Practice question

FGF8 secreted by optic vesicle neuroepithelium acts as second-phase inducer after BMP priming. Vogel-Hopker experiments demonstrated FGF8 transcripts localize to distal optic vesicle, and beads soaked in FGF8 implanted under head ectoderm ectopically activate L-Maf, a large Maf bZIP transcription factor specific to lens. L-Maf directly binds LCE enhancer of delta-crystallin and transactivates lens fiber differentiation genes including Prox1 and connexins. Inhibiting FGFR signaling with dominant-negative receptor abolishes L-Maf expression, proving FGF8 initiates transcriptional cascade for fiber elongation and crystallin accumulation.