Intraembryonic vasculogenesis 7.0 to 7.5 dpc.

The upper panels (A-F) show the immunofluorescence patterns of TAL1, Flk1, PECAM, CD34, VE-cadherin, and Tie2 in 8.2- to 8.3-dpc embryos; the lower panels (G-L), show expression at 8.35 to 8.5 dpc. At 8.2 to 8.3 dpc TAL1 and Flk1 immunostaining (panels A and B) is associated with the aortic primordia (bracket), the endocardial primordia (arrows), and the lateral vascular networks (asterisk). TAL1 expression is down-regulated as part of endocardiogenesis. Comparison of the immunofluorescence patterns of TAL1 (panel A) and Flk1 (panel B) at 8.2 dpc indicates that these proteins are coexpressed by cells of the endothelial lineage. At 8.5 dpc endocardial expression of TAL1 and Flk1 diverge; although Flk1 (panel H, arrow) continues to be expressed, TAL1 immunostaining (panel G, arrow) is no longer evident. PECAM, CD34, and VE-cadherin expression is restricted to the primordia of larger vessels. Comparison of PECAM, CD34, and VE-cadherin immunofluorescence (panels C-E and I-K) to that of TAL1 and Flk1 (panels A, G and B, H) shows colabeling of the aortic primordia (bracket), the endocardial primordia (arrowsheads), and the endocardium (arrow). Conspicuously absent in these embryos is colabeling in the lateral vascular networks (asterisk). Tie2 is rapidly up-regulated. At 8.2 to 8.3 dpc, Tie2 immunofluorescence is weak as compared to that of the other proteins evaluated in this study (compare panel F to panels A-E); however, by 8.5 dpc (panel L) clear Tie2 expression is associated with the aortic primordia (bracket) and the endocardium (insert). Magnification bar = 100 μm.