Apoptosis | Cell division | Biology | Khan Academy

Hello Emily, hello David.

So we're here today to talk about apoptosis. Uh, I was going to ask you some questions about it; you were going to explain what it even is to me.

Absolutely. Okay, talk apoptosis.

So, this word apoptosis—I did a little bit of etymology research. I broke the word apart because it comes from Greek particles that I identified. So we have, here's the limit of what I know right now. So it comes from these two pieces: the "oppo" meaning away and "ptosis" which means like a falling. So it's really this kind of—it means like "to the falling away."

And my understanding is that this programmed cell death, because that's what it is—that's what you've written—is kind of analogous to leaves falling away from a tree; that it's something that's supposed to happen. And the cells just die in this prescribed way when they're supposed to. This isn't random; this is something that the cell embarks on for a particular purpose.

Yeah, I think that's a really good way of putting it. And I think that is why it was named apoptosis by some of the first folks who studied it. They really wanted to underscore that this is a form of cell death that is a normal, healthy part of an organism's development, its maintenance of its body, even its prevention of things like viral infection or cancer.

Um, so it's very much a normal and healthy thing that your body is actually doing right now. Right now. Right now.

Oh man! So, okay, so what are the ways in which a cell can die? What are— I mean, because we have this diagram here. Here's a normal, healthy cell, and then here we've got this what looks like the aftermath of some kind of explosion. What is this?

So that's sort of showing two very broad categories of ways that a cell can die. The simplest way to label them would be messy and tidy. But more formally, the bursting-looking cell is undergoing necrosis, which is a form of cell death where the cell basically swells up and explodes, and it releases its contents, which is not very good because those can damage other cells in the area. They can attract cells of the immune system that will cause inflammation.

And then on the flip side, you have the tidy approach: apoptosis. Um, and here you actually see the cell shrinking down and kind of breaking up its DNA, breaking up—those nuclear, these little orange things in here are the chromosomes being cut apart. So those are like really little fragments of chromosome; they'd actually be much littler even than what I've drawn there. But what the cell's going to do is it's just kind of going to come apart into little fragments, encircling different cell components.

So you can sort of see those starting to bud off, and then cells from the immune system, whose job is basically to gobble up debris, they’re going to come and eat those little fragments. It's going to be like nothing ever happened.

So this is kind of the difference between disposing of your garbage in trash bags and disposing of your garbage by just dumping it out the window of your apartment.

Yes, I think that's true. But maybe even taking it a step further: if you dispose of your garbage in trash bags, or by apoptosis, you can actually reuse what was in the garbage.

Interesting! So, other cells can use those components for their own purposes, and they won't suffer any damage from having nasty stuff floating around outside.

So what are the circumstances under which apoptosis happens? Like, how, how common is this, and how common is necrosis?

Um, so necrosis is usually something that your body does not want to happen. That's going to happen when a cell is perhaps exposed to a chemical toxin, when a cell is actually mechanically damaged. So those are circumstances where a cell has basically received an insult that has caused it to die in a not very controlled way.

Okay, and apoptosis is kind of something that would be going on basically from the time a human being, or another vertebrate, is a tiny embryo throughout its life.

So when you're developing—when you're developing your hands, your hand actually kind of starts out as this chunk of tissue that's kind of like a paddle.

Okay, and it's actually apoptosis that is going to whittle your fingers out of that block.

So I've got this hand, right? I've got this kind of like webby hand, and you're telling me that as the hand develops, the tissue gets reabsorbed into the other cells that are going to make up the rest of my hand.

Yeah, so the cells, they'll first die by apoptosis, and then basically the blebs will get scavenged up.

I'm sorry, the what?

I guess I never named them, but these are called the little protrusions—that's a very technical term.

I love that!

You'll see it in scientific contexts.

That's a science word! It's totally scientific! I love that!

All right, okay, you can also see them in the picture, right?

What is this? What is this image here?

So those are basically just the same thing that the line diagram is showing: healthy cells, which is the left panel, and then blebby cells undergoing apoptosis, which is the right panel. So they're having all of their cell components repackaged into garbage bags to be—or if we want, recycling bags—then to be sent off to other cells where their components can be reused as stuff.

Exactly. Okay, now what's this—a tadpole beast here? What is this about?

So that's kind of the same thing that we were talking about with the hand. But another place where you see apoptosis happening in development is when you have a tadpole metamorphosing into a frog.

Um, so tadpoles—this is actually kind of part of the way they’re already—the tadpoles have a very long tail, and frogs generally don't have much of a tail to speak of. And the way that the frog loses its tail is through apoptosis.

Um, and again, it's said to resorb the tail so that it can make use of the cellular components.

Cool! So the tail just sort of shoots up and becomes hindquarters?

Yeah, I don't actually know exactly, you know, if it starts from the end and eats its way inward, but somehow it gets removed developmentally.

Man, that's really cool! So it's not really like leaves falling off a tree; it's not like this is a three-toed skink or something. If you grabbed a metamorphosing tadpole by the tail, it would just break off.

It's really more that it gradually gets subsumed into the tissue of the growing frog. I mean, at the cellular level, I guess the cells do—they pull away from their neighbors, so in that sense it’s a falling off. But it's not necessarily a falling off that you see at the level of a whole organ; it would be more of falling off of an individual cell.

So, so far we've given examples of apoptosis that are happening within developing creatures, but you're saying that right now apoptosis is happening in my body?

And in fact, so does that mean—is that just to keep the number of cells in my body constant?

That's a big part of the role that apoptosis plays. Um, like your blood system in particular is continually producing new cells, and if you produce cells but you never got rid of cells, you would eventually end up with too many cells. And too many cells in general is not a great thing to have in the human body; that's the kind of thing that you might get in cancer if you're having cells accumulating too much.

Um, so part of it is just keeping a healthy balance. But since we also just brought up cancer, that's actually another wonderful favor that all of our cells are doing for us. If they suffer DNA damage, which could predispose them to become cancerous, first they'll try to fix it. But if they can't fix it, they will actually, under normal circumstances, undergo apoptosis so there's no chance of them passing that damage on and becoming cancerous.

And that's actually a really important protective role that apoptosis plays in an adult human, or in a human at any stage of their life.

So it's kind of like a big red button that, you know, it's like a self-destruct button. If a cell comes to appreciate that it is developing cancerous symptoms, then it just hits the button and begins this cascade of enzymes that cause it to undergo apoptosis.

Yeah, I mean, I think that's the general way to think of it, for sure. Um, so there are actually kind of different ways that cells that are progressing towards cancer might be stopped, and some of those involve internal mechanisms—so the cell doing its own surveillance and observing, "Wow, my DNA does not look good, and I can’t fix it."

Okay, but also you could have a cell that might be observable from the outside by another cell as a potential cancer cell. So, like, an immune cell could come by and like stick a protein on the outside that also triggers the same thing.

Yeah! Yeah!

I don't know exactly what the mechanism of communication there is, but it would be an interaction between the two cells where one of them would tell the other, "You know, okay, I see there's something wrong with you. You know, time to wrap this up."

Cool! So apoptosis can happen in a couple of ways. But in pretty much all cases, it is a normal, healthy part of the cell life cycle.

Do all cells die this way?

Um, I mean, there are cells that will undergo necrosis, so certainly in that sense not every cell in your body is going to die by apoptosis.

Um, I would imagine that there are also other ways of recycling used cells that are not exactly considered apoptosis. Um, like skin cells—some of them will undergo a kind of similar process, but it's not technically apoptosis even though it is a regulated form of cell death.

Um, so I don't think that I would go as far as to say that all of your cells were eventually going to die this way, but it's sort of a very common maintenance way for cells to die and be replaced.

Cool, thanks Emily.

Thanks, David.