In a loudly bubbling laboratory at Brigham and Women’s Hospital in Boston, about 2,800 of the salamanders called axolotls drift in tanks and cups, filling floor-to-ceiling shelves. Up close, axolotls are just on the cute side of alien. They have fleshy pink bodies and guileless, wall-eyed faces. Unlike most salamanders, which metamorphose into land-dwellers as they grow up, axolotls usually keep their youthful aquatic form for their whole lives. They wear their gills on the outside, a set of three feathery horns on each side of the head. Their four-fingered hands with black nails are delicate and vaguely human — but perhaps it’s best not to dwell on that, given the work that goes on here.
One of the animals in view is missing a limb that was amputated 11 days earlier. The stump has a reddish bull’s-eye visible at its center. It’s the bud of a new arm.
Salamanders are champions at regenerating lost body parts. A flatworm called a planarian can grow back its entire body from a speck of tissue, but it is a very small, simple creature. Zebra fish can regrow their tails throughout their lives. Humans, along with other mammals, can regenerate lost limb buds as embryos. As young children, we can regrow our fingertips; mice can still do this as adults. But salamanders stand out as the only vertebrates that can replace complex body parts that are lost at any age, which is why researchers seeking answers about regeneration have so often turned to them.
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