(A) H3K27me3 tracks at the Cxcl9, Cxcl10, Cxcl12, Cxcl16, and Csf1 loci. The tracks are the pileups of the 3 independent replicates. The log₂ concentration for each called peak and the TSS with direction of transcription (arrow) are indicated. Sequencing reads from total input chromatin were homogeneous across these loci (not shown). (B–J) H3K27me3 silences Csf1 in DSCs, limiting macrophage accumulation in the decidua. (B–D) Immunostaining for F4/80⁺ macrophages (red) in the undecidualized E3.5 uterus 1 day prior to implantation (B), an interimplantation site on E6.5 (C), and an E6.5 implantation site (D). Note the dramatically lower tissue density of macrophages within the decidua as compared with the undecidualized endometrium, consistent with previous results at later gestation (9). Representative images from 3 mice/group; panels C and D show sections near those in Figure 1, H and I, respectively. (E) qRT-PCR determination of Csf1 mRNA expression in stromal cells isolated from E7.5 pregnant mice and cultured for 24 hours (mean ± SEM; n = 3 samples/group). DSCs express lower levels of Csf1 than MSCs, at least in part explaining the lower level of Csf1 expression in the whole decidua (9). (F–J) Effect of ectopic CSF-1 expression within the decidua. Artificially decidualized uteri were injected with Csf1-expressing or empty vector control lentivirus on the day corresponding to E5.5 and the mice were sacrificed 2 days later. Viral preparations included aliquots of EGFP reporter lentiviruses to identify transduced areas. Representative images of serial sections immunostained for F4/80 or GFP (F–I) and mean ± SEM of quantified F4/80⁺ cell densities in infected (GFP⁺) and uninfected (GFP^–) decidual areas (J) from 4 control virus–infected mice and 5 Csf1 virus–infected mice. Note that decidual areas infected with Csf1-expressing lentiviruses accumulate macrophages, demonstrating a CSF-1 deficit within this tissue layer. Asterisk indicates decidual lumen.