A salamander grows the limb back, bones and all

An axolotl that loses a leg regrows the leg: skeleton, muscle, nerves, and skin, in the right pattern, without a scar. The cells at the stump remember what tissue they were.

After amputation, the wound surface is covered within hours by a layer of cells, and under it a blastema forms — a mound of proliferating cells that will become the missing parts. The regenerate is not a blob of generic flesh. A cut at the wrist builds a hand. A cut at the shoulder builds a shoulder, an arm, and a hand. Nerves have to be present for the bud to grow. The pattern, including the joints, comes back. Scarring, which in a mammal locks the wound into a patch of collagen, is the road not taken.

A 2009 lineage-tracing study found that the cells in the blastema are not blank. Muscle cells make muscle, skin cells make skin, and cartilage stays with the skeleton. They dedifferentiate enough to divide and to travel, and not enough to forget their trade. That killed the tidy story in which the stump returns to a fully embryonic state. What remains is stranger in practice: an adult limb, cut, replaced by residents of the stump who still know their jobs and collectively know where the cut was. A human fingertip in a small child can close over. A human limb cannot. The difference is not a moral. It is a mechanism we do not have.