Acquisition of maturational competence in in vitro grown mouse oocytes

The maturational competence of the in vitro grown mouse oocytes of various sizes was examined, and was compared to that of oocytes of equivalent chronological age which grew in vivo. Oocyte-granulosa cell complexes isolated from 10-day-old female mice were cultured for 10 days in vitro in the presence of 2 mM hypoxanthine to maintain the meiotic arrest. When the in vitro grown oocytes were denuded and cultured for further 24 h in the hypoxanthine-free medium, 9, 37, and 100% of oocytes of 60-64.5, 65-69.5, and over 70 microns in diameter underwent germinal vesicle breakdown (GVBD), respectively. Of those oocytes which underwent GVBD, 19, 37 and 68% of oocytes larger than 60, 65, and 70 microns progressed to metaphase II, respectively. No oocyte less than 60 microns resumed meiosis. The parallelism found in the oocyte size and maturational competence was similar to that of their in vivo counterparts collected from 20-day-old mice. A significant proportion of the in vitro grown oocytes, however, formed a pronucleus-like structure(s) after maturation culture, indicating that they could not maintain a normal arrest at metaphase II.