The liver and the pancreas are specified from the foregut endoderm, but how early foregut precursors form remains unknown. Aaron Zorn's group now report that in Xenopus, the Wnt pathway links early gastrula-stage endoderm patterning to organ specification. Their results, on , show that while in anterior endoderm, Wnt/β-catenin activity is repressed, inducing liver and pancreas specification, it's upregulated in the posterior endoderm,where foregut fate is inhibited and intestinal development occurs. Their experimental repression of β-catenin activity in the embryonic posterior endoderm induced ectopic organ buds to form that express early hepatic and pancreatic markers; increased β-catenin activity in the foregut endoderm repressed liver and pancreas formation. The early foregut marker, hhex, is a target of this β-catenin activity and is repressed indirectly via the transcriptional repressor Vent2. Wnt signalling later enhances liver development. Thus, turning Wnt signalling on/off at the right moment is essential for these organs' proper formation, a finding that should advance the differentiation of liver and pancreatic tissue from stem cells.
