Organizer formation in Xenopus requires:
Organizer formation requires intersection of dorsal Wnt-beta-catenin pathway providing dorsal competence and vegetal VegT-Nodal pathway providing mesodermal competence. Dorsal signal stabilizes beta-catenin activating Siamois, while VegT and Vg1 induce mesodermal genes such as Xbra and Xnrs. Only cells at dorsal marginal zone receiving both Siamois and VegT-dependent Nodal achieve high Goosecoid and Chordin expression marking functional organizer. Single dorsal signal produces endoderm only, mesoderm signal alone ventral mesoderm. Intersection ensures single organizer at dorsovegetal marginal
Ref: Wolpert, Principles of Development, 5th ed., Chapter 5: Organizer requires dorsal and mesodermal signals intersection.
