What an axolotl can regrow
The axolotl (Ambystoma mexicanum) is a salamander that keeps its larval body for life. It is also one of the best regenerators among animals with a backbone. It regrows a lost limb, its tail, parts of its spinal cord, and parts of its heart, and it does this without a scar.
The brain is on that list too. The part that scientists have studied most is the telencephalon, the front section of the brain. In people, the matching region includes the cortex.
How the regrowth happens
The work is done by ependymoglial cells. These cells line the fluid-filled spaces inside the brain. In an adult axolotl they stay able to divide, and they act as the brain’s stem cells.
A 2022 study in Science (Lust and others) followed the process cell by cell. It found three stages:
- The cells wake up. After the injury, ependymoglial cells near the wound switch into a state that is seen only after injury, and they multiply.
- They make young neurons. The new cells become neuroblasts, the stage before a neuron.
- The young neurons mature. They become the same types of neuron that were lost.
A second study in the same issue (Wei and others) mapped where each cell type sits in the brain during regrowth. It also found progenitor cells that appear in response to injury. The two teams used different methods and reached a similar picture.
What the studies found
| Year | Study | What it found |
|---|---|---|
| 1916 | Burr, Journal of Comparative Neurology | The first report: young salamanders of the genus Ambystoma regrew a removed forebrain. |
| 2013 | Maden and others, Neural Development | Mapped the zones of the axolotl brain where cells still divide, and showed that a removed piece of the telencephalon grows back. |
| 2016 | Amamoto and others, eLife | Adult axolotls rebuilt the original mix of neuron types. But the tissue was not arranged as before, and long-distance connections were not fully rebuilt. |
| 2022 | Lust and others, Science | Followed regrowth cell by cell over about 12 weeks. Lost neuron types came back, and the study reported that connections with other brain regions were re-established. |
| 2022 | Wei and others, Science | Mapped cell types in place during regrowth and found progenitor cells that appear after injury. |
What is still not known
The regrowth is real, but “good as new” is not proven. The 2016 study found that the new tissue had the right cell types in the wrong arrangement. The 2022 study found more recovery than that. The two results are not yet fully reconciled.
Nobody has shown what happens to what the animal knew. A regrown region has new neurons. Whether it carries the old memories, or works exactly as the old one did, has not been tested well.
The reason an axolotl can do this, and a mouse or a person cannot, is also open. The axolotl genome was published in 2018 (Nowoshilow and others, Nature). It is about 32 billion base pairs, roughly ten times the size of the human genome, and researchers now use it to look for the genes behind regeneration.
Can a human brain do this?
No. An adult human brain makes very few new neurons, and it closes an injury with a scar. It does not rebuild the lost tissue.
What a human brain can do is change the connections it already has. That is how learning works. Each time you pull a fact out of memory, the memory gets stronger, and a review at the right moment slows how fast it fades. This is the part of “regrow your brain” that is literally available to you.
It is also why the mascot of this app is an axolotl. The animal rebuilds what it lost. Gillo, the app, helps you rebuild a habit of attention and memory, five minutes at a time, with questions and spaced review.
Where axolotls live
In the wild, the axolotl lives in one place: the canals of Lake Xochimilco in Mexico City. It is critically endangered there. Almost every axolotl alive today lives in a laboratory, an aquarium or a home. The pink ones are a captive-bred color. Wild axolotls are dark and mottled.