Expression of growth factor receptors by CD34⁺ aorta-associated cells from 5-week human embryos. (A) All cells within hematopoietic clusters (arrow) are uniformly labeled upon hybridization of a c-kit³⁵S-labeled riboprobe (original magnification × 260). (B) higher magnification of (A; original magnification × 650). (C) Flk-1/KDR transcripts are detectable in aorta endothelial cells upon hybridization of a DIG-labeled riboprobe (arrow), whereas intra-aortic hematopoietic clusters are unlabeled (arrowhead; original magnification × 200). (D) Hybridization of a radioactive flk-1/KDR probe allows to evidence positive cells in the innermost layer within hematopoietic foci (arrowhead; original magnification × 650). No signals were observed upon hybridization of sense riboprobes (not shown).