Endosialin expression in E12.5 mouse embryos. Brain sections from E12.5 embryos were double-labeled for endosialin and VEGFR-2 (A); endosialin and PDGFR-β (B) and endosialin and collagen type IV (C). Panel A illustrates the expression of endosialin (green) and VEGFR-2 (red) by the endothelial cells of brain capillaries in longitudinal and cross-sections (insert). Endothelial cells appeared to co-express both markers as seen in the merged images (yellow). Endosialin and PDGFR-β are expressed by distinct cell populations within the brain capillaries as indicated in panel B. Endosialin positive endothelial cells (green) are in close contact with PDGFR-β positive pericytes (red, arrowheads) from the adluminal side of the vessel wall, yet are spatially separated as revealed by the merged images (insert, right panel). In panel C, more mature vessels of the perineural plexus endothelial cells expressing endosialin (green) are embedded in the basal membrane, identified by labeling of collagen type IV (red). Cell nuclei were counterstained with DAPI (blue). Asterisks indicate the presence of nucleated erythrocytes within the vessels. Scale bars: 20 µm.