Differential effect of recipient cytoplasm for microtubule organization and preimplantation development in rat reconstituted embryos with two-cell embryonic cell nuclear transfer

In the present study, we examined the developmental ability of enucleated zygotes, MII oocytes, and parthenogenetically activated oocytes at pronuclear stages (parthenogenetic PNs) as recipient cytoplasm for rat embryonic cell nuclear transfer. Enucleated zygotes as recipient cytoplasm receiving two-cell nuclei allowed development to blastocysts, whereas the development of embryos reconstituted with MII oocytes and parthenogenetic PNs was arrested at the two-cell stage. Previous observations in rat two-cell embryos suggested that the distribution of microtubules is involved in two-cell arrest. Therefore, we also examined the distribution of microtubules using immunofluorescence. At the two-cell stage after nuclear transfer into enucleated zygotes, microtubules were distributed homogeneously in the cytoplasm during interphase, and normal mitotic spindles were observed in cleaving embryos from the two- to four-cell stage. In contrast, embryos reconstituted with MII oocytes and parthenogenetic PNs showed aberrant microtubule organization. In enucleated zygotes, fibrous microtubules were distributed homogeneously in the cytoplasm. In contrast, dense microtubules were localized at the subcortical area in the cytoplasm and strong immunofluorescence intensity was observed at the plasma membrane, while very weak intensity was detected in the central part of enucleated MII oocytes. In enucleated parthenogenetic PNs, high-density and fibrous microtubules were distributed in the subcortical and central areas, respectively. Pre-enucleated parthenogenetic PNs also showed lower intensity of microtubule immunofluorescence in the central cytoplasm than zygotes. In conclusion, the results of the present study showed that zygote cytoplasm is better as recipient than MII oocyte and parthenogenetic PNs for rat two-cell embryonic cell nuclear transfer to develop beyond four-cell stage. Furthermore, microtubule organization is involved in the development of reconstituted embryos to overcome the two-cell arrest.