In this video, we're going to talk about the second commonly recognized characteristic of epithelial tissue, which is that epithelial tissue is tightly pressed tissue that is anchored to a basement membrane. One of the key features of epithelial tissue is that the cells are held really tightly together through cell junctions. Because these epithelial tissue cells are held really tightly together, that leaves very little room between the cell, and that leaves very little room for the extracellular matrix for the ECM. Epithelial tissue is going to have really tightly packed cells with very little ECM. Epithelial tissue is pretty much analogous to a brick wall, and that's why we're showing you a brick wall right here.

What you'll notice is that these red bricks that are so tightly packed together represent the cells of the epithelial tissue. The cement that's holding these red bricks together here represents the extracellular matrix. Like a brick wall, the bricks or the cells are going to be really tightly packed together with very little ECM, very little extracellular matrix. If we take a look at our diagram down below, you'll notice that we're showing you all of these different cell junctions right here. Right here, we're showing you a tight junction.

You might recall from some of our previous lesson videos that tight junctions are going to hold 2 neighboring cells really tightly together to create a leak-proof barrier. There are plenty of tight junctions throughout this epithelial tissue holding the epithelial tissue cells really tightly together creating a leak-proof barrier. That means that there can be some liquid here in the open space above the epithelial tissue here, and that liquid will not be able to leak through the epithelial tissue as effectively because of these tight junctions, creating that leak-proof barrier. Over here, we're showing you desmosomes, which you might recall from our previous lesson videos, are more structurally complex cell junctions that are going to really firmly anchor these neighboring epithelial tissue cells together. We have a few desmosomes throughout, and that helps the epithelial tissue to remain really tightly and, anchored firmly.

Down below right here, we're showing you some gap junctions, which you might recall creates a gap here, essentially linking the cytoplasm of the neighboring epithelial tissue cells to allow them to exchange nutrients with each other, for example. You might recall from our previous lesson video that we briefly mentioned that this tightly pressed epithelial tissue is going to be anchored to an extracellular structure called the basement membrane. The basement membrane is really going to consist of 2 thin extracellular layers. The first thin extracellular layer of the basement membrane is going to be the basal lamina. The second layer of the basement membrane is going to be the reticular lamina.

The basal lamina is actually going to be produced by the epithelial tissue or by the epithelia for short. Whereas the reticular lamina is going to be produced by the underlying connective tissue. You'll notice here is that in our brick wall analogy, our brick wall is anchored to a basement membrane that you can see down below that has those 2 thin extracellular layers, the basal lamina and the reticular lamina. Let's take a look at our diagram to visualize this a little bit better. Once again, in our diagram, we have the epithelial tissue up above shown here.

Down below, we have the connective tissue in a bluish color. Separating the epithelial and connective tissue is this structure here that we are calling the basement membrane. You can see the basement membrane right here in our diagram. What you'll notice is that this basement membrane actually consists of 2 thin extracellular layers. We have this pink extracellular layer right here and then we have this semi-purplish extracellular layer immediately beneath. The thin extracellular layers are going to be the basal lamina and the reticular lamina. One thing that can help you remember that the basal lamina is going to be produced by the epithelial tissue and the reticular lamina is going to be produced by the connective tissue is the B's here to help remind you of something important and that is that the basal surface of the epithelial tissue is going to be facing the basement membrane and you can think that the basal surface of the epithelial tissue is going to be producing that basal lamina layer of the basement membrane. Of course, that means that the reticular lamina must be produced by the underlying connective tissue. This here concludes our lesson on the second commonly recognized characteristic of epithelial tissue, and we'll get to talk about the third one in our next video.

So I'll see you all there.