Cyclic appearance of cytoplasmic NOR-silver-stained particles in sea urchin embryos

When sea urchin embryos were subjected to nucleolar organizer region (NOR)-silver staining, densely stained particles were observed in the cytoplasm. The appearance of these cytoplasmic particles (CPs) was cell-cycle dependent. During early development, the CPs were detected at interphase, but not during mitosis; they disappeared at metaphase and reappeared at telophase. The CPs appeared periodically even when embryos were treated with cytochalasin B or aphidicolin, which inhibits the progression of cytokinesis and nuclear division, respectively. By contrast, CPs were not detected in the colchicine-treated embryos in which both cytokinesis and nuclear divisions were prevented. The CPs were observed only in the embryos whose stage was early blastula (about 6th to 7th cleavage) or earlier; no CPs were detected even at interphase in the embryos at late blastula (about 8th to 9th cleavage) or later. Electron microscopic evaluation showed CPs to be granular structures, similar to heavy bodies. Also, sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) showed that 95-kDa and 38-kDa proteins were the NOR-silver-staining proteins in sea urchin embryos. These proteins existed during the course of the cell cycles. These results suggest that (1) the cyclic appearance of the CPs or heavy bodies is closely related to the cell cycle as well as the programming of the embryogenesis, but independent of the cycle of cytokinesis and nuclear division; (2) 95-kDa and 38-kDa proteins are the major NOR-silver-staining proteins in sea urchin embryos.