During the period of embryonic development, methylation patterns of the germline and somatic cell lineages are established (Maccani & Marsit, 2009). During the cleavage phase, or the early cell divisions that occur as a fertilized egg begins to develop into an embryo, methylation in the zygote’s genome is almost completely removed. After implantation, as the cells produced during the cleavage phase begin to organize themselves, a process called gastrulation, the organism’s methylation patterns are reestablished by de novo methylation (Jaenisch, 1997; Kafri et al., 1992; Monk, Boubelik, & Lehnert, 1987; Razin & Shemer, 1995). Such patterning and repatterning of methylation marks also occurs in trophoblast lineages, the various specialized cells comprising the placenta (Jaenisch, 1997). Proper setting and resetting of methylation marks throughout development is key for the proper health and development of the embryo.
