Spatiotemporal relationship of tissue macrophages and blood vessels during angiogenesis in the developing hindbrain. Eighty-micron transverse sections (A-C) and whole mounts (D-H) of 10.5-, 11.5-, and 12.5-dpc wild-type hindbrains, labeled for IB4 (red) and F4/80 (green). Solid white arrows indicate macrophages positive for both markers; clear arrows, macrophages positive for IB4 only; curved arrows, macrophages embracing vascular intersections; arrowheads indicate endothelial tip cells. (G-H) Higher magnifications of the SVP illustrate the interaction of tip cells (arrowheads) with macrophages (arrows) and the bridging of neighboring tip cells by a macrophage (wavy arrow in G; note that one of the 2 vessel sprouts emerges from a deeper plane of section); panel H shows a higher magnification of the boxed area in panel E. Scale bars: panels A through F, 100 mm; panels G and H, 25 mm. V indicates ventricular brain surface; p, pial brain surface; rv, radial vessels; hb, hindbrain; SVP, subventricular vascular plexus; and PNP, perineural vascular plexus. (I) Quantitation of macrophages (Mϕ, green) and vascular intersections in the SVP (red) between 10.5 and 12.5 dpc; n ≥ 15. Error bars represent SD of the mean.