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Early Development Leading to Axis Formation

At ten hours post fertilization, the zebrafish embryo has clearly recognizable anterior-posterior and dorsal-ventral axis. To generate this basic body plan, the zebrafish, the embryo undergoes rapid development and morphogenetic changes. A blastodisc is then formed on top of the yolk after fertilization, during the following three hours of development, rapid, synchronous cleavage occurs within the blastodisc to generate a blastula embryo of around 1000 cells. After the mid blastula transition, at about four hours post fertilization (hpf), cell rearrangements reshape the blastoderm into what will be a characteristic vertebrate body plan. In the process known as epiboly, cells interpolate radially, thinning the blastoderm over the yolk. After gastrulation, epiboly movements have spread the blastomeres so that the blastoderm covers the entire yolk cell. There are also three other movements that contribute to the formation of the axis. At five hours post fertilization, cells at the margin internalize and form the hypoblast, the precursor of the mesoderm and endoderm.

By six hpf, movements of convergence and extension have begun, which results in dorsal accumulation of cells moving from lateral and ventral regions of the blastoderm (convergence). Converging cells then insert with dorsal blastomeres and spread them along the animal-vegetal acis, which leads to the lengthening of the anterior-posterior axis (extention). This convergance of all these cells to the dorsal side leads to the first clearly apparent break in the radial symmetry and forms the shield, a thickening at the dorsal blastoderm margin that is equivalent of the amphibian Spemann-Mangold organizer.