Practice question

Nieuwkoop center comprises dorsovegetal endodermal blastomeres enriched with nuclear beta-catenin which activates transcription of Siamois, Twin and Nodal-related genes Xnr1,2,5,6 through TCF binding. These secreted Nodals act vertically on overlying dorsal marginal mesoderm to induce organizer genes Goosecoid, Chordin and Noggin, effectively creating mesoderm from ectodermal marginal zone. Vegetal cells themselves remain endoderm expressing Sox17 and Mix. Without Nieuwkoop signals, no dorsal mesoderm forms and embryo ventralized with excess blood. Ventral vegetal cells lacking beta-catenin induce ventral mesoderm via BMPs. Thus Nieuwkoop center primarily induces dorsal mesoderm organizer precursors.