Practice question

Sonic hedgehog produced by prechordal mesendoderm and ventral forebrain plays crucial role in separating single eye field into two bilateral fields. Early eye field expresses Rx and Pax6 broadly; SHH induces midline repression of Pax6, promotes proximal optic stalk character, and allows lateral optic vesicle outgrowth retaining Pax6. This midline clearance and proximal-distal patterning prevent cyclopia. SHH does not directly induce lens, activate Pax6 broadly, or drive neural crest migration, but patterns forebrain midline essential for paired eye formation and bilateral optic vesicle evagination.