The Axolotl Regrows Entire Limbs, Spinal Cord Included

The axolotl (Ambystoma mexicanum) is the only vertebrate known to maintain full limb regeneration into adulthood. Salamanders in general have more regenerative capacity than mammals, but the axolotl goes furthest: sever a limb, and over roughly 40 to 60 days, a complete replacement grows back — bones, cartilage, muscle, skin, and functioning nerve connections included. Damage to the spinal cord heals similarly, something that does not happen in any mammal.

The mechanism relies on dedifferentiation. When tissue is lost, the mature cells at the wound site partially revert to an earlier, more plastic state, forming a structure called a blastema. The blastema cells proliferate and then redifferentiate into whatever cell types the missing structure needs. They don't become fully pluripotent stem cells — they retain a kind of positional memory and generally only give rise to the appropriate tissue type — but the degree of reversion is extraordinary by mammalian standards.

In 2018, a team at Harvard and the Medical University of Vienna sequenced the axolotl genome — at 32 billion base pairs, it's ten times larger than the human genome — and identified several gene families that appear to be axolotl-specific and active during regeneration. One, called PROD1, controls how cells read their position along the limb axis.

In the wild, the axolotl lives in exactly one place: the canal system around Lake Xochimilco in Mexico City. Urbanization, water pollution, and invasive tilapia have reduced the wild population to somewhere between 50 and a few hundred individuals. Most axolotls alive today are lab or pet specimens descended from 34 animals exported to Paris in 1864.