Primitive erythropoietic progenitors are enriched in PECAM-1– and Tie2-positive cells. (A) Flow cytometric analysis of a 7.75 dpc wild-type embryo at the late–head fold (LHF) stage using anti–PECAM-1 or Tie2 antibody. (B) Primitive hematopoietic colony assay. Cells from fractions positive or negative for the expression of endothelial markers shown in panel A were subjected to the primitive hematopoietic colony assay.