Adult regenerative defects arise from discordant scaling of signal dependent growth and patterning

Orders of magnitude distinguish an organs size in adulthood from its size when patterning was established during embryonic development. The prospect of engineering adult organ regeneration raises the fundamental question of whether regenerative stem cell patterning should be elicited at embryonic or adult scale. Axolotls regenerate their limbs at all stages of post-embryonic life, encompassing an order of magnitude increase in animal size, but the mechanisms allowing this robust capacity remain unclear. Limb regeneration occurs by the formation of an embryonic-like progenitor zone, the blastema, whose dimensions increase with animal size, suggesting that at least some aspects of limb regeneration must scale. Here, by combining spatial transcriptomics, biophysical modelling, quantitative imaging, and functional perturbations, we found distinct scaling signatures among key signaling pathways, arguing against a body-size-dependent hormonal scaling mechanism. SHH signaling showed partial scaling that saturated in largest adult sizes, with a correspondingly early termination of blastema growth while Wnt9a-modulated digit patterning scaled at all animal sizes. As a result of this differential scaling, digit patterning wavelength was mismatched with respect to domain growth in the largest blastemas, preventing the addition of the last digit. In agreement with this model, regenerative failure can be rescued by timed supplementation of SHH. Our results show that coordinated scaling of morphogen signaling is a key requirement for adult organ regeneration.