Practice question

Ventral neural tube patterning relies on Sonic hedgehog morphogen interpreted through Gli transcription factors, particularly Gli2 activator and Gli3 repressor forms, to establish nested expression of homeodomain proteins forming progenitor code. Shh gradient represses Class I factors Pax6, Dbx1, Irx3 ventrally and induces Class II factors Nkx2.2, Olig2, Nkx6.1 in threshold-dependent manner. Cross-repressive interactions between classes sharpen boundaries creating five ventral progenitor domains that generate distinct interneuron and motor neuron subtypes essential for circuitry. Sox2 maintains neural progenitors, Nanog and Oct4 regulate pluripotency not dorsoventral patterning. Therefore Shh signaling through Gli is central specifier.