Simplified cryopreservation of porcine cloned blastocysts

Abstract

Recently, a non-invasive delipation (lipid removal) method combined with ultrarapid vitrification has been used successfully for in vitro produced (IVP) porcine embryos. In the present study, this method was combined with parthenogenesis and a recent form of somatic cell nuclear transfer (SCNT)—handmade cloning (HMC)—to establish a simplified and efficient cryopreservation system for porcine cloned embryos. In Experiment 1, zonae pellucidae of oocytes were partially digested with pronase, followed by centrifugation to polarize lipid particles. Ninety percent (173/192) oocytes were successfully delipated in this way. Parthenogenetic activation (PA) after complete removal of zona resulted in similar blastocyst rates in delipated vs. control oocytes (28 ± 7% vs. 28 ± 5%, respectively). Subsequent vitrification of produced blastocysts with the Cryotop technique resulted in higher survival rates in the delipated group compared to the control group (85 ± 6% vs. 32 ± 7%, respectively; P < 0.01). In Experiment 2, delipated oocytes were used for HMC with normal oocytes as control. Partial zona digestion was further applied before enucleation both in delipated and control groups, to bisect oocyte successfully. Although the blastocyst rate of reconstructed embryos was similar between groups derived from delipated vs. control oocytes (21 ± 6% and 23 ± 6%, respectively), after vitrification higher survival rates were achieved in the delipated groups than in controls (79 ± 6% vs. 32 ± 8%, respectively). Our results prove that porcine embryos produced from delipated oocytes by PA or HMC can be cryopreserved effectively by ultrarapid vitrification. Further experiments are required to assess the in vivo developmental competence of the cloned-vitrified embryos.