Conserved anterior/posterior tissue asymmetry encodes proximal/distal positional information during axolotl limb regeneration.

While mammalian limb regeneration is limited to the distal fingertips, axolotls can regenerate entire limbs, providing a model system to uncover conserved patterning mechanisms. Although anterior-posterior (A/P) signaling is required for limb regeneration, how A/P patterning is coordinated across limb segments remains unclear. Using spatial transcriptomics and our SpatialFlux analysis pipeline, we found that A/P genes are asymmetrically distributed along the proximal-distal (Pr/Di) axis of both uninjured and regenerating axolotl limbs. This asymmetric boundary coincides with distally enriched AP-1 and ERK signaling, with ERK inhibition disrupting both posterior and distal patterning programs, linking A/P organization to Pr/Di positional identity. Remarkably, a similar asymmetric boundary is present in fetal human limbs, revealing an evolutionarily conserved patterning paradigm. These findings challenge the long-standing model of a symmetrical A/P limb midline boundary and support a revised model, in which shifting A/P ratios regulate Pr/Di patterning.