The role of Wnt pathway proteins in dorsal-ventral axis specification. (A-D) Differential translocation of β-catenin into Xenopus blastomere nuclei. (A) Early 2-cell stage of Xenopus, showing β-catenin (orange) predominantly at the dorsal surface. (B) Presumptive dorsal side of a Xenopus blastula stained for β-catenin shows nuclear localization. (C) Such nuclear localization is not seen on the ventral side of the same embryo. (D) β-catenin dorsal localization persists through the gastrula stage. (E) Dorsal axis formation caused by the injection of both blastomeres of a 2-cell Xenopus embryo with dominant inactive GSK-3. Dorsal fate is actively suppressed by wild-type GSK-3. (F) Irenic model whereby the Nieuwkoop center (characterized by siamois gene expression and the ability to induce dorsal mesoderm) is created by the synergy of the activation of β-catenin dorsally and the activation of the TGF-β signal vegetally. (A, D from R. T. Moon; B and C from Schneider et al. 1996, photographs courtesy of P. Hausen; E from Pierce and Kimelman 1995, photograph courtesy of D. Kimelman.)