Extracellular signals and transcriptional programs controlling neurogenesis

The assembly of neuronal circuits requires the generation of various types of neurons at appropriate positions. Numerous extracellular signals and tra nscription factors controlling the fate of progenitor cells of the nervous system have recently been identified. Extrinsic factors control a number of critical checkpoints for neural progenitors, such as the commitments towar ds proliferation versus differentiation, between a glial versus a neuronal fate, and also the nature of the neurons generated at different dorso-ventr al positions and at different times during neurogenesis. Transcription fact ors of the bHLH and homeodomain classes function in regulatory cascades to control the determination of neural progenitors and the acquisition of gene ric and specific aspects of the neuronal phenotype. The mecanisms by which extrinsic signals and transcriptional programs get integrated so that a neu ron adopts a differentiated phenotype appropriate to its position remain to be established.