Blood clots and thrombi undergo platelet-driven contraction/retraction followed by structural rearrangements. We have established quantitative relationships between the composition of blood clots and extent of contraction to determine intravital contraction of thrombi and emboli based on their content. The composition of human blood clots and thrombi was quantified using histology and scanning electron microscopy. Contracting blood clots were segregated into the gradually shrinking outer layer that contains a fibrin-platelet mesh and the expanding inner portion with compacted red blood cells (RBCs). At 10% contraction, biconcave RBCs were partially compressed into polyhedral RBCs, which became dominant at 20% contraction and higher. The polyhedral/biconcave RBC ratio and the extent of contraction displayed an exponential relationship, which was used to determine the extent of intravital contraction of ex vivo thrombi, ranging from 30% to 50%. In venous thrombi, the extent of contraction decreased gradually from the older (head) to the younger (body, tail) parts. In pulmonary emboli, the extent of contraction was significantly lower than in the venous head but was similar to the body and tail, suggesting that the emboli originate from the younger portion(s) of venous thrombi. The extent of contraction in arterial cerebral thrombi was significantly higher than in the younger parts of venous thrombi (body, tail) and pulmonary emboli but was indistinguishable from the older part (head). A novel tool, named the “contraction ruler,” has been developed to use the composition of ex vivo thrombi to assess the extent of their intravital contraction, which contributes to the pathophysiology of thromboembolism.

Background. COVID-19 is a predominantly respiratory disease caused by the SARS-CoV-2 virus that has spread as a pandemic since 2020; characterized by high mortality and unexplored long-term outcomes. Aim. To describe the morphological changes in the lungs and other organs in severe forms of COVID-19. Material and methods. A macro- and microscopic examination of the lungs, kidneys, liver, brain and heart of 45 patients who died from complications of COVID-19 was carried out. Histological preparations were stained with hematoxylin and eosin, by the Picro-Mallory stain, as well as with fluorescent antibodies and dyes to visualize neutrophil extracellular traps. Results. The leading pathomorphological manifestations of COVID-19 were inflammation and microthrombosis in the lungs. At the same time, diffuse alveolar damage often developed as a manifestation of acute respiratory distress syndrome in an adult. Diffuse leukocyte infiltration of the lungs was combined with the formation of neutrophil extracellular traps in the lumens of the blood vessels. Diffuse microthrombi were found in most lung tissue samples (87%), while macroscopic thrombi were much less common (16%). In single cases, the cause of death was thromboembolism of the branches of the pulmonary artery, originating from deep vein thrombi of the lower extremities or the right atrial appendage. In the tissues of the liver, heart, kidneys and brain, moderate alterative (degenerative and necrobiotic) microscopic changes were observed, as well as morphological signs of concomitant chronic diseases. Conclusion. COVID-19 is characterized by pathogenically interrelated morphological manifestations of inflammation and primary pulmonary (micro)thrombosis in the form of accumulation of leukocytes both in blood clots and in the alveoli, while regional pulmonary immunothrombosis is associated with diffuse damage to the alveoli; in the heart, brain, liver and kidneys, predominantly dystrophic and necrobiotic changes occur, which are typical of secondary injuries and/or concomitant pathology.

Immunothrombosis can substantially affect the course and outcomes of severe infections and immune-mediated diseases. While inflammatory thrombi are neutrophil-rich, impact of neutrophils on clot contraction, a key modulator of thrombus stability and obstructiveness, was unknown. This study investigated how neutrophils and neutrophil extracellular traps (NETs) affect the rate and extent of platelet-driven clot contraction. Isolated human neutrophils were stimulated with phorbol-12-myristate-13-acetate (PMA) to induce NETosis, confirmed by fluorescence microscopy and scanning electron microscopy. Thrombin-induced clots, formed from whole blood or platelet-rich plasma, were supplemented with non-activated or PMA-activated neutrophils. Clot contraction kinetics and viscoelasticity were analyzed. PMA-activated neutrophils significantly enhanced the rate and final extent of clot contraction compared to controls. This promoting effect was abolished by deoxyribonuclease (DNAse) I, confirming that it was mediated by NETs embedded in the fibrin network. The factor Xa inhibitor rivaroxaban also abrogated this effect, indicating a role for NET-induced endogenous thrombin generation and platelet hyperactivation. Thromboelastography revealed that NETs made clots softer and more deformable. We conclude that activated neutrophils promote clot contraction via NETs embedded in the fibrin network, which enhance platelet contractility via endogenous thrombin production and increase clot deformability, suggesting that inflammatory thrombosis may require treatments addressing this enhanced contractility.

The spiny mouse (Acomys species) is capable of scarless wound regeneration through largely yet unknown mechanisms. To investigate whether this capacity is related to peculiarities of the hemostatic system, we studied the blood of Acomys cahirinus in comparison to Mus musculus (Balb/c) to reveal differences in blood composition and clotting in both males and females. In response to surgical manipulations, blood clots formed in wounds of Acomys comprised a stronger hemostatic seal with reduced surgical bleeding in comparison with Balb/c. Acomys demonstrated notably shorter tail bleeding times and elevated clottable fibrinogen levels. Histological analysis revealed that clots from Acomys blood had densely packed fibrin-rich clots with pronounced fibrin segregation from erythrocytes. Acomys exhibited superior plasma clot stiffness as revealed with thromboelastography. The latter two characteristics are likely due to hyperfibrinogenemia. Light transmission platelet aggregometry demonstrated that ADP-induced platelet aggregates in Acomys males are stable, unlike the aggregates formed in the plasma of Balb/c undergoing progressive disaggregation over time. There were no apparent distinctions in platelet contractility and baseline expression of phosphatidylserine. Hematological profiling revealed a reduced erythrocytes count but increased mean corpuscular volume and hemoglobin content in Acomys. These results demonstrate the distinctive hemostatic potential of Acomys cahirinus, which may contribute to their remarkable regenerative capacity.