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

© CSIRO 2005Follicular 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.K.M. Banwell, M. Lane, D.L. Russell, K.L. Kind and J.G. Thompso