Axolotls possess an impressive capacity to robustly regenerate, and can regrow lost limbs following injury. This occurs irrespective of the overall body size, which can vary up to five-fold between juvenile and adult stages. During regeneration, two signalling molecules known as morphogens are produced at opposite ends of the limb bud (or blastema), called Sonic Hedgehog (Shh) and Fibroblast Growth Factor 8 (FGF8). Yet, how the two morphogens ensure size-adaptive growth of the axolotl limb is unknown. Through novel findings published in PNAS, researchers from the Friedrich group at the Cluster of Excellence Physics of Life (PoL) and the Sandoval-Guzmán group at the Center for Regenerative Therapies (CRTD) propose a new model for robust morphogen scaling to promote limb growth.
In the axolotl and other vertebrates, tissue growth is induced by spatial overlap of opposing SHH and FGF8 morphogen gradients, which reinforce each other. As the tissue expands, the distance between the two morphogen sources increases, which in turn reduces the overlap of their signaling areas. This decrease in overlap acts as a brake, eventually halting the combined morphogen activity and arresting tissue growth precisely at the correct size. Recent experimental data of in situ stainings of SHH and FGF8 source regions in the developing blastema suggested potential scaling of morphogen regions with size. Inspired by these observations, the authors explored two theoretical scenarios for how the morphogen gradients may scale during growth: dynamic scaling with blastema size, or static scaling with animal size.
The authors analyzed growth arrest and proportional growth for both scaling scenarios, to determine which model is consistent with experimental observations. The dynamic scaling model, which resulted in continuous changes in morphogen parameters as the blastema grows, could not ensure proportional growth alone. Instead, static scaling, was found to be sufficient for proportional growth; where morphogen gradient parameters (like the size of the source region) were established statically based on the animal's overall size and remained constant during limb regrowth. Comparison of the model predictions to new experimental data on SHH and FGF8 gradients suggested that at least a subset of parameters, such as the FGF8 source size, exhibits similar static scaling behavior.
Although the model only accounts for the first phase of blastema regeneration, this is an essential step to ensure limb patterning, and the correct number of bone elements in the limb. The new model suggests a potentially general principle for size control during development and regeneration by coupling the spatial patterns of two opposing morphogen gradients with tissue growth. These findings advance concepts underlying regeneration that are applicable to salamanders and other species that can regrow limbs proportional to a wide variety of body sizes. In addition, connections between development and regeneration are highlighted, outlining general conditions for proportional growth, with potential implications for morphogenesis in non-regenerative species.
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Original publication
Natalia Lyubaykina, Dunja Knapp, Pietro Tardivo, Maximilian Kotz, Tatiana Sandoval-Guzmán, Benjamin M. Friedrich. Static morphogen scaling enables proportional growth in tissue growth model inspired by axolotl limb regeneration. PNAS (2025). DOI: https://doi.org/10.1073/pnas.2503086122