339 OXYGEN CONCENTRATION DURING IN VITRO MATURATION OF MURINE OOCYTES INFLUENCES SUBSEQUENT FETAL AND PLACENTAL OUTCOMES

Abstract

Although the oxygen environment of the ovarian follicle is thought to influence oocyte developmental competence, little is known of the optimal oxygen environment for oocyte in vitro maturation (IVM). Previously, we found that oxygen concentration (either 2, 5, 10, or 20% O₂; 6% CO₂; and balance of N₂) during IVM of murine oocytes had no effect on maturation rate or subsequent fertilization, cleavage, and blastocyst development rates. However, 2% O₂ results in blastocysts with a higher (P < 0.05) trophectoderm cell number (mean ± SEM, 35.1 ± 2.3) when compared to 20% (19.4 ± 1.7), with 5 and 10% O₂ yielding similar but intermediate cell numbers. When examined for cell apoptosis by TUNEL labelling, the 2% O₂ IVM-derived embryos were also found to have a significantly higher percentage of cells undergoing apoptosis compared to the 5% O₂ IVM-derived embryos and embryos derived from in vivo matured oocytes (unpublished data). Although the blastocyst development rate is not affected by varying oxygen environment during oocyte maturation, the resultant blastocysts exhibited signs of differing quality. The aim of this study was to investigate the effect of varying oxygen during IVM on post-transfer outcomes. Immature cumulus-oocyte complexes (COCs) were collected from the ovaries of eCG-stimulated CBAB6F1 females (21 days) and cultured for 17–18 h under 2, 5, or 20% O₂, whereas in vivo-matured COCs were also collected post-hCG. After IVF/C (both under 5% O₂), 6 blastocysts were transferred to each uterine horn of pseudopregnant Swiss recipients. Fetal and placental parameters were measured on Day 18 of pregnancy. The ability of the embryos to implant or develop was not altered by IVM oxygen concentration. However, the average weight of fetuses derived from 5% O₂ matured oocytes was reduced (823.3 ± 28.1 mg, P < 0.05) compared to those in the 20% O₂ group (928.5 ± 26.1 mg). The average weight of the placenta in the 5% O₂ group was also reduced (87.4 ± 4.0 mg) compared to those derived from in vivo-matured oocytes (104.5 ± 5.4 mg). In contrast, the fetal:placental weight ratio was unchanged in the 5% O₂ treatment, suggesting these placentae, although small, are still efficient. This is the first evidence that programming of fetal/placental growth occurs from treatments applied during oocyte maturation.