Not everyone is onboard with the new Facebook timeline, but here’s a timeline you can really get into. Researchers are reporting the first genome-scale, base-resolution timeline of DNA methylation in the mouse embryo, including an analysis of the DNA methylation of oocytes and sperm.
To do this, the team analyzed mouse oocytes, sperm, zygotes, early embryos, and embryos at the early inner-cell-mass (ICM) stage and post-implantation stage with reduced representation bisulfite sequencing (RRBS). Here are a few things they learned:
- Oocytes are hypomethylated compared to sperm, and the oocyte DNA methylation levels closely resemble those of the early embryo.
- The most dramatic changes happen at fertilization (going from a sperm cell to a fertilized zygote means massive demethylation) and after the ICM stage (massive remethylation).
- In zygotes, paternal methylation decreases, while maternal CpGs remain unmethylated.
- There’s a big reduction in methylation going from sperm cell to zygote in long interspersed elements (LINEs), specifically L1Md_T and L1Md_Gf.
- In the zygote, the researchers identified 376 differentially methylated regions (DMRs) from oocytes and 4894 DMRs from sperm. DMRs from oocytes are mostly in CpG island promoters, whereas sperm-contributed ones are mostly intergenic.
The scientists say that this analysis expands on the classical model, which was based on immunohistochemistry observations. They also say that this timeline will help researchers address unanswered questions about early mouse development.
Check out the DNA methylation schedule at Nature, March 2012.