Fig. 2

Epigenetic modification and retrotransposon expression changes in embryogenesis. a Different stages of embryonic development process are showed (from left to right): 1-, 2-, 4-, 8-cells, morula, and blastocyst stages. Embryonic stem cells (ESCs) are derived from the inner cell mass (ICM) of the early blastocysts, and have similar developmental potential as ICM. Inducible pluripotent stem cells (iPSCs) come from differentiated somatic cells and together with ESCs comprise in vitro PSCs; b DNA methylation (green) and H3K9me3 (yellow) levels decrease after fertilization, then increase to normal levels in somatic cells; c The expression of retrotransposon subclasses changes in the embryonic development process, and some typical elements are shown such as MuERV-L(green), Alu(yellow), LINE1(purple), and LTR7-HERVH(blue); d The summary of expression patterns of retrotransposons demonstrated in (c) and related references