Figure 1.

Schematic representation of the changes occurring in the chromatin of the zygote. The blastomeres of the two-cell embryo are the first truly totipotent cells. They originate from two transcriptionally repressed cells and acquire their peculiar capacity through the changes occurring in the zygote. The main necessary modifications concerning the remodeling of the chromatin of the two parental genomes are: erasure of the epigenetic marks that typify the genome of gametes; maintenance of epigenetic marks in the imprinted genes. This extensive reprogramming is carried out by the maternal factors stored in the oocyte.