Fig. 2. Temporal regulation of ß-catenin/Tcf activity during endoderm pattering. (A) At the 32-cell stage, Xenopus embryos were injected in the anterior D1 cells with RNA encoding the fusion protein GR-LEFN-ßCTA (800 pg), which constitutively activates ß-catenin target genes in the presence of dexamethasone (Dex). Dex (1 µM) was added to the media of injected embryos at the indicated stages and embryos were assayed by for1, pdx1 and endocut in situ hybridization at stage 35. (B) Addition of Dex to GR-LEFN-ßCTA-injected embryos from stage 30 to 42, followed by hhex in situ, revealed enlarged liver buds. (C) 32-cell stage embryos were injected in posterior D4 cells with RNA encoding GR-NTcf3 (800 pg), which represses ß-catenin/Tcf target genes when activated. Dex (1 µM) was added to the media of injected embryos at the indicated stages and embryos were assayed by for1, pdx1 and endocut in situ hybridization at stage 35. (D) GR-NTcf3 was injected into D1 cells at the 32-cell stage, and when Dex was added from stages 30 to 42 some embryos exhibited smaller liver buds based on for1 in situ hybridization. No effect was observed in uninjected embryos treated with Dex.