Old and new models for yolk sac primitive hematopoiesis. (A) Previous models indicated that primitive (P) erythrocytes develop directly from a yolk sac vascular-endothelial precursor termed the hemangioblast.²  New data extend the hematopoietic repertoire of yolk sac hematopoiesis by defining a bipotent primitive MEP that gives rise to erythrocytes (Ery) or megakaryocytes (Meg), which then mature into platelets (not shown). Data by Tober et al indicate that the primitive MEP is hemangioblast derived.⁶  (B) Bi-lineage erythro-megakaryocytic colonies arising from single primitive MEPs. The right panel shows a colony derived from human ESCs (adapted from Figure 4 of the article beginning on page 1506). Megakaryocytes are stained with anti-CD41 (pink). The left panel shows a colony derived from mouse yolk sac. Erythrocytes are stained with anti-βH1 globin (blue); megakaryocytes are stained with anti-GP1bβ (red). Adapted with permission from Tober et al.⁶  Professional illustration by Marie Dauenheimer.