Phenotype of the ACE⁺ cells in the P-Sp/AGM of the early human embryo. (A) Cross-sections of a 38-day human embryo immunostained with ACE/BB9 (green) and anti–Oct-4A (red) Abs. The majority of the mesodermal ACE⁺ cells express the transcription factor Oct-4A, which is also expressed by other ACE⁻ cells dispersed in the AGM region and localized in the ventral wall of the aorta (arrow). (B) Transversal sections of a 26- and a 34-day human embryo were co-stained with ACE/BB9 (green) and anti–SSEA-1 (red) Abs. ACE⁺ cells dispersed in the paraaortic mesoderm co-express SSEA-1 (arrow). At 34 days, ACE also stained the endothelial cells and the hematopoietic cell clusters associated with the ventral aortic endothelium. Some of these hematopoietic progenitors also express SSEA-1. (C-D) Cross-sections of a 32-day embryo were stained with ACE and CXCR4 Abs (C), and ACE and SDF-1 Abs (D), respectively. Some ACE⁺ cells dispersed in the paraaortic mesoderm co-express the receptor CXCR4 (arrow, C). At these stages, this region presents a polarized expression of the SDF-1 chemokine (D). Scale bar indicates 50 μm. Ao indicates aorta; G, gut; DM, dorsal mesentery; GR, genital ridges; Mn, mesonephros; NC, notochord; and NT, neural tube.