 Okay, what is the tissue? Let's get a definition of tissue to start us off. A tissue, it's a diverse group of cells with an ordered structure working together, that says working together for a common function. Okay, really? Let's break it down. What exactly does that mean? A diverse group of cells. We know that the cell is the first level of organization that displays the characteristics of life. We also know that in the human body, there are a ridiculous number of different types of cells. So think about it for a second. How many different cells do you think? Yes, how many different cells do you think there are in the human body? You're thinking and you're guessing, and now I'm going to tell you the answer. Although this number is in debate, 200 is the number that's reported in the biology book that I've been using. Seriously? 200 in the human body. Now, that number is a lot higher. If you look it up on Wikipedia, it's a lot higher. And we think that the reason why it's a lot higher is because there are different kinds of, say, lymphocytes. And there are different kinds of, say, T lymphocytes. And then there's different kinds of killer T lymphocytes. And you break all those down. And maybe some classifications say, hey, man, it's a lymphocyte. Let's just call it good. Some might say, no, each of those separate types of lymphocytes count as a single new kind of cell. Seriously? 200 kinds of cells. It's not a tissue if the cells are all the same. So you have to have some diversity of the cells. And they're working together. They're working together to carry out a common function. So if you think about the immune system, you have just a jillion different kinds of cells. And they are working together to keep invaders out of your body. Thank you very much for doing that. The other thing that's necessary in a tissue is that it has to have an ordered structure. In your body, you have in humans. And I wonder if there is any variation if there are other kinds of tissues and other critters. In you, there are four types of tissues. Now in this class, I mean today, we are doing epithelial tissues. Epithelial tissues cover surfaces and line spaces. Done. That's what they are. And we're going to spend this whole lecture talking about these tissues that cover things and line things. We also have connective tissue. Connective tissues are unbelievably diverse. And our entire lecture next time is on connective tissues. So we're going to look out what are they made of? Where do you find them? Connective tissues, blood is a connective tissue. Bone is a connective tissue. Fat is a connective tissue. A fluffy, like, I don't know, it's like feathery tissue called a realer connective tissue is a connective tissue. Connective tissue is really, really diverse. We also have muscle tissue. This is one of the four types of tissue you'll find. Muscle tissue is contractile. What that means is that it shortens. And then there are processes that help it lengthen so that it can shorten again. And we have multiple different kinds of muscle tissue. And we'll talk about that later in the semester. And our last type of tissue is nervous tissue. Nervous tissue is a communicative tissue. It's sending messages. And it does so in a really unique way. So these are our four types of tissue. We will see them all the way through. It probably scares the crap out of you that we have a comprehensive final in this course. But honestly, you get to the end and you're, like, amazed at how much you've learned. And you will be able to identify all these different kinds of tissues given a slide of Joe anything, like an unknown slice of body part. And you'll be able to identify all the tissues inside. So don't get overwhelmed. This is a slow and fun process. Let's focus in on our friend, the epithelial tissue.