WORLD OF REPRODUCTIVE BIOLOGY Human

After fertilization, the methylation status of the genome changes substantially when it undergoes rapid global demethylation. Methyl marks are re-established as cells begin to differentiate after implantation. These events, crucial to gene expression and regulation, are understood mainly from experiments in mice and a few other mammals. Studies in humans have been limited because of a scarcity of gametes and embryos. Two research groups—one led by Jie Qiao [1] and the other by Alexander Meissner [2]—now overcome this limitation with new methods that assess genome-wide methylation using small numbers of cells. The result is an atlas of methylation dynamics in early human embryos. Together, the studies reveal many parallels with mouse DNA methylation. For instance, the DNA of sperm was found to be highly methylated, and that of oocytes moderately methylated. And, like mice, the sperm-derived genome underwent more complete demethylation than the maternal genome. However, the results also revealed some differences between the species—for instance, in mice, demethylation occurs mainly at the 1-cell stage, whereas in human embryos this occurs from fertilization to the 2-cell stage