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  • - [Voiceover] Hello, Emily.

  • - [Voiceover] Hello, David.

  • - [Voiceover] So we're here today to talk about apoptosis.

  • I was gonna ask you some questions about it,

  • you were going to explain what it even is to me.

  • - [Voiceover] Absolutely. - [Voiceover] Okay.

  • - [Voiceover] Let's talk apoptosis.

  • - [Voiceover] So this word, apoptosis.

  • I did a little bit of etymology research.

  • I broke the word apart,

  • 'cause 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 "apo" meaning "away,"

  • and "ptosis," which means, like, a falling.

  • So it's really this kind of,

  • it means, like, to be falling away.

  • And my understanding is that this programmed cell death,

  • 'cause 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.

  • - [Voiceover] Yeah, I think that's a really,

  • a good way of putting it,

  • and I think that is why it was named apoptosis

  • by some of the first folks who studied it,

  • is that 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.

  • So it's very much a normal and healthy thing

  • that your body is actually doing right now.

  • - [Voiceover] So, right now?

  • - [Voiceover] Right now. - [Voiceover] Oh man.

  • (laughing)

  • So, okay, so what are the ways in which a cell can die?

  • What are, I mean, 'cause 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?

  • - [Voiceover] So that's sort of showing

  • two very broad categories of ways that a cell can die,

  • and the simplest way to label them

  • would be messy and tidy.

  • - [Voiceover] Okay. - [Voiceover] But more,

  • 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,

  • and here you actually see the cell shrinking down

  • and kind of breaking up its DNA,

  • breaking up its nucleus--

  • - [Voiceover] And that's these little orange things in here

  • are the chromosomes being cut apart--

  • - [Voiceover] Yes, 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 gonna 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 gonna come and eat those little fragments,

  • and it's gonna be like nothing ever happened.

  • - [Voiceover] 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.

  • - [Voiceover] 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.

  • - [Voiceover] Interesting.

  • - [Voiceover] 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.

  • - [Voiceover] So what are the circumstances

  • under which apoptosis happens?

  • Like, how common is this,

  • and how common is necrosis?

  • - [Voiceover] So necrosis is usually something

  • that your body does not want to happen.

  • That's gonna 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.

  • - [Voiceover] Okay.

  • - [Voiceover] 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. - [Voiceover] Okay.

  • - [Voiceover] And it's actually apoptosis

  • that is gonna whittle your fingers out of that block.

  • - [Voiceover] 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 were gonna make up the rest of my hand?

  • - [Voiceover] Yeah, so the cells,

  • they'll first die by apoptosis,

  • and then basically the blebs will get scavenged up.

  • - [Voiceover] I'm sorry, the what?

  • - [Voiceover] I guess I never named them, but the little--

  • - [Voiceover] These are called blebs?

  • - [Voiceover] The little protrusions,

  • that's a very technical term.

  • That's a bleb. - [Voiceover] Bleb.

  • I love that. (giggles)

  • - [Voiceover] You'll see it in scientific--

  • - [Voiceover] That's a science word, is bleb?

  • - [Voiceover] Totally scientific.

  • - [Voiceover] I love that, all right.

  • - [Voiceover] It's blebs. - [Voiceover] Okay.

  • - [Voiceover] You can also see them

  • in the picture up at right.

  • - [Voiceover] So what is this image here?

  • - [Voiceover] So those are, that's 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.

  • - [Voiceover] So they're having all of their cell components

  • repackaged into garbage bags to be,

  • or, if we want, recycling bags,

  • that'd be sent off to other cells

  • where their components can be reused as stuff?

  • - [Voiceover] Exactly. - [Voiceover] Okay.

  • Now what's this tadpole beast here?

  • What is this about?

  • - [Voiceover] 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.

  • 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.

  • And again, it's said to resorb the tail,

  • so that it can make use of the cellular components.

  • - [Voiceover] Cool.

  • So a tail just sort of shoots up and becomes hindquarters?

  • - [Voiceover] 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.

  • - [Voiceover] Aw man, that's really cool.

  • So it's not, it's not, 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.

  • - [Voiceover] 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 a falling off of an individual cell.

  • - [Voiceover] So so far we've given examples

  • of apoptosis that are happening within developing creatures,

  • but you're saying that within, like,

  • right now, apoptosis is happening in my body.

  • - [Voiceover] That's a fact.

  • - [Voiceover] So does that mean,

  • is that just to keep the number of cells

  • in my body constant?

  • - [Voiceover] That's a big part

  • of the role that apoptosis plays,

  • 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-- - [Voiceover] Okay.

  • - [Voiceover] Cells accumulating too much.

  • 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,

  • is 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?

  • - [Voiceover] Yeah, I mean, I think that's

  • the general way to think of it for sure.

  • 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, gee, my DNA does not look good,

  • and I can't fix it.

  • - [Voiceover] Okay. - [Voiceover] But also,

  • you could have a cell that might be observable

  • from the outside by another cell

  • as a potential cancer cell--

  • - [Voiceover] So like an immune cell

  • could come by and like, stick a protein on the outside

  • that also triggers the same thing?

  • - [Voiceover] Yeah, yeah, I don't,

  • you know, 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.

  • - [Voiceover] 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 lifecycle.

  • Do all cells die this way?

  • - [Voiceover] 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.

  • I would imagine that there are also

  • other ways of recycling used cells

  • that are not exactly considered apoptosis.

  • Like skin cells, some of them will undergo

  • kind of a similar process,

  • but it's not technically apoptosis,

  • even though it is a regulated form of cell death.

  • - [Voiceover] Okay.

  • - [Voiceover] So I don't think that I would go as far

  • as to say that all of your cells

  • were eventually gonna die this way.

  • But it's sort of a very common maintenance way

  • for cells to die and be replaced.

  • - [Voiceover] Cool.

  • Thanks, Emily.

  • - [Voiceover] Thanks, David.

- [Voiceover] Hello, Emily.

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細胞分裂 - 生物學 - 可汗學院 (Apoptosis | Cell division | Biology | Khan Academy)

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    Study English 發佈於 2021 年 01 月 14 日
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