Chromosome remodeling and cytoplasmic distribution during embryonic development in fused pair embryos

Cell fusion is now widely employed as an in vitro model for inducing and investigating cell fate changes. In early embryo research, fusion of embryos is also a way to probe the mechanisms of mammalian oogenesis and preimplantation development. To establish a novel model for porcine early embryo studies and investigate its developmental mechanisms, pair of zona pellucida (ZP)-free oocytes were electro fused to produce the fused pair (FP) embryos, which were further in vitro cultured to the blastocyst stage. Firstly, developmental competence was assessed, revealing a cleavage rate of 92.27 ± 5.59 % and a blastocyst rate of 26.12 ± 6.61 %, which were similar to the parthenogenetic activation (PA) embryos (p > 0.05). Subsequently, nuclear and spindle staining was performed on FP embryos collected at 14-22 h, with 67.18 ± 3.18 % of the embryos spindle reorganization occurred and nuclei fusion, whereas a few displayed independent division of the two nuclei or tripolar spindle. Lastly, the distribution of lipid droplets (LDs) and mitochondria in FP embryos was assessed via fluorescent staining. Results showed that the even distribution of LDs from one oocyte was observed in each blastomere of 4-cell to blastocysts. A similar distribution pattern was observed for mitochondria, which was being observed at the 2-cell stage, a relatively earlier developmental stage than that of LDs. Results suggested that cytoplasm including mitochondria and LDs could redistribute once two oocytes fused into a single embryo. More studies are needed for the underlying mechanism and potential impact on the developmental ability of FP embryos.