Oxygen concentration during mouse oocyte in vitro maturation affects embryo and fetal development

Background: Little is known of how the oxygen environment in the ovarian follicle affects oocyte and embryo development, but this has an important impact on the conditions used for in vitro maturation (IVM) of oocytes. We investigated the effect of varying oxygen concentrations during IVM on subsequent pre and post- implantation development. METHODS: IVM of mouse cumulus- oocyte complexes (COCs) was performed under 2, 5, 10 or 20% O-2 (6% CO2, balance N-2). In vivo- matured COCs were collected post ovulation. Embryos were generated by IVF and culture. Blastocyst development, cell number and apoptosis were assessed, and fetal and placental outcomes analysed following embryo transfer at day 18 of pregnancy. Results: Oxygen concentration during IVM did not affect oocyte maturation or subsequent fertilization, cleavage and blastocyst development rates. Maturation of oocytes under 2% O-2 increased blastocyst trophectoderm cell number compared with all groups and numbers at 5% were higher than 20% (both P < 0.05). Percentage of apoptotic cells was increased in blastocysts developed from 2% O-2-matured oocytes, compared with maturation at 5% O-2 or in vivo (P < 0.05). Rates of embryo implantation and development into a viable fetus were not altered by IVM oxygen. However, fetal weight was reduced following oocyte maturation at 5% O-2 compared wiht 20% O-2 and maturation at 5% O-2 also reduced placental weight, when compared with in vivo- matured oocytes (both P < 0.05). Conclusions: Level of O-2 exposure during oocyte maturation can alter the cellular composition of blastocysts, but these changes in cell number do not correlate with the altered fetal and placental outcomes after transfer.