Biomechanical analysis of cerebral arteries

The intent of this thesis is to study the function of elastin in cerebral arteries, its contribution into the aging process and its implication into different cerebral diseases and in particular cerebral aneurysms. As of today, little is known on the genesis, growth and rupture of cerebral aneurysm. It is widely thought that these lesions rupture when hemodynamically induced wall stress exceeds wall strength. Therefore, there is a clear need for biomechanical analysis of the aneurysmal wall, which would include precise characterization of the stress and strain field experienced in vivo. The characterization of the stress and strain field requires knowledge of geometry, loading conditions as well as precise knowledge of the elastic properties of the aneurysmal and adjacent parent vessel walls. Because elastin is relatively absent in the aneurismal wall and elastin destruction is thought to be part of the aneurysm formation process, one of the main goals of this thesis was to characterize the effects of elastin degradation on the mechanical properties of arteries while assessed with a constituent-based strain energy function developed by Zulliger et al (). The first chapter had as primary objective to assess the contribution of elastin on the biomechanical properties of the rabbit common carotid wall, by selectively destructing elastin with elastase. The second objective was to assess the interaction of elastin with the other structural elements and in particular with collagen and its fiber engagement profile. A number of earlier studies have reported that aging process affects elastin, a key component of the arterial wall integrity and functionality as shown in chapter one. Thus, the part two of this thesis focused on the effects of aging on the biomechanical properties human cerebral arteries and in particular on the contribution of elastin. Cerebral arteries from human cadavers were analyzed biomechanically and morphologically in order to quantify the structural remodeling with aging. The effects of aging on functionality of elastin were assessed via selective enzymatic degradation and structural analysis of the elastin. Furthermore, quantification of desmosine and fibrillin cross-linking were evaluated. The main conclusion is that elastin, although physically present as evidenced by histology, is not functional with minimal contribution to the elasticity of aged cerebral wall. Reduced elasticity and non-functional elastin may affect the metabolism of endothelial and medial cells with important clinical implications in relation to the development of cerebrovascular disease. In the third part of this thesis, we were concerned with methods to collect aneurysmal tissues from different clinical centers for later mechanical testing. Thus, the effect of cryopreservation on the arterial matrix was analyzed in the absence of vascular smooth muscle cells. The experiments provided evidence cryopreservation affects only marginally extracellular matrix (ECM), suggesting thus that changes observed in arterial properties after cryopreservation are likely attributed to VSM cells, either as a direct result of VSM cells damage or as indirect effects of VSM cells injury on ECM. The study results also suggested that acellular tissues (i.e., cerebral aneurysms) can be stored without damage at -80°C with cryoprotectant agents (CPA) for later mechanical testing. In the fourth part, we sought a correlation between non-invasively measured biomechanical properties of common carotid arteries and the existence of cerebral aneurysms. This was based on earlier studies, which have showed that risk for cardiovascular events may be assessed by noninvasively measured markers such as stiffness or high pulse pressure. To that end, we measured geometry and elasticity of human carotids in patients with and without cerebral aneurysms. The findings suggested that patients carrying a cerebral aneurysm, present significantly higher diameters of the common carotid arteries at diastolic pressures and, furthermore, the diameters exhibit significantly higher asymmetry, as expressed by the difference between the left and right common carotid systolic and diastolic diameters. This asymmetry in diameters (and likely in other biomechanical properties) may be exploited as clinical indicators for the existence or future development of cerebral aneurysms. Larger clinical studies are required to reconfirm our interesting, yet preliminary findings. ---------------------------------------- () Zulliger MA, Rachev A, Stergiopulos N. A constitutive formulation of arterial mechanics including vascular smooth muscle tone. Am J Physiol Heart Circ Physiol. 2004;287:H1335-1343

Arterial in vitro remodeling

Gabriela Montorzi Thorell

Vascular remodeling is defined as any enduring change in the size and composition of an adult blood vessel, allowing adaptation or repair. The vascular remodeling response has been shown to depend on a variety of endogenous and environmental factors. Physiological remodeling is a tightly regulated process that mainly occurs in response to long-term changes in hemodynamic conditions. The adaptation to these hemodynamic changes implies the production of mediators that influence structure as well as function. A loss of regulation in the adaptive response underlies the pathogenesis of major cardiovascular diseases, including hypertension, atherosclerosis, restenosis and arterial aneurismal dilatation. Specialized enzymes called matrix metalloproteinases (MMPs) have been shown to have a predominant participation in the reorganization of the vessel structure, through the degradation of the extracellular matrix scaffold. The aim of this thesis is to gain insight in the biological and mechanical processes taking place in the vascular wall as an adaptive response to different biomechanical stimuli such as blood pressure and blood flow. This work proposes a new model for the study of vascular remodeling where physical factors acting on the arterial wall can be dissociated and analyzed, individually, in relation to the biological response. The results are presented in form of an introduction, three scientific papers and a conclusion section. The investigation has been designed around three different approaches: adaptation of a non-uniform artery to an in vitro environment, vascular adaptation to steady and pulsatile pressure and vascular adaptation to unidirectional and oscillatory flow. The adaptive response to each one of the variables chosen has been analyzed through biological and biomechanical remodeling indicators of the arterial wall. The introduction is an overview of the biological and biomechanical characteristics of arterial wall in relation to the remodeling process. The contribution of vascular smooth muscle cells and extracellular matrix to physiological and pathological arterial remodeling is discussed. Paper I assesses the relative remodeling of a non-axisymmetric artery in relation to its environment. The study considers the native circumferential asymmetry of the porcine right common carotid, which results from the non-homogenous mechanical and hemodynamic native environment. The adaptive response of the artery to an in vitro perfusion environment is analyzed. The study shows that in vitro perfusion leads, through remodeling, to a circumferentially uniform scleroprotein distribution and to a change in arterial compliance. This study emphasizes the link between structural changes, the biomechanical response and the enzymatic implication in the adaptive response. In paper II we investigate the role of continuous and cyclic stretch, produced by steady or pulsatile pressure acting on the arterial wall, on the remodeling response. The study shows that exposure to continuous and cyclic stretch differentially affects the relative scleroprotein content and leads to a change in arterial wall stiffness. The adaptive outcome is studied through an integrative approach taking into consideration the geometrical and structural adaptation, the biomechanical behavior and the enzymatic agents implicated in extracellular matrix turnover. Paper III analyzes the influence of different flow patterns on the arterial adaptive response. We investigate the differential effects of oscillatory flow, mimicking a plaque-prone hemodynamic environment, and unidirectional flow, to mimicking a physiologically protective (plaquefree) environment. The effect of these hemodynamic forces on the remodeling response are characterized through the study of endothelial and smooth muscle cell function as well as through assessment of agents influencing extracellular matrix turnover. The conclusions section presents a synthesis of the results and contribution of this thesis and proposes perspectives for future studies.

EPFL2004

Effect of intimal shear and cyclic strech on arterial segments perfused in vitro

Veronica Gambillara

Hemodynamics have crucial role in the control of vascular tone, in the regulation of arterial remodeling, and in the production of mediators that maintain arterial function. Changes in hemodynamics forces induce profound alterations in vascular cell metabolism and function, in the extracellular matrix, and in smooth muscle cell (SMC) phenotypes, all being part of vessel wall remodeling. Vessel wall remodeling plays a key role in arterial wall homeostasis but also in the initiation and development of arterial disease. This aim of this work is to study the effects of two distinct biomechanical stimuli, oscillatory shear and cyclic stretch on vessel wall function. Our 71 investigation is performed using an improved in vitro artery perfusion system, which allows to study the exact contribution of each biomechanical stimulus on isolated arterial segments, where the biological cell responses of endothelial and SMCs, and the extracellular matrix interactions are well preserved. The present thesis is divided into two main sections. The first part focuses on the effects of oscillatory shear stress on arterial segments, a mechanism presumed to be involved in the localization and initiation of atherosclerotic plaques. The second section investigates the influence of reduced cyclic stretch on the arterial wall, a procedure that is occurring in arterial stiffening. In the first and second papers, we examined the effects of three different shear stress conditions on the endothelium, SMC, and arterial wall responses, in porcine carotid arteries perfused in vitro for three days. We demonstrated that oscillatory shear stress, which is usually present in plaque-prone areas, decreased the bradykinin-induced vasorelaxation after 3 days of perfusion. The endothelial dysfunction was correlated to an impaired endothelial nitric oxide synthase gene expression and bradykinin-mediated activation. The potential lack of nitric oxide production and/or bioavailability observed in arteries exposed to oscillatory shear stress may also be the cause of the start of matrix turnover. Expression and activation of matrix metalloproteinase-2 and-9 were increased in arteries exposed to oscillatory shear, suggesting that this type of shear stress triggers a remodeling process. This part of the study provides a new insights on the role of shear stress in the control of arterial function and supports the hypothesis that oscillatory shear stress is a determining factor predisposing the arterial wall to atherosclerosis. The third paper focuses on the effects of cyclic stretch on vascular smooth muscle function, phenotype and wall remodeling. Arteries were wrapped by an external banding, which drastically decreased compliance and therefore the cyclic stretch. Arteries were perfused for one day under a unidirectional shear stress (6±3 dyn/cm2) in combination with a mean pressure of 80 mmHg and a pulse pressure of +/-10 mmHg. We demonstrated that exposure to reduced cyclic stretch induced a rapid modulation of SMC phenotype which was combined with a decrease in norepinephrine-induced vasocontraction. SMC dedifferentiation was associated to a decrease of plasminogen activator inhibitor-1 expression, which, combined with enhanced matrix metalloproteinase-2, may promote wall remodeling and SMC migration. These findings give a further approach into possible consequences of an arterial stiffening on large vessel wall patho-physiology.

EPFL2005

Vascular physiology

Sylvain Roy

This thesis aims at better understanding the role played by vascular smooth muscle cells (VSM) in the biomechanics of the arterial wall. Current models assume that under maximal vasodilation VSM carry no load, or the so-called "active stress" is zero. Biomechanical tests on arterial segments, where VSM cells were either chemically removed or structurally weakened have shown that VSM are playing a determinant role in the biomechanical properties of the arterial wall and carry important load even when maximally dilated. Based on these results a new model of the arterial wall biomechanics is proposed that takes into account the passive structure of the arterial wall constituents. The entire thesis consists of three distinct studies, either published or submitted and under review for publication. The first paper presents the effects of decellularization on the biomechanical properties of porcine common carotid arteries. The decellularization was performed by a detergent-enzymatic procedure, which preserves the extracellular matrix scaffold. Rupture tests were conducted. Inner diameter and wall thickness were measured by echotracking during pressure inflation from 10 to 145 mmHg. Distensibility and incremental elastic modulus were computed. At 10 mmHg, mean inner diameter of decellularized arteries was substantially higher than controls, whereas both decellularized and control arteries reached the same internal diameter value at 145 mmHg. Removing cellular elements from media lead to changes in arterial dimensions. The incremental elastic modulus increased, and the collagen fibers engaged more rapidly during inflation, thus yielding a stiffer vessel. Distensibility and compliance were therefore significantly lower in decellularized vessels, reaching a factor three reduction in the physiological range of pressures. The conclusion of this paper is that decellularization yields vessels able to withstand high inflation pressures with, however, markedly different geometrical and biomechanical properties. The second paper proposes an interpretation of the data collected in the experiments conducted in the first paper. This initial work on decellularized arteries revealed the existence of significant residual stresses within the arterial wall, which are released upon chemical removal of vascular smooth muscle in normal arteries causing substantial radial expansion. Hence, the often-used Hill's model describing active and passive stresses within the wall does not hold true, because the existence of prestresses precludes the fundamental assumption of zero active stress when the vascular smooth muscle is inactive. A new mathematical model based on a modified Hill's model was developed, where the total wall elastin is partitioned in two parts: one in-parallel to vascular smooth muscle and collagen and one connected in-series with vascular smooth muscle. Based on experimental evidences, compressive prestresses were assumed to exist on the parallel elastic component and tensile prestresses on the series elastic component. Further, it was assumed that the elastic constants of elastin and collagen and the statistical description of collagen engagement are not affected by decellularization. Excellent fits of the pressure-diameter curves of normal and decellularized arteries were obtained. The model predicts that the majority of elastin is in-series with the vascular smooth muscle (74±8%) and thus only about one-fourth of elastin acts in parallel to the vascular smooth muscle. The conclusion of this paper is that correct biomechanical modeling of the arterial wall requires the knowledge of the zero stress state of both the series and parallel elastic components. The third and last paper analyzes the effects of cytoskeleton destruction, after administration of Cytochalasin D, on the biomechanical properties of porcine common carotids. Compared to untreated, maximally dilated controls, Cytochalasin D-treated arteries have shown a marked increase in compliance in the elastin-dominated, 10-150 mmHg, pressure range. After weakening the vascular smooth muscle stress-bearing cytoskeleton by Cytochalasin D the artery would expand, reaching a new equilibrium state. This study brings further evidence that VSM is under tension, even when it is under zero load and at maximal vasodilation. This residual tension was released upon partial destruction of the cytoskeleton with Cytochalasin D. From a biomechanical standpoint, this means that the Zero Stress States of the in series and parallel elastic components are substantially different.

EPFL2008