Platelet packing density is an independent regulator of the hemostatic response to injury

Abstract

Background Hemostasis studies performed in vivo have shown that hemostatic plugs formed after penetrating injuries are characterized by a core of highly activated, densely packed platelets near the injury site, covered by a shell of less activated and loosely packed platelets. Thrombin production occurs near the injury site, further activating platelets and starting the process of platelet mass retraction. Tightening of interplatelet gaps may then prevent the escape and exchange of solutes. Objectives To reconstruct the hemostatic plug macro- and micro-architecture and examine how platelet mass contraction regulates solute transport and solute concentration in the gaps between platelets. Methods Our approach consisted of three parts. First, platelet aggregates formed in vitro under flow were analyzed using scanning electron microscopy to extract data on porosity and gap size distribution. Second, a three-dimensional (3-D) model was constructed with features matching the platelet aggregates formed in vitro. Finally, the 3-D model was integrated with volume and morphology measurements of hemostatic plugs formed in vivo to determine how solutes move within the platelet plug microenvironment. Results The results show that the hemostatic mass is characterized by extremely narrow gaps, porosity values even smaller than previously estimated and stagnant plasma velocity. Importantly, the concentration of a chemical species released within the platelet mass increases as the gaps between platelets shrink. Conclusions Platelet mass retraction provides a physical mechanism to establish steep chemical concentration gradients that determine the extent of platelet activation and account for the core-and-shell architecture observed in vivo.

Platelets and Fibrinogen: Emerging Complexity in Trauma-Induced Coagulopathy

“…Therefore, densely packed platelet cores having strong agonist activation may behave differently than partially activated platelet shells that are available for blood sampling. 18,19 Taken together, these findings suggest that there exist multiple phenotypic subtypes of dysfunctional platelets after trauma (including exhausted and not) or that platelet activation machinery remains intact and the functional lesion lies elsewhere.…”

RGS10 shapes the hemostatic response to injury through its differential effects on intracellular signaling by platelet agonists

“…In contrast, thrombin is the main driver of platelet activation in the thrombus core where close packing of the platelets traps thrombin and raises its local concentration. [31][32][33] The lack of an effect of the knockout on fibrin accumulation suggests that the thrombin concentration in the core has not been markedly affected. We infer from the lack of an effect of deleting RGS10 on the core that the local thrombin concentration is sufficient to produce a maximum PAR4mediated response.…”

“…[29][30][31] These gradients are agonist specific, determined in part by the hindered transport of molecules in the narrow gaps between platelets, especially in the core region. 32 As a result, platelets in different regions of hemostatic plugs are exposed to different combinations and concentrations of agonists. Thrombin and fibrin are limited to the thrombus core, whereas ADP and TxA 2 are the major drivers of events in the thrombus shell.…”

In Silico Hemostasis Modeling and Prediction

“…2), either particle-based 36,37 or continuous. 38 The origin of such heterogeneity has been a subject of several computational studies, 39,40 which were focused on the impact of agonist transport parameters within thrombus on the local platelet density and corresponding geometry of interplatelet space. Using the continuous viscoelastic model of the preformed heterogeneous clots, it was recently shown that the stability of such thrombi against the flow is largely dependent on the internal permeability distribution.…”