Practice question

Normal FGF4 expressed posteriorly maintains Wnt8c domain and primitive streak position by suppressing anterior streak genes. Experimental application of exogenous FGF8 beads or FGF-soaked grafts to anterior marginal zone reprograms epiblast competence, inducing ectopic Brachyury and repressing endogenous posterior streak via inhibitory feedback loop involving Sprouty and BMP antagonists. Because epiblast remains plastic early, gradient reversal relocates Nodal signaling and streak now initiates anteriorly extending posteriorly, effectively reversing orientation of embryonic axis. Demonstrates FGF instructive role not merely permissive for axis polarity.