Human haematopoiesis occurs at various anatomical sites throughout development, including the yolk sac, the aorta-gonad-mesonephros region, the liver, the placenta and the bone marrow. Cells marked by high expression of CD34 and low CD45 – suggestive of possible HSCs – have been reported in human fetal membranes; however, their exact niche as well as their functional capacity remain untested. In this issue (p. 1399), Alicia Bárcena and colleagues isolate and interrogate this putative HSC population, and demonstrate for the first time that the human chorion contains transplantable, definitive HSCs. The authors carefully separate the chorion and the amnion, and show via fluorescence-activated cell sorting that only the chorion contains the putative HSCs, and only from 15 weeks of gestation. The cells display markers of HSC and primitive haematopoietic progenitors, such as little CD38 and CD133, low levels of CD117 and CD4, and medium to high levels of HLA-DR, CD31, CD90, CD95, TIE2 and CD71. Cells co-expressing CD34 and CD45 antigens are found either in association with mesenchymal stromal cells or with endothelial cells of chorion vasculature . Using in vivo xenotransplantations, the authors demonstrated that the CD34++ CD45low cells possess multilineage long-term HSC activity specifically between weeks 15 and 24 of gestation. This study reveals novel insight into an unexpected niche for HSCs during human development.
