Practice question

Specification of single eye field on anterior neural plate depends on inhibition of posteriorizing Wnt and BMP signals and activation of FGF signaling from anterior endoderm and mesoderm. FGF through Ras-MAPK phosphorylates and represses Wnt effectors, while also indirectly upregulating Otx2 and Sox2 in rostral plate. Explants treated with FGF maintain Rx expression, marker of eye field, whereas Wnt activation caudalizes neural plate to telencephalon. Thus FGF acts as permissive cue ensuring competent anterior neuroectoderm remains eye-capable before Shh-mediated splitting, distinguishing early field specification from later optic vesicle patterning phases.