Meg-CFCs and 2 novel bipotential MEPs emerge during embryonic hematopoiesis. (A) Number (+SEM) of acetylcholinesterase-positive (megakaryocyte) colonies grown from individual embryonic tissues by stage. The inset (i) shows an acetylcholinesterase-positive colony. (B) Immunohistochemical staining of erythroid and megakaryocyte progenitor–derived colonies. (Top) Primitive erythroid (i), megakaryocyte (ii), and bipotential primitive MEP (iii) colonies from neural plate–stage cultures stained with primitive erythroid-specific βH1-globin (blue) and megakaryocyte-specific GP1bβ (pink) antibodies. (Bottom) Erythroid (v), megakaryocyte (vi), and bipotential definitive MEP (vii) colonies from E9.5 to E10.5 yolk sac stained with panerythroid Ter119 (blue) and with GP1bβ (pink) antibodies. Boxed areas in panels iii and vii highlight proplatelet formation at 100 × shown in panels iv and viii, respectively. Arrowheads indicate proplatelet formation in panels ii and vi. Scale bars represent 10 μm. (C) Average number (+SEM) of primitive erythroid, megakaryocyte, and bipotential primitive MEP colonies from plated whole embryos (PS-LNP) and yolk sac (HF-46 sp) as determined by immunohistochemistry. (D) Primitive MEP–derived colony derived from a neural plate–staged embryo contains cells positive for βH1-globin and CD41. (E) Number (+SEM) of βH1-globin/GP1bβ-double positive colonies from a dilution series of 4 to 5 sp yolk sacs, in triplicate. (F) Spatial and temporal distribution of Meg-CFCs as determined by GP1bβ-stained colonies per tissue. (G) Spatial and temporal distribution of definitive MEPs, as determined by Ter119/GP1bβ double-positive colonies per tissue. sp indicates somite pair; PS, preprimitive streak; MS, mid primitive streak; LS, late primitive streak; ENP, early neural plate; MNP, mid neural plate; LNP, late neural plate; HF, head fold; and LHF, late head fold. Approximate embryonic day (E) is provided below the developmental stages.