Practice question

Shh knockout illustrates midline patterning defects. Normally Shh from prechordal plate suppresses Pax6 medially and promotes hypothalamic and optic stalk identities, splitting single eye field. In Shh null mice described by Chiang et al., failure of this suppression leaves Pax6 expressed across midline, preventing invagination of two vesicles. Forebrain also fails to divide, resulting in holoprosencephaly spectrum with most severe manifestation as cyclopia, single median eye with fused retina and single lens, alongside loss of ventral forebrain structures, confirming Shh requirement for bilateralization of eye primordia.