Vasculogenesis in 7.0- to 7.8-dpc embryos.

TAL1 (panel A) and Flk1 (panel B) immunofluorescences are associated with extraembryonic blood islands (arrows), vascular networks (braces), and the allantois (bracket) of the 7.0- to 7.3-dpc embryo. TAL1 and Flk1 are differentially expressed by cells of the forming blood islands. At 7.0 dpc strong TAL1 (panel A, arrows) and weak Flk1 (panel B, arrows) expressions are associated with the blood islands. Intraembryonic vasculogenesis is initiated at 7.3 dpc in the primordia of the endocardium. In panel B, Flk⁺ mesodermal cells located cranially define the endocardial primordia (doubleheaded arrow). The dorsal aortae are first discernible at 7.6 dpc. TAL1⁺ cells arranged along the embryonic axis define the aortic primordia (panel C, arrowheads). PECAM expression is confined to the primordia of “larger” vessels—PECAM⁺ cells are detected in the 7.8-dpc aortic primordia (arrowheads); note the absence of PECAM expression in regions lateral to the aortae (compare panels C and D). Dashed lines in panel C suggest the boundary between the intraembryonic and extraembryonic regions. Magnification bar = 100 μm.