Tortuosity triggers platelet activation and thrombus formation in microvessels.

Tortuous blood vessels, including microvessels, are often seen in humans in association with various conditions such as thrombosis [1], atherosclerosis [1], abdominal aortic aneurism [2], hypertension [3,4], aging [2], retinopathy [5], and arterial tortuosity syndrome [6]. Vessel tortuosity (e.g., curvature, coils, twists, kinks) can restrict or completely occlude blood flow [7], which may result in stroke [8], transient ischemic attack [8], or hindrance of arteriogenesis and tissue regeneration [9]. Thrombosis, the formation of a blood clot (thrombus) within a blood vessel, may lead to vessel occlusion [10] with subsequent myocardial infarction or stroke [11]. Thrombosis has been observed in tortuous blood vessels in patients; for example, in internal carotid arteries [1] and in umbilical cords [12]. Thus it is of clinical importance to understand the mechanisms of thrombus formation in tortuous vessels. Thrombosis can be triggered by chemical and physical mechanisms. Chemical mechanisms include injury to the endothelium [13], rupture of atherothrombotic plaque [10], and hypercoagulable states [13]. A physical mechanism is high fluid shear stress [10]. Tortuosity alters blood flow that can cause high fluid shear stress, likely promoting thrombosis in the absence of chemical mechanisms. Despite the association between thrombosis and tortuosity, studies that investigate the physical mechanisms are lacking. Yet one recent in vivo study in rats observed thrombus formation in uninjured bent venules but not in straight venules [14]. Based on computational fluid dynamics simulations, the authors suggested that thrombosis was caused by increased fluid shear stress due to altered fluid flow from bending of the initially straight venules. However, the underlying physical mechanisms and microscale processes are still poorly understood. Microvessels are embedded within all tissue and organs, distribute blood throughout the entire body, and play a role in inflammation [15]. Thus the study of pathological processes in microvessels, such as thrombosis, is essential to understand associated diseases and develop new therapies. For instance, microvascular thrombosis has been observed in patients with disseminated intravascular coagulation in clinical, experimental, and autopsy settings [16]. Autopsies have shown microthrombi in various parts of the body including lung, liver, kidney, heart, brain, pancreas, adrenal gland, gastrointestinal tracts, and skin [16]. As well, irreversible thrombosis in the microvasculature has been shown to be a lethal response to inflammatory stress due to E. coli in the baboon [17]. In the microcirculation in both arterioles and venules, animal models have shown that thrombi formed in vivo are mainly composed of platelets [18]. Hence, platelets are crucial in thrombosis within the microvasculature in both arterioles and venules. Under thrombogenic conditions, platelets become activated and adhere to each other and to the endothelium through glycoproteins in the platelet surface membrane, various plasma proteins, and a fibrin network [10]. High shear stresses, along with chemical agonists, induce platelet activation [10]. Activated platelets activate additional platelets by releasing and catalyzing platelet agonists [10]. Curvature of tortuous vessels modifies fluid shear stress and fluid velocity field and thus likely initiates shear-induced platelet activation and increases platelet collisions. These two physical mechanisms (i.e., shear-induced activation and collisions) can promote platelet adhesion and aggregation. Therefore understanding the interactions of individual platelets is critical to elucidate physical mechanisms of tortuosity-induced thrombus formation. Tracking individual platelets in large numbers subject to aggregation is difficult by in vivo or in vitro testing but is more feasible in silico. Several computational studies have predicted regions within the flow where platelets may become activated and/or probable sites of thrombus formation in various nonparallel flows (e.g., curved, stenosis, artificial heart valve) [14,19–27]. However, these studies simulated fluid flow either without individual platelets or with platelets that were assumed to follow streamlines and neglected platelet-platelet or platelet-wall collisions. Conversely, simulations that include colliding, adhesive, individual platelets have been accomplished in straight vessels and chambers [28–32]. A few of these simulations with colliding platelets also include platelet activation due to chemical factors [28,31,32]. Though these computational studies provide effective approaches to investigate thrombus formation and platelet activation, thrombus formation in tortuous vessels with shear-induced platelet activation of colliding platelets has not been investigated. Because vessel tortuosity is associated with thrombosis and because shear-induced activation and collision of platelets are likely increased in tortuous vessels, inclusion of these attributes in a model is necessary to study thrombosis in silico. Accordingly, the objectives of this study were to develop a new approach to simulate thrombosis in tortuous vessels using a discrete element computational model and to use this model to determine the effects of venule tortuosity and fluid velocity on the thrombus initiation process.