When a Limb Is Lost, the Whole Body Awakens: A New Secret of Salamander Regeneration

In the field of regenerative biology, the Mexican axolotl (Ambystoma mexicanum) has long stood as one of the most fascinating organisms. These salamanders possess the remarkable ability to regenerate an entire lost limb. That ability alone has captivated scientists for decades. Yet recent research has uncovered an even deeper layer of this phenomenon: when an axolotl loses a limb, cells throughout the rest of its body also become activated, entering a temporary state that prepares the animal for regeneration.

Mexican axolotl（Image source：Amandasofiarana， CC BY-SA 4.0 ）

This response is not merely a side effect limited to the injured tissue. Instead, it represents a coordinated, body-wide physiological program. Following amputation, many cells in distant tissues re-enter the cell cycle for a short period. During this window, the formation of the regeneration bud—known as the blastema—occurs more rapidly, the differentiation process begins earlier, and key regeneration-associated genes such as Prrx1 are expressed more strongly.

Importantly, this activation is temporary. After several weeks, the heightened cellular activity gradually subsides and tissues return to their usual state. However, during the brief period when this activation persists, the axolotl gains a remarkable advantage: if another limb is injured during this time, regeneration proceeds significantly faster than usual.

This phenomenon appears closely connected to the ecological realities salamanders face in the wild. Many species of salamanders, including the axolotl's close relative the tiger salamander (Ambystoma tigrinum), frequently suffer repeated limb injuries due to predation or aggressive encounters with other individuals. Field observations have documented juvenile tiger salamanders regenerating multiple limbs simultaneously, with each limb at a different stage of regrowth. Such patterns suggest that injuries often occur in rapid succession.

Under these circumstances, the ability to prepare the entire body for potential future injuries would provide a clear survival advantage. Once the first injury occurs, other limbs are already primed for faster regeneration should they also be lost.

At the cellular level, most of the cells activated during this body-wide response are not unusual or newly created cell types. Instead, they are stem cells or progenitor cells that already exist within normal tissues and typically maintain routine tissue renewal. These include muscle satellite cells and various fibroblast populations. Rather than forcing cells into an abnormal state, the systemic response appears to temporarily boost the activity of an existing repair system that normally operates at a lower level.

Growth process of the blastema（Image source：McCusker C et al. (2015)， CC BY 4.0 ）

The nervous system plays a central role in coordinating this process. Experiments show that if the nerves supplying an uninjured limb are severed, that limb no longer participates in the systemic activation response—even when another limb is amputated. Multiple tissue types, including the epidermis, skeletal structures, and internal soft tissues, require intact peripheral nerves in order to activate.

Further investigation reveals that the sympathetic branch of the nervous system is especially important. This system releases the neurotransmitter norepinephrine, which acts as a signaling molecule that connects the site of injury with distant tissues throughout the body. When sympathetic nerve activity is chemically blocked, systemic activation fails to occur, and regeneration at the amputated limb itself is also severely impaired. The blastema forms more slowly, cell division decreases, and overall regenerative capacity declines.

Norepinephrine exerts its effects by binding to adrenergic receptors on target cells. In the case of distant tissues, a receptor known as the α2A adrenergic receptor acts as a key sensor of these signals. When norepinephrine activates this receptor, cells such as satellite cells and fibroblasts accelerate their entry into a regenerative program.

Interestingly, a different set of receptors—β-adrenergic receptors—plays a separate role. These receptors are not essential for the body-wide activation response itself. Instead, they operate locally at the site of injury, where they help drive cell proliferation and the formation of the blastema. Blocking β-adrenergic signaling does not prevent distant tissues from becoming activated, but it does reduce cell division within the regenerating limb.

Example of the mTOR pathway（Image source：Marafie SK et al. (2024)， CC BY 4.0 ）

Both signaling pathways ultimately converge on a central cellular regulator known as the mTOR pathway, a molecular system that governs cell growth, metabolism, and proliferation. When mTOR activity is experimentally inhibited, both systemic activation and limb regeneration slow dramatically. This indicates that mTOR acts as a downstream hub integrating signals from the nervous system and directing cells toward regenerative behavior.

Within the cells activated by this pathway, researchers have also observed increased activity of transcription factors associated with epithelial-mesenchymal transition (EMT). These factors—including Twist, Egr1, Etv1, Smad3, and AP-1—are widely known for their roles in wound healing and tissue remodeling. Their presence suggests that systemic activation places cells into a "ready state," primed for the structural changes required during regeneration. However, the full EMT process itself remains localized to the actual site of injury.

For salamanders that grow up in dense larval populations where biting and cannibalism are common, such a strategy makes evolutionary sense. A first injury triggers a body-wide alert system, ensuring that additional wounds can be repaired more rapidly if they occur soon afterward.

From the perspective of other animals—including humans—this discovery offers an intriguing insight. Our limitation may not lie solely in the absence of regenerative cells. Instead, we may also lack the systemic signals that switch those cells into a regeneration-ready state. In the axolotl, the sympathetic-nerve–norepinephrine–mTOR pathway functions as that global switch, awakening cells throughout the body to prepare for rebuilding lost tissues.

In other words, regeneration is not merely the work of cells at the wound itself. Across the entire organism, tissues quietly prepare for the possibility that they too may soon need to rebuild.

Author: Shui-Ye You

References:

1. Payzin-Dogru D et al. (2025). Adrenergic signaling coordinates distant and local responses to amputation in axolotl. Cell.

2. Marafie SK et al. (2024). mTOR: Its Critical Role in Metabolic Diseases, Cancer, and the Aging Process. Int J Mol Sci.

3. McCusker C et al. (2015). The axolotl limb blastema: cellular and molecular mechanisms driving blastema formation and limb regeneration in tetrapods. Regeneration.

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