Global Chromatin Reprogramming in Fetal Germ Cells KDM1B is required for the establishment of maternal methylation imprints at some DMRs in oocytes [31]. The DNA de novo methylation cofactor, DNMT3L [34] requires a DNA substrate associated with unmethylated H3K4 [35]. It is not known if the presence of any histone mark is required for global de novo methylation or for imprint establishment. We hypothesized that chromatin composition may provide a framework for global de novo methylation and may trigger imprint establishment in the germ line. We used immunocytochemistry to examine global changes of DNA methylation and chromatin composition in male and female mouse fetal germ cells and somatic cells of the gonad. We expected that fetal germ cells exhibit dynamic changes in chromatin composition compared to somatic cells of the gonad because the latter are not known to undergo global epigenetic Figure 1. Global changes of DNA methylation in fetal germ cells. Germ cells (marked by arrowheads) distinctly stain positive using germ cell markers, Oct3/Oct4 or Ddx4. The Oct3/Oct4 marker was used at 11.5 dpc and the Ddx4 marker was used at later stages. Anti-5mC antibody was used to stain methylated DNA. Anti-DNA antibody was used to visualize nuclei. Cells were obtained from (A) Male gonads (B) Female gonads. Duplicates are shown for each developmental stage, indicated to the left in dpc. (C) The relative staining intensities of female and male germ cells (GC) to the respective gonadal somatic cells (SC) are depicted in the chart with standard deviations. doi:10.1371/journal.pone.0023848.g001 PLoS ONE | www.plosone.org 2 August 2011 | Volume 6 | Issue 8 | e23848