Why Can't Humans Regenerate Body Parts?

Snail fur can regrow their parts -- so why can't we?

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.

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.

A common hermit crab is shown covered in snail fur ( | Getty Images

But humans can't make an entire new arm. "There must be something in humans that prevents the regenerative process from going that far," Gardiner says.

Some scientists think it may be an evolutionary tradeoff.

"If an amphibian chews off one of its arms, it could hide away for weeks without eating and regenerate," Enrique Amaya, a developmental biologist at the University of Manchester in Great Britain, recently told BBC News. "That's out of the question for an animal whose high metabolic rate requires it to feed constantly. It has to heal quickly and dirtily."

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NUI-Galway's Frank, whose research examines heat-shock proteins and Wnt signaling, which are found in both stem cells and tumors, thinks that humans and other mammals may lose their embryo-like regenerative ability because it would make them more vulnerable to developing cancers.

"Because these (embryonic-like stem) cells are so versatile, it is difficult to keep them under control," Frank explains. "They are more likely to 'misbehave' or form tumors than differentiated cells. We hypothesize that only animals that have very simple body plans, like Hydractinia, can manage this problem because they have less complex organs and 'misbehaving' cells are less of a problem. But complex animals, like humans, need better control of their cells to maintain their highly complex organs. They have to get rid of them during early development before they become too complex."

Gardiner, however, suspects that humans may still have the latent capability to grow limbs and organs -- and he thinks that scientists eventually may come up with a way to turn certain switches on or off to reactivate their regenerative capabilities, without unleashing the cancer process. He says that recent advances, such as researchers' discovery in 2007 of how to convert differentiated cells back into induced pluripotent stem cells, have eliminated what many once thought were insurmountable barriers to regeneration.

As Gardiner explains, growing new human limbs or organs may be a matter of providing a different set of genetic instructions to our cells -- essentially, a blueprint that would show guide them in creating various types of differentiated cells and organizing them into a structure.

"When you look at blastemas (at the cellular level), they look just like tumors -- except that they stop and differentiate, and form an arm," Gardiner says. The difference, he explains, " is the information that's controlling the growth and patterns. "

Some skeptics have argued that growing a new human arm would be such a time-consuming process that it would be impractical. Gardiner, however, disagrees.

"Salamander arms are just as complex as human arms," he notes. "What's important is that you need structure to regenerate. The fibroblasts (a type of cell that forms the framework for tissue) make the blueprint in the salamander ... I think that over the long term we'll be able to regenerate something like a salamander can, but to do that, we'll need to figure out the information grid."

Skeptics also have argued that growing an adult human arm or another organ might take so many years that it would be impractical. Gardiner, again, disagrees. "How long does it take to make a baby's arm to grow? Probably, a couple of months. What would happen would be that you'd grow back a baby-sized arm-there seems to be a limit on how big you can regenerate."

Afterward, however, the arm might be programmed at the cellular level to grow to adult size more rapidly than the rest of your body parts did. "A salamander only regenerates a small arm, too, but it grows back faster than the rest of the animal grows, so that it catches up."

It's difficult to say how soon scientists might be able to unlock and program the human regenerative process, Gardiner says, because no one yet knows how many stages are involved. "There might be only two or three steps, in which case it might take us 10 years," he speculates. "But if there turn out to be a lot more steps, it might be 50 years." But he expects that someday it will happen.