Practice question

Lens induction relies heavily on fibroblast growth factor signaling for proliferation, placode maintenance and fiber differentiation. FGF1, FGF2 from neural retina and vitreous humor activate FGFR2b and FGFR3 in lens epithelium via Ras-MAPK and PI3K-Akt pathways, upregulating L-Maf, Prox1, c-Maf and crystallins. Exogenous FGF beads mimic optic vesicle inducing activity in chick explants, while dominant-negative FGFR abolishes lens vesicle invagination. Unlike Hedgehog or JAK-STAT which pattern other tissues, FGF provides sustained dosage-dependent cue that promotes lens placode thickening, survival and terminal differentiation into transparent fiber cells.