Loss of Endosialin results in aberrant vascular sprouts in the aortic ring outgrowth assay. Thoracic aortae were dissected from Endosialin^+/+ or Endosialin^−/− 129/SvJ mice, embedded in a collagen I matrix and cultured for 6 days in the presence of VEGF-A. (A) Fixed wild-type aortae stained with FITC–isolectin B4 (green) and anti–mouse Endosialin Ab followed by Alexa Fluor 555–anti–rabbit Ig (red). Nuclei were counterstained with DAPI. Image shows a vascular sprout with expression of Endosialin on pericytes (arrowheads) but not endothelial cells (arrows). Scale bar indicates 25 μm. (B) Phase-contrast images illustrating increased vascular branching in Endosialin^−/− outgrowths (arrows). Scale bar indicates 200 μm. Quantification of aortic ring vascular sprout number (n = 12), sprout length (n = 5), and branch point number (n = 5) per aortic ring. Data shown are means ± SEM.