Expression of ACE in the human embryo. (A-B) Transverse sections of a 19-day human embryo. (A) In the extraembryonic YS, ACE/BB9 (green) and CD34 (red) are expressed in the hemangioblastic structures, blood islands (arrows), and in some endothelial cells bordering newly formed blood vessels (arrowheads). (B) In the embryo proper, ACE marks rare cells scattered in the caudal portion of the splanchnopleura (arrows). No co-localization of the CD34 or CD45 Ags is observed in these ACE⁺ cells. (C-D) Cross-sections through 23- and 26-day human embryos. At these stages, ACE expression is limited to some migrating cells concentrated in the mesoderm of the dorsal mesentery, which surrounds the dorsal aorta (Ao), in the caudal portion of the embryo. These ACE⁺ cells express neither CD34 nor CD45 (arrowheads in the inset on the right, which represents a high magnification of the splanchnopleura dotted in panel C). At 26 days of development, some endothelial CD34⁺ cells, which border the lumen of the aorta, also express ACE (white arrowheads). (E) Cross-sections through the dorsal aorta in a 34-day embryo. ACE is expressed by hematopoietic CD34⁺CD45⁺ cell clusters associated with the endothelium on the ventral side of the aorta (arrowheads), as well as by surrounding endothelial CD34⁺ cells (white arrowheads). The diagrams on the right in panels B through E indicate the position of the respective sections. Scale bar in panel A represents 20 μm; and panels B through E, 50 μm. NG indicates neural groove; NT, neural tube.