Fig. 1. Repression of ß-catenin signaling in the endoderm is necessary and sufficient for liver and pancreas development. (A) 32-cell stage Xenopus embryos were injected with either a pCSKA-Wnt8 plasmid (250 pg) or stabilized pt-ß-catenin RNA (250 pg) in the D1 anterior endoderm cells. Other embryos were injected with RNA encoding Dkk1 (500 pg) or Gsk3ß (500 pg) into D4 posterior endoderm cells to repress Wnt signaling. (B) In situ hybridization at stage 35 with the liver marker for1, or with a combination of pancreas/duodenum marker pdx1/xlhbox8 and the lung marker nkx2.1, or with the intestinal marker endocut. Some embryos were hybridized with just pdx1. Arrowheads indicate ectopic or repressed gene expression. The solid red line indicates the relative size of the foregut domain. Gut tubes were isolated at stage 42 to visualize organ bud morphology. The dashed red line outlines the liver bud. L, liver; P, pancreas; Lu, lungs. (C) In situ hybridization to Gsk3ß-injected guts with liver markers for1, ambp, the early pancreas marker ptf1a and the exocrine pancreas marker elastase. (D) A sectioned embryo co-injected with Gsk3ß and ß-gal RNA shows ß-gal-staining nuclei (blue) and for1 expression (brown) localized to the endoderm.