Practice question

High Activin concentration mimics peak Nodal signaling near Nieuwkoop center, strongly phosphorylating Smad2/3 which translocates nucleus with FoxH1 and Mixer, binding Goosecoid promoter activating organizer head mesoderm program. Goosecoid is transcription factor specifying prechordal plate and dorsal organizer, repressing Brachyury and ventral genes, inducing Chordin expression. Low doses induce Xbra for pan-mesoderm, high switches to Goosecoid. This dose-specific induction explains why deep endoderm adjacent to strong Nodal source becomes organizer, while marginal cells exposed to moderate levels become trunk mesoderm forming somites and lateral plate.