Fig. 3. The neural tube-derived vascular patterning signal induces target cells at a distance. Mouse presomitic mesoderm grafts were placed in quail embryos adjacent to host medial presomitic mesoderm, in a cavity made by removal of quail lateral presomitic mesoderm. Grafts were either Flk1^+/– (lacZ in Flk1 locus) (B-D) or wild type (E-G), and after incubation for 48 or 72 hours, chimeric embryos were fixed and stained for ß-gal (B-D) or PECAM (E-G). (A) In ovo manipulations. The orange arrow indicates the medial quail presomitic mesoderm that is situated between the mouse graft (purple rectangle) and the host neural tube (dark blue). (B) Chimera incubated for 72 hours and stained for ß-gal. Arrows indicate mouse graft-derived vascular cells (blue) in the trunk and limb vessels of the quail host. (C,D) Sections through the trunk of the chimeric embryo in B. ß-Gal-positive vessels are highlighted by arrows in the intersomitic and limb vascular beds (C) and the kidney rudiment (D). (E) Chimera incubated for 72 hours and stained for PECAM. Arrows indicate graft-derived vascular cells (brown) in host intersomitic and limb vessels. (F,G) Sections through the neural tube of the chimera in E stained with PECAM antibody (brown) to visualize mouse graft-derived vascular cells, and QH1 antibody (purple) to visualize host vessels. (F) PECAM-positive cells (brown) identified in the ventral region of the PNVP. The inset shows a higher magnification of this vessel (arrow indicates PECAM-positive graft-derived cells). (G) A different region of the neural tube shows PECAM-positive endothelial cells (arrow) in a quail PNVP vessel. Scale bar: 80 µm in C; 40 µm in D,F; 20 µm in G.