Endosialin expression by developing brain capillaries in E10.5 and E12.0 mouse embryos. Sections of forebrain from an E10.5 (A, B) mouse embryo were stained with anti-collagen type IV (A) or Ab 171 (B). Collagen type IV maps to blood vessels of the perineural plexus and angiogenic sprouts in the brain parenchyma (arrows in A). Most endosialin-positive vessels at this stage are localized to the perineural plexus (arrows in B), while most vessels in the brain lack endosialin (arrowheads in B). At E12.5 endosialin (C) localizes to small and medium-sized blood vessels, including those (double arrows) inside the brain, and the number and pattern of endosialin-positive vessels is similar to those expressing VEGFR-2 (D, double arrows). By contrast, PDGFR-β (E) is seen in subsets of vessels with a distinct, discontinuous pattern (arrows in E). Scale bars: 25 µm.