Practice question

Ventral neural tube identity is controlled by Sonic hedgehog secreted from notochord and floor plate. Shh diffuses dorsally establishing a ventral-to-dorsal gradient that antagonizes dorsal BMP and Wnt signals from roof plate. Different thresholds activate graded transcription factor codes: high Shh induces Nkx2.2 and floor plate genes, intermediate induces Olig2 and motor neurons, low induces Pax6 and interneurons. Gli activator-repressor ratio translates Shh dose into position-specific gene expression, compartmentalizing motor neuron, V0-V3 interneuron domains along dorsoventral axis essential for functional spinal cord circuitry and locomotor output.