A characterization of axolotl digit regeneration: conserved mechanisms, divergent patterning, and a critical role for hedgehog signaling.

Axolotl digits offer an experimentally versatile model for studying complex tissue regeneration. Here, we provide a comprehensive morphological and molecular characterization of digit regeneration, revealing both conserved features and notable divergences from classical limb regeneration. Digit blastemas progress through similar morphological stages, are nerve-dependent, contain key regenerative cell populations, and express many canonical morphogens and mitogens. However, they exhibit minimal expression of the A-P patterning genes Shh, Fgf8, and Grem1; suggesting distal outgrowth and patterning occur independently of these signals. Joint regenerative fidelity varies significantly across digits and cannot be explained by differences in nerve supply, cell proliferation, or differential expression of any patterning genes assessed in this study. Furthermore, functional experiments reveal Hedgehog signaling is essential for interphalangeal joint regeneration, but activation alone is insufficient to improve fidelity in less robust digits. This system combines experimental accessibility with intrinsic variation in regenerative outcomes, making it an ideal platform to identify critical determinants of successful tissue regeneration and refine models of appendage patterning.