FGF signaling is active in the foregut endoderm during organ induction. Confocal immunostaining of bisected Xenopus embryos show active FGF-MEK signaling with di-phospho ERK1/2 (pErk) (white) in the developing foregut tissue at the indicated stages (anterior left, ventral down). Images are 10X magnification with independent scans of the boxed regions at 20X magnification. (A) Stage NF15 embryos show a low level of FGF signaling in the migrating anterior mesendoderm (ame). (B) Stage NF19, (C) NF23 and (D) NF28 show pErk staining in the ventral foregut endoderm (fg) as well as the underlying cardiac mesoderm (cm). The dashed yellow line indicates the boundary between the endoderm and mesoderm. (E) Mid-sagittal and (F) transverse optical sections of stage NF35 embryos show pErk staining in the liver epithelium (lv), heart (ht) and nascent lung buds (lu) and lateral plate mesoderm (lpm). (G and H) Stage NF42 gut tubes show pErk in liver bud (lv), gal bladder (gb), dorsal (dp) and ventral pancreatic buds (vp), the stomach (st) and the distal tips of the lung buds (lu). (I) Stage NF23 control embryos with no primary antibody.