Extracellular structures and cell-cell junctions
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- [Voiceover] We've given a lot of importance to individual cells and that's for good reason. Cells are the basic unit of life and they are fascinating in their own right. As we've seen in other videos, there's whole universes inside of cells. There's a lot more complexity than many of us might have guessed before really studying cells. But how do we go from cells to tissues? So when you know if you look even if you look at your skin, that tissue of your skin, or your tendons or, if you think about heart's tissues or the different organs' tissues. How do you go from a different tissue, which of course, eventually, will then get you to a full multi-cellular organism. So all the tissues and organs together, you're going to get the whole organism. How do the cells get together, coordinate, structure themselves to form me or you? And the answer is, or at least it involves, something called the extracellular matrix. Extracellular matrix. And just as we've talked about the insides of a cell, not just being a bunch of organelles floating around that we have a cytoskeleton that gives the inside of the cell structure and allows it to even, potentially, move and divide and transport things. So if you assume that this blob right over here is a cell, what I just drew in yellow, that would be its cytoskeleton on the inside, but there's also an analogous thing on the outside that helps coordinate how the cells all relate to each other and that's what we're talking about when we talk about the extracellular matrix. And it's made up of a bunch of different types of fibers and proteins and glycoproteins and probably the most notable of these is collagen. So I'll do colagen in yellow right over here. So these could be strands of collagen right over here and collagen is actually the most common protein in mammals. It makes up approximately, so this is collagen in the yellow right over there, and it makes approximately, I've seen estimates of 25 to 35%, but I'll just go with it makes approximately 30% of the proteins in mammals. 30% of mammalian, I'll say mammal, mammal protein. So roughly 30% of the protein in your body is collagen and a lot of it is making up these strands that help make up, it's not the only protein found, that help make up the extracellular matrix. And you see these cells here, these things that I've drawn, they're kind of embedded in this the way I've drawn it. They can be fixed. They look like they're a little bit, they're attached to this matrix and it helps position them. And it is true that the extracellular matrix, the collagen fibers and other things that we find there help attach the cells and structure the cells into tissues. They also help inform the cell, let the cell know when to grow, when to divide, even potentially when to die or when to produce different types of molecules. And to get a little bit deeper, to understand what's actually going on, how the cell actually attaches, if we were to zoom in, let's say we were to zoom in right over there on that square so I'm zooming in on the cellular membrane, we could get to this bigger picture that is taking up most of the screen right over here, so a view of this is like zoomed in representation. So right over here this is inside the cell. This is inside the cell here and we an see that we even have an actin microfilament right over here that helps form, this is part of the cytoskeleton and then have the collagen fibers which is making up part of the extracellular matrix and then we see that it all gets attached with these proteins and these proteins, they're a class of proteins called integrins. Integrins. And they're embedded in the membranes of cells and through other fibers, it's something like a fibronectin, they can be attached to the broader extracellular matrix and this is fascinating because it obviously structurally connects this extracellular, I guess you could say structure, this extracellular matrix to the inside of the cell, to the cytoskeleton, through these proteins and as I mentioned, these proteins help kind of lodge things together, lock them in place, but they can also be used to signal, they can sense tension depending on what type of cells you have, they can signal for the cell itself to get active or deactivate in some interesting ways. So it's a fascinating thing and I wanna make it very clear to you, a lot of times when you're studying biology even an introductory biology class you'll see things like this in textbooks, it's like, "Oh, of course, we have intogren proteins "that are going across our cellular membrane "and they're attached to things like fibronectin "and they're attached to the cytoskeleton "and they get attached to the collagen fibers "throughout the extracellular matrix." And it seems like oh, all of the biology is already figured out, but the real answer is how all of these things actually work together and how they signal to each other and how cells know what to do based on how much stress or tension or how crowded a certain area is. These are areas of open research. In fact everything that I'm talking about, if you were to delve a little bit deeper, and I encourage you to do web searches on these, you'll find current research papers where people are saying, "Well, how exactly "does an intogren know what to do?" or "How does it exactly signal to the cell?" or "How does it exactly bind itself "to either the cytoskeleton or the extracellular matrix?" These are all interesting areas of research and people are gonna be researching them for some time because there's always more questions on how these incredibly complex proteins and glycoproteins, fibronectin is a glycoprotein, it's proteins where the side chains have carbohydrate chains, saccharide chains, branching off of them. And so how do all of these thing interact and how do they kind of "know what to do" and how do all of these complex signaling mechanisms work? So it's a fascinating area of research. But, hopefully this gives you an appreciation for even a further appreciation for the complexity that makes you you. We've already talked about cells themselves being complex, but now they're lodged in this extracellular matrix which helps us better define tissues and helps kind of let the cells live in this community and know how to relate to each other and have a little bit of signaling from their outside environment. And I've just drawn one kind of intogren complex right over here, but you would have many along the cell or along the membrane and these aren't the only proteins. The fascinating things about cellular membranes, you'll often just see them drawn as this lipid bilayer. They have all sorts of proteins that are lodged inside of them that are used as receptors that allow certain molecules in and certain molecules out. So they really almost cities unto themselves and then they interact with their broader environment as well.
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