Savvy salamanders
The salamander is the only vertebrate species able to completely re-create a limb as an adult. It takes 40 days for the new limb to form, and three more months for it to grow to the same size and shape of its predecessor. Regeneration depends on cells in the stump of a lost limb de-differentiating to return to the “blank-slate” state, and becoming blastema cells.
In higher vertebrates, including mammals, damaged muscles can regenerate using a reserve of satellite cells-similar to stem cells, but found in mature muscle-that create imperfect replicas of the tissue. Healing wounds produce scar tissue in humans, which some believe is a byproduct of healing quickly to avoid infection. Regenerated salamander tissue, on the other hand, is methodic and scar-free, even in cases of heart injury.
Picking apart Planaria
One of the classical examples of regeneration is found in the Planaria genus, a group of primitive freshwater flatworms that have been the subject of close to a century of scientific research. Experiments have shown that single planarians can be cut into as many as 279 pieces, each of which re-grows into an entire, healthy organism. Unlike salamanders, planarians form regenerative blastema cells using adult stem cells that are distributed throughout its body. Tapping stem cells for planarian regeneration is akin to returning to the embryonic path all animals follow in early development. Humans have, perhaps, lost the ability to use this technique during adulthood because of a high risk of the cells turning cancerous, though adult stem cells do maintain our blood, skin, and bones. Ongoing research on planarians-such as sequencing their genome-may reveal how stem cells work during embryonic development, tissue maintenance and regeneration, and suggest how similar processes might function in humans.
Letting loose
The evidence for regeneration in other animals is widespread: deer antlers, turtle shells, crocodile jaws, bat wings, and even snail penises can grow back after damage or a complete loss. Deer antlers, one of the fastest-growing animal tissues known, re-grow to maximum size in three to four months each spring. New evidence from Professor Joanna Price of the Royal Veterinary College suggests that stem cells may be responsible for this phenomenon.
Insects and crustaceans are well-known for their ability to shed their exoskeletons and reveal entirely regenerated structures beneath. Should a moult go badly and body parts get stuck in the dead exoskeleton, they can be left there and regenerated later. Likewise, lizards, crabs, and spiders are known for their ability to drop a limb in anticipation of a predator’s attack. Living life, even for a short while, without a tail or leg comes with its own costs, though-increased predation, decreased overall growth, and the inability to compete for mates or defend territory are just some of the problems to be faced.
Only human, unfortunately
No more than a few isolated instances exist of human tissue regeneration. Our internal organs, like the liver or the lining of the intestine, replenish their cells regularly, but the mechanisms employed in these cases differ greatly from those required for the complete regeneration of a limb. Several extreme cases of fingertip re-growth have been documented in children under eleven years of age where bone, skin and cartilage were regenerated perfectly. But while comic-book super-healing would be a desirable reality for many, we first need to solve the mysteries of regeneration in both humans and animals. Regeneration research could be instrumental in treating diseases like Parkinson’s and multiple sclerosis.
-Sandy Huen
