The specification of neuronal identity by graded Sonic Hedgehog signalling

During the development of vertebrate nervous system, distinct classes of motor neurons and interneurons are generated at distinct dorsoventral positions in the ventral neural tube. The differentiation of these neuronal subtypes is directed by the secreted protein Sonic Hedgehog (Shh). Shh acts in a graded manner to establish different neural progenitor cell populations, defined by the expression of homeodomain transcription factors. These factors are critical for the interpretation of graded Shh signalling and act initially both to refine progenitor domain boundaries and to maintain their integrity. Subsequently, these factors direct the expression of genes that confer neuronal subtype identity to post-mitotic neurons.