Generation of Ectopic Morphogen Gradients in the Zebrafish Blastula

In the zebrafish embryo, cells of the early blastula animal pole are all equivalent and are fully pluripotent until the midblastula transition that occurs at the tenth cell cycle (512 to 1K cells). This naive territory of the embryo is therefore perfectly suited to assay for morphogen activity. Here we describe different methods to generate ectopic morphogen gradients, either in vivo at the animal pole of the embryo, or in vitro in animal pole explants or in aggregates of animal pole blastomeres (also named embryoid bodies). These methods include injection of mRNA coding for growth factor(s) into animal pole blastomere(s), transplantation of growth factor(s) secreting cells, implantation of beads coated with purified protein(s), and various combinations of these different approaches. Our comparative study reveals that all these methods allow to generate morphogen gradient(s) that are able to induce, both in vivo and in vitro, the formation of a well-patterned embryonic axis.