Global Chromatin Reprogramming in Fetal Germ Cells Figure 6. Global changes of H3K27me3 in fetal germ cells. Gonad preparations were stained with anti-H3K27me3 antibody. DAPI was used to visualize nuclei (blue). Other details are as in legend to Figure 1. doi:10.1371/journal.pone.0023848.g006 signals. Male fetal germ cells have less clear chromocenters [10], but the weak DAPI spots largely colocalized with H3K79me3 and did not colocalize with H3K79me2 (Figure 9). Discussion Dynamic global chromatin changes in fetal germ cells The major finding of this study is that chromatin undergoes dynamic global changes in male fetal germ cells at the time of global DNA methylation reprogramming (Figure 10). The dramatic chromatin changes in prospermatogonia occurred at 15.5 dpc and thus preceded the wave of CpG methylation peak at 17.5 dpc. Importantly, global chromatin changes were not apparent in female fetal germ cells that do not accumulate DNA methylation until later, at the growing oocyte stage. The global increase in prospermatogonia was not limited to heterochro- matin components, but also included marks that are typically found in euchromatin. Particularly interesting is that global PLoS ONE | www.plosone.org 7 August 2011 | Volume 6 | Issue 8 | e23848