Oxygen concentration during in vitro maturation of murine oocytes affects blastocyst cell lineage

Abstract

Follicular antral oxygen tension is thought to influence subsequent oocyte developmental competence. Despite this, in vitro maturation (IVM) is routinely performed in either 5 or 20% O2 and while low O₂ has been shown to be beneficial to embryo development in many species, the effect of altering O₂ concentration during IVM has not been adequately investigated. Here we investigated the effects of a range of O₂ concentrations during IVM on meiotic maturation and subsequent embryo development after IVF. Ovaries from eCG-stimulated CBA F1 female mice (21 days) were collected and intact cumulus oocyte complexes (COCs) cultured for 17–18 h under 2, 5, 10 or 20% O₂ (6% CO₂ and balance of N2). Matured COCs were denuded of cumulus cells, fixed and stained (1% aceto-orcein) for visualisation of maturation status. No significant difference in maturation rates between treatment groups was observed. Following IVF (performed under 5% O₂, 6% CO₂ and balance of N₂), no difference in fertilisation rates between treatment groups was observed in a randomly selected cohort 7 h post-fertilisation. There was also no significant difference in cleavage rates after 24 h or ability to reach blastocyst stage after 96 h, with a tendency (P = 0.079) for more blastocysts in 2% O₂. However there was a significant increase in the number of trophectoderm cells present in the resulting blastocysts (P < 0.05) in the 2% O₂ group (35 ± 2.1) compared to 20% O₂ (25 ± 2.8). Our data suggests that O₂ concentration during IVM does not influence nuclear maturation or subsequent fertilisation, cleavage and blastocyst development rates. However, maturation in 2% O₂ significantly alters subsequent cell lineage within blastocysts to favour trophectoderm development. Such skewed trophectoderm cell number may influence embryo viability.