A. The 1st wave of YS hematopoiesis (YS1) is initiated on E7.5. pri-HSCs generated in this wave primarily produce nucleated RBCs and Mks/platelets to support embryonic survival up to E15.5. In addition, Mφ produced in this wave generate microglia in the brain for the remainder of life. B. The 2nd wave of YS hematopoiesis (YS2) starts on E8.5. EMPs and LMPs generated in this wave migrate to FL and then fetal BM to produce RBCs, platelets, innate immune cells and granulocytes. This wave of hematopoiesis can support the survival of the embryo up until birth. In addition, Mφ and innate immune cells infiltrate the embryo’s tissues and renew themselves, maintaining tissue resident Mφ and immune cells for the remainder of life.