DNA methylation dynamics during oocyte and embryo development and its association with environmental induced alterations

Epigenetic mechanisms are fundamental to successful gametogenesis and development. The fertilized egg undergoes global DNA demethylation to facilitate remodelling from two differentiated gametespecific states to a pluripotent embryonic state. Maintenance of appropriate levels of DNA methylation during preimplantation development is essential to embryo viability. Recent advances in epigenetic research have highlighted the susceptibility of foetal epigenetic programming to maternal health and nutritional status, particularly, at the time of conception. There is much evidence that maternal stress impacts on ovarian function, leading to compromised oocytes presented for fertilization in a suboptimal environment. Similarly, declining fertility has become a substantive issue in western countries, where it is primarily associated with high mean ages at childbearing. Thus the use of assisted reproduction technologies (ART) interventions to overcome low fertility is increasing steadily across the globe. In addition, the use of prolonged in vitro culture following the removal and storage of oocytes and/or ovarian tissue in advance of cancer treatment, or to circumvent ovarian aging, is increasing rapidly. ART is associated with compromised pre and post -natal outcomes, including premature birth, low birth weight, congenital abnormalities and elevated risk of epigenetic disorders. There is extensive evidence from studies in cattle that embryos produced by conventional ART protocols are susceptible to errors in epigenetic programming. The present review discusses the impact of intrinsic physiological status and external environments on oocyte and embryo DNA methylation with regard to data available from mouse, human and bovine models.