The effect of sperm preparation on the timing of penetration in bovine in vitro fertilization.

Five hundred and seventy three follicular bovine cumulus oocyte complexes were matured in vitro and in vitro fertilization (IVF) carried out with the native semen of a brown bull. Half the cultures were fertilized with transmigrated spermatozoa (TM), the other with sperm suspensions prepared using the swim-up (SU) process. Five, six and seven h after IVF the oocytes were fixed and stained. Using a phase-contrast microscope, both the incidence of penetration and polypenetration and the stage of development of a pronucleus were determined. With the TM samples, five hours after IVF penetration was observed in 39.3 per cent of the oocytes, whereas with the SU samples the first instances of penetration were observed only two hours later (4.2%). By this time a normal-looking pronucleus was observed in 16.4 per cent of the oocytes in the TM group. Polypenetration was seen only after seven hours of incubation. This study showed that TM spermatozoa penetrate the oocyte earlier than SU spermatozoa.