Implication of glucose 6 phosphate isomerase (EC 5.3.1.9) activity in blocking segmentation of the mouse ovum at the 2 cell stage in vitro

The mouse embryo, when grown in media containing glucose, synthesizes and accumulates glycogen. In certain strains, glucose could be responsible for the 2 cell block; it can be replaced for a short time by glutamine, but with an increase in embryo degeneration. We have tested the hypothesis according to which the problems of glycogen accumulation and the developmental arrest could be due to a metabolic lock involving glucose 6 phosphate isomerase (EC 5.3.1.9). Fructose is located immediately downstream from this metabolic lock. We have exactly replaced the glucose in Whittingham M16 culture medium by fructose: Medium F.M. With this culture medium, Swiss one cell embryos, which normally block in M16 medium (greater than 90%), do not block any longer at the 2 cell stage (11.6%). They can be cultured for 4 days with high rates of blastocyst formation (53.6%). Embryo degeneration (23.6%) is lower than that observed in CZB (31%), and not different from our control degeneration in vivo. Moreover fructose seems to be adequate all through the culture period as there is no need for further addition of metabolites, to obtain blastocysts, as observed for CZB. This demonstrates that a lock at the Glucose phosphate isomerase level is responsible for glycogen accumulation and the 2 cell block in the mouse embryo in vitro.