Systemic platelet activation markers, and local platelet deposition and angiogenesis in the tumor microenvironment. (A) Plasma PF4 levels at the time of euthanasia were quantified using an ELISA to assess systemic markers of platelet activation. (B) Representative images of platelet accumulation and vessel density in mammary tumors. Platelets and vascular endothelial cells were visualized with anti-CD41 (green) and anti-CD34 (red) antibodies, respectively. Nuclei were visualized with Hoechst 33342 (blue). Scale bars, 50 µm. (C) Percentage of intratumoral vessels with deposited platelets. Fifteen fields of view per tumor were analyzed for quantification. Each tumor was evaluated from 6 mice per treatment. (D) Percentage of CD34-positive area in the field of view of samples from saline-treated (n = 6) or THPO-ASO–treated (n = 6) mice. Fifteen fields of view per tumor were analyzed for quantification. Every tumor was evaluated in each mouse. A 2-tailed, unpaired Student t test was used for comparison of means. *P < .05; ***P < .001.