Expression of hematopoietic transcription factors in CD34⁺ aorta-associated cells from 5-week human embryos. (A) Immunostaining with an anti-CD34 antibody. Arrowheads point to hematopoietic cell clusters in the aorta. Hematoxylin counter-staining (original magnification × 200). (B) Hybridization on an equivalent section of a tal-1/SCL radioactive probe. All cells in the intra-aortic cluster are specifically labeled (original magnification × 260). (C through E) Hybridization of DIG-labeled riboprobes (purple staining) specific for c-myb (C),GATA-2 (D), and GATA-3 (E). Further incubation of these sections with an anti-CD34 antibody (red staining) shows the coexpression of both markers in aorta-associated blood cell progenitors (original magnification × 260). Arrows in (D) point to circulatingGATA-2⁺ cells in the lumen of the aorta and those in E to a faint GATA-3 signal in mesenchymal cells subjacent to the ventral endothelium of the aorta. (F) GATA-3 labeling in mesenchymal cells underlying the ventral wall of the aorta is obvious using a ³⁵S-radiolabeled probe (arrow). Note the higher expression of GATA-3 mRNA in the innermost cells within a large hematopoietic cluster (original magnification × 650). No signals were observed upon hybridization of sense riboprobes (not shown).