DNA methylation in the preimplantation embryo: the differing stories of the mouse and sheep.

In mammals, active demethylation of cytosine methylation in the sperm genome prior to forming a functional zygotic nucleus is thought to be a function of the oocyte cytoplasm important for subsequent normal development. Furthermore, a stepwise passive loss of DNA methylation in the embryonic nucleus has been observed as DNA replicates between two-cell and morula stages, with somatic cell levels of methylation being re-established by, or after the blastocyst stage when differentiated lineages are formed. The ability of oocyte cytoplasm to also reprogram the genome of a somatic cell by nuclear transfer (SCNT) has raised the possibility of directing reprogramming of a somatic nucleus ex ovo by mimicking the epigenetic events normally induced by maternal factors from the oocyte. Whilst examining DNA methylation changes in normal sheep fertilization, we were surprised to observe no demethylation of the sheep male pronucleus at any point in the first cell cycle. Furthermore, using quantitative image analysis, we observed limited demethylation of the sheep embryonic genome only between the two- and eight-cell stages and no evidence of remethylation by the blastocyst stage. We suggest that the dramatic differences in DNA methylation between the sheep and other mammalian species examined call in to question the requirement and role of DNA methylation in early mammalian embryonic development.