Cut off the leg of most animals and the wound simply closes over with scar tissue. Cut off the leg of an axolotl and, within weeks, a new one grows back with bones, muscle, blood vessels, and skin arranged exactly as they were before, and researchers studying that process think it could eventually reshape how doctors think about healing damaged tissue in people.
A Salamander That Never Grows Up
The axolotl, a Mexican salamander formally named Ambystoma mexicanum, keeps its larval body plan for its entire life instead of transforming into a land-dwelling adult the way most salamanders do, a trait called neoteny. That permanently juvenile state, along with an unusually large genome that has made it a favorite subject for lab study, sits alongside its best-known trait: axolotls can regenerate not just limbs but also portions of the heart, spinal cord, and brain. In the wild, the species survives only in the canals of Lake Xochimilco outside Mexico City, and the International Union for Conservation of Nature classified it as critically endangered in 2006 because of pollution, habitat loss, and invasive fish that prey on its young, even as captive populations remain common in research labs and the pet trade.
The Blastema: A Temporary Construction Site
When an axolotl loses a limb, skin cells rapidly migrate to seal the wound, and mature cells beneath that skin, muscle, bone, and connective tissue among them, begin to lose their specialized identity and revert to a more flexible state. Those cells cluster into a structure called a blastema, a mass of proliferating cells that carries the instructions to rebuild everything the limb lost. The first phase of regeneration produces a new limb with the correct joints and proportions relative to itself, but at a fraction of full size, so a second growth phase has to expand that miniature limb until it matches the rest of the animal’s body.
Why the Limb Cannot Regrow Without Its Nerves
Decades of experiments have shown that regeneration in axolotls depends on an intact nerve supply at the amputation site. If researchers strip the nerves from a freshly amputated stump, the wound simply scars over and no blastema ever forms, and a minimum threshold of nerve fibers has to reach the wound surface before regrowth can proceed at all. A study of neural control published in the journal eLife found that varying how many nerves connect to the new limb changes its ultimate size, with a denser nerve supply producing a larger regenerate and a sparser one producing a smaller one, which indicates that the nervous system does not just permit regeneration but actively sets its scale.
Researchers eventually traced part of that nerve dependence to a specific protein called Neuregulin-1, which neurons at the wound site secrete directly into the forming blastema. A study in the journal Development showed that implanting beads soaked in Neuregulin-1 into a denervated limb could rescue regeneration up through the digits, essentially standing in for the missing nerves, while blocking the protein’s signaling in a normally innervated limb reduced cell proliferation, prevented the blastema from forming properly, and caused disorganized collagen buildup instead of a clean regrowth.
Other Signals That Guide the Rebuild
Neuregulin-1 is not the only nerve-derived cue at work. Researchers have also identified a protein called anterior gradient, along with several growth factors known as BMPs and FGFs, as additional nerve-dependent signals that help sustain the blastema once it forms, indicating that regeneration relies on a layered set of chemical instructions rather than a single trigger. Separately, adrenergic signaling, the same family of pathways activated by stress hormones, has been shown to help kick off the earliest response to amputation, promoting the cell growth and proliferation needed to build the blastema in the first place. Together these findings describe a process where nerves, growth factors, and hormonal signals all have to align in sequence for a new limb to take proper shape.
What the Research Could Mean Beyond Salamanders
Humans and axolotls share most of the same genes involved in limb development, which is part of why researchers keep returning to this animal rather than treating its regenerative ability as a biological curiosity unique to amphibians. The working hypothesis driving much of this research is that mammals, including people, may retain a dormant version of this regenerative machinery that simply fails to activate the way it does in salamanders, whether because of how mammalian wounds heal, immune responses that favor scarring, or a lack of the right nerve signals reaching the injury site in time. Scientists have used a technique called the Accessory Limb Model, which surgically creates the conditions for a blastema to form on an intact limb, as a way to isolate exactly which signals are necessary and sufficient for regeneration to begin. Each new nerve-derived factor identified through that kind of work adds a piece to a molecular map that researchers hope will eventually clarify why axolotls can rebuild a limb from scratch while most other vertebrates cannot.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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