Anti-BMP9 treatment increases vascularization of the retina. (A-C) Visualization of blood vessels by isolectin B4 (iB4) staining of OF1 retinas at P6 after treatment with BMP9-blocking antibodies (5 mg/kg) or recombinant ALK1ecd (5 mg/kg) injected at P1 and P3. (D) Number of branching points at the capillary plexus and at the front of migration. (E) Bar graphs show the relative distance covered by the vascular plexus calculated as the ratio of the vascular radius over the retinal radius in percentage. (F-H) Pericyte coverage visualized by NG2 immunostaining of OF1 retinas at P6. Panels A through H show data from 2 litters containing 10 PBS treated pups, 9 anti-BMP9 treated pups and 5 ALK1ecd treated pups. Error bars show SEM. **P < .01 versus PBS group. (A-C) Scale bar = 200 μm.