Male and female embryos differ in their response to oxygen concentration

Abstract

Male and female preimplantation embryos exhibit differences in their physiologies due to the presence of a second active X-chromosome in female embryos prior to implantation. Here we evaluate the impact of oxygen concentration on the morpho-kinetic development of male and female mouse embryos. Time-lapse analysis of embryo development, followed by determination of embryo sex and blastocyst cell number after embryo culture in either 5% or 20% oxygen. Male C57BL/6 mice with an X-linked enhanced green fluorescent protein (eGFP) were used as studs to create females that expressed eGFP from the 8-cell embryo stage. Pronucleate oocytes (n=500) were cultured individually for 48 h in 2 μL drops of G1, followed by 48h in G2, under paraffin oil at 37°C in a multi-gas imaging incubator (Sanyo MCOK-5M[RC]) set to 6% CO2, 5% O2, 89% N2 or 6% CO2 in air (20% oxygen). Morphology of each embryo was assessed every 15 minutes and resultant blastocyst cell number determined. In 5% oxygen the times between divisions of male embryos were similar to females. However, in 20% oxygen female embryo development was characterized by significant delays in cleavage times from the 7-cell stage onwards (P< 0.05), culminating in delayed time of blastocoel formation (P<0.01). Mean blastocyst cell number for male and female embryos in 5% oxygen were 113.6 ± 2.1 and 107.5 ± 2.1 respectively (P<0.05; a difference of 3.6%). At 20% oxygen, blastocyst cell numbers for male and female embryos were 40.3 ± 2.0 and 34.7 ± 2.1 respectively (P<0.01; a difference of 14.0%). Confirming previous reports, it was found that atmospheric oxygen had an adverse effect on the developmental kinetics of the preimplantation mouse embryo. It was further determined that development of female embryos not only lagged behind their male counterparts, but that female embryos appear to be more sensitive to a high oxygen concentration.