GPIIb-IIIa expression in intraembryonic hematopoietic sites. (A) Transverse section of an E3.5 to 4 embryo: immunoperoxidase staining with the anti–GPIIb-IIIa MoAb. In the embryo proper, GPIIb-IIIa immunoreactivity is concentrated on cells located ventral to the aorta. (B) Higher magnification from the field boxed in A. Intraaortic clusters are GPIIb-IIIa⁺ (arrow) and a few positive cells are scattered in the mesenchyme beneath the aortic endothelium (arrowhead). (C) The same section, double stained with anti-CD45MoAb, HISC7, and shown by indirect immunofluorescence. Numerous isolated CD45⁺ cells were distributed throughout the embryo, in the vessels and in the mesenchyme. Bar = 119 μm. (D) Higher magnification from the field boxed in C, showing the same aortic area as in B. The staining patterns of the two antibodies are similar at the level of the intraaortic clusters, showing coexpression of GPIIb-IIIa and CD45 antigens at this site. Bar = 59 μm. (E) Transverse section of an E6 embryo. GPIIb-IIIa immunoperoxidase staining. Bar = 297 μm. (F) Higher magnification from the field boxed in E, showing a paraortic foci containing GPIIb-IIIa⁺ cells. Bar = 119 μm. Ao, aorta; PCV, posterior cardinal vein; PA, pulmonary artery; NC, notochord; NT, neural tube; O, esophagus.