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Regionalisation of the amphibian embryo is classically thought to involve induction by the Spemann organiser, itself induced by the Nieuwkoop centre. This model has now been extended to teleosts, with the identification of a gene that appears to define the zebrafish equivalent of the Nieuwkoop centre.
Figure 1.
A model for the establishment of the dorsal organiser and axis formation in Xenopus and zebrafish embryos. (a) In Xenopus, cortical rotation moves dorsal determinants towards the future dorsal side of the embryo, creating a large domain where, starting at the 32-cell stage, β-catenin undergoes nuclear translocation. In the classical two-step model for organiser formation, the Nieuwkoop centre derives from dorsal-vegetal blastomeres (D), and the Spemann organiser forms in the progeny of dorsal marginal blastomeres (C). In an alternative model, at the mid-blastula stage, the domain where β-catenin is nuclear defines the blastula organiser and expresses Siamois, and at the gastrula stage, Siamois and other factors further define the vegetal head organiser and marginal trunk organiser. Note that Siamois and goosecoid are co-expressed. (b) In zebrafish, dorsal determinants are transported towards the future dorsal side of the embryo and enter the blastoderm. At the mid-blastula transition, β-catenin is translocated into dorsal yolk syncytial layer (YSL) nuclei. At this stage dharma/nieuwkoid is expressed in the dorsal blastoderm. A few minutes later, β-catenin appears in dorsal blastoderm nuclei, while dharma/nieuwkoid expression fades out in the blastoderm and can now be detected in the dorsal YSL. At the shield stage, dharma/nieuwkoid and goosecoid are expressed in the YSL and blastoderm, respectively, with no apparent overlap. In both species, the dorsal organiser forms where β-catenin is translocated into the nuclei, but downstream molecular events may differ significantly.
