Contractile actin belt and mesh structures provide the opposite dependence of epithelial stiffness on the spontaneous curvature of constituent cells.

Actomyosin generates contractile forces within cells, which have a crucial role in determining the macroscopic mechanical properties of epithelial tissues. Importantly, actin cytoskeleton, which propagates actomyosin contractile forces, forms several characteristic structures in a 3D intracellular space, such as a circumferential actin belt lining adherence junctions and an actin mesh beneath the apical membrane. However, little is known about how epithelial mechanical property depends on the intracellular contractile structures. We performed computational simulations using a 3D vertex model, and demonstrated the longitudinal tensile test of an epithelial tube, whose inside and outside are defined as the apical and basal surfaces, respectively. As a result, these subcellular structures provide the contrary dependence of epithelial stiffness and fracture force on the spontaneous curvature of constituent cells; the epithelial stiffness increases with increasing the spontaneous curvature in the case of belt, meanwhile it decreases in the case of mesh. This qualitative difference emerges from the different anisotropic deformability of apical cell surfaces; while belt preserves isotropic apical cell shapes, mesh does not. Moreover, the difference in the anisotropic deformability determines the frequency of cell rearrangements, which in turn effectively decrease the tube stiffness. These results illustrate the importance of the intracellular contractile structures, which may be regulated to optimize mechanical functions of individual epithelial tissues.

Effects of arterial blood flow on walls of the abdominal aorta: distributions of wall shear stress and oscillatory shear index determined by phase-contrast magnetic resonance imaging.

Although abdominal aortic aneurysms (AAAs) occur mostly inferior to the renal artery, the mechanism of the development of AAA in relation to its specific location is not yet clearly understood. The objective of this study was to evaluate the hypothesis that even healthy volunteers may manifest specific flow characteristics of blood flow and alter wall shear or oscillatory shear stress in the areas where AAAs commonly develop. Eight healthy male volunteers were enrolled in this prospective study, aged from 24 to 27. Phase-contrast magnetic resonance imaging (MRI) was performed with electrocardiographic triggering. Flow-sensitive four-dimensional MR imaging of the abdominal aorta, with three-directional velocity encoding, including simple morphological image acquisition, was performed. Information on specific locations on the aortic wall was applied to the flow encodes to calculate wall shear stress (WSS) and oscillatory shear index (OSI). While time-framed WSS showed the highest peak of 1.14 ± 0.25 Pa in the juxtaposition of the renal artery, the WSS plateaued to 0.61 Pa at the anterior wall of the abdominal aorta. The OSI peaked distal to the renal arteries at the posterior wall of the abdominal aorta of 0.249 ± 0.148, and was constantly elevated in the whole abdominal aorta at more than 0.14. All subjects were found to have elevated OSI in regions where AAAs commonly occur. These findings indicate that areas of constant peaked oscillatory shear stress in the infra-renal aorta may be one of the factors that lead to morphological changes over time, even in healthy individuals.

Total Cavopulmonary Connection is Superior to Atriopulmonary Connection Fontan in Preventing Thrombus Formation: Computer Simulation of Flow-Related Blood Coagulation.

The classical Fontan route, namely the atriopulmonary connection (APC), continues to be associated with a risk of thrombus formation in the atrium. A conversion to a total cavopulmonary connection (TCPC) from the APC can ameliorate hemodynamics for the failed Fontan; however, the impact of these surgical operations on thrombus formation remains elusive. This study elucidates the underlying mechanism of thrombus formation in the Fontan route by using a two-dimensional computer hemodynamic simulation based on a simple blood coagulation rule. Hemodynamics in the Fontan route was simulated with Navier-Stokes equations. The blood coagulation and the hemodynamics were combined using a particle method. Three models were created: APC with a square atrium, APC with a round atrium, and TCPC. To examine the effects of the venous blood flow velocity, the velocity at rest and during exercise (0.5 and 1.0 W/kg) was measured. The total area of the thrombi increased over time. The APC square model showed the highest incidence for thrombus formation, followed by the APC round, whereas no thrombus was formed in the TCPC model. Slower blood flow at rest was associated with a higher incidence of thrombus formation. The TCPC was superior to the classical APC in terms of preventing thrombus formation, due to significant blood flow stagnation in the atrium of the APC. Thus, local hemodynamic behavior associated with the complex channel geometry plays a major role in thrombus formation in the Fontan route.

Blood flow dynamic improvement with aneurysm repair detected by a patient-specific model of multiple aortic aneurysms.

Aortic aneurysms may cause the turbulence of blood flow and result in the energy loss of the blood flow, while grafting of the dilated aorta may ameliorate these hemodynamic disturbances, contributing to the alleviation of the energy efficiency of blood flow delivery. However, evaluating of the energy efficiency of blood flow in an aortic aneurysm has been technically difficult to estimate and not comprehensively understood yet. We devised a multiscale computational biomechanical model, introducing novel flow indices, to investigate a single male patient with multiple aortic aneurysms. Preoperative levels of wall shear stress and oscillatory shear index (OSI) were elevated but declined after staged grafting procedures: OSI decreased from 0.280 to 0.257 (first operation) and 0.221 (second operation). Graftings may strategically counter the loss of efficient blood delivery to improve hemodynamics of the aorta. The energy efficiency of blood flow also improved postoperatively. Novel indices of pulsatile pressure index (PPI) and pulsatile energy loss index (PELI) were evaluated to characterize and quantify energy loss of pulsatile blood flow. Mean PPI decreased from 0.445 to 0.423 (first operation) and 0.359 (second operation), respectively; while the preoperative PELI of 0.986 dropped to 0.820 and 0.831. Graftings contributed not only to ameliorate wall shear stress or oscillatory shear index but also to improve efficient blood flow. This patient-specific modeling will help in analyzing the mechanism of aortic aneurysm formation and may play an important role in quantifying the energy efficiency or loss in blood delivery.

Three-dimensional modulation of cortical plasticity during pseudopodial protrusion of mouse leukocytes.

Leukocytes can rapidly migrate virtually within any substrate found in the body at speeds up to 100 times faster than mesenchymal cells that remain firmly attached to a substrate even when migrating. To understand the flexible migration strategy utilized by leukocytes, we experimentally investigated the three-dimensional modulation of cortical plasticity during the formation of pseudopodial protrusions by mouse leukocytes isolated from blood. The surfaces of viable leukocytes were discretely labeled with fluorescent beads that were covalently conjugated with concanavalin A receptors. The movements of these fluorescent beads were different at the rear, central, and front surfaces. The beads initially present on the rear and central dorsal surfaces of the cell body flowed linearly toward the rear peripheral surface concomitant with a significant collapse of the cell body in the dorsal-ventral direction. In contrast, those beads initially on the front surface moved into a newly formed pseudopodium and exhibited rapid, random movements within this pseudopodium. Bead movements at the front surface were hypothesized to have resulted from rupture of the actin cytoskeleton and detachment of the plasma membrane from the actin cytoskeletal cortex, which allowed leukocytes to migrate while being minimally constrained by a substrate.

Numerical simulation of the vascular structure dependence of blood flow in the kidney.

A numerical simulation was performed to clarify renal blood flow determination by the vascular structures. Large and small vessels were modeled as symmetric and asymmetric branching vessels, respectively, with simple geometries to parameterize the vascular structures. Modeling individual vessels as straight pipes, Murray's law was used to determine the vessel diameters. Blood flow in the vascular structure was calculated by network analysis based on Hagen-Poiseuille's law. Blood flow simulations for a vascular network segment demonstrated that blood flow rate and pressure vary within the same-generation vessels because of an asymmetric vessel branch while they generally tend to decrease with vessel diameter; thus, the standard deviation of flow rate relative to the mean (relative standard deviation [RSD]) increased from 0.4 to 1.0 when the number of the daughter vessels increased from 3 to 10. Blood flow simulations for an entire vascular network of a kidney showed that the vessel number and branching style, rather than Strahler order, are major parameters in successfully reproducing renal blood flow measured in published experiments. The entire vascular network could generate variation in the physiological flow rate in afferent arterioles at 0.2-0.38 in RSD, which is at least compatible with 0.16 by diameter variation within the same-generation vessels.

Blood cell distribution in small and large vessels: Effects of wall and rotating motion of red blood cells.

A two-dimensional computer simulation of blood flow between two parallel plates as the tube was performed to understand the distribution of red blood cells (RBCs) and platelets (PLTs) according to the blood vessel size. The motion of the blood cells (BCs) was directly calculated using the particle method. The tube diameter and hematocrit were set as 20-500 µm and 0-0.4, respectively. In simulations with tank-treading (TT) RBCs under the planar Poiseuille flow, RBCs moved from the tube wall to form a cell-free layer (CFL). Then, the PLTs moved into the CFL, and the RBCs concentrated around the tube center, excluding the PLTs. By comparing the BC distribution between the Couette and Poiseuille flows, the range of the wall effect was estimated to be ≤50-100 µm at the hematocrit of 0.4. Tumbling (TB) RBCs uniformly distributed inside the tube, while forming rouleaux-like aggregates on the wall at 0.4 in hematocrit; at hematocrit ≤0.3, the TB RBCs tended to be excluded from the tube center as known to the tubular pinch effect. The mechanical interaction among the RBCs and tube wall facilitated TT motion even if the apparent shear rate was so small that an RBC in a dilute suspension would exhibit TB motion. These results indicate that the TT motion of RBCs combined with the wall effect plays a major role in forming CFL and avoiding aggregation of BCs and that TB motion helps BCs to distribute uniformly in large vessels where the shear rate is relatively low.

Simulation of platelet adhesion and aggregation regulated by fibrinogen and von Willebrand factor.

We propose a method to analyze platelet adhesion and aggregation computationally, taking into account the distinct properties of two plasma proteins, von Willebrand factor (vWF) and fibrinogen (Fbg). In this method, the hydrodynamic interactions between platelet particles under simple shear flow were simulated using Stokesian dynamics based on the additivity of velocities. The binding force between particles mediated by vWF and Fbg was modeled using the Voigt model. Two Voigt models with different properties were introduced to consider the distinct behaviors of vWF and Fbg. Our results qualitatively agreed with the general observation of a previous in-vitro experiment, thus demonstrating that the significant development of thrombus formation in height requires not only vWF, but also Fbg. This agreement of simulation and experimental results qualitatively validates our model and suggests that consideration of the distinct roles of vWF and Fbg is essential to investigate the physiological and pathophysiological mechanisms of thrombus formation using a computational approach.

Computational study on effect of red blood cells on primary thrombus formation.

The primary thrombus formation is a critical phenomenon both physiologically and pathologically. It has been seen that various mechanical factors are involved the regulation of primary thrombus formation through a series of physiological and biochemical processes, including blood flow and intercellular molecular bridges. However, it has not been fully understood how the existence of red blood cells contributes to the process of platelet thrombus formation. We computationally analyzed the motions of platelets in plasma layer above which red blood cells flow assuming a background simple shear flow of a shear rate of 1000 s(-1) using Stokesian dynamics. In the computation, fluid mechanical interactions between platelets and red blood cells were taken into account together with the binding forces via plasma proteins between two platelets and between a platelet and injured vessel wall. The process of the platelets aggregation was significantly dependent on whether red blood cells were present. When red blood cells were absent, the aggregate formed grew more vertically compared to the case with red blood cells. Conversely, when red blood cells were present, the aggregate spread more horizontally because the red blood cells constrained the vertical growth when the height of the aggregate reached the level of the red blood cells. Our results suggest that red blood cells mechanically play a significant role in primary thrombus formation, which accelerates the horizontal spread of the thrombus, and point out the necessity of considering the presence of red blood cells when investigating the mechanism of thrombus formation.

In vitro confocal micro-PIV measurements of blood flow in a square microchannel: the effect of the haematocrit on instantaneous velocity profiles.

A confocal microparticle image velocimetry (micro-PIV) system was used to obtain detailed information on the velocity profiles for the flow of pure water (PW) and in vitro blood (haematocrit up to 17%) in a 100-microm-square microchannel. All the measurements were made in the middle plane of the microchannel at a constant flow rate and low Reynolds number (Re=0.025). The averaged ensemble velocity profiles were found to be markedly parabolic for all the working fluids studied. When comparing the instantaneous velocity profiles of the three fluids, our results indicated that the profile shape depended on the haematocrit. Our confocal micro-PIV measurements demonstrate that the root mean square (RMS) values increase with the haematocrit implying that it is important to consider the information provided by the instantaneous velocity fields, even at low Re. The present study also examines the potential effect of the RBCs on the accuracy of the instantaneous velocity measurements.

Differentiation of stenosed and aneurysmal arteries by pulse wave propagation analysis based on a fluid-solid interaction computational method.

Pulse Wave Velocity (PWV) is recognized by clinicians as an index of the mechanical properties of human blood vessels. However, the measured PWV of real human blood vessels will not always obey the Moens-Korteweg equation, which describes the PWV in ideal elastic tubes. Waveform analysis has been studied as an alternative diagnosis for cardiovascular disease, and reflected waves that occur in the diseased region may be a key for the estimation of the severity of disease. In this study, we modeled stenosed and aneurysmal arteries in a three-dimensional coupled fluid-solid interaction scheme, and analyzed the pulse wave propagation in order to assess the reflected waves that occurred in the diseased region. A commercial code (Radioss, MECALOG, France) was used to solve the fluid-solid interactions. A steady flow with Reynolds number 1000 was imposed at the inlet of the artery as the basic flow, then a single rectangular pulse with Reynolds number 4000 was imposed upon the basic flow to produce a propagating wave. We showed that the reflected waves from the stenosis and the aneurysm are different in their phase, and the wavelength of the reflected waves from the aneurysm is affected by the aneurysm length.

Particle method for computer simulation of red blood cell motion in blood flow.

A particle method for the computer simulation of blood flow was proposed to analyze the motion of a deformable red blood cell (RBC) in flowing blood plasma. The RBC and plasma were discretized by particles that have the characteristics of an elastic membrane and a viscous fluid, respectively. The membrane particles were connected to their neighboring membrane particles by springs, and the motion of the particles was determined on the basis of the minimum energy principle. The incompressible flow of plasma that was expressed by the motion of the fluid particles was determined by the moving-particle semi-implicit (MPS) method. The RBC motion and plasma flow were weakly coupled. The two-dimensional simulation of blood flow between parallel plates demonstrated the capability of the proposed method to express the blood flow phenomena observed in experiments, such as the downstream motion of the RBC and the deformation of the RBC into a parachute shape.

Dynamics of Actin Stress Fibers and Focal Adhesions during Slow Migration in Swiss 3T3 Fibroblasts: Intracellular Mechanism of Cell Turning.

To understand the mechanism regulating the spontaneous change in polarity that leads to cell turning, we quantitatively analyzed the dynamics of focal adhesions (FAs) coupling with the self-assembling actin cytoskeletal structure in Swiss 3T3 fibroblasts. Fluorescent images were acquired from cells expressing GFP-actin and RFP-zyxin by laser confocal microscopy. On the basis of the maximum area, duration, and relocation distance of FAs extracted from the RFP-zyxin images, the cells could be divided into 3 regions: the front region, intermediate lateral region, and rear region. In the intermediate lateral region, FAs appeared close to the leading edge and were stabilized gradually as its area increased. Simultaneously, bundled actin stress fibers (SFs) were observed vertically from the positions of these FAs, and they connected to the other SFs parallel to the leading edge. Finally, these connecting SFs fused to form a single SF with matured FAs at both ends. This change in SF organization with cell retraction in the first cycle of migration followed by a newly formed protrusion in the next cycle is assumed to lead to cell turning in migrating Swiss 3T3 fibroblasts.

Spatial and temporal regulation of cancellous bone structure: characterization of a rate equation of trabecular surface remodeling.

A computer simulation of trabecular surface remodeling was carried out to investigate the spatial and temporal regulation of the cancellous bone structure caused by bone cellular activities responding to a local mechanical environment. In the remodeling simulation, the rate of trabecular surface movement was directly related to stress at the trabecular level. Two model parameters, the threshold value of the lazy zone and the sensing distance of the mechanical environment, were introduced into the remodeling rate equation to express the sensitivity of bone cells to mechanical stimuli. A rectangular cancellous bone model under simple and nonuniform compressive loads was constructed using pixel finite elements. A simulation result revealed that the trabecular structure underwent a temporal and spatial change depending on the loading condition. It was found that the threshold value of the lazy zone regulates the rate of structural changes in time, and that sensing distance regulates the spatial distribution of the trabecular structure. The results demonstrate the possibility that the spatial and temporal regulation of the trabecular structure is determined by the sensitivities of bone cells to mechanical stimuli.

The application of computer simulation in the genesis and development of intracranial aneurysms.

The genesis and development of intracranial aneurysm have long been of interest but remain not understood. In the present study we simulate the progression of intracranial aneurysms by constructing a computational model of a curved artery. It is hypothesized that high local wall shear stress above a threshold value will lead to degeneration of the arterial wall mechanical properties. And this degenerative effect may continue even after the wall shear stress has become lower than the threshold, which is referred to as the "time remaining effect" in this study. We performed several groups of studies using both assumptions and aneurysm development is observed as result of the interplay between high wall shear stress, wall degeneration and wall deformation. In the growth of aneurysms with "time-remaining effect", the increase of aneurysmal height accelerates in later steps. It is concluded that computer simulation can yield insight into the understanding of the pathophysiology of aneurysmal initiation and growth, and help in clarifying the role of certain hemodynamic parameters.

Applications of computational models to better understand microvascular remodelling: a focus on biomechanical integration across scales.

Microvascular network remodelling is a common denominator for multiple pathologies and involves both angiogenesis, defined as the sprouting of new capillaries, and network patterning associated with the organization and connectivity of existing vessels. Much of what we know about microvascular remodelling at the network, cellular and molecular scales has been derived from reductionist biological experiments, yet what happens when the experiments provide incomplete (or only qualitative) information? This review will emphasize the value of applying computational approaches to advance our understanding of the underlying mechanisms and effects of microvascular remodelling. Examples of individual computational models applied to each of the scales will highlight the potential of answering specific questions that cannot be answered using typical biological experimentation alone. Looking into the future, we will also identify the needs and challenges associated with integrating computational models across scales.

Functional adaptation of cancellous bone in human proximal femur predicted by trabecular surface remodeling simulation toward uniform stress state.

Two-dimensional simulation of trabecular surface remodeling was conducted for a human proximal femur to investigate the structural change of cancellous bone toward a uniform stress state. Considering that a local mechanical stimulus plays an important role in cellular activities in bone remodeling, local stress nonuniformity was assumed to drive trabecular structural change to seek a uniform stress state. A large-scale pixel-based finite element model was used to simulate structural changes of individual trabeculae over the entire bone. As a result, the initial structure of trabeculae changed from isotropic to anisotropic due to trabecular microstructural changes caused by surface remodeling according to the mechanical environment in the proximal femur. Under a single-loading condition, it was shown that the apparent structural property evaluated by fabric ellipses corresponded to the apparent stress state in cancellous bone. As is observed in the actual bone, a distributed trabecular structure was obtained under a multiple-loading condition. Through these studies, it was concluded that trabecular surface remodeling toward a local uniform stress state at the trabecular level could naturally bring about functional adaptation phenomenon at the apparent tissue level. The proposed simulation model would be capable of providing insight into the hierarchical mechanism of trabecular surface remodeling at the microstructural level up to the apparent tissue level.

Changes in the fabric and compliance tensors of cancellous bone due to trabecular surface remodeling, predicted by a digital image-based model.

Fabric and compliance tensors of a cube of cancellous bone with a complicated three-dimensional trabecular structure were obtained for trabecular surface remodeling by using a digital image-based model combined with a large-scale finite element method. Using mean intercept length and a homogenization method, the fabric and compliance tensors were determined for the trabecular structure obtained in the computer remodeling simulation. The tensorial quantities obtained indicated that anisotropic structural changes occur in cancellous bone adapting to the compressive loading condition. There were good correlations between the fabric tensor, bone volume fraction, and compliance tensor in the remodeling process. The result demonstrates that changes in the structural and mechanical properties of cancellous bone are essentially anisotropic and should be expressed by tensorial quantities.

Patient Record Information System (PaRIS) for primary health care centers in Indonesia.

This study explores the Patient Record Information System (PaRIS) for primary health care centers in a developing country such as Indonesia. The specific geography of the thousand islands country Indonesia is the reason for transportation difficulties as well as communication problems. This causes a serious adverse effect on the public healthcare service especially in the rural area within the country. Hence, a sustainable system is required that makes use of appropriate Information and Communication Technology (ICT). We developed a clinical information system with modest communication technology combined with a unique database distribution system. The Internet and its free software are the main tools for this system. It is a good opportunity for a developing country such as Indonesia to apply open free software in regard to the healthcare sector. This cost effective and sustainable system can enhance the work of physicians in order to provide better and applicable public health care service.

Elastic behavior of a red blood cell with the membrane's nonuniform natural state: equilibrium shape, motion transition under shear flow, and elongation during tank-treading motion.

Direct numerical simulations of the mechanics of a single red blood cell (RBC) were performed by considering the nonuniform natural state of the elastic membrane. A RBC was modeled as an incompressible viscous fluid encapsulated by an elastic membrane. The in-plane shear and area dilatation deformations of the membrane were modeled by Skalak constitutive equation, while out-of-plane bending deformation was formulated by the spring model. The natural state of the membrane with respect to in-plane shear deformation was modeled as a sphere ([Formula: see text]), biconcave disk shape ([Formula: see text]) and their intermediate shapes ([Formula: see text]) with the nonuniformity parameter [Formula: see text], while the natural state with respect to out-of-plane bending deformation was modeled as a flat plane. According to the numerical simulations, at an experimentally measured in-plane shear modulus of [Formula: see text] and an out-of-plane bending rigidity of [Formula: see text] of the cell membrane, the following results were obtained. (i) The RBC shape at equilibrium was biconcave discoid for [Formula: see text] and cupped otherwise; (ii) the experimentally measured fluid shear stress at the transition between tumbling and tank-treading motions under shear flow was reproduced for [Formula: see text]; (iii) the elongation deformation of the RBC during tank-treading motion from the simulation was consistent with that from in vitro experiments, irrespective of the [Formula: see text] value. Based on our RBC modeling, the three phenomena (i), (ii), and (iii) were mechanically consistent for [Formula: see text]. The condition [Formula: see text] precludes a biconcave discoid shape at equilibrium (i); however, it gives appropriate fluid shear stress at the motion transition under shear flow (ii), suggesting that a combined effect of [Formula: see text] and the natural state with respect to out-of-plane bending deformation is necessary for understanding details of the RBC mechanics at equilibrium. Our numerical results demonstrate that moderate nonuniformity in a membrane's natural state with respect to in-plane shear deformation plays a key role in RBC mechanics.