The specification of neuronal identity by graded sonic hedgehog signalling

During the development of vertebrate nervous system, distinct classes of mo tor neurons and interneurons are generated at distinct dorsoventral positio ns in the ventral neural tube. The differentiation of these neuronal subtyp es is directed by the secreted protein Sonic Hedgehog (Shh). Shh acts in a graded manner to establish different neural progenitor cell populations, de fined by the expression of homeodomain transcription factors. These factors are critical for the interpretation of graded Shh signalling and act initi ally both to refine progenitor domain boundaries and to maintain their inte grity. Subsequently, these factors direct the expression of genes that conf er neuronal subtype identity to post-mitotic neurons.