Abstract: The objectives of the present study were first to carry out the cumulus-oocyte complexes (COCs) divided into levels A, B, C and D according to cumulus cells, which levels A and B were cultured in vitro for 24, 32, 38, 44, 48 and 54 h, and C was cultured in vitro for 24, 32, 38, 44, 48, 54 and 60 h, respectively. Subsequently, the oocytes with extruded polar body at different time intervals were induced by application of an AC pulse of 10 V for 5 s followed by 1.6 kV/cm for 60 μs using an Electro Cell Manipulator 2001. These results showed that the maturation rate of COCs of levels A and B arrived at the highest period was no significant difference (83.2% and 78.0%, P＞0.05) in vitro for 48 h, but both were significantly higher than the same level COCs cultured for 24, 32 and 38 h. Different levels of COCs culture same time in vitro, the maturation rate of levels A and B were not significant (P＞0.05), but both were significantly higher than the levels C. The rate of cleavage (78.2%) in vitro culture for 44 h was significantly higher(P＜0.05) than COCs cultured for 24 to 38 h. In conclusion, COCs of levels A and B were more suitable for oocytes cultured in vitro, obtaining a high maturation rate and cleavage, granule cells had significant influence on oocyte maturation. COCs cultured in vitro before 38 h, some oocytes may be observed the polar body, but the developmental rate to cleavage were extremely lower, indicating that oocytes were no really mature. At the same time，oocytes of the developmental maturation could not judge only by excluding the polar body. Oocytes cultured in vitro 44 to 48 h was the best period of in vitro maturation(IVM) and provide a number of high-quality metaphase-II (MⅡ) oocytes for production of parthenotes and somatic cell nuclear transfer(SCNT) embryos.

Key words: COCs; IVM; polar body; parthenogenetic activation