Wnt signaling in gut organogenesis.

Wnt signaling regulates some aspect of development of nearly all endoderm-derived organs and Wnts mediate both differentiation and proliferation at different steps during visceral organogenesis. Wnt2b induces liver formation in zebrafish 1 and may combine with other inducers, Fibroblast Growth Factors 1 & 4 and Bone Morphogenetic Protein 4, to specify the mammalian liver.2-5 Later in development, Wnts are critical for liver expansion and, finally, for terminal hepatocyte differentiation,6-12 as reviewed elsewhere in this issue (Monga). Likewise, in the pancreas, Wnts drive proliferation of exocrine and endocrine cells13,14 and promote acinar cell differentiation,13,15 as reviewed in the chapter by Murtaugh. Here we examine the intricate involvement of Wnt signaling in growth and differentiation of the digestive tract.