Fig. 1. Immunolocalization of endothelial and hematopoietic cells in the AGM region. (A) Transverse section through a 31-day human embryo immunostained for CD34. The boxed area is the AGM region, including the aorta, gonads and mesonephros. (B-D) Higher magnifications of the same AGM region stained for CD34, CD45 and CD31, respectively. (B) Anti-CD34 stains the endothelium lining the dorsal aorta (da), cardinal veins (cv) and capillaries around the mesonephros (mn). The arrowhead indicates the hematopoietic CD34⁺ cell progenitors associated with the ventral endothelium of the aorta. (C) CD45 is expressed on all hematopoietic cells present in the embryo, including the aortic stem cells (arrowhead), but is absent from endothelial cells. Rare CD45⁺ hematopoietic cells scattered in the mesoderm underlying the dorsal aorta are CD34-negative. (D) CD31 stains both endothelial cells and adherent intra-aortic hematopoietic cells. Scale bars: 100 µm; nt, neural tube; g, gonad.

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