Bipartite axiation follows incomplete epiboly in zebrafish embryos treated with chemical teratogens

Medial clefts in the axis of the trunk region are malformations known from many chordates and are mostly referred to as rachischisis anterior. In teleosts, rachischisis was previously ascribed either to secondary rifting of a single uniform axial rudiment, or to the establishment of two (half) axes and body halves physically separate from the very beginning. In order to decide between these conflicting interpretations, we treated zebrafish embryos during blastodisc stages and epiboly with several chemical teratogens causing rachischisis anterior. Treatment with ethanol, Colcemid, hydroxyurea, or cycloheximide was found to delay the proliferation and movements of the deep cells more strongly than the timing of cell differentiation, so that the deep cells embark on organogenesis before having reached their destinations in the uniform germ shield. Treatment with alpha-amanitin, on the other hand, seems primarily to affect the periblast and enveloping layer; the incomplete epiboly observed in these layers appears to restrain deep cell epiboly physically and thus to cause rachischisis. In both instances, the split condition of the embryo's trunk region is clearly due to the ectopic formation of physically separate body halves right from the beginning, a mode we call bipartite axiation. We also describe secondary anomalies specific for individual teratogens, and briefly discuss the possible origins of rachischisis anterior among other chordates including man.