Birds and Mammals
In order for the zygotic genes to be activated, the parental chromatin undergoes many changes. New histones are placed on the DNA during the early cell divisions, and the gamete-specific DNA methyl groups are removed (except for those on imprinted genes). In both mice and human embryos, the DNA methylation of sperm and egg chromatin is almost entirely removed. While some “imprinted gene” methylation remains, that which is concerned with cell differentiation appears to be removed. This allows an almost “clean slate” for the newly forming blastocyst cells. New DNA methylation patterns characteristic of totipotent and pluripotent cells are established (Abdalla et al. 2009; Guo et al. 2014; Smith et al. 2014). Thus, by the 16-cell stage, the genome of each cell is hypomethylated and each of these 16 cells appears to be pluripotent (Tarkowski et al. 2010). The stage is now set for cell differentiation to take place.
Abdalla, H., Y. Yoshizawa and S. Hochi 2009. Active demethylation of paternal genome in mammalian zygotes. J. Reprod. Dev. 55: 356–360.
Guo, H. and 24 others. 2014. The DNA methylation landscape of human early embryos. Nature 511: 606–610.
Smith, Z. D. and 7 others. 2014 DNA methylation dynamics of the human preimplantation embryo. Nature 511: 611–615.
Tarkowski, A. K., A. Suwiska, R. Czolowska and W. Ozdzeski. 2010. Individual blastomeres of 16- and 32-cell mouse embryos are able to develop into foetuses and mice. Dev. Biol. 348: 190–198.