A sonic hedgehog-independent, retinoid-activated pathway of neurogenesis in the ventral spinal cord

Sonic hedgehog (Shh) is thought to control the generation of motor neurons and interneurons in the ventral CNS. We show here that a Shh-independent pa thway of interneuron generation also operates in the ventral spinal cord. E vidence for this parallel pathway emerged from an analysis of the induction of ventral progenitors that express the Dbx homeodomain proteins and of Ev x1/2 (V0) and En1 (V1) neurons. Shh signaling is sufficient to induce Dbx c ells and V0 and V1 neurons but is not required for their generation in vitr o or in vivo. Retinoids appear to mediate this parallel pathway. These find ings reveal an unanticipated Shh-independent signaling pathway that control s progenitor cell identity and interneuron diversity in the ventral spinal cord.