Now, it has surfaced once again in 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."
Immortality, though, 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. It turns out, though, that what Frank actually is talking about is the snail fur's ability to fully regenerate lost body parts.
"It sounds gruesome, but if it has its head bitten off, it simply grows another one within a few days," the Irish scientist explained in a recent Irish Times article about the research.
That's a little less like the immortal guy on "Star Trek" and a little more like claiming that your 1985 Pontiac Fiero will last forever, as long as you gradually replace all of its parts. But even so, being able to grow your own replacement parts whenever you needed one would a pretty nifty trick.
The snail fur is far from the only creature on the planet to have this ability. Earthworms, starfish, lobsters, snails, salamanders and scores of other creatures can produce their own replacement organs and/or limbs as well. A few 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.
So if a zebra fish can grow a new tail, why can't we regenerate an arm or leg -- or a kidney or heart -- whenever we need a new one?
For the answer, we have to look at how we grow our bodies in the first place. 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--AKA 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. They also can grow scar tissue to seal off a wound. Humans do have some limited regenerative ability as well. 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.
Why we and other mammals lose most of our ability to grow new limbs or organs remains unclear. Some researchers believe that it's a natural consequence of our greater complexity as organisms.
It may be, for example, that the genetic mechanisms in our bodies that try to keep cancers from developing also would prevent a blastema from forming. Enrique Amaya, a developmental biologist at the University of Manchester in Great Britain, recently offered a different possible explanation.
Read more at Discovery News
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