Brain angiogenesis is impaired in the absence of macrophages. Angiogenesis in hindbrains lacking PU.1 (Pu.1^−/−) or heparin-binding VEGF isoforms (Vegfa^120/120) at 12.5 dpc (A-L) and 11.5 dpc (M-P). (A-G) Whole mount view onto the SVP (A-C) or radial vessels (RV) diving into the brain parenchyma (E-G), visualized by PECAM immunohistochemistry (IHC) in a 0.25-mm² area. (I-K) Transverse sections (100 μm) of IB4-labeled hindbrains show vascular bridges (white arrowheads) between neighboring radial vessels. (M-O) IB4-positive endothelial tip cells (clear arrowheads) and macrophages (clear arrows) in the whole-mounted SVP. Scale bars represent 100 μm (A-K) and 50 μm (M-O). (D,H,L,P) Quantitation of the number of vascular intersections in the SVP (D; n = 5), the number of radial vessels (H; n = 5), and the number of vascular bridges between radial vessels (L; n = 3). Error bars represent SD of the mean. P values were determined by comparing the measurements obtained for both types of mutants to the control, which contained the measurements obtained for wild-types from both groups.