Decreased microvessel sprouting from Rap1b^−/− aortic rings. (A) Aortic rings prepared from normal (WT) or Rap1b^−/− mice were embedded in growth factor–deprived Matrigel and cultured for 4 days in the presence of 50 ng/mL bFGF (top row) or 50 ng/mL VEGF (bottom row). (B) Quantitation of the average number of sprouts per ring from normal (blue bars) or Rap1b^−/− (red bars) aortas indicates reduced ex vivo response of Rap1b^−/− aortas to both factors. Data from 6 independent experiments; 8 to 14 rings per experiment. Error bars are SEM.