Organizing pattern formation in the embryo of the zebrafish, Danio rerio

During gastrulation, the functions of a Spemann organizer-type region are essential for the specification of the vertebrate body plan. Functioning as an inductive center, this specialized group of cells dorsalizes ventral-lateral tissues and induces the overlying ectoderm to form nervous system. While the importance of this type of organizing center has been demonstrated in all vertebrate species, how the Spemann organizer region is established is best understood in the embryos of amphibians. Analysis of the early events underlying specification of the amphibian Spemann organizer has indicated that the process entails essentially two mechanisms. First, maternally controlled localizations in the egg are rearranged after fertilization generating an initial dorso-ventral asymmetry. This initial asymmetry establishes a primary inducing center, the Nieuwkoop center. The Nieuwkoop center then induces the Spemann organizer region in the overlying tissue. This sequence of events has been adopted to form a framework to explain the patterning of all vertebrate embryos. This thesis investigated how well this two step sequential model could be applied to the development of the zebrafish. Lineage tracing demonstrated that in the zebrafish, cytoplasm streams form the yolk cell into early blastomeres. This streaming combined with the spatial pattern of blastomere cellularization leads to a potential differential distribution of yolk cytoplasm. Removing the yolk cell during after the first cleavage division, which blocks the redistribution of yolk cytoplasm, did not block dorso-posterior specification, however it did perturb the specification of dorso-anterior fates in a progressive stage dependent manner. These findings lend support to the prediction that cytoplasmic localizations function in the process of dorsal specification in vertebrate embryos outside of amphibia. Analysis of the expression and activity of the zebrafish nieuwkoid homeobox gene led to the identification of its organizer inducing or Nieuwkoop center-like activity, suggesting that the concept of the amphibian Nieuwkoop center can be extended to teleosts. Genetic inactivation of nieuwkoid demonstrated that in addition to being sufficient, it is necessary for organizer induction. Examination of molecular nature of nieuwkoid activity revealed an unexpected role of the Nieuwkoop center, namely to promote organizer formation indirectly by antagonizing ventralizing influences prior to gastrulation.