Figure 1

General view of the main epigenetic reprogramming pathways in preimplantation development. (A) DNA methylation reprogramming in the mouse embryo. Active demethylation happens in paternal PN(pronucleus) followed by passive demethylation of the entire genome during cleavage stage. Then comes de novo methylation when the embryo commences cell differentiating. (B) Histone modification reprogramming, from fertilization to syngamy. In the paternal PN, sperm nuclear protamines are gradually replaced by highly acetylated histones, which are immediately substituted by monomethylated ones. Dimethylated and trimethylated histones appear later. In the maternal PN, all the modifications listed can be detected in this period. H3K9me3 is found in centromeric major satellites, whereas H3K9me2 is found in minor satellites.