Figure 2.

Macrophage polarization in the non-pregnant and pregnant uterus. The number and function of macrophages in the non-pregnant uterus changes during the menstrual cycle with numbers peaking during menses where they play a role in the breakdown of the endometrial functional layer through secretion of MMPs as well as debris clearance through phagocytosis. Upon exposure to seminal fluid, the endometrium is thought to induce an inflammatory response resulting in the recruitment of macrophages that display an M1-like phenotype. As pregnancy proceeds and extravillous trophoblasts (EVTs) invade the decidual stroma, decidual macrophages (DMs) shift towards a mixed M1/M2 profile and eventually a predominantly M2 phenotype to prevent fetal rejection. Placental macrophages (PMs) additionally display an M2 phenotype and aid in regulation of angiogenesis in the feto-placental vasculature through secretion of VEGF and fibroblast growth factor (FGF)2. Term labor is associated with an increased number of M1 macrophages and inflammatory cytokines in the decidua and it is this inflammatory environment which is thought to induce labor.