 Hi everybody, Dr. O here. This video I want to cover are tissue membranes. So let's first of all, let's find what a membrane is. A membrane is a layer of epithelial tissue with an underlying layer of connected tissue. So there are four tissue membranes that make up that are in the human body. We have mucous membranes, cirrus membranes, cutaneous membrane, which is a fancy way of naming your skin, and the synovial membranes. So the mucous membranes, so you just simply look up, mucous membranes are going to align cavities that are exposed to the outside environment. So they're going to be aligned with epithelial cells and they're going to turn out mucous. Cirrus membranes are going to align the cavities that are inside your body that aren't exposed to the exterior. These would be your ventral body cavities like your pericardial cavity, peritoneal cavity, and your pleural cavity. The cutaneous membrane, like I already mentioned, is your skin. That's going to be the epithelial cells of your epidermis plus the underlying dermis. We'll cover that in much more detail later. In your synovial membranes, they're going to align your synovial joints. So these are going to be most of your large complex joints, will be synovial joints. So let me just quickly show you an example of each one, but we'll dive into them more when they matter later. We're just talking about them from a basic histological standpoint at this point. So here we see a mucous membrane. So we have the epithelial cells and the key thing there is the goblet cells. So goblet cells are going to produce the mucous that are going to coat the outside of this mucous membrane. That mucous is going to have secretory IgA antibodies. Mucous itself is going to slow microbial growth and slow their movement. So it keeps their passageways lubricated and moistened, but also plays a role in keeping out pathogens, hopefully. So that is a mucous membrane. Here we see one of your cirrus membranes. So the biggest difference is mucous is a thick, sticky secretion. Cirrus membranes are full of cirrus fluid, which is very watery. So here we see the actual pericardial cavity. So I covered the ventral body cavities in a separate video. Here we see the cutaneous membrane, your skin, the epidermis with the underlying dermis and hypodermis, also known as a subcutaneous layer. And then lastly, here's a typical example of a synovial joint with synovial membranes that produces synovial fluid. So you keep hearing synovial. So this is going to be anywhere two of your long bones come together. This is probably going to be like between your fingers or toes or something. So that would be a synovial membrane. All right. So those, let's go back to the beginning real quickly. Those are your four membrane types. Mucous membranes that line cavities exposed to the exterior. Cirrus membranes that line cavities that are not exposed to the exterior. The cutaneous membrane, which is your skin, and then your synovial membranes, which lines your synovial joints inside your body. All right. Those are your membranes. Have a wonderful day. Be blessed.