During pregnancy the trophoblast cells of the placenta are the only fetal cells in direct contact with maternal blood and decidua. Their functions include transport of nutrients and oxygen, secretion of pregnancy hormones, remodelling the uterine arteries, and communicating with maternal cells. Despite the importance of trophoblast cells in placental development and successful pregnancy, little is known about the identity, location and differentiation of human trophoblast progenitors. We identify a proliferative trophoblast niche at the base of the cytotrophoblast cell columns in first trimester placentas that is characterised by integrin α2 (ITGA2) expression. Pulse-chase experiments with 5-Iodo-2′-deoxyuridine (IdU) imply that these cells can contribute to both villous (VCT) and extravillous (EVT) lineages. These proliferating trophoblast cells can be isolated using ITGA2 as a marker by flow cytometry and express genes from both VCT and EVT. Microarray expression analysis shows that ITAG2+ cells display a unique transcriptional signature including NOTCH signalling and a combination of epithelial and mesenchymal characteristics. ITGA2 thus marks a niche allowing the study of pure populations of trophoblast progenitor cells.

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