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. 2016 Oct 24;7:182. doi: 10.3389/fgene.2016.00182

FIGURE 1.

FIGURE 1

The genome goes through two cycles or waves of epigenetic reprogramming. At fertilization the paternal genome is actively demethylated (blue line), whereas the maternal genome is passively demethylated (red line). As a result most of the blastocyst genome is demethylated by implantation. However, some regions of the genome, like imprinted differentially methylated regions (DMRs) and intracisternal A particles (IAP’s), maintain methylation allowing some epigenetic information to be transmitted to the next generation (gray line). Subsequent to implantation the genome of the developing zygote goes through de novo methylation completing the first wave of reprogramming. The degree of purple shading indicates the degree of methylation in the genome. After the primordial germ cells (PGC) within the developing embryo are specified they will proliferate and migrate to the genital ridge and in doing so will go through genome demethylation. This marks the beginning of the second wave of reprogramming. Upon sex determination and gametogenesis the genome is remethylated at different times in each sex. Germ cells in the male gonad are remethylated in the developing embryo while in utero and the degree of methylation is denoted by blue shading. The prospermatogonia proliferate and at puberty continually progress through meiosis to eventually produce mature spermatozoa. Conversely, female germ cells entire meiosis unmethylated in utero and arrest in prophase 1 just before or at birth. During the follicular growth phase the oocyte genome will be remethylated, germinal vesicle will extrude the first polar body and if fertilized will complete MII. As a result of oocyte remethylation occurs during the follicular growth phase of only the specific oocytes when they are recruited. This figure was generated by combining and modifying images from the following related works: Data from Dean, W., Santos, F., and Reik, W., Seminars in Cell & Developmental Biology, 14, 93–100, Elsevier, 2002; Data from Cowley, M., and Oakey, R., Molecular Cell, 48, 819–821, 2012; Data from Smallwood, S. A. and Kelsey, G. Trends in Genetics, 28, 33–42, 2011; Data from Saadeh, H., and Schulz, R. Epigenetics and Chromatin, 7, 1–15, 2014.