Impact of oxygen tension during in vitro maturation : a sibling-oocyte prospective double-blinded study.

To determine whether oxygen tension (20% versus 5%) has an impact on oocyte maturation rates and morphology during in vitro maturation. A prospective, observational, monocentric, sibling-oocyte study. A total of 143 patients who underwent in vitro maturation for fertility preservation purposes from November 2016 to April 2021 were analyzed. Patients were included when ≥ 2 cumulus-oocyte complexes were retrieved. The cohort of cumulus-oocyte complexes obtained for each patient was randomly split into two groups: Group 20% O2 and Group 5% O2. Cumulus-oocyte complexes were incubated for 48h either under 5% O2 or 20% O2. After 24h and 48h of culture, every oocyte was assessed for maturity and morphology, to estimate oocyte quality. Morphology was evaluated considering 6 parameters (shape, size, ooplasm, perivitelline space, zona pellucida and polar body characteristics), giving a total oocyte score ranging from -6 to +6. Maturation rates and total oocyte scores were compared using paired-sample analysis between Group 20% O2 and Group 5% O2. Patient median age was 31.4 year-old [28.1-35.2]. Mean serum AMH levels and antral follicle count were 3.2 ± 2.3 ng/mL and 27.2 ± 16.0 follicles, respectively. A mean of 10.7 cumulus-oocyte complexes per cycle were retrieved, leading to 6.1 ± 2.4 metaphase II oocytes vitrified (total maturation rate = 57.3%; 991 metaphase II oocytes / 1 728 cumulus-oocyte complexes). A total of 864 cumulus-oocyte complexes were included in each group. Oocyte maturation rates were not different between the two groups (Group 20% O2: 56.82% vs. Group 5% O2: 57.87 %, respectively, P = 0.27). Regarding oocyte morphology, the mean total ooocyte score was significantly higher in Group 5% O2 compared to Group 20% O2 (3.44 ± 1.26 vs. 3.16 ± 1.32, P = 0.014). As culture under low oxygen tension (5% O2) improves oocyte morphology during in vitro maturation, our results suggest that culture under hypoxia should be standardized. Additional studies are warranted to assess the impact of oxygen tension on oocyte maturation and the benefit of in vitro maturation under low oxygen tension for embryo culture after utilization of frozen material.

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