Effects of Platelets and Erythrocytes on the Dynamic Size and Mechanical Properties of Blood Clots during Contraction

Clot contraction is a final step of blood clotting and plays a key role in hemostasis and restoring blood flow past obstructive thrombi. The volume shrinkage of clots is driven by the contractile forces generated by activated platelets and propagated by the platelet-attached viscoelastic fibrin fibers throughout the entire clot. We have recently shown that blood clot contraction results in the formation of compressed. tightly packed, polyhedral erythrocytes (polyhedrocytes) and in the redistribution of platelets and fibrin to the surface of the contracted clot as a result of the complex interplay between platelets, fibrin, and erythrocytes. This study further investigates the role of these major blood cells in the dynamic mechanical (or viscoelastic) properties of the clot and the kinetics of clot contraction. Platelet and erythrocyte levels were varied through the use of partially reconstituted blood. Samples of platelet-containing plasma with or without added erythrocytes were recalcified and activated with thrombin. The viscoelastic properties and the force of contraction of the resultant clot were determined using high precision rheology. The kinetics of contraction was analyzed using a Thromboimager (HemaCore, Moscow, Russia), which allows continuous tracking and quantitative characterization of dynamic clot size by sensing changes in the light scattering of the clot over time. As predicted, the rate and degree of clot contraction depended linearly on the platelet count over a broad range (R²=0.9881). Increased platelet concentration of greater than 500 k/μl resulted in a more than 30% increase (p<0.001) in the percentage of clot contraction at 30 minutes when compared to the lowest platelet concentration (<75 k/μl). There was a significant increase in the rate and a ~15% increase (p<0.001) in the percentage of clot contraction seen in samples with 250-300k/μl, however, and no difference in samples with 125-150k/μl when compared to the lowest platelet concentration. It was observed that increasing the hematocrit level also affected the degree of contraction with a 30% decrease (p<0.001) in the percentage of contraction seen as the erythrocyte level was increased to hematocrit >40% when compared to <10% hematocrit. There was a 10-15% decrease in the percentage of contraction seen at intermediate hematocrit levels (p<0.05). In addition to decreasing the degree of contraction, changing the cellular composition also affected the rate of contraction. Increasing the concentration of either erythrocytes or platelets resulted in a relative increase in the viscous (or plastic) properties when compared to elastic (or stiffness) properties of the clot (p<0.01), showing a complex dependence of the viscoelastic behavior of the contracting clot as a result of the addition of cells. The presence of erythrocytes resulted in a 63% increase (p<0.05) in the contractile forces that were generated by the platelet-fibrin network when compared to platelets alone. We interpret these results as a profound effect of erythrocytes on the course of clot contraction and on the final size and mechanical properties of contracted blood clots. These results reveal that the concentration of cellular components critically affects the ability of the platelet-fibrin network on the outside of the clot to generate forces needed to reduce the clot size and to compact the erythrocytes, resulting in the formation of a stiff, dense hemostatic plug with low permeability.