Practice question

Pax6 knockout mouse homozygous null exhibits complete failure of eye morphogenesis including absence of lens, cornea, and retina. Optic vesicle forms initially but remains small rudiment failing to contact surface ectoderm and regresses because reciprocal maintenance cues absent. Lens placode never thickens due to lack of ectodermal competence factor, cornea does not differentiate transparently, but primary defect is total eyeless phenotype not limited to cornea or lens alone. Presumptive retina fails to expand into cup, demonstrating Pax6 requirement upstream of entire eye program governing both optic cup and lens lineages and survival.