Fig. 1

Embryo development and epigenetic reprogramming cycle. Epigenetic modifications take place in two phases during the embryo development. Fertilization signals the reprogramming during preimplantation development. I-A In preimplantation development just after fertilization, DNA demethylation takes place in male pronucleus but female pronucleus remains unchanged. I-B Thereafter, both genomes are passively demethylated, except at imprinted genes and some transposons, for several rounds of cell division. This demethylation occurs due to disruption of maintenance methylation mechanism. I-C The genome is de novo methylated around the blastocyst stage, which responsible for differentiation of the first two lineages of the blastocyst stage, the inner cell mass (ICM) and the trophectoderm. All embryonic lineages differentiate from the ICM. II-A PGCs arise from somatic tissue and develop into mature gametes during gametogenesis stage. Their genome undergoes DNA demethylation in the embryo between day 11.5 and 12.5, including all imprinted genes. II-B Following demethylation, the genomes of the gametes are de novo methylated and acquire imprints; this process continues up to 18.5 in males and in maturing oocytes before ovulation in females