Graded sonic hedgehog signaling and the specification of cell fate in the ventral neural tube

The patterning of cell types in vertebrate embryos de-pends on the function of organizing centers, specialized cell groups that direct the fate of adjacent cells through the secretion f inductive factors (Gurdon 1987). During early development of the vertebrate nervous ystem, the identity and pattern of cell types generated in the ventral half of the neural tube are controlled by signals provided initially by an axial mesodermal organizing center, the notochord (for review, see Tanabe and Jessell 1996). As-says of neural cell differentiation i vitro and analyses of mutant mouse and zebrafish embryos (for review, see Placzek 1995) have provided vidence that the notochord is the source of two operationally distinct inductive sig-nals: a local signal that induces the differentiation of floorplate cells at the ventral midline of the neural tub