Blocking anti-NRP1 antibody inhibits HGF-induced angiogenesis in a mouse Matrigel model. Mice were injected with Matrigel with or without an angiogenic cocktail and 20 μg/mL anti-NRP1 or nonimmune IgG. (A) Matrigel sections were analyzed by histology after HES staining and by immunochemical staining with an anti–von Willebrand factor or anti-CD31 antibody, using an Axioskop 2 plus fluorescence microscope (Carl Zeiss) equipped with Achroplan 4×/0.10 (left panel), Plan Neofluar 5×/0.15 (middle panel), or Plan Neofluar 40×/0.75 (right panel) objectives. Scale bar equals 100 μm. (B) Quantification of the hemoglobin content of plugs by spectrophotometry. **P < .01 (Student t test).