Expression of ACE in the human fetal liver and BM. (A) Transverse section through a 7.5-week liver stained by anti-CD45 (red) and ACE (green) Abs. ACE marks numerous endothelial cells of the sinusoidal capillaries (arrows), some of which are also CD45⁺ (white arrowheads). No expression of ACE is observed in the endothelial cells bordering the larger hepatic veins (black arrowheads). (B) Adjacent sections through an 11-week liver stained for ACE (red) and CD31 (green), and for CD45 (red) and CD31 (green), respectively. Overlap of adjacent sections: ACE in red, CD45 in blue and CD31 in green. Hematopoietic CD45⁺CD31⁺ progenitors, also expressing ACE, develop as cell aggregates adhering to sinusoid endothelial cells (arrows). Scale bar in panel A represents 50 μm; and B, 100 μm. V indicates vein; S, sinusoidal capillary. (C-E) FACS analysis of ACE expression during BM development. At 8.5 weeks of development (C), ACE is only expressed by endothelial and hematopoietic CD45⁺CD34⁻ cells in the BM, as shown by the absence of cells expressing ACE in the CD34⁺CD45⁺-gated population. Starting from 9.6 weeks of gestation, when blood cell colonization has taken place in the BM, some CD34⁺CD45⁺ progenitors expressing ACE are detected in the medullar cavity (D). This population is also detected at later stages when hematopoiesis is already active inside the BM (E; 14 weeks).