and they keep growing back
it is Mexican salamander
https://www.sciencealert.com/axolotl-discovery-brings-us-closer-than-ever-to-regrowing-human-limbs
A team of biologists from Northeastern University and the University of Kentucky has found one of the key molecules involved in axolotl regeneration. It's a crucial component in ensuring the body grows back the right parts in the right spot: for instance, growing a hand, from the wrist. "The cells can interpret this cue to say, 'I'm at the elbow, and then I'm going to grow back the hand' or 'I'm at the shoulder... so I'm going to then enable those cells to grow back the entire limb'," biologist James Monaghan explains.
That molecule, retinoic acid, is arranged through the axolotl body in a gradient, signaling to regenerative cells how far down the limb has been severed. Closer to the shoulder, axolotls have higher levels of retinoic acid, and lower levels of the enzyme that breaks it down. This ratio changes the further the limb extends from the body. The team found this balance between retinoic acid and the enzyme that breaks it down plays a crucial role in 'programming' the cluster of regenerative cells that form at an injury site. When they added surplus retinoic acid to the hand of an axolotl in the process of regenerating, it grew an entire arm instead.
In theory, the human body has the right molecules and cells to do this too, but our cells respond to the signals very differently, instead forming collagen-based scars at injury sites. Next, Monaghan is keen to find out what's going on inside cells -- the axolotl's, and our own -- when those retinoic acid signals are received.
