Practice question

Requirement of Pax6 in surface ectoderm for lens formation was demonstrated by tissue recombination: grafting wild-type optic vesicle onto Pax6-deficient head ectoderm from Small eye mutant fails to induce lens placode or crystallin expression, whereas Pax6-deficient optic vesicle grafted onto wild-type ectoderm still induces lens. Knockout of Pax6 in optic vesicle alone does not block lens, overexpression in cup does not address ectodermal necessity, neural crest knockout irrelevant. Mosaic recombinant experiment proves cell-autonomous competence role of Pax6 within responding surface ectoderm, not inducing tissue itself.