ANTEROPOSTERIOR PATTERNING IN THE ZEBRAFISH, DANIO-RERIO - AN EXPLANTASSAY REVEALS INDUCTIVE AND SUPPRESSIVE CELL-INTERACTIONS

We report the first extended culture system for analysing zebrafish (D anio rerio) embryogenesis with which we demonstrate neural induction a nd anteroposterior patterning. Explants from the animal pole region of blastula embryos ('animal caps') survived for at least two days and i ncreased in cell number. Mesodermal and neural-specific genes were not expressed in cultured animal caps, although low levels of the dorsoan terior marker otx2 were seen. In contrast, we observed strong expressi on of gta3, a ventral marker and cyt1, a novel type I cytokeratin expr essed in the outer enveloping layer. Isolated 'embryonic shield', that corresponds to the amphibian organizer and amniote node, went on to e xpress the mesodermal genes gsc and ntl, otx2, the anterior neural mar ker pax6, and posterior neural markers eng3 and krx20. The expression of these genes defined a precise anteroposterior axis in shield explan ts. When conjugated to animal caps, the shield frequently induced expr ession of anterior neural markers. More posterior markers were rarely induced, suggesting that anterior and posterior neural induction are s eparable events. Mesodermal genes were also seldom activated in animal caps by the shield, demonstrating that neural induction did not requi re co-induction of mesoderm in the caps. Strikingly, ventral marginal zone explants suppressed the low levels of otx2 in animal caps, indica ting that ventral tissues may play an active role in axial patterning. These data suggest that anteroposterior patterning in the zebrafish i s a multi-step process.