Separation of trophoblastic and vascular cell lineages and vascular maturation in early human placental development

The trophoblast and vascular cell lineages have been studied by immunohistochemistry at 6 and 12-14 weeks of pregnancy. Perivillous and extravillous cytotrophoblasts were specifically stained by anti-cytokeratin 7 whereas endothelial cells were labelled by anti-CD34 at these two stages of pregnancy. Perivillous and extravillous cytotrophoblasts together with erythroblasts showed mitotic figures and anti-Ki67 positive nuclei at the 6th week. In the perivillous cytotrophoblast, the number of Ki67 positive nuclei decreased by 12-14 weeks and the staining was limited to the proximal extravillous trophoblast of cell islands. Some endothelial and perivascular cells were labelled with anti-Ki67 at 12-14 weeks. Erythroblasts did not stain at all at this stage. Endothelial cells bound lectin UEA1 and vessels exhibited a fluorescent signal after anti-myosin staining at 12-14 weeks. These data showed that the cytotrophoblast and endothelial cell lineages are not related from 6 to 12-14 weeks of pregnancy. Because mitotic figures or anti-Ki67 staining were not observed in endothelium or perivascular cells at the 6th week, it seems likely that the endothelial cells committed to vasculogenesis derived from stromal cells. By 12-14 weeks, endothelial cells had nearly achieved their maturation by acquiring 1-fucosyl-binding sites revealed by UEA1-lectin binding and insuring their own renewal by mitosis. The maturation of perivascular cells at 12-14 weeks was shown by the anti-sm-myosin staining. We conclude that placental vasculogenesis involves mesenchymal cells rather than trophoblast. The differenciation of vessels organizing from random plexi to a vascular arborescence may involve paracrine regulatory loops with the trophoblast.