Angioblasts are TAL1⁺/ Flk1⁺and PECAM⁻ cells.

PECAM expression (panel B) is associated with the aortic endothelial cells (arrowheads) but is absent in the more lateral mesoderm (brackets), which is populated by TAL1⁺ cells (panel A, brackets). Aortic morphogenesis is bidirectional. In panel C, a segment of an 8.3-dpc aortae is labeled with PECAM antibodies. Labeling is most intense in the cranial and caudal regions, the more mature segments of the aortae (arrowheads), and diminishes in intensity in the network forming region (bracket). Protein expression in the lateral vascular networks distinguishes this vascular bed from others. Panel D depicts a segment of an aorta and the associated lateral mesoderm immunolabeled using Flk1 antibodies. Vessel primordia (arrows) and angioblasts (asterisks) are evident in the lateral region, a site where no PECAM, CD34, or VE-cadherin immunofluorescence was detected (see Figure 6, I-K, asterisk). Cells of the aortic primordia differ in their temporal and spatial expression of PECAM, CD34, and VE-cadherin. Panels E and F show PECAM immunofluorescence associated with the aortae of 8.2- and 8.5-dpc embryos. Panels G and H show, respectively, CD34 and VE-cadherin immunofluorescence associated with the aortae of 8.5-dpc embryos. Magnification bar = 50 μm.