Schematic of DNA methylation dynamic and epigenetic stability during gametogenesis and embryogenesis in humans. DNA methylome reprogramming in germ cells: primordial germ cells (PGCs) in the human become demethylated early in development; from gonadal differentiation to gametogenesis, DNA methylation of spermatocyte and oocyte cells rises gradually until fertilization, at which point sperm reaches a higher methylation level than the oocyte. DNA methylome reprogramming during embryonic development: after fertilization, within the first week, the methylation level of the zygote decreases to the lowest level at the early blastocyst period, during which DNA methylation of the paternal genome reaches a lower level; subsequently, de novo methylation occurs in somatic cell lineages, until they develop into mature somatic cells with stable methylation levels. Epigenetic stability during development: epigenetic stability is proportional to DNA methylation levels. The blue line represents the paternal genome. The red line indicates maternal genome. The gray dashed line denotes mature somatic cells. From red to green, signifies from vulnerable to robust of the epigenome. PGCs, primordial germ cells. Adapted and used with permission from Guo et al. , Hemberger and Pedersen  and Zhu et al.