Quantification of the Passive Mechanical Properties of the Resting Platelet

Abstract

Sudden coronary artery occlusion is one of the leading causes of death. Several in vitro models have been used to study the relationship between hemodynamic forces and platelet function. However, very few in vivo studies exist that fully explore this relationship due to the lack of rheologic data for the platelet. For this purpose, micropipette aspiration techniques were used in the present study to determine the mechanical properties of platelets. The data were analyzed by two mathematical models: (1) an erythrocyte-type membrane model which yielded a platelet shear modulus of 0.03+/-0.01 dyn cm[-1] (mean+/-SD) and a viscous modulus of 0.12+/-0.04 dyn s cm[-1]. (2) An endothelial-type cell model which approximated the platelet Young's modulus to be 1.7+/-0.6 x 10(3) dyn cm(-2) with a viscous modulus of 1.0+/-0.5 x 10(4) dyn s cm(-2). The endothelial-type cell model more accurately describes the mechanics occurring at the micropipette tip and permits more appropriate assumptions to be made in quantifying the rheologic properties of a platelet. Results from this study can be integrated into numerical models of blood flow in stenosed coronary arteries to elucidate the impact of local hemodynamics on platelets and thrombus formation in coronary artery disease.

A mechanobiological investigation of platelets

“…This behavior must then be integrated into a comprehensive mathematical model that accurately simulates micropipette aspiration of a platelet. The theoretical foundations of the model must also be validated, employing complementary data sets from the published works of Haga et al (11) and data collected presently. The presence of mechanotransduction pathways and the limits of platelet membrane strength must also be investigated for their future relevance in clinical applications.…”

“…Cellular integrity is maintained by the cytoskeletal components of the platelet, which act to withstand the forces generated by blood flow over the endothelium (14). The structural elements of this system include a spectrin framework (15) that interfaces with the cytoplasmic side of the plasma membrane, a microtubule coil that lies along the circumference of the cell (11) and finally a stable network of cross-linking actin-filaments that spans the entire cytoplasm The outermost layer of the platelet, known as the glycocalyx or external coat, is a surface coat exposed to the aqueous plasma medium that consists of glycoproteins (GP) and is 15 -20 nm thick (11,16), a magnitude which makes this exterior coat much thicker than in other blood cells (14). Inferior to the glycocalyx is the platelet cellular membrane with a thickness of 10 nm (11), with a phospholipid component (notably phosphatidylserine) that promotes the progression of the coagulation cascade by providing a catalytic surface for…”

“…The organelle centralization is a characteristic feature of the degranulation event, in which respective platelet aggregation and autocrine agonists such as ADP and serotonin are released from the dense bodies, while a-and lysosomal granules release other components to further facilitate the activation of adjacent platelets (14). Degranulation is a result of a-granule and dense body fusion to the open canalicular system (OCS) of channels that interface with the outer membrane of the platelet (11,18) . The fusion of platelet organelles facilitates the translocation of the intracellular metabolites to the extracellular plasma medium, leading to exponential 8 activation of neighboring platelets.…”

“…Characterization of the mechanical properties of platelets and potential activation can be performed using micropipette aspiration of a single cell (11). This type of mechanical manipulation that investigates cellular and subcellular biomechanical properties dates back to the middle of the twentieth century.…”

“…With this resolved, micropipette aspiration could potentially enable the characterization of the variable stress and strain information in the membrane, and the determination of the magnitude of stress and/or strain required for platelet activation or ultimate lysis. Although micropipette experiments on platelets have been performed, they concentrated on the general deformation index, defined as the ratio of the magnitude of platelet deformation to imposed pressure without considerations to the time history of platelet tongue deformation (86,87,88,11). In other early studies, platelet viscosity was assumed to be constant and independent of the rate and type of deformation, as allowing the platelet's aspirated cross-section to return to its original geometry in the absence of applied external forces enables a force balance between the viscous resisting forces and the elastic restoring forces (89,90).…”