Shh expression in developing and regenerating limb buds of Xenopus laevis.

The zone of polarizing activity (ZPA) is a specialized region involved in the antero-posterior (A-P) axis formation in chick and mouse limb buds. The existence of ZPA in the posterior margin is suggested in Xenopus hindlimb buds because 180 degree rotation of the distal limb tip induces the supernumerary limb. In this study, we investigated the expression of Sonic hedgehog (shh), a molecular marker for ZPA, in Xenopus developing limb buds and regenerating blastemas by whole-mount in situ hybridization. Although shh was expressed in the posterior margin of the limb bud like in chicks, its expression domain did not correspond to the ZPA map of Xenopus hindlimb buds. shh expression was distant from the ZPA at stage 53 in particular. To clarify the difference between the shh expression domain and ZPA, we examined shh expression in 180 degree rotated limb buds. As a result, ectopic shh expression was newly induced in the proximal region to its original expression domain. These results suggest that ZPA is accompanied by shh expression as in chick limb buds. Furthermore we examined shh expression in regenerating blastemas. shh was reexpressed in the posterior margin of the blastema. This result supports the possibility that ZPA also exists in the regenerating blastema.