Fertilization and subsequent development in vitro of pig oocytes inseminated in a modified Tris-buffered medium with frozen-thawed ejaculated spermatozoa

Abstract

The present study examined the penetrability of pig oocytes by frozen- thawed ejaculated boar spermatozoa, prepared by the pellet method, coincubated in a modified Tris-buffered medium. Subsequent embryonic development of fertilized oocytes was also determined. Porcine oocyte- cumulus complexes were cultured in BSA-free North Carolina State University (NCSU) 23 medium containing porcine follicular fluid (10%), cysteine (0.1 mg/ml), and hormonal supplements (eCG and hCG: 10 IU/ml each) for 20-22 h. They were then cultured in the same medium but without hormonal supplements for an additional 20-22 h. After culture, cumulus cells were removed and oocytes were coincubated for 12 h with three different (1 x 105, 5 x 105, and 1 x 106/ml) sperm concentrations (experiment 1). In experiment 2, oocytes were coincubated with sperm (5 x 105/ml) for 3, 6, 9, and 12 h. In experiment 3, at 6 h after coincubation with sperm at 5 x 1065/ml concentration, oocytes were transferred into NCSU 23 + 0.4% BSA medium. At 48 and 144 h, cleavage and blastocyst formation rates, respectively, were evaluated. Insemination with 1 x 105/ml resulted in a 40% sperm penetration rate of oocytes with 16% polyspermy. Mean number of sperm (MNS) per oocyte was 1.2 ± 0.1. At 5 x 105 and 1 x 1066/ml, penetration rate (84-87%) and polyspermy (57-64%) increased (p < 0.001), with no difference between the two concentrations. However, MNS per oocyte increased (p < 0.05) with increasing sperm concentration. Penetration rate was 31% at 3 h and increased (p < 0.001) at 6-12 h (80-88%), with no difference between these time points. Polyspermy increased (p < 0.05) in a time-dependent manner up to 9 h, with no difference between 9 and 12 h. Compared to 3 h, MNS per oocyte increased (p < 0.05) at 9 and 12 h, with no mean difference at 6 h. At 48 after culture, the cleavage rate was 40%, and at 144 h, the blastocyst rate was 19%. This study describes the cryopreservation of ejaculated boar semen by the pellet method and the successful in vitro fertilization of pig oocytes by frozen-thawed spermatozoa with subsequent development to the blastocyst stage.