Inhibition of FGF signaling in Xenopus embryos. (A) Confocal immunostaining of pErk (red) and a GFP lineage tracer (green) in the foregut region of bisected stage NF23 Xenopus embryos that were injected into the presumptive foregut endoderm cells at the 16-cell stage with RNA encoding either β-gal (3 ng) and GFP or dnFGFR (3 ng) and GFP. The β-gal/GFP injected embryos show pErk in GPF + cells whereas in dnFGFR/GFP injected embryos pErk is undetectable in GFP + foregut cells. (B) A representative stage NF42 gut tube from βgal/GFP and dnFGFR/GFP injected embryos assayed by in situ hybridization for gfp RNA to show mosaic contribution of labeled cells to different organ buds. p; pancreas, lv; liver, s; stomach, i; intestine. (C) Summary showing percentage of organ buds containing labeled cells indicates a decreased contribution to the lung and liver buds in dnFGFR injected embryos (n = 120) compared to βgal controls (n = 70).