The role of yolk syncytial layer and blastoderm movements during gastrulation in zebrafish

During gastrulation, a set of highly coordinated morphogenetic movements creates the shape and internal organization of the embryo. In teleostean fishes, these morphogenetic movements involve not only the embryonic progenitor cells (deep cells) but also two extra-embryonic tissues: an outer sheet of epithelial cells (EVL) and a yolk syncytial layer (YSL). Epiboly is characterized by the spreading of the blastoderm (deep cells and EVL) to cover the large yolk cell, whereas convergence and extension leads, respectively, to mediolateral narrowing and anteroposterior elongation of the embryo. Recent studies have shown that the nuclei of the YSL undergo epiboly and convergence and extension movements similarly to the overlying deep cells, suggesting that these tissues interact during gastrulation. However, it is so far not clear whether and how the movements of YSL nuclei and deep cells influence each other. In the first part of this thesis, the convergence and extension movement of YSL nuclei was quantitatively compared to the movement of the overlying mesendodermal progenitor (or “hypoblast)” cells. This revealed that, besides the similarity in the overall direction of movement, YSL nuclei and hypoblast cell movements display differences in speed and directionality. Next, the interaction between YSL and hypoblast was addressed. The movement of the blastoderm was analyzed when YSL nuclei movement was impaired by interfering with the YSL microtubule cytoskeleton. We found that YSL and blastoderm epiboly were strongly reduced, while convergence and extension were only mildly affected, suggesting that YSL microtubules and YSL nuclei movement are required for epiboly, but not essential for convergence and extension of the blastoderm. We also addressed whether blastodermal cells can influence YSL nuclei movement. In maternal-zygotic one-eyed pinhead (MZoep) mutant embryos, which lack hypoblast cells, YSL nuclei do not undergo proper convergence movement. Moreover, transplantation of wild type hypoblast cells into these mutants locally rescued the YSL nuclei convergence phenotype, indicating that hypoblast cells can control the movement of YSL nuclei. Finally, we propose that the hypoblast influences YSL nuclei movement as a result of shape changes caused by the collective movement of cells, and that this process requires the adhesion molecule E-cadherin