 we're going to start with epithelial tissue. And I already told you that epithelium, it's defined by the fact that it is found next to a space. And before we really look at the different kinds of epithelial tissues or like what they do, I want you to think about like, why would there be a special tissue type lining spaces? Where are these spaces that we speak of? Anything that is a space is a space. For example, out here, my fingers are in a space and not me. And my fingers, there's all sorts of spaces everywhere. This space is sort of defining my body. My skin defines my body. And my skin is made, has an outer layer of stratified squamous epithelial tissue. Think about why you need skin to be lined with epithelium. I have a space, rotler, and everything except for my teeth in that space is all epithelial tissue. If I stick my finger in my mouth, everything I touch, truly everything I touch except for my teeth, I'm touching epithelial tissue. If I take my finger and I continue down, because you know that sounds like fun, everything that my finger is touching inside that body tube, my esophagus, dude, that's all epithelium. What about, we know that that's actually outside the body, what about inside your body? Are there any tubes or spaces inside your body? Dude, your heart, your heart chambers are lined with epithelium, your blood vessels lined with epithelium, every body cavity lined with epithelium, every organ covered with epithelium. It's in every place that you have a space. Why? Why would we need such a tissue? Well, let's look at the functions of epithelium. Think about your skin first. What would be the function of skin? Like, why do we have that stuff? Dude, get rid of it. Nobody needs skin. Just kidding. Dude, skin is all about protection. Epithelium can be protective. Your esophagus is lined with a kind of epithelium, the same kind of epithelium, a little bit different molecularly, as your skin. They serve the same function of protection. Epithelium is also a tissue that is really good at secreting stuff. And if you think about the kinds of structures that secrete glands, secrete stuff. For example, sweat glands in your skin, secrete sweat. And it's epithelial tissue that makes up that sweat gland. And the epithelial cells are actually doing the work of producing the sweat. When you produce, when a cell produces something and then barfs it into the space that it is lining, that's the process of secretion. Epithelial cells are also responsible for absorption. And that's a big job. What kinds of places do you think you would be absorbing stuff? How about digestive system? Absorption is when you take something that is in a tube and you pull it into a different space. Whether you are absorbing it into the body, whether you're absorbing it into the blood, or you're absorbing it into a space that's actually outside the body, absorption is going from the space somewhere else. Now, there's some characteristics, some structural characteristics of epithelium that enable those three functions. So let's look at some characteristics of epithelium. First of all, I'm going to draw these characteristics. I'm not going to write it down. And the reason why is because you can't read my handwriting anyway. So epithelial cells, I'm going to draw my cells like this. Not all of them are shaped this way. Usually, I'm trying to make them kind of regular, whatever. Usually epithelial cells, they're not square. What cells actually are square? No. Or pointy on the edges, not so much. But they're closely packed together. That is a characteristic that you will see of epithelium. The fact is that sometimes it's a single layer of cells that are packed closely together. But other times, there's multiple layers of cells. And I'm going to make this little tissue here consist of multiple layers of cells. Multiple meaning two, because I'm not going to draw this forever. Done. How do you know it's epithelium? Well, dude, you don't unless I tell you this is a space. You see a space? You can pretty much assume that there's going to be epithelium next to it. I'm going to draw another structure that's really important that is associated with epithelial tissues. And it's, I sort of feel unfortunate about my choice of color. This is a structure that is associated with epithelial tissue. I'm going to jot down that it's a structure, just so you know that that's how I would ask you about it. I could point to this structure on an exam or quiz and ask you, name this structure. And I wouldn't say name this cell or I wouldn't say name this thing. Actually, who knows? I might say that actually. Sorry about that. But it is a structure and the best way for me to say it is to ask you to name the structure. Okay, what is this structure? It's called the basement membrane. And it's almost like a placenta between epithelial tissue and the tissue that lies beneath epithelium. And I'm going to draw the tissue that lies beneath. And this is a certain kind of tissue. It's real. That tissue does exist. And I'm going to draw something else going through this one. What do you think this is? It's a blood vessel dogs. Let's play Pictionary. This is a blood vessel. So did I draw any blood vessels going through my epithelial tissue? I did not and that was actually on purpose and it's because epithelium is avascular. There isn't a blood supply for epithelial tissues. Interesting. Epithelium is connected to this underlying tissue by a basement membrane. The basement membrane is produced by the epithelium and the tissue that lies beneath. And what tissue do you think that is? What kind of tissue? I made it messy. There are cells in it. It's like connecting epithelium to something else. Because otherwise your epithelial tissue would just fall off. Like think about it. If your skin wasn't, if the epidermis, which is made of a certain kind of epithelium, wasn't attached to the underlying tissue, you just would be like, and your epithelium would slough off. You'd be really disturbing, so don't do that. Let's have a basement membrane and let's have connective tissue underneath. Varying types of connective tissue are associated with varying types of epithelium to make different structures in your body. The basement membrane is produced by both the epithelial tissue layer and the connective tissue. They work together. That's kind of why I was envisioning it like a placenta because both parties are producing this structure. It's a protein-ish structure and the purpose of it is to sort of anchor the epithelium onto that connective tissue layer. It's also a place where materials pass through the basement membrane in both directions. If we're feeding the epithelial tissues from the blood, isn't that the whole purpose of blood? Yes, pick up garbage and deliver nutrients and goodies. If there is no blood supply in the epithelium, then somehow the good stuff has to diffuse through the basement membrane and into those epithelial cells because they're going to die. If this epithelium was in the digestive system, you can imagine that nutrients and stuff that you are eating needs to be absorbed into the blood to be delivered to the rest of your body. And so the basement membrane is sort of a wall that stuff has to go through. Well, look at my connective tissue. Connective tissue is a mess. Connective tissue connects things and surrounds things and basically is like packing peanuts and that's awesome. But epithelium is directional. There's a space edge and there's a basement membrane edge, right? And every epithelial tissue has this. So we actually have directional terms to describe epithelial tissues. You have the luminal or apical edge of a cell or a layer of tissue and you also have the basal or basolateral layer or direction edge of a tissue. My epithelial tissue, this whole thing is my epithelial tissue. And so the luminal edge of my epithelium touches the space and the basal edge of my epithelium touches the basement membrane. You can always have, well, just like all directional terms are relative, B point B is luminal to point C but basal to point A. So these directional terms are relative. Okay. The last thing that I haven't, I'm going to have to write it down because I don't have a way of drawing this, although it could be interesting to try. Epithelial tissues are highly regenerative. What's that mean? They replace the cells, replace themselves quickly. In fact, I want to say that the epithelium lining, your digestive tract, replaces itself completely every 12 days. So the entire lining completely regenerates itself every 12 days. That's fast, like that. Your skin does not regenerate itself every 12 days. Why? Why would epithelium be so quick to replace itself? Well, think about it, dude, you're lining a space, you are basically the border to the outside constantly and stuff is constantly having to cross or slough by you. Think about your digestive tract, man. Food, bacteria, crazy fibers, whatever. Acids and enzymes, like your cells are taking a beating in the digestive tract and so they got to heal themselves. They got to handle that. So those are some characteristics of epithelia. And now let's talk about how we classify these things.