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Why Can We Regrow a Liver, but Not a Limb?

5 minutes

(Describer) An animated ball becomes an amoeba and a rocket. Title:

(singers) ♪ Science ♪

♪ Out Loud ♪♪

(Describer) MIT K-12 Videos.

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 awesome if we could regrow limbs, but we can't. The closest we can get is regenerating our livers.

(Describer) Another lizard stands on a stem while a third stands on soil.

Let's back up. Our bodies are made of organs, which are made of tissues made of many cells, from nerve cells to bone cells to skin cells. Our cells are constantly dying, so we need to be able to make more of them. So we have stem cells, which can differentiate into, say, a liver cell or a blood cell. When we're embryos, our stem cells are super-powered. They're pluripotent and can become any sort of cell our body might need and help grow everything from our stomach lining to our skin. As adults, our stem cells lose this superpower. Adult stem cells can't become 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 must become intestinal cells. Neither of these will become a liver cell or a nerve cell.

(Describer) A sign says “Jabberwock Reptiles”. Snakes and lizards are inside different enclosures.

Lizards never lose their stem cell superpowers. When a lizard's tail falls off, 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 cell the lizard might need to regenerate a tail. Why can't humans regrow an arm? Scientists think it's an evolutionary trade-off. Lizards have small bodies and need a couple weeks to regrow the tail. It would take more time and energy for a human to regrow an arm, plus the added energy needed to keep pluripotent stem cells in reserve. Instead, we grow scar tissue over the wound and learn to live without an arm.

(Describer) She holds a lizard that flicks its tongue.

But our livers are a bit different. The liver is our biggest internal organ. It helps with digestion, stores nutrients and immune signals, and filters blood waste. If your liver shut down completely, you'd die quickly. 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 "regrown "liver isn't coming from stem cells, so the structure won't match the original. This isn't true regeneration, like with the lizard's tail, but the liver will function and keep you alive.

(Describer) In a lab...

What if your body can't fix your liver? Couldn't you take part of your sister's healthy liver, grow one in this tissue culture lab and transplant it into you? Unfortunately, liver cells don't survive long enough outside the body to grow into enough tissue to transplant. Scientists at M.I.T.'s Lab for Multiscale Regenerative Technologies are mimicking the human body's environment in the lab to allow liver cells to grow into functional livers. And livers are just the beginning. For example, organ cells 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.

(Describer) Holding lizards...

For now, our reptile friends have one-upped us, but we're catching up. Watch out, little buddies. Accessibility provided by the U.S. Department of Education.

(Describer) Title: Made with love at MIT.

I'm Sari. Thanks for watching Science Out Loud. If you liked this video, check out other ones. For more information, look at our website. Visit our website.

[crew chattering, laughing]

(Describer) Title: k12videos.mit.edu. Accessibility provided by the U.S. Department of Education.

[laughs]

(director) Cut. Good? Yep.

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Unlike lizards, humans cannot regrow limbs. But humans can regenerate their livers. A MIT student explains how and why. Part of the "Science Out Loud" series.

Media Details

Runtime: 5 minutes

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