Why Can't Humans Regenerate Body Parts?

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A common hermit crab is shown covered in snail fur (Hydractina echinata).
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The idea that some humble life forms on our planet -- jellyfish, corals and the like -- are actually immortal seems to be as compelling as that vintage 1969 "Star Trek" episode about the 5,000-year-old man, born in ancient Mesopotamia, who was still around to meet Captain Kirk and his crew because he was unable to die.

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It’s surfaced again in a recent Irish Times article that’s creating a buzz on Reddit and elsewhere across the web, about research at the National University of Ireland-Galway’s Regenerative Medicine Institute on Hydractinia echinata, a.k.a. the snail fur.

The snail fur is a pinkish mass of spines, tentacles and polyps just 20 to 30 millimeters in length, which makes it small enough to attach itself to the shells of hermit crabs along the Irish and British coast. The snail fur would seem unremarkable, except for one quality: According to Uri Frank, a scientist at the institute, the creature “in theory -- lives forever.”

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Immortality, of course, is a concept that’s largely in the eye of the beholder, as we learned from the brouhaha that erupted after a New York Times Magazine article trumpeted a Japanese scientist’s assertion that Turritopsis dohrnii, a species of jellyfish, also lived forever.

In an email, Frank explains Hydractinia can indeed die but -- like many other clonal invertebrates -- it does not seem to suffer from age-related deterioration. What Frank’s research really focuses upon is Hydractinia’s ability to fully regenerate lost body parts -- or even an entire new body from a tissue fragment of itself. If a Hycractinia’s head is bitten off, it can simply grow another one in a few days.

Hydractinia echinata -- and other clonal invertebrates who reproduce asexually -- possess extreme regenerative abilities, but they’re not the only creatures on the planet who can grow back missing body parts. Earthworms, starfish, lobsters, snails, salamanders and scores of other creatures can produce their own replacement organs and/or limbs as well. Some mammals can regenerate themselves to a lesser degree as well; two species of African spiny mice, for example, have the ability to regrow lost sweat glands, fur, and cartilage.

And that leads to a question that has long puzzled scientists. So if a zebra fish can grow a new tail, why can’t we regenerate a new arm or leg -- or a kidney or heart -- whenever we need a new one?

“Nobody really knows the answer,” says David M. Gardiner, a professor of developmental and cell biology at the University of California-Irvine, who is a principal investigator in the UCI Limb Regeneration research program. “Regeneration is a fundamental, basic, biological property, just like reproduction.”

As Gardiner explains, humans actually do have regenerative abilities. Our bodies continually rebuild themselves at the cellular level, and have an impressive capability to fix damage and heal wounds. We can’t grow back a lost limb, but as a 2013 article in Nature documented, children sometimes are able to grow back fingertips that have been accidentally amputated. And an adult human can regenerate a portion of his or her liver, if that organ is damaged.

“If we didn’t have the ability to repair ourselves, we couldn’t survive,” Gardiner notes. “But if we can regenerate in pieces, then why can’t we make organs?”

What’s frustrating is that we all had that ability when we were in the womb. Humans are built, piece by piece, by embryonic stem cells, which are highly pluripotent -- that is, able to divide and differentiate into various other sorts of cells, from nerve cells to muscle cells to blood cells.

Creatures that regenerate limbs and organs have stem cells that keep this ability throughout their life cycles. If a salamander’s leg is cut off, for example, its stem cells rush into action and form a fast-growing mass of undifferentiated cells called a regeneration blastema, which eventually will differentiate and form the various structures of a new limb.

But like most mammals, by the time we’re born, those pluripotent cells are replaced by somatic -- adult stem cells, which can maintain and to a limited degree repair the part of the body in which they’re found. Adult stem cells in bone marrow, for example, can make new blood cells, and adult stem cells in the skin can help to replenish its layers, or grow scar tissue on an arm to seal off a wound.