Female blastocysts have been shown to consume more glucose than male blastocysts in both mouse and human following superovulation and in vitro culture. Here we examine the carbohydrate metabolism of the in vivo developed blastocyst to determine if this difference between the sexes is induced by ART.

Female wild-type mice were mated naturally with transgenic males with X-linked eGFP to produce female embryos that express eGFP from the morula stage onwards. Embryos were flushed from the uterus on the morning of day 4 and their morphology and total cell number were recorded. One hundred morphologically similar male and female embryos were selected for metabolic analysis. Individual blastocysts were incubated in a drop of modified G2 media buffered with MOPS and serial samples of the medium were taken over 4 hours. Glucose and lactate concentrations were measured using ultramicrofluorescence.

While mean glucose uptake (male 2.34 ± 0.18 and female 2.70 ± 0.18 pmol/hr) and lactate production (male 1.80 ± 0.20 and female  1.63 ± 0.19 pmol/hr) were not significantly different between males and females, the rate of glycolysis was found to be higher in males (P<0.01; 48.7% ± 6.2% and 30.6% ± 6.0%). Cell number was not different (57.44 ± 1.44 and 59.52 ± 1.43). The sex ratio was not significantly skewed, and males and females appeared to be morphologically similar.

These results indicate that the higher glucose uptake in female blastocysts previously reported may be caused by either superovulation or in vitro culture. The higher cell number in male blastocysts often reported from in vitro culture was also not seen in these in vivo produced embryos. However, males did exhibit a different metabolic pattern to females, which is consistent with female embryos having a different proteome.