SONIC HEDGEHOG - A COMMON SIGNAL FOR VENTRAL PATTERNING ALONG THE ROSTROCAUDAL AXIS OF THE NEURAL-TUBE

The vertebrate hedgehog-related gene, sonic hedgehog, is expressed in ventral domains along the entire rostrocaudal length of the neural tub e, including the forebrain. Shh induces the differentiation of ventral neuronal cell types in explants derived from prospective forebrain re gions of the neural plate. Neurons induced in explants derived from bo th diencephalic and telencephalic levels of the neural plate express t he LIM homeodomain protein Islet-l, but these neurons possess distinct identities that match those of the ventral neurons normally generated in these two subdivisions of the forebrain. These results, together w ith other studies of neuronal differentiation at caudal levels of the neural tube, suggest that a single inducing molecule, Shh, mediates th e induction of distinct ventral neuronal cell types along the entire r ostrocaudal extent of the embryonic central nervous system.