LYVE1 is expressed in yolk sac endothelium and hematopoietic stem and progenitor cells Lyve1-Cre marks yolk sac definitive hematopoiesis but bypasses primitive erythropoiesis LYVE1 lineage cells initiate fetal liver definitive erythropoiesis A subset of fetal and adult hematopoietic stem cells are derived from the LYVE1 lineage