ORDER AND COHERENCE IN THE FATE MAP OF THE ZEBRAFISH NERVOUS-SYSTEM

The zebrafish is an excellent vertebrate model for the study of the ce llular interactions underlying the patterning and the morphogenesis of the nervous system, Here, we report regional fate maps of the zebrafi sh anterior nervous system at two key stages of neural development: th e beginning (6 hours) and the end (10 hours) of gastrulation, Early in gastrulation, we find that the presumptive neurectoderm displays a pr edictable organization that reflects the future anteroposterior and do rsoventral order of the central nervous system. The precursors of the major brain subdivisions (forebrain, midbrain, hindbrain, neural retin a) occupy discernible, though overlapping, domains within the dorsal b lastoderm at 6 hours, As gastrulation proceeds, these domains are rear ranged such that the basic order of the neural tube is evident at 10 h ours, Furthermore, the anteroposterior and dorsoventral order of the p rogenitors is refined and becomes aligned with the primary axes of the embryo, Time-lapse video microscopy shows that the rearrangement of b lastoderm cells during gastrulation is highly ordered, Cells near the dorsal midline at 6 hours, primarily forebrain progenitors, display an terior-directed migration, Cells more laterally positioned, correspond ing to midbrain and hindbrain progenitors, converge at the midline pri or to anteriorward migration, These results demonstrate a predictable order in the presumptive neurectoderm, suggesting that patterning inte ractions may be well underway by early gastrulation, The fate maps pro vide the basis for further analyses of the specification, induction an d patterning of the anterior nervous system, as well as for the interp retation of mutant phenotypes and gene-expression patterns.