High incidence of polyspermy is still a major problem in the in vitro fertilization (IVF) of porcine oocytes matured in vitro. This study was designed to examine whether embryo cryopreservation straws can be used to conduct IVF in porcine oocytes. The efficiency of this system was further compared with traditional microdrop IVF. Immature oocytes were aspirated from antral follicles and matured in vitro. After maturation, oocytes were inseminated either in straws or in microdrops with frozen-thawed boar spermatozoa. For straw IVF, sperm concentration and the presence of air columns between insemination segment and oil column were examined. Sperm-oocyte binding and cortical granules (CGs) before and after sperm penetration were examined by confocal microscopy. When various sperm concentrations were used for IVF in the straws with air columns, it was found that 5 × 106 cells/ml of sperm concentration was the optimal concentration; a high penetration rate (94.0%) and normal fertilization (oocytes with both male and female pronuclei) rate (38.2%) were obtained. Increasing sperm concentration to 10 × 106 cells/ml increased polyspermic penetration (61.9%) without affecting sperm penetration (86.9%). Reducing sperm concentration to 1 × 106 cells/ml reduced polyspermic penetration (25.6%), but sperm penetration rate (69.9%) was also reduced. When IVF was conducted in the straws with or without air columns, and in the microdrops, it was found that sperm penetration in the straws with air columns (96.5%) was significantly (p < 0.05) higher than that in the straws without air columns (81.7%) and in the microdrop (72.9%). However, the incidence of polyspermic penetration in the straws with air columns (34.2%) and without air columns (36.6%) was significantly (p < 0.05) lower than that (52.4%) in the microdrops. The number of spermatozoa bound to the oocytes was increased gradually in the straws but not in the microdrops in which more spermatozoa bound to the oocytes soon after insemination. CG exocytosis was more complete and faster in the oocytes inseminated in the straws than in the microdrops. These findings indicate that IVF of porcine oocytes in the straws provides a better condition in which more oocytes are fertilized normally than that in the microdrop IVF.
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