Practice question

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 boundary rather than entire embryo, essential for normal body axis patterning.