Primitive erythropoiesis from mesodermal precursors expressing VE-cadherin, PECAM-1, Tie2, endoglin, and CD34 in the mouse embryo.

Vascular endothelial (VE) cadherin, PECAM-1 (platelet endothelial cell adhesion molecule-1, CD31), Tie2, CD34, and endoglin are established markers for adult and embryonic endothelial cells (ECs). Here, we report that the expression of these EC markers is initiated in the extraembryonic region at the late-streak stage (nominal stage E6.75). Immunohistochemical analysis shows that EC marker-positive cells arise in a subset of Flk1 (VEGF-R2) mesodermal cells. In contrast, GATA1, a marker for primitive erythropoietic progenitors, is expressed in a more restricted subset of Flk1-positive cells. Using flow cytometry, we observed that the GATA1-positive cell population existed as a subset of the EC marker-positive cell. Consistent with this notion, we showed with the primitive hematopoietic colony assay that primitive erythropoietic progenitors are enriched in PECAM-1- and Tie2-positive cells. These results suggest that primitive hematopoietic cells arise from EC marker-positive cells. Thus, VE-cadherin, PECAM-1, CD34, endoglin, and Tie2 are expressed not only in adult and embryonic ECs but in extraembryonic Flk1-positive cells during gastrulation. The latter cell population includes progenitors that give rise to primitive hematopoietic cells, suggesting that primitive and definitive hematopoietic cells in the mouse embryo arise from EC marker-positive cells.