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Fig. 4. Whole mount in situ hybridization analysis of flk-1 expression. Embryos in A, B, and F were photographed at 70X, C at 35X, and D and E at 30X. A: Lateral view of a stage 21 embryo showing staining in the endocardial precursor region (small arrow), in the aortic arches (out of the plane of focus, small arrowhead), and in the head mesenchyme underly- ing the prosencephalon (large arrow) and overlying the tip of the notochord (open arrowhead). B: Dorsal view of a stage 23 embryo showing flk-1 expression at the midline in the head mesenchyme (large arrow) and more laterally in the aortic arches (small arrowheads). C: In a stage 27 embryo, flk-1 is prominently expressed in groups of endothelial precursor cells located at the position of the developing cardinal veins (large concave arrow) and of the developing vitelline veins (open arrowhead). Expression is also detected in the endocardium (smallarrow), the aortic arches (small arrowhead) and the head mesenchyme (large arrow). D: Ventral view of a stage 28 embryo showing the ring of flk-1 expression surrounding the region of the ventral blood islands. Arrowheads indicate the vitelline veins and a short arrow indicates the endocardium. E: Staining marks the posterior cardinal veins (large arrow), the common cardinal vein (or the Duct of Cuvier; small arrow), the sinus venosus (open arrow), and the ventral aorta (arrowhead). Note that the cardinal veins show lower levels of flk-1 transcripts anteriorly where the vessels are mature. Posteriorly, the endothelial cells have not yet fully coalesced into mature vessels and are still in a loose association. F: Lateral view of the larval tail of a mid tailbud embryo (stage 34) showing flk-1 staining in the posterior cardinal veins (large arrow), the rectal diverticulum (white arrow), and the dorsal aorta (small arrow). |
