A Single Trophoblast Layer Acts as the Gatekeeper at the Endothelial-Hematopoietic Crossroad in the Placenta

During embryonic development the placental vasculature acts as a major hematopoietic niche, where endolthelial to hematopoietic transition ensures emergence of hematopoietic stem cells (HSCs). However, the molecular mechanisms that regulate the placental hematoendothelial niche are poorly understood. Using a parietal trophoblast giant cell (TGC)-specific knockout mouse model and single-cell RNA-sequencing, we show that the paracrine factors secreted by this single layer of TGCs are critical in the development of this niche. Disruptions in the TGC specific paracrine signaling leads to the loss of HSC population and the concomitant expansion of a KDR+/DLL4+/PROM1+ hematoendothelial cell-population in the placenta. Combining single-cell transcriptomics and receptor-ligand pair analyses, we also define the parietal TGC-dependent paracrine signaling network and identify Integrin signaling as a fundamental regulator of this process. Our study elucidates novel mechanisms by which non autonomous signaling from the primary parietal TGCs maintains the delicate placental hematopoietic-angiogenic balance and ensures embryonic and extraembryonic development. Gata2f/f;Gata3f/f; Pl1Cre +/- males were crossed to Gata2f/f;Gata3f/f females and the embryos were analyzed at E13.5. Single cell RNA sequencing of the placental samples were generated by deep sequencing using the 10x Genomics Chromium Single Cell Gene Expression Solution (10xgenomics.com).