Histopathology of brain tumor 7 to 11 days after systemic cell transplant. (A) CD31 (platelet endothelial cell adhesion molecule 1 [PECAM-1]) and Prussian blue stains of fixed brain in a mouse receiving unlabeled cells. Slide is positive for CD31 (brown) and negative for iron (Prussian blue). (B) CD31 and Prussian blue stain of fixed brain in a mouse receiving FE-PLL-labeled cells. Iron-labeled (blue) cells are distributed around the tumor periphery. The CD31 stain could not be seen simultaneously with PB except in iron-negative areas. (C) Fixed 6-μm section from a labeled mouse stained for CD31 only. (D) Same CD31 slide after staining for Prussian blue. Cells that are both CD31⁺ and PB⁺ are circled. (E) Fixed 6-μm section stained with VWF and Prussian blue. Arrows indicate cells that are both VWF⁺ and PB⁺.