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

To elucidate the developmental differences occurring after in vitro fertilization (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\p=n-\4h from 2 to 32 h after in vitro insemination to study sperm penetration, male and female pronucleus formation, synkaryosis and first cleavage. The penetration rate was significantly higher (P < 0.05) for in vivo matured oocytes (69.8%) than for in vitro matured oocytes (35.0%). Penetration of spermatozoa into the ooplasm was first recorded 6 h (in vitro matured oocytes) and 4 h (in vivo matured oocytes) after addition of the spermatozoa to the oocytes. For both in vivo and in vitro matured oocytes, 2 h were required for sperm head decondensation. However, maximum sperm head decondensation occurred 2 h later in in vitro matured oocytes. Within 6 h, 41.7 \m=+-\5.6 % of the in vivo matured oocytes had completed second meiotic division, whereas only 20.8 \m=+-\6.5 % of the in vitro matured oocytes reached this developmental stage (P < 0.01). For in vitro matured oocytes, male pronucleus formation was retarded 2\p=n-\4h after onset of insemination and develop-ment of the female pronucleus was enhanced compared with in vivo matured oocytes. Synchronized opposing pronuclei were observed 14 h after insemination in in vitro matured oocytes and after 8 h in in vivo matured oocytes. Synkaryosis was first observed at 16 and 18 h in in vivo and in vitro matured oocytes, respectively. First cleavage was observed 32 h(in vitro matured oocytes) and 28 h (in vivo matured oocytes) after insemination. It is concluded that under our IVF conditions, oocytes matured in vitro display lower penetration and cleavage rates and asynchronous pronucleus development, as well as delayed cleavage, compared with oocytes matured in vivo