Effects of physiological oxygen tension on human cumulus-oocyte-complex metabolism during in vitro maturation: an exploratory study

In vitro oocyte maturation (IVM) is an internationally recognized, patient-friendly assisted reproductive technology, particularly valuable for women with polycystic ovary syndrome (PCOS). Biphasic IVM protocols, incorporating a pre-IVM step to promote cytoplasmic maturation under meiotic arrest, outperform standard IVM yet remain inferior to conventional methods involving controlled ovarian stimulation. Considering suboptimal culture conditions, such as supraphysiological oxygen, might be responsible for lower IVM oocyte competence, in this study, we investigated energy metabolism in human sibling cumulus-oocyte-complexes (COCs) through culturing them under different oxygen concentrations during pre-IVM. COCs (n = 239) from follicles < 10 mm, collected in non-PCOS hyper-responder research oocyte donors (n = 21) with minimal (or no) stimulation prior to oocyte retrieval, were allocated for the study. COCs per donor were randomly cultured in both 20% (atmospheric) and 5% (physiological) oxygen. After 24-hours pre-IVM culture, the samples were processed to (i) assess COC metabolic function with Seahorse ATP Production Rate Assay and MitoStress Test, (ii) measure COC ATP concentrations, (iii) analyze spent medium (glucose, lactate, amino acids), and (iv) evaluate oocyte mitochondrial and antioxidative functions (CellROX, JC-1, and mBCl stainings). COCs cultured under 5% oxygen contained higher ATP despite lower mitochondrial respiration, accompanied by steady glucose uptake and increased lactate production, while mitochondrial potential and antioxidative markers remained unchanged. These findings suggest reliance on an alternative energy substrate, such as fatty acids, under low oxygen conditions. By demonstrating improved metabolic efficiency in human COCs under physiological oxygen, this study highlights the need to re-evaluate IVM culture conditions to better mimic the in vivo environment.