Practice question

Sonic hedgehog mutant embryos exhibit severe midline defects including cyclopia due to failure to suppress Pax6 centrally and split eye field. In SHH-null mouse or zebrafish cyclops mutants, two optic vesicles remain fused, resulting in single median eye chamber with merged retinae and absence of optic stalk and hypothalamus. SHH normally promotes ventral forebrain identity and proximal optic stalk Nkx2 expression. Dorsalization of neural tube reflects BMP excess, optic vesicle still forms but unfused, neural crest migration defect unrelated. Cyclopia thus hallmark of SHH pathway disruption during forebrain patterning and midline development.