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 girth of the central nervous system (CNS) is nearly uniform along its length. Changes in expression patterns of homeobox genes and neuronal markers 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 few wild-type cells transplanted to the midline of a mutant embryo can rescue the forebrain phenotype, including cyclopia. Establishment of forebrain ventral positional coordinates may thus require inductive signaling by forebrain midline cells whose specification depends upon the cyclops gene product.