Dependence of alignment direction on magnitude of strain in esophageal smooth muscle cells

The response of cells in vitro to mechanical forces has been the subject of much research using devices to exert controlled mechanical stimulation on cultured cells or isolated tissue. In this study, esophageal smooth muscle cells were seeded on flexible polyurethane membranes to form a confluent cell layer. The cells were then subjected to uniform cyclic stretch of varying magnitudes at a frequency of approximately five cycles per minute in a custom made mechatronic bioreactor, providing similar strains experienced in the in vivo mechanical environment of the esophagus. The results show that the orientation response is dependent on the magnitude of cyclic stretch applied. Smooth muscle cells showed parallel alignment to the force direction at low cyclic strains (2%) compared to the hill‐valley morphology of static controls. At higher strains (5% and 10% magnitude), the cells exhibited a consistent alignment perpendicular to the strain. To our knowledge, this is the first time that the alignment direction's dependence on strain magnitude has been demonstrated. MTS analysis indicated that cell metabolism was reduced when mechanical strain was applied, and proliferation was inhibited by mechanical strain. Protein expression indicates a decrease in smooth muscle α‐actin, indicative of changes in cell phenotype, an increase in vimentin, which is associated with increased cell motility, and an increase in desmin, indicating differentiation in stimulated cells. Biotechnol. Bioeng. 2009;102: 1703–1711. © 2008 Wiley Periodicals, Inc.     

Design of a novel rotating wall bioreactor for the in vitro simulation of the mechanical environment of the endothelial function

Endothelial cells (ECs), besides being a permeability barrier between the blood and vessel wall, perform many important functions, e.g., cell migration, remodeling, proliferation, and the production, secretion and metabolism of biochemical substances, as well as the regulation of contractility of vascular smooth muscle cells (SMCs). Their function is modulated by chemical ligands as well as mechanical factors. The mechanical stresses acting on the vessel wall include the normal and circumferential stresses that result from the action of blood pressure, the shear stress that acts parallel to the luminal surface of the vessel due to blood flow and the magnitude and orientation of the gravitation field. The aim of this work was to design and construct a novel bioreactor for the stimulation of endothelial cells in vitro with a combination of mechanical factors that simulates their in vivo environment.     

Stepwise morphological changes and cytoskeletal reorganization of human mesenchymal stem cells treated by short-time cyclic uniaxial stretch

This study aimed to investigate stepwise remodeling of human mesenchymal stem cells (hMSCs) in response to cyclic stretch through rearrangement and alignment of cells and cytoskeleton regulation toward smooth muscle cell (SMC) fate in different time spans. Image analysis techniques were utilized to calculate morphological parameters. Cytoskeletal reorganization was observed by investigating F-actin filaments using immunofluorescence staining, and expression level of contractile SMC markers was followed by a quantitative polymerase chain reaction method. Applying cyclic uniaxial stretch on cultured hMSCs, utilizing a costume-made device, led to alteration in fractal dimension (FD) and cytoskeleton structure toward continuous alignment and elongation of cells by elevation of strain duration. Actin filaments became more aligned perpendicular to the axis of mechanical stretch by increasing uniaxial loading duration. At first, FD met a significant decrease in 4 h loading duration then increased significantly by further loading up to 16 h, followed by another decrease up to 1 d of uniaxial stretching. HMSCs subjected to 24 h cyclic uniaxial stretching significantly expressed early and intermediate contractile SM markers. It was hypothesized that the increase in FD after 4 h while cells continuously became more aligned and elongated was due to initiation of change in phenotype that influenced arrangement of cells. At this point, change in cell phenotype started leading to change in morphology while mechanical loading still caused cell alignment and rearrangement. Results can be helpful when optimized engineered cells are needed based on mechanical condition for functional engineered tissue and cell therapy.     

Endothelial actin cytoskeleton remodeling during mechanostimulation with fluid shear stress

Fluid shear stress stimulation induces endothelial cells to elongate and align in the direction of applied flow. Using the complementary techniques of photoactivation of fluorescence and fluorescence recovery after photobleaching, we have characterized endothelial actin cytoskeleton dynamics during the alignment process in response to steady laminar fluid flow and have correlated these results to motility. Alignment requires 24 h of exposure to fluid flow, but the cells respond within minutes to flow and diminish their movement by 50%. Although movement slows, the actin filament turnover rate increases threefold and the percentage of total actin in the polymerized state decreases by 34%, accelerating actin filament remodeling in individual cells within a confluent endothelial monolayer subjected to flow to levels used by dispersed nonconfluent cells under static conditions for rapid movement. Temporally, the rapid decrease in filamentous actin shortly after flow stimulation is preceded by an increase in actin filament turnover, revealing that the earliest phase of the actin cytoskeletal response to shear stress is net cytoskeletal depolymerization. However, unlike static cells, in which cell motility correlates positively with the rate of filament turnover and negatively with the amount polymerized actin, the decoupling of enhanced motility from enhanced actin dynamics after shear stress stimulation supports the notion that actin remodeling under these conditions favors cytoskeletal remodeling for shape change over locomotion. Hours later, motility returned to pre-shear stress levels but actin remodeling remained highly dynamic in many cells after alignment, suggesting continual cell shape optimization. We conclude that shear stress initiates a cytoplasmic actin-remodeling response that is used for endothelial cell shape change instead of bulk cell translocation. atherosclerosis; cytoskeletal dynamics; endothelial cells; mechanotransduction     

Exogenous and endogenous force regulation of endothelial cell behavior

Endothelial cells live in a dynamic environment where they are constantly exposed to external hemodynamic forces and generate cytoskeletal-based endogenous forces. These exogenous and endogenous forces are critical regulators of endothelial cell health and blood vessel maintenance at all generations of the vascular system, from large arteries to capillary beds. The first part of this review highlights the role of the primary exogenous hemodynamic forces of shear, cyclic strain, and pressure forces in mediating endothelial cell response. We then discuss the emergent role of the mechanical properties of the extracellular matrix and of cellular endogenous force generation on endothelial cell function, implicating substrate stiffness and cellular traction stresses as important mediators of endothelial cell health. The intersection of exogenous and endogenous forces on endothelial cell function is discussed, suggesting some of the many remaining questions in the field of endothelial mechanobiology.     

Effect of combined cyclic stretch and fluid shear stress on endothelial cell morphological responses

Endothelial cells in vivo are normally subjected to multiple mechanical stimuli such as stretch and fluid shear stress (FSS) but because each stimulus induces magnitude-dependent morphologic responses, the relative importance of each stimulus in producing the normal in vivo state is not clear Using cultured human aortic endothelial cells, this study first determined equipotent levels of cyclic stretch, steady FSS, and oscillatory FSS with respect to the time course of cell orientation. We then tested whether these levels of stimuli were equipotent in combination with each other by imposing simultaneous cyclic stretch and steady FSS or cyclic stretch and oscillatory FSS so as to reinforce or counteract the cells' orientation responses. Equipotent levels of the three stimuli were 2% cyclic stretch at 2%/s, 80 dynes/cm2 steady FSS and 20 +/- 10 dynes/cm2 oscillatory FSS at 20 dyne/cm2-s. When applied in reinforcing fashion, cyclic stretch and oscillatory, but not steady, FSS were additive. Both pairs of stimuli canceled when applied in counteracting fashion. These results indicate that this level of cyclic stretch and oscillatory FSS sum algebraically so that they are indeed equipotent. In addition, oscillatory FSS is a stronger stimulus than steady FSS for inducing cell orientation. Moreover, arterial endothelial cells in vivo are likely receiving a stronger stretch than FSS stimulus.     

Microtubule dynamics regulate cyclic stretch-induced cell alignment in human airway smooth muscle cells

Microtubules are structural components of the cytoskeleton that determine cell shape, polarity, and motility in cooperation with the actin filaments. In order to determine the role of microtubules in cell alignment, human airway smooth muscle cells were exposed to cyclic uniaxial stretch. Human airway smooth muscle cells, cultured on type I collagen-coated elastic silicone membranes, were stretched uniaxially (20% in strain, 30 cycles/min) for 2 h. The population of airway smooth muscle cells which were originally oriented randomly aligned near perpendicular to the stretch axis in a time-dependent manner. However, when the cells treated with microtubule disruptors, nocodazole and colchicine, were subjected to the same cyclic uniaxial stretch, the cells failed to align. Lack of alignment was also observed for airway smooth muscle cells treated with a microtubule stabilizer, paclitaxel. To understand the intracellular mechanisms involved, we developed a computational model in which microtubule polymerization and attachment to focal adhesions were regulated by the preexisting tensile stress, pre-stress, on actin stress fibers. We demonstrate that microtubules play a central role in cell re-orientation when cells experience cyclic uniaxial stretching. Our findings further suggest that cell alignment and cytoskeletal reorganization in response to cyclic stretch results from the ability of the microtubule-stress fiber assembly to maintain a homeostatic strain on the stress fiber at focal adhesions. The mechanism of stretch-induced alignment we uncovered is likely involved in various airway functions as well as in the pathophysiology of airway remodeling in asthma.     

Analysis of the mechanical behavior of chondrocytes in unconfined compression tests for cyclic loading

Experimental evidence indicates that the biosynthetic activity of chondrocytes is associated with the mechanical environment. For example, excessive, repetitive loading has been found to induce cell death, morphological and cellular damage, as seen in degenerative joint disease, while cyclic, physiological-like loading has been found to trigger a partial recovery of morphological and ultrastructural aspects in osteoarthritic human articular chondrocytes. Mechanical stimuli are believed to influence the biosynthetic activity via the deformation of cells. However, the in situ deformation of chondrocytes for cyclic loading conditions has not been investigated experimentally or theoretically. The purpose of the present study was to simulate the mechanical response of chondrocytes to cyclic loading in unconfined compression tests using a finite element model. The material properties of chondrocytes and extracellular matrix were considered to be biphasic. The time-histories of the shape and volume variations of chondrocytes at three locations (i.e., surface, center, and bottom) within the cartilage were predicted for static and cyclic loading conditions at two frequencies (0.02 and 0.1 Hz) and two amplitudes (0.1 and 0.2 MPa). Our results show that cells at different depths within the cartilage deform differently during cyclic loading, and that the depth dependence of cell deformation is influenced by the amplitude of the cyclic loading. Cell deformations under cyclic loading of 0.02 Hz were found to be similar to those at 0.1 Hz. We conclude from the simulation results that, in homogeneous cartilage layers, cell deformations are location-dependent, and further are affected by load magnitude. In physiological conditions, the mechanical environment of cells are even more complex due to the anisotropy, depth-dependent inhomogeneity, and tension-compression non-linearity of the cartilage matrix. Therefore, it is feasible to speculate that biosynthetic responses of chondrocytes to cyclic loading depend on cell location and load magnitude.     

Stretch-induced hypertrophy activates NFkB-mediated VEGF secretion in adult cardiomyocytes

Hypertension and myocardial infarction are associated with the onset of hypertrophy. Hypertrophy is a compensatory response mechanism to increases in mechanical load due to pressure or volume overload. It is characterized by extracellular matrix remodeling and hypertrophic growth of adult cardiomyocytes. Production of Vascular Endothelial Growth Factor (VEGF), which acts as an angiogenic factor and a modulator of cardiomyocyte function, is regulated by mechanical stretch. Mechanical stretch promotes VEGF secretion in neonatal cardiomyocytes. Whether this effect is retained in adult cells and the molecular mechanism mediating stretch-induced VEGF secretion has not been elucidated. Our objective was to investigate whether cyclic mechanical stretch induces VEGF secretion in adult cardiomyocytes and to identify the molecular mechanism mediating VEGF secretion in these cells. Isolated primary adult rat cardiomyocytes (ARCMs) were subjected to cyclic mechanical stretch at an extension level of 10% at 30 cycles/min that induces hypertrophic responses. Cyclic mechanical stretch induced a 3-fold increase in VEGF secretion in ARCMs compared to non-stretch controls. This increase in stretch-induced VEGF secretion correlated with NFkB activation. Cyclic mechanical stretch-mediated VEGF secretion was blocked by an NFkB peptide inhibitor and expression of a dominant negative mutant IkBα, but not by inhibitors of the MAPK/ERK1/2 or PI3K pathways. Chromatin immunoprecipitation assays demonstrated an interaction of NFkB with the VEGF promoter in stretched primary cardiomyocytes. Moreover, VEGF secretion is increased in the stretched myocardium during pressure overload-induced hypertrophy. These findings are the first to demonstrate that NFkB activation plays a role in mediating VEGF secretion upon cyclic mechanical stretch in adult cardiomyocytes. Signaling by NFkB initiated in response to cyclic mechanical stretch may therefore coordinate the hypertrophic response in adult cardiomyocytes. Elucidation of this novel mechanism may provide a target for developing future pharmacotherapy to treat hypertension and heart disease.     

A physically motivated constitutive model for cell-mediated compaction and collagen remodeling in soft tissues

Collagen is the main load-bearing component of many soft tissues and has a large influence on the mechanical behavior of tissues when exposed to mechanical loading. Therefore, it is important to increase our understanding of collagen remodeling in soft tissues to understand the mechanisms behind pathologies and to control the development of the collagen network in engineered tissues. In the present study, a constitutive model was developed by coupling a recently developed model describing the orientation and contractile stresses exerted by cells in response to mechanical stimuli to physically motivated collagen remodeling laws. In addition, cell-mediated contraction of the collagen fibers was included as a mechanism for tissue compaction. The model appeared to be successful in predicting a range of experimental observations, which are (1) the change in transition stretch of periosteum after remodeling at different applied stretches, (2) the compaction and alignment of collagen fibers in tissue-engineered strips, (3) the fiber alignment in cruciform gels with different arm widths, and (4) the alignment of collagen fibers in engineered vascular grafts. Moreover, by changing the boundary conditions, the model was able to predict a helical architecture in the vascular graft without assuming the presence of two helical fiber families a priori. Ultimately, this model may help to increase our understanding of collagen remodeling in physiological and pathological conditions, and it may provide a tool for determining the optimal experimental conditions for obtaining native-like collagen architectures in engineered tissues.     

The elongation and orientation of cultured endothelial cells in response to shear stress

Vascular endothelial cells appear to be aligned with the flow in the immediate vicinity of the arterial wall and have a shape which is more ellipsoidal in regions of high shear and more polygonal in regions of low shear stress. In order to study quantitatively the nature of this response, bovine aortic endothelial cells grown on Thermanox plastic coverslips were exposed to shear stress levels of 10, 30, and 85 dynes/cm2 for periods up to 24 hr using a parallel plate flow chamber. A computer-based analysis system was used to quantify the degree of cell elongation with respect to the change in cell angle of orientation and with time. The results show that (i) endothelial cells orient with the flow direction under the influence of shear stress, (ii) the time required for cell alignment with flow direction is somewhat longer than that required for cell elongation, (iii) there is a strong correlation between the degree of alignment and endothelial cell shape, and (iv) endothelial cells become more elongated when exposed to higher shear stresses.     

Endothelial cells as mechanical transducers: Enzymatic activity and network formation under cyclic strain

Although it is established that endothelial cells can respond to external mechanical cues (e.g., alignment in the direction of fluid shear stress), the extent to which mechanical stress and strain applied via the endothelial cell substrate impact biomolecular and cellular processes is not well-understood. This issue is particularly important in the context of inflammation, vascular remodeling, and cancer progression, as each of these processes occurs concurrently with localized increases in strain and marked changes in molecules secreted by adjacent cells. Here, we systematically vary the level and duration of cyclic tensile strain applied to human dermal microvascular and bovine capillary endothelial cells via substrate deflection, and then correlate these cues with the secretion of extracellular matrix-degrading enzymes and a morphological transition from confluent monolayers to well-defined multicellular networks that resemble capillary tube-like structures. For a constant chemical environment, we find that super-physiological mechanical strain stimulates both endothelial cell secretion of latent matrix metalloprotease-2 and multicellular networks in a time- and strain-dependent manner. These results demonstrate coupling between the mechanical and biochemical states of microvascular endothelial cells, and indicate that elevated local stress may directly impact new capillary growth (angiogenesis) toward growing tumors and at capillary wall defect sites.     

Cyclic mechanical compression increases mineralization of cell-seeded polymer scaffolds in vivo

Despite considerable documentation of the ability of normal bone to adapt to its mechanical environment, very little is known about the response of bone grafts or their substitutes to mechanical loading even though many bone defects are located in load-bearing sites. The goal of this research was to quantify the effects of controlled in vivo mechanical stimulation on the mineralization of a tissue-engineered bone replacement and identify the tissue level stresses and strains associated with the applied loading. A novel subcutaneous implant system was designed capable of intermittent cyclic compression of tissue-engineered constructs in vivo. Mesenchymal stem cell-seeded polymeric scaffolds with 8 weeks of in vitro preculture were placed within the loading system and implanted subcutaneously in male Fisher rats. Constructs were subjected to 2 weeks of loading (3 treatments per week for 30 min each, 13.3 N at 1 Hz) and harvested after 6 weeks of in vivo growth for histological examination and quantification of mineral content. Mineralization significantly increased by approximately threefold in the loaded constructs. The finite element method was used to predict tissue level stresses and strains within the construct resulting from the applied in vivo load. The largest principal strains in the polymer were distributed about a modal value of -0.24% with strains in the interstitial space being about five times greater. Von Mises stresses in the polymer were distributed about a modal value of 1.6 MPa, while stresses in the interstitial tissue were about three orders of magnitude smaller. This research demonstrates the ability of controlled in vivo mechanical stimulation to enhance mineralized matrix production on a polymeric scaffold seeded with osteogenic cells and suggests that interactions with the local mechanical environment should be considered in the design of constructs for functional bone repair.     

Biomechanics of Schlemm's canal endothelial cells: influence on F-actin architecture

Aqueous humor drains from the eye through Schlemm's canal, a small endothelial-lined collecting duct. Schlemm's canal endothelial cells may be important in controlling the pressure within the eye (and hence are of interest in glaucoma), and are subject to an unusual combination of shear stress and a basal-to-apical pressure gradient. We sought to characterize this biomechanical environment and determine its effects on F-actin architecture in situ. A theoretical model of flow in Schlemm's canal was used to estimate shear stresses applied to endothelial cells by flowing aqueous humor. Alignment of Schlemm's canal endothelial cells in human eyes was quantified by scanning electron microscopy. F-actin architecture was visualized by fluorescent labeling and compared for closely adjacent cells exposed to different biomechanical environments. We found that, despite the relatively low flow rate of aqueous humor, shear stresses experienced by Schlemm's canal endothelial cells could reach those in the arterial system. Schlemm's canal endothelial cells showed a statistically significant preferential alignment, consistent with a shear-mediated effect. Schlemm's canal endothelial cells subjected to a basal-to-apical pressure gradient due to transendothelial flow showed a prominent marginal band of F-actin with relatively few cytoplasmic filaments. Adjacent cells not subject to this gradient showed little marginal F-actin, with a denser cytoplasmic random network. We conclude that Schlemm's canal endothelial cells experience physiologically significant levels of shear stress, promoting cell alignment. We speculate that this may help control the calibre of Schlemm's canal. F-actin distribution depends critically on the presence or absence of transendothelial flow and its associated pressure gradient. In the case of this pressure gradient, mechanical reinforcement around the cell periphery by F-actin seems to be critical.     

Specificity of endothelial cell reorientation in response to cyclic mechanical stretching

Evidence suggests that cellular responses to mechanical stimuli depend specifically on the type of stimuli imposed. For example, when subjected to fluid shear stress, endothelial cells align along the flow direction. In contrast, in response to cyclic stretching, cells align away from the stretching direction. However, a few aspects of this cell alignment response remain to be clarified: (1) Is the cell alignment due to actual cell reorientation or selective cell detachment? (2) Does the resulting cell alignment represent a response of the cells to elongation or shortening, or both? (3) Does the cell alignment depend on the stretching magnitude or rate, or both? Finally, the role of the actin cytoskeleton and microtubules in the cell alignment response remains unclear. To address these questions, we grew human aortic endothelial cells on deformable silicone membranes and subjected them to three types of cyclic stretching: simple elongation, pure uniaxial stretching and equi-biaxial stretching. Examination of the same cells before and after stretching revealed that they reoriented. Cells subjected to either simple elongation or pure uniaxial stretching reoriented specifically toward the direction of minimal substrate deformation, even though the directions for the two types of stretching differed by only about 20°. At comparable stretching durations, the extent of cell reorientation was more closely related to the stretching magnitude than the stretching rate. The actin cytoskeleton of the endothelial cell subjected to either type of stretching was reorganized into parallel arrays of actin filaments (i.e., stress fibers) aligned in the direction of the minimal substrate deformation. Furthermore, in response to equi-biaxial stretching, the actin cytoskeleton was remodeled into a “tent-like” structure oriented out of the membrane plane—again towards the direction of the minimal substrate deformation. Finally, abolishing microtubules prevented neither the formation of stress fibers nor cell reorientation. Thus, endothelial cells respond very specifically to the type of deformation imposed upon them.     

A mechanical apparatus with microprocessor controlled stress profile for cyclic compression of cultured articular cartilage explants

An apparatus was designed for mechanical compression of cultured articular cartilage explants with acylindrical plain-ended loading head (diameter 2-5 mm) driven by a stepping motor. A load cell under the culture dish was applied for feedback regulation utilizing a microprocessor-based control unit. The operating programs allowed either continuous or cyclic loading, the latter with adjustable loading/resting ratio. The improvements in the present design compared with previously described apparatuses for similar purposes include: (1) the accurately controlled compression by a load cell and a rapid feedback circuit; (2) the wide range of selectable stresses (25 kPa-12.5 MPa) with both continuous and cyclic loading modes; (3) the ability to handle cycles as short as 1 s with 15 ms peak loading phase. Using a 4 s cycle and 0.5 MPa load for 1.5 h resulted in a significantly enhanced incorporation of radiosulphate in cultured bovine articular cartilage explants, suggesting a stimulation of proteoglycan synthesis. Light and scanning electron microscopic examinations revealed a slight depression and superficial alterations in cartilage structure at the impact site following high pressures. We expect that this apparatus will help in revealing how articular cartilage tissue and chondrocytes respond to external mechanical stimuli.     

Strain-induced orientation response of endothelial cells: effect of substratum adhesiveness and actin-myosin contractile level

Endothelial cells subjected to cyclic stretching change orientation so as to be aligned perpendicular to the direction of applied strain in a magnitude and time-dependent manner. Although this type of response is not the same as motility, it could be governed by motility-related factors such as substratum adhesiveness and actin-myosin contractile level. To examine this possibility, human aortic endothelial cells (HAEC) were uniaxially, cyclically stretched on silicone rubber membranes coated with various concentrations of fibronectin, collagen type IV and laminin to produce differing amounts of adhesiveness (measured using a radial flow detachment assay). Cells were subjected to 10% pure cyclic uniaxial stretching for three hours at a rate of 10%/sec. Time-lapse images revealed that cells underwent large morphological changes without moving. For each type of protein there was a parabolic dependence on initial adhesiveness with optimal cell orientation occurring at very similar adhesive strengths. The effect of actin-myosin contractile level was examined by stretching cells treated with different doses of 2,3-butanedione monoxime (BDM) and Blebbistatin. Each drug induced a dose-dependent decrease in orientation angles after three hours of cyclic stretching. Furthermore, cell and stress fiber orientations were tightly coupled for untreated and Blebbistatin-treated cells but were uncoupled for BDM-treated cells. Even though orientation response to cyclic stretching is not a spontaneous motile response, it is determined, in large part, by the same factors that affect spontaneous motility--the cell-substratum adhesiveness and actin-myosin contractile level.     

Effects of pulsatile flow on cultured vascular endothelial cell morphology

Endothelial cells (EC) appear to adapt their morphology and function to the in vivo hemodynamic environment in which they reside. In vitro experiments indicate that similar alterations occur for cultured EC exposed to a laminar steady-state flow-induced shear stress. However, in vivo EC are exposed to a pulsatile flow environment; thus, in this investigation, the influence of pulsatile flow on cell shape and orientation and on actin microfilament localization in confluent bovine aortic endothelial cell (BAEC) monolayers was studied using a 1-Hz nonreversing sinusoidal shear stress of 40 +/- 20 dynes/cm2 (type I), 1-Hz reversing sinusoidal shear stresses of 20 +/- 40 and 10 +/- 15 dynes/cm2 (type II), and 1-Hz oscillatory shear stresses of 0 +/- 20 and 0 +/- 40 dynes/cm2 (type III). The results show that in a type I nonreversing flow, cell shape changed less rapidly, but cells took on a more elongated shape than their steady flow controls long-term. For low-amplitude type II reversing flow, BAECs changed less rapidly in shape and were always less elongated than their steady controls; however, for high amplitude reversal, BAECs did not stay attached for more than 24 hours. For type III oscillatory flows, BAEC cell shape remained polygonal as in static culture and did not exhibit actin stress fibers, such as occurred in all other flows. These results demonstrate that EC can discriminate between different types of pulsatile flow environments.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanical effects on endothelial cell morphology: In vitro assessment

Summary Endothelial cells are subjected to fluid mechanical forces which accompany blood flow. These cells become elongated and orient their long axes parallel to the direction of shear stress when the cultured cells are subjected to flow in an in vitro circulatory system. When the substrate is compliant and cyclically deformed, to simulate effects of pressure in the vasculature, the cells elongate an orient perpendicular to the axis of deformation. Cell shape changes are reflected in the alignment of microtubule networks. The systems described provide tools for assessing the individual roles of shear stress, pressure, and mechanical strain on vascular cell structure and function. This work was partially supported by grants HL 17437, HL 18072, and HL 23016 from the National Institutes of Health, Bethesda, MD, and grant C-938 from the Robert A. Welch Foundation.     

Biorheological views of endothelial cell responses to mechanical stimuli

Vascular endothelial cells are located at the innermost layer of the blood vessel wall and are always exposed to three different mechanical forces: shear stress due to blood flow, hydrostatic pressure due to blood pressure and cyclic stretch due to vessel deformation. It is well known that endothelial cells respond to these mechanical forces and change their shapes, cytoskeletal structures and functions. In this review, we would like to mainly focus on the effects of shear stress and hydrostatic pressure on endothelial cell morphology. After applying fluid shear stress, cultured endothelial cells show marked elongation and orientation in the flow direction. In addition, thick stress fibers of actin filaments appear and align along the cell long axis. Thus, endothelial cell morphology is closely related to the cytoskeletal structure. Further, the dynamic course of the morphological changes is shown and the related events such as changes in mechanical stiffness and functions are also summarized. When endothelial cells were exposed to hydrostatic pressure, they exhibited a marked elongation and orientation in a random direction, together with development of centrally located, thick stress fibers. Pressured endothelial cells also exhibited a multilayered structure with less expression of VE-cadherin unlike under control conditions. Simultaneous loading of hydrostatic pressure and shear stress inhibited endothelial cell multilayering and induced elongation and orientation of endothelial cells with well-developed VE-cadherin in a monolayer, which suggests that for a better understanding of vascular endothelial cell responses one has to take into consideration the combination of the different mechanical forces such as exist under in vivo mechanical conditions.