Cell mixing during early epiboly in the zebrafish embryo.

Descendants of early blastomeres in the zebrafish come to populate distinctive regions of the fate map. We present a model suggesting that the distribution of cells in the early gastrula (the fate map stage) results from the passive response of cells to reproducible forces that change the overall shape of the blastoderm just prior to gastrulation. We suggest that one of the morphogenetic changes that accompanies epiboly, the upward doming of the yolk cell into the overlying blastoderm, could be responsible for cell mixing. In support of the model, we show that the timing, extent, and directions of cell mixing in the embryo accurately reflect the expectations of the model. Finally, we show that one portion of the gastrula, a marginal region that later gives rise to many of the mesendodermal derivatives, experiences little cell mixing during the doming process. As a result, this region in the gastrula is populated by the descendants of the subset of the early blastomeres that were originally at the margin. The finding that cytoplasm initially at the edge of the 1-celled blastodisc is transmitted specifically to mesendodermal precursors at the fate map stage raises the possibility that maternal determinants may contribute to initiation of embryonic patterning in the zebrafish embryo.