Generation of Hematopoietic Stem Cells from Purified Embryonic Endothelial Cells by a Simple and Efficient Strategy

Recent progress by versatile approaches supports the new hypothesis that multi-potent hematopoietic stem cells（HSCs） are directly formed from a rare population of endothelial cells in mid-gestation mouse embryos.This process is therefore known as the endothelial-tohematopoietic transition（EHT）.Nevertheless,there is no functional evidence that documents the HSC transition from purified endothelial cells.In this study,we developed an OP9-DL1-based co-culture system that was able to facilitate the HSC specification and/or expansion in vitro of mouse embryonic day 10.5（E10.5） Tie2+ cells remarkably.Then,the immunophenotypically defined endothelial cells were harvested by a combination of surface markers（Flk1+CD31+CD41-CD45-Ter119-） from the caudal half of E10.0-E11.0 mouse cmbryos.The transplantation of the endothelia/OP9-DL1 co-cultures led to long-term,high-level,multi-lineage,and multi-organ hematopoietic reconstitution in the irradiated adult recipients.The induced HSC activity was initially observed at E10.5,and a significant increase was detected at E11.0,which suggests a temporally specific regulation.Taken together,for the first time,we provide functional evidence showing the HSC potential of purified embryonic endothelial cells,which is indispensable for the emerging EHT concept. Moreover,the newly defined co-culture system will aid the exploration of the key molecules governing the HSC transition from embryonic and even postnatal endothelial cells,which has enormous significance in basic and translational research.