Fig. 8. Signaling combinations that influence cell fate in the early Xenopus embryo. Mid-blastula cells are pluripotent, and many factors determine the fate of their progeny. Of major importance in regulating the expression of proneural, myogenic, endodermal and epidermal transcription factors, are the BMP, Xnr (+activin+Vg1), FGF and Wnt signaling pathways. As described in the text, loss-of-function experiments support the combinations of signals shown here as being crucial for ectodermal (neural and epidermis), mesodermal (head mesoderm, notochord, somite) and endodermal fates, at the late blastula and gastrula stages. The specification of blood islands, heart, intermediate and lateral plate mesoderm are not considered in this review. Arrowhead indicates required repression of the activity of the ligand.