Dorsoventral-mediated Shh induction is required for axolotl limb regeneration

Axolotls (Ambystoma mexicanum) exhibit a remarkable ability to regenerate limbs after amputation. Classical experiments have suggested that contact between cells derived from distinct orientations--dorsal, ventral, anterior, and posterior--within the regenerating blastema is necessary for accurate limb pattern formation. However, the molecular basis for this requirement has remained largely unknown. Here, we demonstrate that both dorsal and ventral tissues are required for limb formation via induction of Shh expression, which plays a crucial role in limb patterning. Using the accessory limb model (ALM), we induced position-specific blastemas lacking cells derived from a single orientation (anterior, posterior, dorsal, or ventral). Patterned limb formation occurred only in blastemas containing both dorsal- and ventral-derived cells. We further observed that Shh expression requires dorsoventral contact within a blastema, highlighting the necessity of dorsoventral contact for inducing Shh expression. In addition, we identified WNT10B and FGF2 as dorsal- and ventral-mediated signals, respectively, that create the inductive environment for Shh expression. Our findings clarify the role of dorsal and ventral cells in inducing Shh, a mechanism that has rarely been studied in the context of limb regeneration and pattern formation. This model provides new insights into how cells with different positional identities drive the regeneration process.