Platelet string formation on stimulated murine endothelium. (A-C) Individual examples of strings of adherent platelets on endothelium of exposed mesenteric venules following stimulation with exogenously administered A23187. (D) Example of progressive string resorption over an interval of 15 seconds. The black arrow (length = 50 μm) represents the direction of the blood flow. (E) Distribution of numbers of mesenteric vessels, demonstrating platelet string formation (▪) or not (□), following vessel stimulation by A23187, for the indicated number of mice (m); indicated P values were calculated by 2 × 2 contingency table analysis and Fisher exact test with 2-tailed P value. C indicates nontreated controls; +TSP-1, bolus of 80 μg/kg + infusion of 80 μg/kg/h; +4N1K, injection with calcein-labeled platelets preactivated with 100 μM of the CD47-activating peptide 4N1K; and +Tsp1^-/- platelets and +WT platelets, cross-design experiments with platelet injection in WT and TSP-1^-/- mice, respectively.