So this is the second part of a short video series that I'm calling tissues, membranes, glands, and skin. And here I want to focus specifically on membranes. We find membranes lining and covering, lining organs and covering organs.
And generally they're composed of more than one tissue. Although Saladin does mention that some membranes can actually be composed only of epithelial tissues and some can be composed of only connective tissues. In the situation where we have membranes that are composed of more than one tissue, they are really simple organs. So let's look at these in more detail.
So we want to start with the learning objectives. We're going to be describing the general structure of cutaneous, serous, and mucous membranes. And I've also got the synovial membranes in there too, just for good measure.
We want to identify the tissue types that are present in the skin. And in Saladin here, you're going to be looking for section 5.5. And the important pages are pages 160 to 166. And again, Saladin, the histology chapter, has got many excellent and very clear photomicrographs and diagrams. So the textbook will always be an excellent resource for you. All right.
So we want to start with mucous membranes. Let me move this up just a little bit. There we go. Sorry about that.
Let's go back. Let me see if I can move this up. There we go.
That's a little better. So mucous membranes, or mucosae, are found in cavities that open to the outside. They usually secrete mucus, hence mucous membranes, and this keeps the surface of the membrane moist.
The tissues that are composed of, or that comprise, I should say, a mucous membrane are going to be a simple epithelium, right, so a single layer of epithelial tissue, and then a layer of aerial or connective tissue that's called the lamina propria. Because the mucus or some other secretion keeps the surface of the membrane moist, we often call these wet membranes, wet membranes. And so we find mucus membranes in places where mucus is useful for lubrication, like in the respiratory system or in the GI tract. Cirrus membranes, not serous membranes, serous membranes or serosa are double layered membranes and we find them in closed body cavities. So these are cavities that do not open to the outside.
Each layer of the serosa is composed of a layer of simple squamous epithelium, this is called the mesothelium, and then a layer of areolar connective tissue. In between the two layers we have a thin layer of serous fluid which serves to lubricate so that the two layers can sort of slide past each other. We find serous membranes, as we said, lining closed body cavities, and typically these are going to be cavities that contain organs that are in motion. So we find serous membranes around organs like the lungs, the heart, and the organs of the digestive tract, and they have special names.
the pleura will be the serous membrane that surrounds the lung the pericardium is going to be the serous membrane that surrounds the heart and the peritoneum is the serous membrane that surrounds the digestive organs furthermore our double layer we can't see it here in this cartoony picture but each layer of the serous membrane has its own name so the layer that is adhering closely to the organ is called the visceral layer And the layer that is adhering usually to the body wall is called the parietal layer, the visceral and the parietal layer. And then in between these two layers, this is where the serous fluid is going to be. All right, so serous membranes. The third membrane that we want to talk about is the cutaneous membrane.
There's just one example of this, and this is the skin. We call this a dry membrane because it's exposed to the air. And if you just take a look now at your skin, you should be able to see that your skin is actually dry. It's not a moist membrane.
The layers of tissue that form the cutaneous membrane are going to be a keratinized, stratified, squamous epithelium here. This forms the epidermis. And then the deeper layer is going to be a layer of dense irregular connective tissue. And very often, we'll see that there's another connective tissue layer.
Here will be areolar connective tissue layer that sort of serves as a boundary between these two layers, between the epithelial layer and the connective tissue layer, a dry membrane. We're going to be talking a good bit more about the skin going forward. The last membrane that I want to talk about is the synovial membrane.
This is not a true membrane, and Saladin also makes that point. And he makes the point that there's no true epithelial layer. However, we do have a layer of cells that are called synoviocytes here, and they rest on a layer of aerial or connective tissue here, a specialized layer of connective tissue. We find synovial membranes lining the joint cavities in synovial joints.
And synovial joints are going to be these very freely movable joints that you see in most of your extremities, most of the places where bones come together in the limbs. Synovial fluid is a lubricating fluid within the joint cavity, so it protects the ends of the bones as they come together and ensures that you've got that free movement there where they come together. And the synovial fluid serves to lubricate the joint.
It provides nutrients for the cartilage that's also present there on the articular surfaces of the bones, and it removes waste products. Now, I created a table for you. I've posted this for you on ELC, but I want you to sort of compare and contrast mucous membranes and cutaneous membranes, right? So we've got our wet membrane and our dry membrane.
And you can fill this in. I want you to be sure that you understand the tissues that compose each of these membranes, the secretions that are produced, some other characteristics of this membrane. So for example, is it wet?
Is it dry? You know, is it located in a particular unusual place? And then lastly, the thing I want you to be thinking about is why is this membrane well suited for its location and its function? So I think this will be a little something that I hope will help you kind of going forward. And I want to leave you here with a picture of a hellbender.
This is an eastern hellbender. This is a giant salamander actually and it is now the state amphibian of the state of Pennsylvania. As an amphibian it tends to be very susceptible to changing environmental conditions, for example pollution, and it's only found in streams that have rapidly moving water here.
It gets its name. One of its nicknames is actually a snot otter because it secretes a lot of mucus here, and it dwells exclusively in the water. So when I saw this, I was thinking about the mucus here that's secreted by this creature here as being kind of like a mucus membrane.
And so I'm going to ask you a question. Is a mucus membrane well suited as a covering? Why or why not be thinking about that? And I've put a link here so that you can read about Pennsylvania adopting the snot otter as one of its state animals.
All right. So just a little aside. So from here, we're actually going to talk about the skin. So stay tuned for that.