MIDLINE SIGNALING IN THE PRIMORDIUM OF THE ZEBRAFISH ANTERIOR CENTRAL-NERVOUS-SYSTEM

In all vertebrates the brain develops from the enlarged anterior part of the neural plate. However, in the zebrafish mutant cyclops, the gir th of the central nervous system (CNS) is nearly uniform along its len gth. Changes in expression patterns of homeobox genes and neuronal mar kers reveal a massive deletion of the ventral forebrain, particularly the diencephalon, as well as its precursor region in the neural plate. The deletion is due to a nonautonomous action of the mutation: very f ew wild-type cells transplanted to the midline of a mutant embryo can rescue the forebrain phenotype, including cyclopia. Establishment of f orebrain ventral positional coordinates may thus require inductive sig naling by forebrain midline cells whose specification depends upon the cyclops gene product.