that epithelial tissues, this is my sixth one, it's going in another color, they're directional. So we need a word here. That means that they're... most cells, if you look at a fat cell, which we're going to do in the next lecture, if you look at a fat cell, you're like, dude, the thing is round and full of fat and I can't tell one end from the other. Like if I just rolled it across the room, it's not going to look any different no matter how it lands on the other side of the room. Epithelial tissue, if I rolled one of these cells across the room, it has an end, an edge that attaches the base to the basement membrane, and that's the basolateral side. So the basolateral edge is the edge of the tissue that's close to the basement membrane. And then the apical edge is the edge that's close to the space. So the apical edge is also called the luminal edge. And the fact is that you have lots of tubes in your body. And epithelia line those tubes. The space in the center of the tube is called a lumen. So I'm going to do a little cross-section of this blood vessel and show you the lumen. This is the space inside. It's called a lumen. And it's got, you know, the blood vessel is made out of tissue. And what kind of tissue are you going to find lining the lumen? An epithelial tissue. So this would be the apical edge of the tissue, and this would be the basolateral edge of the tissue. Does that work for you? All epithelial tissues have this directional, directional terms that we can use to describe their sides. They're polar. The cells themselves, the tissue itself is polar. It has an apical end and a basolateral end, which means that the cells are polar. They have an apical end and a basolateral end. Okay, good, good. The basement membrane, you know what? The basement membrane attaches the cell to, attaches the epithelial tissue to this mess. What in the world? What is this mess? Welcome to the world of connective tissue. And we are going to talk about, like, what is that? These are cells. These are fibers. Oh, my gosh. Connective tissue is a mess. You're going to love how organized and clean epithelia is because once we go into connective tissue, we're going to be like, ugh, it's a mess in there. And it is. So let's look at the fact that they can form sheets. Think about this. Okay, you ready? If I have a sheet of epithelial tissue, and then I take it and I crumple it up, it still is a sheet. It's still, each piece of that sheet that got crumpled up is still lining a space on some level. And I could stretch it back out again and see the space that it lines. The point in telling you that. And in fact, maybe I'll just show you. I think I have, oh, yeah, no, that's not it. Look at this. This is a space. And this, do you see how, what kind of tissue are you going to expect this to be? I'm going to expect it to be some sort of epithelial tissue because it's lining the space. It's crinkled and crumpled, right? It doesn't have a nice clean straight edge, but that's okay. We wouldn't expect it inside your body to find like any clean straight edges. So we can have them in sheets. Then not only do you have sheets of tissues that can take up different shapes, but then you chop them into different slices. And that, like, you can see why histology, for those of us who are directionally or spatially challenged, it becomes challenging to visualize the original organ or whatever that you took the slice from. It's another reason why I think histology is difficult for people. Let's do the last one. The last one is, I think, I want to tell you about it because I want to make sure that you're cool with cover or line. I want you to think about this. Here's my hat. My hat is covered with epithelium. Do you agree with that? In windy land, that, like, if I have an organ, I'm going to cover it. I'm going to put a blanket on it. I'm going to pull up its covers, tuck it in, and it's a nice little covered heart. What cavity is the heart sitting inside? Yes. I know you remember that. It's sitting inside. Let's call it the pericardial cavity, just so, I mean, we know it's also in the mediastinum, which is also in the thoracic cavity. That's awesome. But let's just go with the pericardial cavity. Look, epithelium lines cavities. Do you see the difference in my wording here? Here, the epithelia is lining the cavity. Here, it's covering an organ. Do you see that? Do you follow it? You know what's really fun to think about sometimes? I look at this and I'm like, whoa, that's crazy. But that's what my notes look like in a traditional lecture. Like, that's what the whiteboard looks like. I love that. All right. Next, we're headed into, we've got their characteristics. We understand things that they're going to do, but now we're going to talk about their functions. This is who they are. Let's see what they do.