Practice question

Lens competence is restricted to head ectoderm. When optic vesicle, normal source of BMP4 and FGF8, is heterotopically transplanted beneath trunk ectoderm that never expressed Pax6, Six3, and lacks preplacodal specification, no induction occurs. Trunk ectoderm fails to activate L-Maf, Sox2 and crystallin reporters, continues epidermal differentiation forming skin. Control grafts into head ectoderm induce ectopic lens. This experiment proves requirement for tissue competence alongside inducer: inductive signal alone insufficient without responsive intracellular context established earlier by anterior patterning programs during gastrulation and neurulation stages of development.