Partitioning of glucose carbon in post-compaction ovine embryos

Day 5 sheep embryos (late morulae and early blastocysts) were surgically recovered from superovulated ewes. Groups of embryos (four to seven) were incubated for 24 h at 39°C under humidified 5% CO2, 5% Oz, 90% N2 in 50-μl microdrops of a substrate-free synthetic oviduct fluid (SOF) medium (12 mg ml-ʹbovine serum albumin, m-SOF) containing 1.0 mM glucose and ± 1.0 mM glutamine. Following incubation, embryos were washed in m-SOF and stored at − 70°C until fractionation. Embryos were fractionated into acid-soluble (glycogen/non-glycogen) and acid-insoluble (desmoglycogen/non-glycogen) fractions. To further characterise glucose carbon partitioning in the acid-soluble fraction, carbohydrates were separated by high performance liquid chromatography (HPLC). Incorporation of glucose into glycogen or desmoglycogen in post-compaction sheep embryos was negligible. The presence of glutamine tended to accelerate morphological development over the 24 h culture period, but had no significant effect on glucose incorporation. Most glucose carbon was associated with the acid-soluble non-glycogen fraction. HPLC revealed that this was predominantly glucose (approximately 40%), in addition to other metabolic intermediates. It is concluded that, unlike the mouse embryo, sheep embryos produce negligible glycogen from exogenous glucose and that most of the glucose carbon incorporated in the embryo is in the form of low molecular weight metabolites or unmetabolised glucose.