Mouse placental structure and expression of MT-MMPs. (A) Schematic representation of mouse placental development between E9.5 and E10.5. At E9.5, the fetal portion of the placenta is of limited size, but CHs rapidly expand the fetal portion of the placenta at this time. Fetal vessels (FVs) make their way into the prospective LA, and after differentiation of CHs, the vessels are ensheathed in 2 layers of syncytiotrophoblasts that separate the FVs from the adjacent maternal blood sinuses (MSs). This structure facilitates nutrient and gas exchange. (B-C) Hematoxylin and eosin (H&E)–stained cross sections of wild-type mouse placentas at E10.5 with abundant FVs containing nucleated red cells and MSs with enucleated erythrocytes. The yellow dotted lines in panels B-C represent the approximate border between the fetal portion of the placenta and the decidua (maternal portion). (D) Darkfield image of section serial with that shown in panel B hybridized to antisense probe (AS) for MT1-MMP. Note the abundant signal in the embryonic mesenchyme, the LA and the decidua. (E) Darkfield section serial with that shown in panel C hybridized to MT2-MMP AS. Note the signal in the trophoblasts and the limited or absent signal in decidua. (F-G) Brightfield images of sections shown in panels D-E. Note that the signal shown here is pseudocoloration of the darkfield signal projected onto the brightfield image. Scale bar (B-G): 200 μm.