Effects of lipid polarisation on survival of in vivo-derived porcine zygotes vitrified by the superfine open pulled-straw method.

This study aimed to evaluate the post-warming in vitro viability of intact porcine zygotes vitrified using the superfine open pulled-straw (SOPS) method and to investigate whether cryotolerance is increased by lipid polarisation before vitrification. In vivo-derived zygotes (n=317) were either untreated before SOPS vitrification or subjected to one of the following pre-treatments: (1) centrifugation (20 min, 15000 g) or (2) equilibration in high-osmolality medium (6 min, 400 mOsm kg(-1)) followed by centrifugation. Vitrified-warmed and non-vitrified fresh zygotes were cultured in vitro for 120 h. There were no differences in the blastocyst formation rates between the vitrification groups (from 35.4±5.3% to 48.2±5.6%), but fresh zygotes exhibited higher (P<0.001) blastocyst formation rates (87.5±5.3%) than did vitrified-warmed zygotes. The total blastocyst cell number was similar among all groups (from 34.9±2.8 to 44.1±2.8). In conclusion, SOPS vitrification is a promising method for the cryopreservation of untreated in vivo-derived porcine zygotes. Neither lipid polarisation by centrifugation nor exposure to a high-osmolality medium followed by centrifugation affected the post-warming in vitro viability of zygotes. Our study also demonstrated that the donor is an important factor in determining the success of vitrification for in vivo-derived porcine zygotes.