Science Out Loud.

Sea stars can replace severed arms.

Flatworms can regenerate over half their bodies.

And if a predator grabbed this lizard's tail,

she could break it off and regrow it later.

It would be really awesome if we could regrow limbs

just like lizards do.

But we can't.

The closest we can get is kind of, sort of

regenerating our livers.

But let's back up.

Our bodies are made of organs, which

are made of tissues, which are made

of lots of different kinds of cells, from nerve cells

to bone cells to skin cells.

And our cells are constantly dying or getting

scraped off or bled out.

So we need to be able to make more of them.

So we have these things called stem

cells which can morph or differentiate

into, say, a liver cell or a blood cell.

When we're embryos, our stem cells are superpowered.

They're pluripotent, meaning that they

can become any sort of cell our body

might need and help grow everything from our stomach

lining to our muscles to our skin.

But as adults, our stem cells lose the superpower.

Adult stem cells can't become just any sort of cell.

While certain stem cells in our bone marrow

have to become either blood or immune cells,

stem cells in our intestine have to become intestinal cells.

There's no way either of these stem cells

will ever become, say, a liver cell or a nerve cell.

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Lizards don't ever lose their stem cell superpowers.

When a lizard's tail falls off, a bunch of pluripotent

stem cells rush to the stump and form this mass called

a regeneration blastema.

The pluripotent stem cells will differentiate

into skin cells, muscle cells, or bone cells--

any type of cell the lizard might

need to regenerate a tail.

But why can't humans regrow an arm?

Scientists think it's an evolutionary trade-off.

Those lizards have small bodies and would take one of them

a couple weeks to regrow the tail.

But it would take a lot more time and energy for one of us

to regrow a whole arm, plus the added energy it

takes to keep the pluripotent stem cells in reserve.

So instead of wasting all that time and energy,

we just grow some scar tissue over the wound

and learn to live without an arm.

But our livers are a little bit different.

The liver is our biggest internal organ.

And it helps with digestion, stores nutrients

and immune signals, and filters waste from our blood.

If your liver shut down completely,

we would die in a couple days.

So we've evolved to protect against that.

Turns out that even if you lost 75% of your liver,

the remaining liver cells could grow and divide

and reform a mass of liver tissue.

The sort of regrown liver isn't coming from stem cells, though.

So the structure won't be quite the same as the original.

So this isn't true regeneration like with a lizard's tail.

But the liver will function well enough to keep you alive.

But what if your liver fails so badly

that your body can't fix it?

Couldn't you just cut off a piece

of, say, your sister's healthy liver,

grow a new one in this tissue culture lab,

and then transplant it into you?

The problem is that liver cells don't survive long enough

outside the body to grow into enough tissue to transplant.

Scientists at MIT's Lab for Multiscale Regenerative

Technologies are trying to solve this problem.

How can we mimic the human body's environment

in the lab to allow liver cells to grow

into fully functional livers?

And livers are just the beginning.

For example, other organ cells, like the ones

in your heart and brain, don't divide like liver cells.

Researchers are finding ways to trick them and stem

cells to someday regenerate those organs and other body

parts, too.

So for now, our reptile friends have one-upped us.

But we're catching up.

Watch out, little buddies.

Hi.

This is Sari.

Thanks for watching Science Out Loud.

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Oh, that's fine.

It's fine.

[LAUGHTER]

[INAUDIBLE]

Cut.

Good?

Yep.