Factors affecting the developmental competence of mouse oocytes grown in vitro: oxygen concentration

The purpose of this study was to assess the effects of oxygen concentration on the developmental competence of mouse oocytes grown in vitro because oxygen has been shown to affect the nuclear maturation of oocytes (Haidri et al., 1971: J Reprod Fertil 26:409-411) and preimplantation embryo development (Whitten, 1971: Adv Biosci 6:129-139). Oocyte--granulosa cell complexes were isolated from the preantral follicles of 12-day-old mice and cultured for 10 days in serum-free medium equilibrated with 5, 10, 15, or 20% O2. Five percent CO2 was used for all groups. Oocytes from all groups were then matured and fertilized, and preimplantation embryos cultured using 5% O2. With increased oxygen tension, there were dramatic decreases in the percentage of (1) oocytes that survived in vitro culture, (2) surviving oocytes that could resume meiosis and undergo germinal vesicle breakdown (GVB), and then cleave to the two-cell stage, and (3) two-cell-stage embryos that completed the blastocyst transition. For example, 42% of the oocytes grown in a 5% O2 atmosphere cleaved to the two-cell stage compared with only 3% when the oocytes grew in an atmosphere of 20% O2. These dramatic effects of elevated oxygen were mitigated by either increased numbers of the oocyte-granulosa cell complexes in the culture or increased concentration of oxygen late in the culture period, after the oocytes became surrounded by greater numbers of granulosa cells. When the culture period was extended for 4 days beyond the standard 10 day cultures used in the experiments described above, there was an increased occurrence of precocious GVB or failure of the somatic cells to maintain meiotic arrest. This was prevented by increased oxygen concentration. Nevertheless, extending the time of culture even in the presence of elevated oxygen failed to increase oocyte growth to the equivalent of in vivo-grown oocytes or to improve the developmental competence of the in vitro-grown oocytes. It was concluded that concentrations of O2 above 5% have a deleterious effect on oocyte development during the early stages of culture. However, increasing the concentration on O2 during the later stages, in conjunction with other treatments to promote normal granulosa cell development and function and their interaction with the oocyte, may be critical to promote normal oocyte development in vitro.