Practice question

Implantation of fibroblast growth factor 8-soaked beads near competent head ectoderm mimics optic vesicle signal and triggers lens transcriptional program. FGF8 activates MAPK signaling inducing L-Maf expression in ectoderm, which directly transactivates delta-crystallin and beta-crystallin enhancers driving lens fiber differentiation. Implantation experiments in chick demonstrated bead-induced ectopic lens placodes expressing Sox2 and crystallins, whereas inhibition of Pax6, suppression of Six3, or activation of Wnt blocks not observed. This bead assay evidences sufficiency of FGF8 as lens inducer upstream of L-Maf-crystallin cascade and Sox2 activation.