Axolotl Armed and Back Again, Thanks to Retinoic Acid
Featured Image Caption: Axolotl and image of limb and skeletal structure being tested for regeneration. Top image by Tiia Monto, CC BY-SA 4.0, via Wikimedia Commons and bottom two images figure 3B from Duerr, T.J., Miller, M., Kumar, S. et al. Retinoic acid breakdown is required for proximodistal positional identity during axolotl limb regeneration. Nat Commun 16, 4798 (2025). https://doi.org/10.1038/s41467-025-59497-5
Primary article:
Duerr, T. J., Miller, M., Kumar, S., Bakr, D., Griffiths, J. R., Gautham, A. K., Douglas, D., Voss, S. R., & Monaghan, J. R. (2025). Retinoic acid breakdown is required for proximodistal positional identity during axolotl limb regeneration. Nature Communications, 16(1), 4798. https://doi.org/10.1038/s41467-025-59497
Secondary article/sources:
Nam, J., Min, B., Baek, A., Lee, S. Y., Ha, J., Cho, M. J., & Kim, J. (2025). Mammalian Blastema: Possibility and Potentials. International journal of stem cells, 18(2), 126–134. https://doi.org/10.15283/ijsc24121
https://www.sciencedirect.com/topics/medicine-and-dentistry/limb-regeneration
When the limb of a salamander grows, how do cells know where to place the elbow, forearm, and fingers?
For centuries, scientists have known that salamanders could regenerate their limbs. However, the precise mechanism for this remains a mystery. The potential for positive applications is limitless in limb regeneration. Wounded soldiers, diabetic patients, burn victims, and people born with no limbs (tetra-amelia) are just a few examples of how limb regeneration could benefit people in need. The benefit doesn’t just extend to humans, though. Injured animals would also benefit.
In a nutshell, genes instruct cells to grow, divide, and become the cell types that make up the limb. Specific molecules act as go-between assistants and messengers to trigger the timing, placement, and transformation of these cells. Sounds simple enough, but how is this all coordinated?
In this particular study by Duerr et al., scientists wanted to find out how an amputated axolotl limb sends signals to remaining cells to grow a clump of cells called the blastema. This blastema acts as the starting point for the new limb, growing specific cells in a specific order to create a brand-new, functioning appendage.
The study discovered that the gene CYP2B1 regulates the breakdown of retinoic acid to facilitate limb regrowth. Retinoic acid is a derivative of vitamin A, a molecule responsible for cell growth, cell differentiation, and gene activity.
The Good and the Bad News: We are not Salamanders
There have been many advances in studying limb regeneration thanks to the axolotl and advancements in science and technology. Scientists now know an exhaustive list of genes that are involved in the interplay of regeneration. In another case example of progress, researchers have discovered a way to the very tips of human fingers.
However, it is not enough to just catalogue the genes and molecules responsible for limb regeneration; there still needs to be a better understanding of the timing and communication taking place between genes and cells.
The scientists in this study wanted to understand how cells coordinate limb patterning, which includes a spatial awareness of limb placement and the body’s knowledge of how to correctly regrow the limb. In order to investigate this phenomenon, investigators amputated the limbs of axolotls at different limb segments. They then put these segments through tests to narrow down the genes and molecules involved in spatial placement and how these components communicate with each other.
The Trick to Cracking a Code: Mess with Genetic Pathways and See What Happens
A common practice in genetic studies is to block a gene, ramp up the gene’s activity, or to delete a gene entirely. In the process, morphological changes are monitored and so are other genes, molecules, or proteins that turn on or off in response to those changes. This study was no different in that regard, with minor adjustments. They first amputated the axolotl’s arm into four segments and cross-referenced any genes that may be active alongside the genes Hox and Meis, genes that were already known to be associated with limb regeneration. Essentially, they wanted to ask: Are other genes active (expressed) alongside these genes in the tissues we are studying? Is there a related pattern?
Insights Discovered from the Study
CYP2B1 increases the breakdown of retinoic acid furthest away from the body. Retinoic acid is broken down in different concentrations along the growing arm axis, which helps cells determine their future cell type(bones, nerves, etc.) Without it, the normal pattern of limb regeneration fails.
They also determined two genes, Shox and Shox2,decreased in activity further down the segment of the arm. Without Shox, limbs that regrew were shorter than normal. This meant that Shox is the gene responsible for maturing bones in the arm.
How does Limb Regeneration Contribute to Environmental Science?
Normally, when wildlife is unable to be rehabilitated, the only options for them are to either be euthanized or taken to a zoo. For example, an adult moose with a serious injury to the limb is usually euthanized by wildlife officials. However, with further research into limb regeneration, terrestrial wildlife like moose could live on after what is presently considered a life-ending injury. Marine animals with missing limbs from boat collisions could benefit as well. Unlocking nature’s recipe for restoring limbs could help undo some of the harm humans have committed against wildlife. Even without wildlife into consideration, domestic animals could benefit from these advancements.
Frustratingly, the dream of growing limbs in the lab isn’t fully realized yet, but each piece of the research puzzle gets society closer in small, but important, incremental steps. Overall, if scientists can get limb regeneration up and running (pun intended), human medicine and animal rehabilitation would have a very bright future.
Can’t get Enough of Axolotls? Check out this Video.
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