2 CRITICAL PERIODS OF SONIC HEDGEHOG SIGNALING REQUIRED FOR THE SPECIFICATION OF MOTOR-NEURON IDENTIFY

Antibodies that block Sonic Hedgehog (SHH) signaling have been used to show that SHH activity is required for the induction of floor plate d ifferentiation by the notochord and independently for the induction of motor neurons by both the notochord and midline neural eels. Motor ne uron generation depends on two critical periods of SHH signaling: an e arly period during which naive neural plate cells are converted into v entralized progenitors and a late period that extends well into S phas e of the final progenitor cell division, during which SHH drives the d ifferentiation of ventralized progenitors into motor neurons. The ambi ent SHH concentration during the late period determines whether ventra lized progenitors differentiate into motor neurons or interneurons, th us defining the pattern of neuronal cell types generated in the neural tube.