Yolk platelets impede nuclear expansion in Xenopus embryos

During metazoan early embryogenesis, the intracellular properties of proteins and organelles change dynamically through rapid cleavage. In particular, a change in the nucleus size is known to contribute to embryonic development-dependent cell cycle and gene expression regulation. Here, we compared the nuclear sizes of various blastomeres from developing Xenopus embryos and analyzed the mechanisms that control the nuclear expansion dynamics by manipulating the amount of intracellular components in a cell-free system. There was slower nuclear expansion during longer interphase durations in blastomeres from vegetal hemispheres than those from animal hemispheres. Furthermore, upon recapitulating interphase events by manipulating the concentration of yolk platelets, which are originally rich in the vegetal blastomeres, in cell-free cytoplasmic extracts, there was slower nuclear expansion and DNA replication as compared to normal yolk-free conditions. Under these conditions, the supplemented yolk platelets accumulated around the nucleus in a microtubule-dependent manner and impeded organization of the endoplasmic reticulum network. Overall, we propose that yolk platelets around the nucleus reduce membrane supply from the endoplasmic reticulum to the nucleus, resulting in slower nuclear expansion in the yolk-rich vegetal blastomeres.