SONIC HEDGEHOG INDUCES THE DIFFERENTIATION OF VENTRAL FOREBRAIN NEURONS - A COMMON SIGNAL FOR VENTRAL PATTERNING WITHIN THE NEURAL-TUBE

The vertebrate hedgehog-related gene Sonic hedgehog (Shh) is expressed in ventral domains along the entire rostrocaudal length of the neural tube, including the forebrain. We show here that SHH induces the diff erentiation of ventral neuronal cell types in explants derived from pr ospective forebrain regions of the neural plate. Neurons induced in ex plants derived from both diencephalic and telencephalic levels of the neural plate express the LIM homeodomain protein Isl-1, and these neur ons possess distinct identities that match those of the ventral neuron s generated in these two subdivisions of the forebrain in vivo. A sing le inducing molecule, SHH, therefore appears to mediate the induction of distinct ventral neuronal cell types along the entire rostrocaudal extent of the embryonic central nervous system.