Re‐Vitrification at the Pronuclear and Blastocyst Stage of Porcine Embryos Obtained From Vitrified Immature Oocytes

The aims of this study were to investigate the effects of re‐vitrification at the pronuclear (PN) stage of porcine embryos generated from vitrified oocytes on subsequent development and to clarify if re‐vitrification is more feasible at the PN stage or at the blastocyst stage. Immature porcine oocytes at the germinal vesicle (GV) stage were vitrified/warmed and subjected to in vitro maturation, parthenogenetic activation (PA), and embryo culture. Subsequent parthenotes were either cultured without re‐vitrification for 6 days (GV‐vit group) or were re‐vitrified 8 h after PA at the PN stage (GV‐vit/PN‐revit group), and after warming, cultured for 6 days. Embryo development to the blastocyst stage was compared with a control group processed without vitrification at any stage. Then, blastocysts obtained in each group were vitrified/warmed and their survival was assessed. Blastocyst formation in the GV‐vit/PN‐revit group decreased dramatically ( p < 0.05) compared with the GV‐vit and control groups (3.4% vs. 10.2% and 22.4%, respectively). However, 80.0% of the blastocysts in the GV‐vit group survived re‐vitrification. Hence, after blastocyst re‐vitrification, 8.2% of the vitrified oocytes re‐expanded to transferable embryos. In conclusion, re‐vitrification at the PN stage was detrimental for subsequent development, whereas that at the blastocyst stage was more advantageous.