The human placenta contains macrophage progenitors before the onset of fetoplacental circulation. (A) IHC of placental sections revealed the progressive migration of FXIII⁺ macrophages (arrows) from the chorionic plate (i) into the villi (ii′-iii′). Images were acquired at 200× original magnification. (B) Methylcellulose colony- forming assays documented the presence of clonogenic macrophage progenitors in the chorionic plate of the human placenta before the onset of fetoplacental circulation. A representative macrophage colony (original magnification, ×100) and May-Grunwald Giemsa–stained cytospin of a macrophage colony (original magnification, ×400) are shown. (C) PCR performed on single colonies from methylcellulose for the Y chromosome was able to demonstrate the fetal origin of placental colonies from male (sample 1, XY) but not female (sample 2, XX) specimens. In the case of an XX specimen, informative microsatellites, defined by differences in the number of repeats contained in alleles (Al1 and Al2) inherited from each parent could be used to verify the fetal origin of placental progenitors. Of 176 colonies picked and analyzed (ranging from precirculation to 8 weeks developmental age), none was found to be maternal (data not shown). (D) Immunofluorescent staining of placental sections demonstrated the presence of macrophage-committed progenitors (CD34⁺CD43⁺FXIII⁺ and CD34⁺CD43⁺CD68⁺ cells; i and i′ insets, respectively) in the chorionic plate of precirculation specimens. Later in development, placental macrophages no longer expressed CD34. V indicates blood vessels; scale bars, 20 μm.