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Planaria: A window on regeneration
The water-dwelling planaria can lose its head and regrow a new one. Watch and listen as planaria researcher Alejandro Sanchez Alvarado explains how a little flatworm’s abilities might teach us about our own regenerative potential.

We humans prefer to avoid getting our limbs cut off. Once your arm is gone, that’s it. But life’s different for a salamander. Lose a leg? Grow a new one. The same thing happens for starfish, lobsters, and a surprising number of other animals. Snails can even regrow their heads—imagine what the world would be like if humans could do that.

But we can’t. Nor can we grow new limbs or even fingers. That’s why some scientists are studying animals that can regrow body parts, that is, regenerate. Regeneration is fueled by stem cells, cells with the ability to become other types of cells. Scientists hope that learning more about stem cells in other organisms will help us make regeneration possible in humans.

Regeneration in nature

Humans aren’t completely without regenerative talents. We heal from wounds and surgeries and we’re always creating new skin, new blood, and new linings for our stomachs, intestines and lungs. To a certain extent, our livers can even regenerate after they’ve been damaged. Amazingly, liver donors can offer up half of their organ and regrow the removed portion. (People have known about these qualities since ancient times. The myth of the Greek god Prometheus says that his liver regenerated every day after eagles ate it as part of a punishment by Zeus.)

Statue of Prometheus
Humans’ ability to regrow their livers has been known since ancient times. This statue shows the Greek god Prometheus who, as myth tells it, regenerated a new liver after an eagle ate part of it each day. (Photo: Jastrow, Wikimedia Commons)
Many different kinds of animals show some form of regeneration, though most of them are limited to the sort a lizard is capable of, like regrowing a lost tail. A cockroach can regrow a new limb, for example, but the limb itself (thankfully) can’t generate a new cockroach. Scientists call this unidirectional regeneration. A few animals with relatively simple body plans, however—hydras, sea stars, and anemones among them—demonstrate bidirectional regeneration; in other words, they can go both ways. Cut a hydra in half, and you’ll get two hydra. Cut it into four pieces, and you’ll get four. But few animals can equal the regenerative magic of the common, pond-dwelling planarian.

Next: Meet the almost-immortal planarian »