Practice question

In normal chick gastrula FGF8 expressed in primitive streak maintains streak cells in epithelial state expressing Brachyury and regulates directional egress of mesoderm. Precise gradient restricts streak width and guides extension via interaction with Wnt3a maintaining stem zone. Excess FGF8 saturates receptors FGFR1/3 causing prolonged MAPK activation, disrupting Wnt-PCP mediated convergent extension and altering beta-catenin localization, leading to ectopic streak formation, altered orientation and failure of node regression. Result is not simple paraxial expansion but altered Wnt signaling and primitive streak orientation reflecting feedback loop disruption between FGF and Wnt pathways.