Fig. 1.

The timing of vitrification (associated or not with oocyte in vitro maturation) coincides with DNA methylation changes taking place during gametogenesis and embryogenesis. DNA methylation changes described in humans are represented here through full lines. The progenitors of the mouse and human germline (PGCs, Primordial Germ Cells) undergo a marked first genome-wide DNA demethylation. The gametic re-methylation will then be different between the two sexes. Indeed, on the male side, de novo methylation of germ cells is initiated and almost complete during prenatal development while on the female side, oocytes remain hypomethylated throughout the fetal period. After the puberty, DNA methylation is then acquired during the growing phase of the oocyte cohort. Following fertilization, maternal and paternal epigenomes introduced by the gametes must be reset a second time (second wave of demetylation) to establish the pluripotency that is required for development embryonic lineages. However, methylation of DNA acquired in the germ line at the ICRs will be maintained after fertilization to ensure sex-specific and monoallelic expression of imprinted genes