Cytoplasmic factors influence mitochondrial reorganization and resumption of cleavage during culture of early mouse embryos

The mitochondrial distribution pattern has been monitored in normally cleaving and developmentally arrested preimplantation mouse embryos in vitro and compared with the distribution found immediately after flushing from the oviduct in vivo. Mitochondria in normally cleaving embryos in vitro and in vivo were found to be homogeneously distributed throughout the cytoplasm of the blastomeres during interphase. In developmentally arrested embryos in vitro the mitochondria became progressively aggregated and localized in the perinuclear region and the area of the cytocortex immediately adjacent to the plasma membrane. Injection of G2 cell cycle cytoplasmic factor(s) from a cycling 2-cell embryo into an arrested embryo resulted in the re-initiation of normal cleavage. Concomitant with the re-initiation of cleavage, a re-distribution of the aggregated mitochondria to the pattern, associated with normally cycling embryos, was observed. Specific mitochondrial translocations to the mitotic spindle were observed during cleavage. The results have shown that observation of the mitochondrial distribution using the vital stain Rhodamine 123, provides an accurate and reliable prediction of an embryo's ability to proceed through the next cleavage stage and develop in vitro and suggests that the specific association of mitochondria with the mitotic spindle is a prerequisite for normal cleavage.