Visualization of vascular platelet aggregation by plastic embedding and light microscopy

Previous studies have indicated that platelets play a role in inflammation and microvascular damage but routine histologic preparations do not permit clear visualization of the platelets in tissues. Plastic embedding was used in this study to demonstrate platelet aggregates in the pulmonary vasculature of mice exposed to complement activation. The degree of platelet aggregation in the excised lungs was graded semiquantitatively in a total of 75 mice. Control Balb/c mice had a mean aggregation score of 0.16 while mice which received 0.3 ml zymosan-activated plasma (ZAP) intravenously (iv) had a score of 2.2. Injection of 0.3 or 0.4 ml of ZAP to which epsilon-aminocaproic acid was added prior to incubation with zymosan resulted in a score of 2.6 or 4.7 respectively. Balb/c (C5 sufficient) and AKR/J (C5 deficient) mice injected iv with 1 mg zymosan had scores of 2.9 and 3.2, respectively. Cobra venom factor (CVF) injected iv to Balb/c mice induced a dose-dependent aggregation. Taken together, these results confirm that complement activation products can mediate intrapulmonary platelet aggregation and they also demonstrate the suitability of plastic embedding for visualization of intravascular platelet aggregates under light microscopy.