Effects of electrical stimulation before or after in vitro fertilization on sperm penetration and pronuclear formation of pig oocytes

The effects of exposure of pig oocytes to an electrical pulse on sperm penetration and pronuclear formation were determined before or after in vitro fertilization (IVF). After in vitro maturation (IVM) or after collection from oviducts of unmated gilts, pig oocytes either were not exposed or were exposed to an electrical pulse (a 10 sec pulse at 4.0 V mm⁻¹ AC followed by a 30 μsec pulse at 120 V mm⁻¹ DC), followed 30 min later by IVF. The incidence of male pronuclear formation of both IVM and in vivo‐matured oocytes at 12 hr after insemination was decreased from 59% and 100%, respectively, to 2% and 36%, respectively, by the electrical pulse, but the penetration rates (88–100%) and polyspermic rates (79–100%) were not affected by exposure to an electrical pulse. Similarly, when pig IVM oocytes were exposed to an electrical pulse at 6 hr after insemination, electrical activation did not decrease penetration rates (93% vs. 90%), polyspermic rates (83% vs. 91%), or number of spermatozoa in penetrated oocytes (4.0 ± 0.5 vs. 4.6 ± 0.5) but did decrease the rate of male pronuclear formation from 58% to 18%. When oocytes were examined at 6 hr after insemination, 75% of them had been penetrated and resumed meiotic progression, but all sperm heads in penetrated oocytes were fully condensed or only partially decondensed. The percentage of penetrated eggs with multiple female pronuclei was increased when oocytes were exposed to an electrical pulse in all experimental series. In summary, electrical activation of pig oocytes before or just after IVF does not prevent sperm penetration but does inhibit male pronuclear formation and increases the formation of multiple female pronuclei. © 1993 Wiley‐Liss, Inc.