Primitive erythropoietic progenitors coexpress PECAM-1, VE-cadherin, Tie2, CD34, and endoglin. (A) Lateral view of a mouse embryo at the EHF stage. Line shows a transverse section (PLAN APO lens, 1.0×/0.125; final magnification, ×40). (B-E) Immunohistochemical staining of a transverse section as shown in panel A using Hoechst 33342 (B blue), anti-Flk1 antibody (C, red), anti-ϵyglobin antibody (D, green), and the merged image (E). (F-K) High magnification of the blood island region stained with Hoechst 33342 (blue), anti-ϵyglobin antibody (green), anti-Flk1 antibody (F, red), anti–VE-cadherin antibody (G, red), anti–PECAM-1 antibody (H, red), anti-Tie2 antibody (I, red), anti-CD34 antibody (J, red), and antiendoglin antibody (K, red). Note that the expression of VE-cadherin, PECAM-1, Tie2, endoglin, and CD34 is observed in globin-producing cells but that the expression of Flk1 is decreased in globin-producing cells. The scale bar indicates 50 μm. Images were captured with a Leica DC500 CCD camera with IM50 Imaging Manager. The image in panel A was taken through a Leica MZFLIII microscope using a PLAN APO 1.0 lens. Images in panels B-K were taken with a Leica DMRXA microscope using a 5×/0.15 NA objective (panels B-E) or a20×/0.50 NA objective.