After fertilization, the paternal pronuclear is actively demethylated in one-cell stage (red line), inversely, the 5hmC level (blue line) increases in the paternal pronuclear.
Whereas the maternal pronuclear maintains a hypermethylation state in one-cell stage and is passively demthylated in a replication-dependent manner (red dotted
line). Immunofluorescence analysis in one-cell stage embryos identifies a consistent expression of several histone modifications, such as H3K4me3, H3K27me3 and
H3K9me2 (green and orange box) in the maternal pronuclear, while the new incorporated histones in the paternal pronuclear acquire these modifications gradually
during the developmental window. From two-cell to blastocyst stage, 5mC and 5hmC are diluted along with replication of cells and reach the lowest point at blastocyst
stage (red dotted line and blue line, respectively). During this peroid, the paternal X chromosome is exclusively inactivated (cyan box). After implantation, genome
in extraembryonic issues maintains a hypomethylation level (dark green line) and the paternal X chromosome is imprintedly inactivated (cyan box). However, ICM/
epiblast cells restore a previous hypermethylation level (purple line). And these cells reactivate the imprinted inactivated X chromosome in 24 hours (E3.4-E4.5)
(sky blue box). Then the two parental X chromosomes in epiblast cells are randomly inactivated (red box); While in PGCs, a second wave of genome-wide DNA
demethylation is initiated, which is catalyzed by Tet1 and Tet2 (black and gray box), with a decline of 5mC (purple line) and an increase of 5hmC (gray line). During
the process, H3K9me2 disappears gradually, and H3K27me3 and H3K9me3 accumulate soon afterwards (blue and yellow box). After entry into the gonads, PGCs
have a concomitant loss of H3K27me3 and H3K9me3 (blue and yellow box). Accompanied with the loss of DNA methylation and repressed histone modifications,
randomly inactivated X chromosome is reactivated in a replication-dependent way (sky blue box). Following gender-determination, de novo DNA methylation takes
places in an asymmetrical pattern in male and female germ cell precursors. In male embryos, new DNA methylation is established before meiosis and is complished
before birth (green line). While in female embryos, de novo DNA methylation is not initiate until birth and is established during the postnatal growth stage of oocytes
(yellow dotted line). |
