HCA distribution in human embryonic and fetal blood-forming tissues. (A) Cross-section in the trunk region of a 5-week human embryo hybridized with a DIG-labeled antisense HCA probe (dark blue) and subsequently stained with the HPCA-1 antibody against the CD34 surface molecule (red). Note HCA-expressing mesenchymal cells (arrows) subjacent to a cluster of CD34⁺ hematopoietic progenitors associated with the ventral aortic endothelium, which also expresses CD34. A few HCA⁺ cells are also present on the lateral aspect of the aorta (arrowhead) (original magnification × 100). (B) Seventeen-week fetal liver. Hematopoietic cells are intermingled with polygonal hepatocytes displaying strong HCA surface staining (original magnification × 100). (C) Eight and one-half–week fetal thymus. The thymic epithelium appears uniformly stained (original magnification × 25). (D) Close-up view of an 8.5-week fetal thymus that shows stronger HCA density on epithelial cells in the outermost region of the rudiment (original magnification × 250). (E) Seventeen-week fetal thymus. Virtually all epithelial cells beneath the capsule are HCA⁺. Stained cells are also present within the inner cortex (original magnification × 100).