Neuronal diversification: development of motor neuron subtypes.

Recent progress has been made in defining the developmental mechanisms that contribute to the diversification of motor neuron subtypes, which differ in transcription factor gene expression and synaptic connections. These studies suggest that progenitor cells acquire specific motor neuron identities through the coordinate actions of multiple factors. Current evidence suggests that Sonic hedgehog initiates a common pathway for motor neuron differentiation, while positionally distributed factors act to assign subtype identities.