Expression of BB9 in the AGM region of the developing human embryo. Cross sections through the dorsal aorta (Ao) in a 34-day human embryo immunostained with CD34 (A), BB9 (B), and CD45 (C). BB9 stains hematopoietic CD34⁺CD45⁺ cell clusters associated with the endothelium on the ventral site of the aorta (white arrowheads), as well as the CD34⁺ endothelial cells (). BB9 also recognizes some cells in the mesoderm underneath the aorta (). No colocalization of CD34 or CD45 antigens is observed in these BB9-positive cells (➤). Sections were observed with a Nikon Eclipse 50i light microscope with 20× magnification (20×/0.50 objective) and figures were acquired by a Nikon Digital Sight DS-2Mv camera (Nikon, Tokyo, Japan). Images were processed using Photoshop 7.0 software (Adobe Systems, San Jose, CA).