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When a kid lops off a fingertip with a cleaver or car door, there's a chance the end of the digit will grow back. The fingerprint will be gone, and the tip may look a bit strange. But the flesh, bone and nail could return.

Now biologists at New York University have figured out just how this lizard-like regeneration happens in mice. There's some secret sauce at the nail cuticle that makes it possible, scientists report Wednesday in the journal Nature.

Doctors have seen the effect in humans without quite understanding how it happens. "Kids will actually regrow a pretty good fingertip, after amputation, if you just leave it alone," says Dr. Christopher Allan, from the University of Washington Medicine Hand Center, who wasn't involved in the research.

The orthopedic surgeon saw this out a few years ago when an 8-year-old girl stuck her finger into the spokes of her brother's bike. The wheel sliced off her middle finger, near the nail cuticle, and her parents rushed to the ER to have it sewn back on.

Allan specializes in hand reconstruction, but he couldn't find the tiny artery he needed to reconnect. So he opted instead for what surgeons call a biological dressing. Just stick the tip back on and hope for the best, he says.

"The girl came back in a few weeks with the old fingertip in a bag and a new one on her hand," Allan tells Shots. "It was far better than anything that I could have given her with a graft or surgery."

Since the 1970s, doctors around the world have reported similar cases in young kids: Chopped off fingertips regrow if the slice occurs before the edge of the nail. Any farther down the digit, and you're probably out of luck.

Scientists see a similar phenomenon with mice paws. But even the elderly rodents can do it, says Mayumi Ito of New York University. "It's totally amazing," she says. "The adult mice totally regenerate the organ to its original form."

But the amputation must leave a little bit of the fingernail — er, claw. And she wanted to figure out why. So she and her team went hunting for the stem cells.

Fingernails are bit like hair: They continue to grow even when we're adults. (Well, at least, we hope they do.) Thus, both body parts need a continual supply of factory-like cells to make the hard, tough structures – nails or strands of hair.

For curly locks, each follicle contains a bunch of stem cells that serve as hair machines. The cells produce all the components of your coiffure — the silky strands, its lustrous color and the little shaft that attaches to your head.

Now Ito and her team have found analogous cell factories in mice fingernails. She calls them "nail stem cells." They sit right near the cuticle. And they do more than give the mouse its scratch.

When a rodent's "fingertip," as Ito calls it, is amputated, the nail stem cells start to regrow the claw. But they also make a signal that brings the bones and nerves to the wound. Healing is a whole new ball game once you get the nerves involved.

"In amphibians, the nerve is sufficient to trigger the whole regeneration process," Ito says. "We don't know yet if that's the case for mammals, but the nerve is essential to produce the bone of the mouse digit."

The signal made by the nail stem cells — called Wnt, for all you bio buffs — can orchestrate growth all over the body. It even coordinates the formation of limbs and some organs during fetal development.

"We think that nail stem cells may a have a special function to induce the whole regeneration process, including nerve attraction and growth of the bone," Ito say.

So could these nail stem cells one day help with more serious amputations? "We'd like to test this hypothesis in mice," Ito says. But she first wants to find these cells in people.

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