Fig. 7

A model for endoderm and mesoderm formation in zebrafish. In late blastula stage zebrafish embryos, there are two steps in endoderm and mesoderm induction. (A) Zygotic oep expression is induced by Fgf signaling. The Nodal-related proteins Cyc and Sqt act through TGF-β type receptors. Oep is maternally supplied and it is an essential co-receptor for Nodal signaling (Gritsman et al., 1999). Nodal signaling induces fgf and this Fgf signaling functions to induce zygotic oep expression (Mathieu et al., 2004). Thus, the combination of Nodal and Fgf signaling is necessary for both endoderm and mesoderm induction. (B) Induction of endoderm and mesoderm by Nodal and Fgf signaling. Nodal signaling activates bon and fau/gata5 expression. Bon, Fau/Gata5 and Eomes cooperatively regulate the expression of cas (Bjornson et al., 2005). *Eomes; eomes is maternally supplied to the zygote. Cas activates sox17 and foxA2 and represses mesodermal genes to determine endodermal fates. On the other hand, both Fgf and Nodal signaling synergistically activate mesodermal genes, such as ntl, spt, papc and myoD. Fgf and MAPK signaling also negatively regulate cas expression and Cas function to determine the mesodermal fate.