Effects of the duration of exposure to hormone supplements on cytoplasmic maturation of pig oocytes in vitro.

The effect of hormone supplements on cytoplasmic maturation in vitro was examined by incubating oocyte-cumulus complexes, in a medium with PMSG (10 iu ml-1), hCG (10 iu ml-1) and oestradiol (1 microgram ml-1) for various periods and then transferring them to medium without added hormones for the remainder of the maturation period. Exposure of oocyte-cumulus complexes to hormone supplements for 2 h improved only germinal vesicle breakdown and maturation rates compared with complexes not exposed to added hormones. The removal of hormone supplements at 20 h after the start of culture enhanced the ability of oocytes to form male pronuclei 10 to 12 h after insemination. Further, the effects of transfer of intact and oocytectomized oocyte-cumulus complexes to hormone-free medium at 20 h on cumulus expansion were examined. The diameter and morphology of the intact oocyte-cumulus complexes were improved after the removal of oocyte-cumulus cell complexes from hormonal exposure. The responses of oocytectomized oocyte-cumulus complexes to hormone were similar to those of intact oocyte-cumulus complexes with the exception of corona radiata expansion. The results suggest that the removal of hormone supplements from maturation media at 20 h after culture enhanced cytoplasmic maturation and cumulus expansion. Further, cumulus expansion does not appear to depend on intercellular communication between cumulus cells and oocytes. Oocytectomy did influence expansion of the corona radiata during culture.