Epithelial and connective tissue

- [Voiceover] There are four different types of animal tissue that are all made up of eukaryotic cells. Epithelial tissue, connective tissue, muscle tissue and nervous tissue. In this video we're gonna talk about epithelial tissue and connective tissue. When you think about epithelial tissue you can think about it as a lining. Both an inner lining and an outer lining. So for example, epithelial tissue makes up the outer layer of our skin. It makes up the outer layer of organs. It lines organs so the lumen of organs will be lined with epithelial tissue and it also lines the inside of cavities, inside of the cavities of the organism. Epithelial tissue also make up glands so that would include both exocrine glands and endocrine glands. And just to remind you, exocrine glands will release their substances directly to the target organ. Whereas endocrine glands usually release hormones but into the bloodstream, not to the target organ directly. And epithelial tissue comes in two forms. It can be simple and that means it's one layer thick. Or it can be stratified which means it can have two or more layers. And where will you expect to find simple epithelium versus stratified epithelium. Well, you'll find simple epithelium in places where substances need to diffuse from two different places. For example the alveoli of the lungs are lined with simple epithelium because carbon dioxide and oxygen need to diffuse from the alveoli into the bloodstream and vice versa. And of course that will be pretty difficult if you had a thick layer of cells. And you'd expect to find stratified epithelium in places that need to resist chemical or a mechanical stress. For example, the esophagus is lined with stratified epithelium. That's because the esophagus will have food coming through it. The food might be sharp, it might be hot and we want a thick layer of cells to protect the underlying tissue of the esophagus. A stratified layer epithelium acts as that protective layer. Let's take a look at a section of simple epithelium and epithelial cells are attached to something known as the basement membrane. The basement membrane is not made up of cells but rather it's made up of different types of fibers. For example one fiber that can be found in the basement membrane is collagen. And the basement membrane is semipermeable to certain substances and that's pretty important because epithelial tissue is avascular. That means that epithelial cells have no blood vessels which then makes us ask the question of how do they get nutrients? They get nutrients from the underlying tissue. What happens is that nutrients will diffuse from the underlying tissue through the basement membrane to the epithelial cells. And that's how epithelial cells get their nutrients. Let's just recap some of the places that you'd expect to find epithelial cells. We already mentioned the outer layer of the skin, the tissue lining the mouth, esophagus and GI tract and of course this is not an exhaust of list. In the tissue lining the kidney tubules, and the tissue lining blood and lymphatic vessels. And in fact the tissue that lines blood vessels and lymphatic vessels has a special name. It's known as endothelium. Let's talk about connective tissue. Connective tissue supports tissues, connects tissues and separates different types of tissues from each other and then there are different types of connective tissue that don't necessarily fall into these neat categories. What are some examples of connective tissue? Bones, cartilage, blood, lymph, adipose tissue which is fat. The membranes covering the brain and the spinal cord and other types of tissues. What does connective tissue look like? What are some characteristics of connective tissue? Basically it has three components. It has cells. It has what's known as a ground substance and then it has fibers. The ground substance and the fibers together make up a matrix. Let's see what this looks like. Here we have the ground substance which is usually a viscous type of fluid. Then interspersed in the ground substance are fibers and then we have cells, and these cells are usually what is producing the matrix. Let's look at some connective tissue in more detail. The first we'll talk about is areolar tissue which is this tissue right over here. Areolar tissue is a very common type of connective tissue. It binds together different types of tissue and it provides flexibility and cushioning, and we can actually see the structure pretty clearly in this picture. You can see the cells over there, those little dots. There's no cell over here. You can see the fibers running through it. Over here and over here. And then the ground substance is the kind of background yellow, viscous liquid that you're seeing. Then we have adipose tissue. Adipose tissue is basically fat, tissue is a fat. it provides cushioning for the body, it stored energy and it actually is an exception to the rule. It does not have fibers like most other connective tissue. Then we have what's called fibrous connective tissue. Fibrous connective tissue is pretty strong. It provides support and shock absorption for bones and organs, and you find it in the dermis which is the middle layer of the skin, tendons and ligaments. Here are some more types of connective tissue. We have blood. Blood is also an exception like adipose tissue and that it does not contain fibers. And the matrix of blood is the plasma and you can see the matrix, this yellowish liquid in which the blood cells are suspended. Then we have osseous tissue or bone tissue. These cells in osseous tissue are known as osteocytes. Those are those brown cells that are kind of forming a pattern and the matrix in osseous tissue is what's known as bone mineral or hydroxyapatite, which is basically collagen fibers with different minerals like phosphates, magnesium, calcium, et cetera. And then we have hyaline cartilage. The cells in hyaline cartilage are chondrocytes. You can see them over here, there are a bunch of these cells. And they're found in surfaces of joints. These are all examples of different types of connective tissue and many of them provide some form or another of support for tissues and organs.