Timing system for the start of gastrulation in the Xenopus embryo

This study examined which component of the egg, the nucleus or cytoplasm, is involved in the timing of the start of gastrulation in the Xenopus embryo, and when it starts to measure time. First, nuclei of cells of 256-cell stage embryos were transplanted to enucleated eggs 60 min after activation. These eggs showed first cleavage 20-30 min later than control eggs fertilized at the same time as the activation of recipient eggs, and started gastrulation 25-35 min later than control embryos (depending on the delay in the first cleavage). Second, eggs whose nuclei were temporarily isolated by the extrusion of the portion containing the nucleus out of the fertilization envelope showed first cleavage 60-90 min later than sibling control eggs, because of delayed introduction of the nucleus from the extruded portion. They started gastrulation 60-90 min later than sibling control embryos (depending on the delay in the first cleavage). The portion inside the envelope underwent two to three rounds of oscillation in cell cycle relevant activities before the first cleavage, while the portion outside underwent the same rounds of cleavage as the inside portion. From the present and previous results it is concluded that the putative timing system for the start of gastrulation in the Xenopus embryo, whether it consists of a single or of multiple clocks, starts measuring time at or around the first cleavage, and that the presence of both the nucleus and the cytoplasm in the same cell and occurrence of mitosis and/or cleavage there are indispensable for the timing system to work, although the role of the cytoplasm is superior to that of the nucleus.