Differences in pronucleus formation and first cleavage following in vitrofertilization between pig oocytes matured in vivoand in vitro

To elucidate the developmental differences occurring after in vitrofertilization (IVF) of pig oocytes matured either in vitro(n= 1934) or in vivo(n= 1128), the present experiment investigated the morphological changes from penetration to the two-cell stage. Oocytes were examined every 2–4 h from 2 to 32 h after in vitroinsemination to study sperm penetration, male and female pronucleus formation, synkaryosis and first cleavage. The penetration rate was significantly higher (P< 0.05) for in vivomatured oocytes (69.8%) than for in vitromatured oocytes (35.0%). Penetration of spermatozoa into the ooplasm was first recorded 6 h (in vitromatured oocytes) and 4 h (in vivomatured oocytes) after addition of the spermatozoa to the oocytes. For both in vivoand in vitromatured oocytes, 2 h were required for sperm head decondensation. However, maximum sperm head decondensation occurred 2 h later in in vitromatured oocytes. Within 6 h, 41.7 ± 5.6% of the in vivomatured oocytes had completed second meiotic division, whereas only 20.8 ± 6.5% of the in vitromatured oocytes reached this developmental stage (P< 0.01). For in vitromatured oocytes, male pronucleus formation was retarded 2–4 h after onset of insemination and development of the female pronucleus was enhanced compared with in vivomatured oocytes. Synchronized opposing pronuclei were observed 14 h after insemination in in vitromatured oocytes and after 8 h in in vivomatured oocytes. Synkaryosis was first observed at 16 and 18 h in in vivoand in vitromatured oocytes, respectively. First cleavage was observed 32 h (in vitromatured oocytes) and 28 h (in vivomatured oocytes) after insemination. It is concluded that under our IVF conditions, oocytes matured in vitrodisplay lower penetration and cleavage rates and asynchronous pronucleus development, as well as delayed cleavage, compared with oocytes matured in vivo.