- [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,