Yolk syncytial layer formation is a failure of cytokinesis mediated by Rock1 function in the early zebrafish embryo.

The yolk syncytial layer (YSL) performs multiple critical roles during zebrafish development. However, little is known about the cellular and molecular mechanisms that underlie the formation of this important extraembryonic structure. Here, we demonstrate by timelapse confocal microscopy of a transgenic line expressing membrane-targeted GFP that the YSL forms as a result of the absence of cytokinesis between daughter nuclei at the tenth mitotic division and the regression of pre-existing marginal cell membranes, thus converting the former margin of the blastoderm into a syncytium. We show that disruption of components of the cytoskeleton induces the formation of an expanded YSL, and identify Rock1 as the regulator of cytoskeletal dynamics that lead to YSL formation. Our results suggest that the YSL forms as a result of controlled cytokinesis failure in the marginal blastomeres, and Rock1 function is necessary for this process to occur. Uncovering the cellular and molecular mechanisms underlying zebrafish YSL formation offers significant insight into syncytial development in other tissues as well as in pathological conditions.