Return of the Lost Limbs

When people lose legs after accidents or illnesses, emergency care and artificial limbs often allow them to walk again. But salamanders and newts in the same situation don’t need doctors or artificial body parts. They can grow limbs back on their own.

Scientists have known for a long time that certain animals can regenerate limbs, but they haven’t quite figured out how these creatures do it.

Newts have the ability to grow back limbs that have been cut off. Even more impressive, the limb seems to “know” whether to grow just a foot (left) or an entire leg (right).

Science

Researchers from University College London have now come up with some new insights. Their work may lead to breakthroughs that could eventually enable people, too, to regrow lost limbs.

The researchers started with two simple observations: When you cut off a newt’s leg at the ankle, only the foot grows back. If you cut off a leg at the base, the whole leg grows back.

In both cases, the regrowth begins with stem cells (see From Stem Cell to Any Cell). Stem cells are unspecialized cells that can develop into nearly any type of cell in the body.

But how do a newt’s stem cells know when to regrow only a foot and when to regrow an entire leg?

This question relates to another mystery: In newts, a severed leg will grow back only if the bundle of nerves in it also grows back. But if something prevents the nerve bundle from growing, the stem cells at the site of the wound won’t multiply to produce a new leg.

In its study, the British team zeroed in on a protein called nAG. When the team prevented nerves in a limb from growing, but added the nAG protein to stem cells in the limb, the limb still regrew.

The scientists suspect that nerves in the stub of a limb signal the release of the nAG protein. That protein seems to guide limb regrowth.

People and other mammals have proteins that are similar to nAG. Further research into these compounds may some day help human limbs and organs heal themselves.