Early neurogenesis in the zebrafish embryo.

Zebrafish embryos represent an attractive system for the study of early vertebrate neurogenesis. The embryos develop outside of the mother and are transparent allowing analysis at the cellular level in the living embryo during all phases of early neurogenesis. The teleostean neural tube is generated by a mechanism different from that of other vertebrates. A massive keel is formed first and the central canal appears by subsequent cavitation. Despite this, however, the organization of the neural plate and the neural keel resembles that of other vertebrates in many aspects. Oriented cell divisions coupled with oriented cell intercalations appear to be involved in the morphogenesis of the neural keel. Embryos mutant in the cyclops gene show deficiencies in the ventral neural tube. They lack the floor plate and the ventral parts of the diencephalon. Two recently cloned genes, axial and sonic hedgehog, have been implicated in the development of the ventral midline of the neural tube. Expression of axial and sonic hedgehog is impaired by the cyclops mutation in the midline of the neural plate. This, together with the effects of ectopic expression of the two cloned genes, suggests that axial, sonic hedgehog, and cyclops are part of the regulatory cascade leading to floor plate formation.