Practice question

Vertebrate eye lens induction relies on optic vesicle as primary inducer of overlying head surface ectoderm. Optic vesicle evaginates from diencephalon, contacts competent head ectoderm expressing Pax6, and secretes BMP4 and FGF signals promoting lens placode thickening, invagination, and expression of Sox2, L-Maf, and crystallins. Lens placode is responding tissue, neural ectoderm becomes retina, surface ectoderm alone is not self-inductive. Classic Spemann and Lewis transplantation showed optic vesicle removal prevents lens while ectopic vesicle induces ectopic lens in competent regions and ages.