Alterations in mouse embryo intracellular pH by DMO during culture impair implantation and fetal growth

The preimplantation embryo is highly susceptible to in-vitro stress, and although this does not necessarily perturb blastocyst development, it can significantly affect embryo physiology and the ability to form a viable pregnancy. This study determined that the preimplantation mouse embryo is highly sensitive to a small decrease in intracellular pH (<0.2 pH units). Embryos cultured in media containing a weak acid (5,5-dimethyl-2,4-oxazolidinedione; DMO) formed blastocysts with decreased cell number and inner cell mass number, as well as increased apoptosis, even though blastocyst development and morphology were unchanged. Interestingly, the effects were similar regardless of whether the pH stress was present for a short-term ‘acute’ exposure (during the zygote to 2-cell, or 2-cell to 8-cell division) or an extended ‘chronic’ period of time (continually from the zygote to the blastocyst stage). Exposure to DMO during the first cleavage division did not alter implantation; however, fetal weight and crown–rump length were significantly decreased (P < 0.05). In contrast, continuous exposure to DMO throughout preimplantation development reduced not only implantation but also fetal weight and crown–rump length. This study highlights the importance of correct intracellular pH and demonstrates that slight deviations can significantly impact embryo development and viability.The early embryo is known to be sensitive to its environment. This study determined the effect of a small alteration to pH of the culture environment on the ability of the embryo to grow and develop into a successful pregnancy. We found that the younger the embryo was the less capable it was to cope with the stress. Also embryos that appeared normal by observation were often not and by closer examination were showing signs of stress within the cells of the embryo resulting in poor pregnancy outcomes.