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06/14/2013

Why your chopped-off fingertip can sometimes grow back

KO mice histology
Top: a mouse digit that regenerated after amputation. Bottom: a mouse digit that failed to regenerate after amputation because it lacked the "Wnt signalling network" described in a new paper published by Nature. Photo credit: Ito Lab, NYU Langone Medical Center

When you cut off your hair, it grows back. Ditto with fingernails. But what about fingertips?

As it turns out, sliced-off fingertips can sometimes grow back on children, as long as enough of the fingernail is left in tact. On NPR's Shots blog, orthopedic surgeon Dr. Christopher Allan recalled an eight-year-old former patient who lost a chunk of her middle fingertip to her brother's bicycle wheel but in just a few weeks, a new one had already grown back and "it was far better than anything that I could have given her with a graft or surgery," Allan told NPR reporter Michaeleen Doucleff.

Mice, scientists have seen, can also regenerate lost "fingertips." But what's behind this phenomenon? Researchers at NYU Langone Medical Center decided to find out and on Wednesday, the results of their study were published in the journal Nature.

In a press release, lead author Mayumi Ito reported that her team discovered a population of stem cells in the nail beds of lab mice that were "rich in nerve endings and blood vessels that stimulate nail growth."  But these cells also seem to drive nearby bone and tissue regeneration thanks to a family of proteins called the "Wnt signaling network" — proteins also responsible for hair and tissue regeneration. The team conducted tests where they blocked this signaling pathway in mice with amputated digits and sure enough, the tips did not grow back.

While this self-healing process pales in comparison to what occurs in wounded amphibians — which can regenerate limbs even after the most dramatic of amputations — there are some parallel mechanisms, Ito told Nature News.

“I was amazed by the similarities,” Ito said in an interview with science writer Ed Yong. “It suggests that we partly retain the regeneration mechanisms that operate in amphibians.”

Ito told NPR's Shots that she will now look for the same stem cells in humans — the hope is that these findings could pave the way towards future therapies that help people regenerate lost limbs.

But humans probably won't be healing salamander-style anytime soon. As regeneration biologist Ashley Seifert said to Yong at Nature News, perhaps mammals' ability to regrow fingertips evolved independently of amphibians, which completely lack a nail organ, and so it's possible our regenerative powers depend on the presence of a nail. Seifert also pointed out that activating the Wnt pathway failed to trigger any fingertip regeneration when an entire nail was missing.

Jennifer Yang is the Star’s global health reporter. She previously worked as a general assignment reporter and won a NNA in 2011 for her explanatory piece on the Chilean mining disaster. Follow her on Twitter: @jyangstar

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