A CD45-negative population of pre-HSCs develops into definitive HSCs in the AGM region of the embryo.

Key Points • The first embryonic platelets are produced by a unique lineage of diploid cells not polyploid MKs. • Diploid platelet-forming cells are produced in the early mouse embryo via a progenitor cell-independent pathway. In this study, we test the assumption that the hematopoietic progenitor/colony-forming cells of the embryonic yolk sac (YS), which are endowed with megakaryocytic potential, differentiate into the first platelet-forming cells in vivo. We demonstrate that from embryonic day (E) 8.5 all megakaryocyte (MK) colony-forming cells belong to the conventional hematopoietic progenitor cell (HPC) compartment. Although these cells are indeed capable of generating polyploid MKs, they are not the source of the first platelet-forming cells. We show that proplatelet formation first occurs in a unique and previously unrecognized lineage of diploid platelet-forming cells, which develop within the YS in parallel to HPCs but can be specified in the E8.5 Runx1-null embryo despite the absence of the progenitor cell lineage. (Blood. 2014;124(17):2725-2729)