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Cellular movement

Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Vishal Punwani.

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    "I don't know much about boats. Just what I learned from Zelda: Windwaker."
    That was amazing.
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    Do we know yet how the "batman" signal works?? Is it through hormones?
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    In both models of movement, the neutrophil seems to extend forward and stretch. But if it is always stretching in the front, does it retract in the back so it moves forward like a slug? Also, how does it do that?
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Video transcript

- [Voiceover] So without explicitly thinking about it, we all probably have some idea that some cells move around, that they have to move around in our bodies to do their jobs properly. And I don't mean like red blood cells. Here's some, in a blood vessel here, here are some red blood cells, right? Red blood cells are actually pumped around in our circulation by our heart. So they just go with the flow. They go with the flow but they don't actually swim around in our blood, right? That'd be a little strange. They just kind of get pumped along. But there are other cells that do swim around a bit. There's other cells that can move around on their own, and not just in our bloodstream. Some can move pretty well throughout our tissues as well. And cells migrating and moving around in our bodies is incredibly important. Actually, in development, cells migrating from one part of the embryo to another is really, really important in forming our tissues and our organ systems. And in our adult bodies, cell movement is critically important for things like our immune system function. So what I want to do now is I just want to look at a few different ways our cells manage to move around. So why don't we start with sperm cells? These guys move around in an interesting way. So here are some sperm, right, produced in the testes. And you probably know that their entire life is essentially all about movement. They have a long way to go in their lifetime. Of course, I'm talking about their epic journey toward an egg for fertilization. So they need a way to move around. And that's where this flagellum comes in. So this is essentially a tail, right, a tail that whips around to propel the sperm along. And in humans, this flagellum is made of, if we zoom in on it a bit, it's made of these little monomers of microtubule proteins that get put together, sort of like Lego. And they work together along with these little connections between them called dynein proteins to whip this flagellum back and forth and cause the sperm to move. And you can actually find flagella in bacteria and archaea as well, but they're made slightly different in these two, of different proteins and with some little motor proteins here, and you could actually have more than one flagellum per cell in bacteria and so on. So a little different, but we won't worry about that too much here. So our flagella are one of the ways our cells can move around. And, in fact, if your sperm couldn't move around properly, let's say if your flagella failed to form properly, like you might see in certain diseases, it'd be a lot harder for your sperm to get to an egg to fertilize it, right? So already, you can see movement in sperm is just critically important. And another way of cell movement that's totally different to sperm is the way that white blood cells move around. So the cells of our immune system. So I'll use a type of white blood cell called a neutrophil for our example. Here's our neutrophil here. It's nice and neutrophil-y. Its neutrophil mom and dad are very proud of how strong it is. So these cells are a major part of our immune system, especially in dealing with bacterial infections. So what they like to do is they sort of travel around our bloodstream, right? Here's our neutrophil in our bloodstream here. So they travel around until they get the signal from somewhere in the tissues that says that their help is needed, right, maybe to take care of a bacterium or something like that. So they get the signal, kind of like the Batman signal that goes up in the sky, except this is either more cool or less cool than the Batman signal, depending on what your tastes are, I guess. I'm kind of on the fence, but, anyway. So they float around, and if they're in range of a signal, they respond to it. So they respond to it by sticking to the endothelium, the lining of the blood vessel that they're in, and they sort of roll along for a bit. And then at some point, they duck between two endothelial cells and continue moving along inside the tissues here, to where they're needed. So the point I'm trying to make is that, wow, that's a lot of movement, right? This thing just moved more than Michael Jackson in his heyday. So how can the cell manage to do that? Well, essentially, it all comes down to its cytoskeleton, a set of proteins that makes up its structure. This literally means cell skeleton here, "cyto" for cell, if you remember your Greek. So let's look at the two major mechanisms that cells are thought to move around by. There are two sort of theories for cell movement. It's about how cells move around. And it's not entirely known yet if cells use one mechanism or just the other mechanism, or maybe both of them to move around. That's not totally known yet. But here they are. So the first mechanism is the cytoskeletal model of movement. So here's our cell, right? Let's say it's our neutrophil again. So essentially, this theory holds that the cell really quickly polymerizes. It puts together these little proteins it has inside called actin proteins. So, again, kind of like putting Lego pieces together, it polymerizes these little filaments made out of actin proteins at the front of the cell here, the "leading edge" of the cell, and this pushes the cell forward a little. This is the main way that the cell's forward edge is advanced. And the back, what happens to the back of the cell? Well, interestingly, our microtubule friends, remember we talked about them forming the majority of our sperm's flagellum, well, in this case, we have microtubules back here that do a few things. And I don't know too much about boats, only what I learned from The Wind Waker Zelda game, but our microtubules back here act as sort of a rudder to steer our cell while the actin at the leading edge is extending away its filaments, right? And the microtubules also act as an anchor to stop our cell from moving. So let me clarify here, the microtubules can take on either a fixed or a super flexible state. So when they're being nice and flexible and dynamic, they can help set the direction that the cell moves in, right, like a rudder on a boat. And when they're being a bit stubborn and fixed, they act like an anchor, and they don't let the cell go anywhere, even if the leading edge is trying to get things going. So that's sort of the idea behind the cytoskeletal model. And the membrane flow model, this is our second theory of movement, this one's pretty cool too. So in this model, here's our new cell, right, the idea is that bits of the plasma membrane, the endocytos, they get internalized as vesicles from all parts of the cell, and then they move toward the front of the cell to exocytose there, and add membrane at the front. But there's two different types of endocytosed vesicles that get produced. There's ones made of just plasma membrane, right, so they just, all they do is they extend the leading edge a bit. And then there's ones with little flip proteins attached called integrins. So when these integrins exocytose, they get deposited at the front of the cell. They sort of stick their foot down and they anchor this bit of cell membrane down here to help the cell crawl along and move in this direction. Then you get a bunch more plasma membrane ones, just adding membrane in this direction, then you might get some more integrin feet coming down here. So the net effect is movement in this direction here.