Orientation of cultured arterial smooth muscle cells growing on cyclically stretched substrates

Cell orientation under stretch: A review of experimental findings and mathematical modelling

The key role of electro-chemical signals in cellular processes had been known for many years, but more recently the interplay with mechanics has been put in evidence and attracted substantial research interests. Indeed, the sensitivity of cells to mechanical stimuli coming from the microenvironment turns out to be relevant in many biological and physiological circumstances. In particular, experimental evidence demonstrated that cells on elastic planar substrates undergoing periodic stretches, mimicking native cyclic strains in the tissue where they reside, actively reorient their cytoskeletal stress fibres. At the end of the realignment process, the cell axis forms a certain angle with the main stretching direction. Due to the importance of a deeper understanding of mechanotransduction, such a phenomenon was studied both from the experimental and the mathematical modelling point of view. The aim of this review is to collect and discuss both the experimental results on cell reorientation and the fundamental features of the mathematical models that have been proposed in the literature.

A Statistical Mechanics Approach to Describe Cell Reorientation Under Stretch

Experiments show that when a monolayer of cells cultured on an elastic substratum is subject to a cyclic stretch, cells tend to reorient either perpendicularly or at an oblique angle with respect to the main stretching direction. Due to stochastic effects, however, the distribution of angles achieved by the cells is broader and, experimentally, histograms over the interval [Formula: see text] are usually reported. Here we will determine the evolution and the stationary state of probability density functions describing the statistical distribution of the orientations of the cells using Fokker-Planck equations derived from microscopic rules for describing the reorientation process of the cell. As a first attempt, we shall use a stochastic differential equation related to a very general elastic energy that the cell tries to minimize and, we will show that the results of the time integration and of the stationary state of the related forward Fokker-Planck equation compare very well with experimental results obtained by different researchers. Then, in order to model more accurately the microscopic process of cell reorientation and to shed light on the mechanisms performed by cells that are subject to cyclic stretch, we consider discrete in time random processes that allow to recover Fokker-Planck equations through classical tools of kinetic theory. In particular, we shall introduce a model of reorientation as a function of the rotation angle as a result of an optimal control problem. Also in this latter case the results match very well with experiments.

Critical Frequency and Critical Stretching Rate for Reorientation of Cells on a Cyclically Stretched Polymer in a Microfluidic Chip

The ability of cells to sense and respond to mechanical signals from their surrounding microenvironments is one of the key issues in tissue engineering and regeneration, yet a fundamental study of cells with both cell observation and mechanical stimulus is challenging and should be based upon an appropriate microdevice. Herein we designed and fabricated a two-layer microfluidic chip to enable simultaneous observation of live cells and cyclic stretching of an elastic polymer, polydimethylsiloxane (PDMS), with a modified surface for enhanced cell adhesion. Human mesenchymal stem cells (hMSCs) were examined with a series of frequencies from 0.00003 to 2 Hz and varied amplitudes of 2%, 5%, or 10%. The cells with an initial random orientation were confirmed to be reoriented perpendicular to the stretching direction at frequencies greater than a threshold value, which we term critical frequency (f_c); additionally, the critical frequency f_c was amplitude-dependent. We further introduced the concept of critical stretching rate (R_c) and found that this quantity can unify both frequency and amplitude dependences. The reciprocal value of R_c in this study reads 8.3 min, which is consistent with the turnover time of actin filaments reported in the literature, suggesting that the supramolecular relaxation in the cytoskeleton within a cell might be responsible for the underlying cell mechanotransduction. The theoretical calculation of cell reorientation based on a two-dimensional tensegrity model under uniaxial cyclic stretching is well consistent with our experiments. The above findings provide new insight into the crucial role of critical frequency and critical stretching rate in regulating cells on biomaterials under biomechanical stimuli.

Ordering of adsorbed rigid rods mediated by the Boussinesq interaction on a soft substrate

Orientational ordering driven by mechanical distortion of soft substrates plays a major role in material transformation processes such as elastocapillarity and surface anchoring. We present a theoretical model of the orientational response of anisotropic rods deposited onto a surface of a soft, elastic substrate of finite thickness. We show that anisotropic rods exhibit a continuous isotropic-nematic phase transition, driven by orientational interactions between surface deposited rods. This interaction is mediated by the deformation of the underlying elastic substrate and is quantified by the Boussinesq solution adapted to the case of slender, surface deposited rods. From the microscopic rod-rod interactions, we derive the appropriate Maier-Saupe mean-field description, which includes the Boussinesq elastic free energy contribution due to the substrate elasticity, derive the conditions for the existence of a continuous orientational ordering transition, and discuss the implication of results in the soft (bio)system context.

Harnessing Mechanosensation in Next Generation Cardiovascular Tissue Engineering

The ability of the cells to sense mechanical cues is an integral component of ”social” cell behavior inside tissues with a complex architecture. Through ”mechanosensation” cells are in fact able to decrypt motion, geometries and physical information of surrounding cells and extracellular matrices by activating intracellular pathways converging onto gene expression circuitries controlling cell and tissue homeostasis. Additionally, only recently cell mechanosensation has been integrated systematically as a crucial element in tissue pathophysiology. In the present review, we highlight some of the current efforts to assess the relevance of mechanical sensing into pathology modeling and manufacturing criteria for a next generation of cardiovascular tissue implants.

Mechanotransduction in the Cardiovascular System: From Developmental Origins to Homeostasis and Pathology

With the term ‘mechanotransduction’, it is intended the ability of cells to sense and respond to mechanical forces by activating intracellular signal transduction pathways and the relative phenotypic adaptation. While a known role of mechanical stimuli has been acknowledged for developmental biology processes and morphogenesis in various organs, the response of cells to mechanical cues is now also emerging as a major pathophysiology determinant. Cells of the cardiovascular system are typically exposed to a variety of mechanical stimuli ranging from compression to strain and flow (shear) stress. In addition, these cells can also translate subtle changes in biophysical characteristics of the surrounding matrix, such as the stiffness, into intracellular activation cascades with consequent evolution toward pro-inflammatory/pro-fibrotic phenotypes. Since cellular mechanotransduction has a potential readout on long-lasting modifications of the chromatin, exposure of the cells to mechanically altered environments may have similar persisting consequences to those of metabolic dysfunctions or chronic inflammation. In the present review, we highlight the roles of mechanical forces on the control of cardiovascular formation during embryogenesis, and in the development and pathogenesis of the cardiovascular system.

Cell Nanomechanics Based on Dielectric Elastomer Actuator Device

As a frontier of biology, mechanobiology plays an important role in tissue and biomedical engineering. It is a common sense that mechanical cues under extracellular microenvironment affect a lot in regulating the behaviors of cells such as proliferation and gene expression, etc. In such an interdisciplinary field, engineering methods like the pneumatic and motor-driven devices have been employed for years. Nevertheless, such techniques usually rely on complex structures, which cost much but not so easy to control. Dielectric elastomer actuators (DEAs) are well known as a kind of soft actuation technology, and their research prospect in biomechanical field is gradually concerned due to their properties just like large deformation (> 100%) and fast response (< 1 ms). In addition, DEAs are usually optically transparent and can be fabricated into small volume, which make them easy to cooperate with regular microscope to realize real-time dynamic imaging of cells. This paper first reviews the basic components, principle, and evaluation of DEAs and then overview some corresponding applications of DEAs for cellular mechanobiology research. We also provide a comparison between DEA-based bioreactors and current custom-built devices and share some opinions about their potential applications in the future according to widely reported results via other methods.

Cyclic stretch-induced mechanical stress to the cell nucleus inhibits ultraviolet radiation-induced DNA damage

Ultraviolet (UV) radiation exerts adverse effects on genome stability, alters the normal state of life, and causes several diseases by inducing DNA damage. Although mechanical stimulation such as stretching has significant effects on the prevention and treatment of diseases, its influence on nuclear morphology and/or intranuclear functions involving resistance to DNA damage remains unknown. Here, we investigated the effects of mechanical stimulation by cyclic stretching on nuclear morphology and resistance of DNA to UV damage in NIH3T3 fibroblasts. Adherent cells on silicone elastic membranes were subjected to ~ 10% cyclic uniaxial stretch at a frequency of 0.5 Hz for 12 h. As a result, the intracellular actin cytoskeleton and nucleus were found to be elongated and aligned in the direction of zero normal strain (~ 62° with respect to the stretch direction) in an actomyosin tension-dependent manner. The nuclei of the stretched cells were dramatically compressed by the reorganized actin stress fibers located on their apical and both sides, and a significant increase in the intranuclear DNA density was observed. Intercellular tension, as assessed with live cell atomic force microscopy imaging, also increased following exposure to cyclic stretch. The UV radiation-induced DNA damage, estimated from the fluorescence intensity of the phospho-histone γ-H2AX, significantly decreased in these stretched cells. These results indicate that the cyclic stretch-induced morphological changes in the nucleus may improve the UV radiation resistance of cells, probably owing to the intracellular force-induced condensation of chromatin. To our knowledge, this is the first study to demonstrate the inhibition of the UV radiation-induced DNA damage by mechanical stimulation.

Multifactorial bottom-up bioengineering approaches for the development of living tissue substitutes

Bottom-up bioengineering utilizes the inherent capacity of cells to build highly sophisticated structures with high levels of biomimicry. Despite the significant advancements in the field, monodomain approaches require prolonged culture time to develop an implantable device, usually associated with cell phenotypic drift in culture. Herein, we assessed the simultaneous effect of macromolecular crowding (MMC) and mechanical loading in enhancing extracellular matrix (ECM) deposition while maintaining tenocyte (TC) phenotype and differentiating bone marrow stem cells (BMSCs) or transdifferentiating neonatal and adult dermal fibroblasts toward tenogenic lineage. At d 7, all cell types presented cytoskeleton alignment perpendicular to the applied load independently of the use of MMC. MMC enhanced ECM deposition in all cell types. Gene expression analysis indicated that MMC and mechanical loading maintained TC phenotype, whereas tenogenic differentiation of BMSCs or transdifferentiation of dermal fibroblasts was not achieved. Our data suggest that multifactorial bottom-up bioengineering approaches significantly accelerate the development of biomimetic tissue equivalents.-Gaspar, D., Ryan, C. N. M., Zeugolis, D. I. Multifactorial bottom-up bioengineering approaches for the development of living tissue substitutes.

A Low-Cost Mechanical Stretching Device for Uniaxial Strain of Cells: A Platform for Pedagogy in Mechanobiology

Mechanical cues including stretch, compression, and shear stress play a critical role in regulating the behavior of many cell types, particularly those that experience substantial mechanical stress within tissues. Devices that impart mechanical stimulation to cells in vitro have been instrumental in helping to develop a better understanding of how cells respond to mechanical forces. However, these devices often have constraints, such as cost and limited functional capabilities, that restrict their use in research or educational environments. Here, we describe a low-cost method to fabricate a uniaxial cell stretcher that would enable widespread use and facilitate engineering design and mechanobiology education for undergraduate students. The device is capable of producing consistent and reliable strain profiles through the use of a servomotor, gear, and gear rack system. The servomotor can be programmed to output various waveforms at specific frequencies and stretch amplitudes by controlling the degree of rotation, speed, and acceleration of the servogear. In addition, the stretchable membranes are easy to fabricate and can be customized, allowing for greater flexibility in culture well size. We used the custom-built stretching device to uniaxially strain macrophages and cardiomyocytes, and found that both cell types displayed functional and cell shape changes that were consistent with the previous studies using commercially available systems. Overall, this uniaxial cell stretcher provides a more cost-effective alternative to study the effects of mechanical stretch on cells, and can therefore, be widely used in research and educational environments to broaden the study and pedagogy of cell mechanobiology.

Engineering Tissues without the Use of a Synthetic Scaffold: A Twenty-Year History of the Self-Assembly Method

Twenty years ago, Dr. François A. Auger, the founder of the Laboratory of Experimental Organogenesis (LOEX), introduced the self-assembly technique. This innovative technique relies on the ability of dermal fibroblasts to produce and assemble their own extracellular matrix, differing from all other tissue-engineering techniques that use preformed synthetic scaffolds. Nevertheless, the use of the self-assembly technique was limited for a long time due to its main drawbacks: time and cost. Recent scientific breakthroughs have addressed these limitations. New protocol modifications that aim at increasing the rate of extracellular matrix formation have been proposed to reduce the production costs and laboratory handling time of engineered tissues. Moreover, the introduction of vascularization strategies in vitro permits the formation of capillary-like networks within reconstructed tissues. These optimization strategies enable the large-scale production of inexpensive native-like substitutes using the self-assembly technique. These substitutes can be used to reconstruct three-dimensional models free of exogenous materials for clinical and fundamental applications.

Active Traction Force Response to Long-Term Cyclic Stretch Is Dependent on Cell Pre-stress

Mechanical stimulation is recognized as a potent modulator of cellular behaviors such as proliferation, differentiation, and extracellular matrix assembly. However, the study of how cell-generated traction force changes in response to stretch is generally limited to short-term stimulation. The goal of this work is to determine how cells actively alter their traction force in response to long-term physiological cyclic stretch as a function of cell pre-stress. We have developed, to our knowledge, a novel method to assess traction force after long-term (24 h) uniaxial or biaxial cyclic stretch under conditions of high cell pre-stress with culture on stiff (7.5 kPa) polyacrylamide gels (with or without transforming growth factor β1 (TGF-β1)) and low pre-stress by treating with blebbistatin or culture on soft gels (0.6 kPa). In response to equibiaxial stretch, valvular interstitial cells on stiff substrates decreased their traction force (from 300 nN to 100 nN) and spread area (from 3000 to 2100 μm(2)). With uniaxial stretch, the cells had similar decreases in traction force and area and reoriented perpendicular to the stretch. TGF-β1-treated valvular interstitial cells had higher pre-stress (1100 nN) and exhibited a larger drop in traction force with uniaxial stretch, but the percentage changes in force and area with stretch were similar to the non-TGF-β1-treated group. Cells with inhibited myosin II motors increased traction force (from 41 nN to 63 nN) and slightly reoriented toward the stretch direction. In contrast, cells cultured on soft gels increased their traction force significantly, from 15 nN to 45 nN, doubled their spread area, elongated from an initially rounded morphology, and reoriented perpendicular to the uniaxial stretch. Contractile-moment measurements provided results consistent with total traction force measurements. The combined results indicate that the change in traction force in response to external cyclic stretch is dependent upon the initial cell pre-stress. This finding is consistent with depolymerization of initially high-tension actin stress fibers, and reinforcement of an initially low-tension actin cytoskeleton.

Stretch-induced actomyosin contraction in epithelial tubes: Mechanotransduction pathways for tubular homeostasis

Many tissues in our body have a tubular shape and are constantly exposed to various stresses. Luminal pressure imposes tension on the epithelial and myoepithelial or smooth muscle cells surrounding the lumen of the tubes. Contractile forces generated by actomyosin assemblies within these cells oppose the luminal pressure and must be calibrated to maintain tube diameter homeostasis and tissue integrity. In this review, we discuss mechanotransduction pathways that can lead from sensation of cell stretch to activation of actomyosin contractility, providing rapid mechanochemical feedback for proper tubular tissue function.

Human Corneal Fibroblast Pattern Evolution and Matrix Synthesis on Mechanically Biased Substrates

In a fibroblast colony model of corneal stromal development, we asked how physiological tension influences the patterning dynamics of fibroblasts and the orientation of deposited extracellular matrix (ECM). Using long-term live-cell microscopy, enabled by an optically accessible mechanobioreactor, a primary human corneal fibroblast colony was cultured on three types of substrates: a mechanically biased, loaded, dense, disorganized collagen substrate (LDDCS), a glass coverslip, and an unloaded, dense, disorganized collagen substrate (UDDCS). On LDDCS, fibroblast orientation and migration along a preferred angle developed early, cell orientation was correlated over long distances, and the colony pattern was stable. On glass, fibroblast orientation was poorly correlated, developed more slowly, and colony patterns were metastable. On UDDCS, cell orientation was correlated over shorter distances compared with LDDCS specimens. On all substrates, the ECM pattern reflected the cell pattern. In summary, mechanically biasing the collagen substrate altered the early migration behavior of individual cells, leading to stable emergent cell patterning, which set the template for newly synthesized ECM.

Simplified Analysis for the Association of a Constrained Receptor to an Oscillating Ligand

The stability of a bond cluster upon oscillated loads under physiological conditions is strongly regulated by the kinetics of association and dissociation of a single bond, which can play critical roles in cell–matrix adhesion, cell–cell adhesion, etc. Here, we obtain a simplified analysis for the bond association process of a constrained receptor to an oscillating ligand due to its diffusion-independence, which can facilitate the potential multiscale studies in the future. Based on the analysis, our results indicate that the mean passage time for bond association intriguingly saturates at high oscillating frequencies, and there can also surprisingly exist optimal bond elasticity for bond association. This work can bring important insights into understanding of the behaviors of bond cluster under cyclic loads at the level of a single bond.

A magnetically actuated cellular strain assessment tool for quantitative analysis of strain induced cellular reorientation and actin alignment

Commercially available cell strain tools, such as pneumatically actuated elastomer substrates, require special culture plates, pumps, and incubator setups. In this work, we present a magnetically actuated cellular strain assessment tool (MACSAT) that can be implemented using off-the-shelf components and conventional incubators. We determine the strain field on the MACSAT elastomer substrate using numerical models and experimental measurements and show that a specific region of the elastomer substrate undergoes a quasi-uniaxial 2D stretch, and that cells confined to this region of the MACSAT elastomer substrate undergo tensile, compressive, or zero axial strain depending on their angle of orientation. Using the MACSAT to apply cyclic strain on endothelial cells, we demonstrate that actin filaments within the cells reorient away from the stretching direction, towards the directions of minimum axial strain. We show that the final actin orientation angles in strained cells are spread over a region of compressive axial strain, confirming previous findings on the existence of a varied pre-tension in the actin filaments of the cytoskeleton. We also demonstrate that strained cells exhibit distinctly different values of actin alignment coherency compared to unstrained cells and therefore propose that this parameter, i.e., the coherency of actin alignment, can be used as a new readout to determine the occurrence/extent of actin alignment in cell strain experiments. The tools and methods demonstrated in this study are simple and accessible and can be easily replicated by other researchers to study the strain response of other adherent cells.

Heading in the Right Direction: Understanding Cellular Orientation Responses to Complex Biophysical Environments

The aim of cardiovascular regeneration is to mimic the biological and mechanical functioning of tissues. For this it is crucial to recapitulate the in vivo cellular organization, which is the result of controlled cellular orientation. Cellular orientation response stems from the interaction between the cell and its complex biophysical environment. Environmental biophysical cues are continuously detected and transduced to the nucleus through entwined mechanotransduction pathways. Next to the biochemical cascades invoked by the mechanical stimuli, the structural mechanotransduction pathway made of focal adhesions and the actin cytoskeleton can quickly transduce the biophysical signals directly to the nucleus. Observations linking cellular orientation response to biophysical cues have pointed out that the anisotropy and cyclic straining of the substrate influence cellular orientation. Yet, little is known about the mechanisms governing cellular orientation responses in case of cues applied separately and in combination. This review provides the state-of-the-art knowledge on the structural mechanotransduction pathway of adhesive cells, followed by an overview of the current understanding of cellular orientation responses to substrate anisotropy and uniaxial cyclic strain. Finally, we argue that comprehensive understanding of cellular orientation in complex biophysical environments requires systematic approaches based on the dissection of (sub)cellular responses to the individual cues composing the biophysical niche.

The many ways adherent cells respond to applied stretch

Cells in various tissues are subjected to mechanical stress and strain that have profound effects on cell architecture and function. The specific response of the cell to applied strain depends on multiple factors, including cell contractility, spatial and temporal strain pattern, and substrate dimensionality and rigidity. Recent work has demonstrated that the cell response to applied strain depends on a complex combination of these factors, but the way these factors interact to elicit a specific response is not intuitive. We submit that an understanding of the integrated response of a cell to these factors will provide new insight into mechanobiology and contribute to the effective design of deformable engineered scaffolds meant to provide appropriate mechanical cues to the resident cells.

Dynamics of Cellular Reorientation on a Substrate under Biaxial Cyclic Stretches

It is widely known that mechanical signals such as force, geometry, and substrate elasticity can be utilized by cells to regulate their structures, functions, and behaviors. However, the exact nature of the underlying mechanisms of cellular mechanosensing is unclear. Recently, extensive experiments on cellular reorientation dynamics on a substrate under biaxial cyclic stretches were performed, and the measured behaviors were found to be incompatible with existing theories. Here, we show that a theoretical model based on both tensile and shearing forces on focal adhesions (FAs) is capable of reproducing the new experimental data. This work provides important mechanistic insights into how behaviors of FAs can strongly affect cellular reorientation dynamics on a cyclically stretched substrate.

Temporal responses of human endothelial and smooth muscle cells exposed to uniaxial cyclic tensile strain

The physiology of vascular cells depends on stimulating mechanical forces caused by pulsatile flow. Thus, mechano-transduction processes and responses of primary human endothelial cells (ECs) and smooth muscle cells (SMCs) have been studied to reveal cell-type specific differences which may contribute to vascular tissue integrity. Here, we investigate the dynamic reorientation response of ECs and SMCs cultured on elastic membranes over a range of stretch frequencies from 0.01 to 1 Hz. ECs and SMCs show different cell shape adaptation responses (reorientation) dependent on the frequency. ECs reveal a specific threshold frequency (0.01 Hz) below which no responses is detectable while the threshold frequency for SMCs could not be determined and is speculated to be above 1 Hz. Interestingly, the reorganization of the actin cytoskeleton and focal adhesions system, as well as changes in the focal adhesion area, can be observed for both cell types and is dependent on the frequency. RhoA and Rac1 activities are increased for ECs but not for SMCs upon application of a uniaxial cyclic tensile strain. Analysis of membrane protrusions revealed that the spatial protrusion activity of ECs and SMCs is independent of the application of a uniaxial cyclic tensile strain of 1 Hz while the total number of protrusions is increased for ECs only. Our study indicates differences in the reorientation response and the reaction times of the two cell types in dependence of the stretching frequency, with matching data for actin cytoskeleton, focal adhesion realignment, RhoA/Rac1 activities, and membrane protrusion activity. These are promising results which may allow cell-type specific activation of vascular cells by frequency-selective mechanical stretching. This specific activation of different vascular cell types might be helpful in improving strategies in regenerative medicine.

Stretching fibroblasts remodels fibronectin and alters cancer cell migration

Most investigations of cancer-stroma interactions have focused on biochemical signaling effects, with much less attention being paid to biophysical factors. In this study, we investigated the role of mechanical stimuli on human prostatic fibroblasts using a microfluidic platform that was adapted for our experiments and further developed for both repeatable performance among multiple assays and for compatibility with high-resolution confocal microscopy. Results show that mechanical stretching of normal tissue-associated fibroblasts (NAFs) alters the structure of secreted fibronectin. Specifically, unstretched NAFs deposit and assemble fibronectin in a random, mesh-like arrangement, while stretched NAFs produce matrix with a more organized, linearly aligned structure. Moreover, the stretched NAFs exhibited an enhanced capability for directing co-cultured cancer cell migration in a persistent manner. Furthermore, we show that stretching NAFs triggers complex biochemical signaling events through the observation of increased expression of platelet derived growth factor receptor α (PDGFRα). A comparison of these behaviors with those of cancer-associated fibroblasts (CAFs) indicates that the observed phenotypes of stretched NAFs are similar to those associated with CAFs, suggesting that mechanical stress is a critical factor in NAF activation and CAF genesis.

A microfabricated, optically accessible device to study the effects of mechanical cues on collagen fiber organization

As the primary structural protein of our bodies, fibrillar collagen and its organizational patterns determine the biomechanics and shape of tissues. While the molecular assembly of individual fibrils is well understood, the mechanisms determining the arrangement of fibers and thus the shape and form of tissues remain largely unknown. We have developed a cell culture model that successfully recapitulates early tissue development and the de novo deposition of collagen fibers to investigate the role of mechanical cues on collagen fiber alignment. The devices used a thin, collagen-coated deformable PDMS membrane inside a tissue culture well built on microscope-grade coverslips. Deformations and strains in the PDMS membrane were quantified by tracking fluorescent bead displacement and through the use of a COMSOL model. Cyclical strains were applied to serum-cultured rabbit corneal cells at 0.5 Hz for 24-48 h and showed a preferred alignment after 36 h of cyclical loading. Cells cultured with ascorbic acid under methylcellulose serum-free conditions deposited a collagenous matrix that was visible under live second harmonic generation microscopy at week 4. Our microfabricated tissue culture system allows for the controllable application of a wide range of stress profiles to cells, and for the observation and quantification of cells and de novo collagen formation in vitro. Future studies will involve the fabrication of models to study the formation and organization of collagen in ocular diseases.

Disentangling the multifactorial contributions of fibronectin, collagen and cyclic strain on MMP expression and extracellular matrix remodeling by fibroblasts

Early wound healing is associated with fibroblasts assembling a provisional fibronectin-rich extracellular matrix (ECM), which is subsequently remodeled and interlaced by type I collagen. This exposes fibroblasts to time-variant sets of matrices during different stages of wound healing. Our goal was thus to gain insight into the ECM-driven functional regulation of human foreskin fibroblasts (HFFs) being either anchored to a fibronectin (Fn) or to a collagen-decorated matrix, in the absence or presence of cyclic mechanical strain. While the cells reoriented in response to the onset of uniaxial cyclic strain, cells assembled exogenously added Fn with a preferential Fn-fiber alignment along their new orientation. Exposure of HFFs to exogenous Fn resulted in an increase in matrix metalloproteinase (MMP) expression levels, i.e. MMP-15 (RT-qPCR), and MMP-9 activity (zymography), while subsequent exposure to collagen slightly reduced MMP-15 expression and MMP-9 activity compared to Fn-exposure alone. Cyclic strain upregulated Fn fibrillogenesis and actin stress fiber formation, but had comparatively little effect on MMP activity. We thus propose that the appearance of collagen might start to steer HFFs towards homeostasis, as it decreased both MMP secretion and the tension of Fn matrix fibrils as assessed by Fluorescence Resonance Energy Transfer. These results suggest that HFFs might have a high ECM remodeling or repair capacity in contact with Fn alone (early event), which is reduced in the presence of Col1 (later event), thereby down-tuning HFF activity, a processes which would be required in a tissue repair process to finally reach tissue homeostasis.

Synergy between Rho signaling and matrix density in cyclic stretch-induced stress fiber organization

Cells adapt in response to mechanical stimulation to ensure adequate tissue functioning. F-actin stress fibers provide a key element in the adaptation process. The high sensitivity and fast adaptation of the F-actin cytoskeleton to cyclic strain have been studied extensively in a 2-D environment; however, 3-D data are scarce. Our previous work showed that stress fibers organize perpendicular to cyclic stretching (stretch-avoidance) in three dimensions. However, stretch-avoidance was absent when cells populated a high density matrix. In this study our aim was to obtain more insight into the synergy between matrix density and the signaling pathways that govern stress fiber remodeling. Therefore we studied stress fiber organization in 3-D reconstituted collagen tissues (at low and high matrix density), subjected to cyclic stretch upon interference with molecular signaling pathways. In particular, the influence of the small GTPase Rho and its downstream effectors were studied. Only at low matrix density does stress fiber stretch avoidance show a stretch-magnitude-dependent response. The activity of matrix metalloproteinases (MMPs), Rho-kinase and myosin light chain kinase are essential for stress fiber reorientation. Although high matrix density restricts stress fiber reorientation, Rho activation can overcome this restriction, but only in the presence of active MMPs. Results from this study highlight a synergistic action between matrix remodeling and Rho signaling in cyclic-stretch-induced stress fiber organization in 3-D tissue.

Cyclic tensile strain controls cell shape and directs actin stress fiber formation and focal adhesion alignment in spreading cells

The actin cytoskeleton plays a crucial role for the spreading of cells, but is also a key element for the structural integrity and internal tension in cells. In fact, adhesive cells and their actin stress fiber–adhesion system show a remarkable reorganization and adaptation when subjected to external mechanical forces. Less is known about how mechanical forces alter the spreading of cells and the development of the actin–cell-matrix adhesion apparatus. We investigated these processes in fibroblasts, exposed to uniaxial cyclic tensile strain (CTS) and demonstrate that initial cell spreading is stretch-independent while it is directed by the mechanical signals in a later phase. The total temporal spreading characteristic was not changed and cell protrusions are initially formed uniformly around the cells. Analyzing the actin network, we observed that during the first phase the cells developed a circumferential arc-like actin network, not affected by the CTS. In the following orientation phase the cells elongated perpendicular to the stretch direction. This occurred simultaneously with the de novo formation of perpendicular mainly ventral actin stress fibers and concurrent realignment of cell-matrix adhesions during their maturation. The stretch-induced perpendicular cell elongation is microtubule-independent but myosin II-dependent. In summary, a CTS-induced cell orientation of spreading cells correlates temporary with the development of the acto-myosin system as well as contact to the underlying substrate by cell-matrix adhesions.

Cyclic stretch induces cell reorientation on substrates by destabilizing catch bonds in focal adhesions

A minimal model of cellular mechanosensing system that consists of a single stress fiber adhering on a substrate via two focal adhesions made of catch bonds is adopted to investigate the phenomena of cell reorientation on substrates induced by an applied uniaxial cyclic stretch. The model indicates that the catch bonds in the focal adhesions experience a periodically oscillating internal force with amplitude and frequency controlled by two intrinsic clocks of the stress fiber, one associated with localized activation and the other with homogeneous activation of sarcomere units along the stress fiber. It is shown that this oscillating force due to cyclic stretch tends to destabilize focal adhesions by reducing the lifetime of catch bonds. The resulting slide or relocation of focal adhesions then causes the associated stress fiber to shorten and rotate to configurations nearly perpendicular to the stretching direction. These predicted behaviors from our model are consistent with a wide range of experimental observations.

The influence of matrix integrity on stress-fiber remodeling in 3D

Matrix anisotropy is important for long term in vivo functionality. However, it is not fully understood how to guide matrix anisotropy in vitro. Experiments suggest actin-mediated cell traction contributes. Although F-actin in 2D displays a stretch-avoidance response, 3D data are lacking. We questioned how cyclic stretch influences F-actin and collagen orientation in 3D. Small-scale cell-populated fibrous tissues were statically constrained and/or cyclically stretched with or without biochemical agents. A rectangular array of silicone posts attached to flexible membranes constrained a mixture of cells, collagen I and matrigel. F-actin orientation was quantified using fiber-tracking software, fitted using a bi-model distribution function. F-actin was biaxially distributed with static constraint. Surprisingly, uniaxial cyclic stretch, only induced a strong stretch-avoidance response (alignment perpendicular to stretching) at tissue surfaces and not in the core. Surface alignment was absent when a ROCK-inhibitor was added, but also when tissues were only statically constrained. Stretch-avoidance was also observed in the tissue core upon MMP1-induced matrix perturbation. Further, a strong stretch-avoidance response was obtained for F-actin and collagen, for immediate cyclic stretching, i.e. stretching before polymerization of the collagen. Results suggest that F-actin stress-fibers avoid cyclic stretch in 3D, unless collagen contact guidance dictates otherwise.

Strain waveform dependence of stress fiber reorientation in cyclically stretched osteoblastic cells: effects of viscoelastic compression of stress fibers

Actin stress fibers (SFs) of cells cultured on cyclically stretched substrate tend to reorient in the direction in which a normal strain of substrate becomes zero. However, little is known about the mechanism of this reorientation. Here we investigated the effects of cyclic stretch waveform on SF reorientation in osteoblastic cells. Cells adhering to silicone membranes were subjected to cyclic uniaxial stretch, having one of the following waveforms with an amplitude of 8% for 24 h: triangular, trapezoid, bottom hold, or peak hold. SF reorientation of these cells was then analyzed. No preferential orientation was observed for the triangular and the peak-hold waveforms, whereas SFs aligned mostly in the direction with zero normal strain (∼55°) with other waveforms, especially the trapezoid waveform, which had a hold time both at loaded and unloaded states. Viscoelastic properties of SFs were estimated in a quasi-in situ stress relaxation test using intact and SF-disrupted cells that maintained their shape on the substrate. The dynamics of tension FSFsacting on SFs during cyclic stretching were simulated using these properties. The simulation demonstrated that FSFsdecreased gradually during cyclic stretching and exhibited a compressive value (FSFs< 0). The magnitude and duration time of the compressive forces were relatively larger in the group with a trapezoid waveform. The frequency of SF orientation had a significant negative correlation with the applied compressive forces integrated with time in a strain cycle, and the integrated value was largest with the trapezoid waveform. These results may indicate that the applied compressive forces on SFs have a significant effect on the stretch-induced reorientation of SFs, and that SFs realigned to avoid their compression. Stress relaxation of SFs might be facilitated during the holding period in the trapezoid waveform, and depolymerization and reorientation of SFs were significantly accelerated by their viscoelastic compression.

Cyclic stress at mHz frequencies aligns fibroblasts in direction of zero strain

Recognition of external mechanical signals is vital for mammalian cells. Cyclic stretch, e.g. around blood vessels, is one such signal that induces cell reorientation from parallel to almost perpendicular to the direction of stretch. Here, we present quantitative analyses of both, cell and cytoskeletal reorientation of umbilical cord fibroblasts. Cyclic strain of preset amplitudes was applied at mHz frequencies. Elastomeric chambers were specifically designed and characterized to distinguish between zero strain and minimal stress directions and to allow accurate theoretical modeling. Reorientation was only induced when the applied stretch exceeded a specific amplitude, suggesting a non-linear response. However, on very soft substrates no mechanoresponse occurs even for high strain. For all stretch amplitudes, the angular distributions of reoriented cells are in very good agreement with a theory modeling stretched cells as active force dipoles. Cyclic stretch increases the number of stress fibers and the coupling to adhesions. We show that changes in cell shape follow cytoskeletal reorientation with a significant temporal delay. Our data identify the importance of environmental stiffness for cell reorientation, here in direction of zero strain. These in vitro experiments on cultured cells argue for the necessity of rather stiff environmental conditions to induce cellular reorientation in mammalian tissues.

A Device to Study the Effects of Stretch Gradients on Cell Behavior

Mechanical forces are key regulators of cell function with varying loads capable of modulating behaviors such as alignment, migration, phenotype modulation, and others. Historically, cell-stretching experiments have employed mechanically simple environments (e.g., uniform uniaxial or equibiaxial stretches). However, stretch distributions in vivo can be highly non-uniform, particularly in cases of disease or subsequent to interventional treatments. Herein, we present a cell-stretching device capable of subjecting cells to controllable gradients in biaxial stretch via radial deformation of circular elastomeric membranes. By including either a defect or a rigid fixation at the center of the membrane, various gradients are generated. Capabilities of the device were quantified by tracking marked positions of the membrane while applying various loads, and experimental feasibility was assessed by conducting preliminary experiments with 3T3 fibroblasts and 10T1/2 cells subjected to 24 h of cyclic stretch. Quantitative real-time PCR was used to measure changes in mRNA expression of a profile of genes representing the major smooth muscle phenotypes. Genes associated with the contractile state were both upregulated (e.g., calponin) and downregulated (e.g., α-2-actin), and genes associated with the synthetic state were likewise both upregulated (e.g., SKI-like oncogene) and downregulated (e.g., collagen III). In addition, cells aligned with an orientation perpendicular to the maximal stretch direction. We have developed an in vitro cell culture device that can produce non-uniform stretch environments similar to in vivo mechanics. Cells stretched with this device showed alignment and altered mRNA expression indicative of phenotype modulation. Understanding these processes as they relate to in vivo pathologies could enable a more accurately targeted treatment to heal or inhibit disease, either through implantable device design or pharmaceutical approaches.

Loss of mechanical strain impairs abdominal wall fibroblast proliferation, orientation, and collagen contraction function

BACKGROUND

Laparotomy wound load forces are reduced when dehiscence and incisional hernia formation occur. The purpose of this study was to determine the effects of strain loss on abdominal fascial fibroblast proliferation, orientation, and collagen compaction function.

METHODS

Cultured rat linea alba fibroblasts were subjected to continuous cyclic strain (CS), CS interrupted at 24 or 48 hours followed by culture at rest (IS-24 and IS-48) or were cultured without mechanical strain (NS). Cell number was measured and images analyzed for cell orientation. Fibroblasts from these groups were seeded onto the surface of (FPCL-S) or mixed into (FPCL-M) a collagen gel matrix and gel area was measured over time.

RESULTS

Continuous strain stimulated proliferation when compared with the nonstrained cells. The loss of strain (IS) delayed proliferation compared with CS throughout (P < .05). CS fibroblasts aligned perpendicular to the direction of strain within 12 hours. Within 12 hours of strain loss, IS-48 fibroblasts became significantly less aligned (P < .0001), and seemed similar to the randomly organized NS fibroblasts 48 hours after strain removal. The CS and IS-24 groups demonstrated faster and greater overall FPCL-M compaction than both the IS-48 and NS groups (P < .0002). The CS group contracted the gel faster than the NS group in FPCL-S (P = .029).

CONCLUSION

Mechanical strain rapidly induces a proliferative, morphologic, and functional response in abdominal wall fibroblasts that is dependent on the continued presence of the strain signal and quickly lost when the load force is removed. The loss of wound edge tension that occurs during laparotomy wound separation and hernia formation may contribute to impaired wound healing through loss of a key stimulatory mechanical signal with important implications for abdominal wall reconstruction.

Influence of cell and collagen concentration on the cell-matrix mechanical relationship in a corneal stroma wound healing model

The effect of different collagen and cell concentrations on the mechanical and remodeling behaviors of corneal stroma wound healing models consisting of collagen hydrogels seeded with human corneal fibroblasts during a 25 day culture period were examined. Human corneal fibroblasts were seeded at 1 × 10(5), 3 × 10(5) or 5 × 10(5) cells per hydrogel, and collagen concentrations of 2.5 mg/ml, 3.5 mg/ml or 4.5 mg/ml were examined. Two non-destructive techniques, spherical indentation and optical coherence tomography, were used to measure the elastic modulus and dimensional changes respectively at several time-points over the culture period. The elastic modulus of the hydrogels increased continuously over 25 days. Hydrogels with higher initial cell seeding densities and lower initial collagen concentrations were found to increase in elastic modulus faster and possessed a higher elastic modulus by the end of the culture period when compared to the other hydrogels. A mathematical equation was applied to accurately fit the change in elastic modulus over time. This study demonstrates a robust in vitro technique able to monitor the effect of different parameters on the cell-matrix mechanical relationship in a corneal stroma model during prolonged culture periods and enhances our understanding on corneal wound healing processes.

Applying controlled non-uniform deformation for in vitro studies of cell mechanobiology

Cells within connective tissues routinely experience a wide range of non-uniform mechanical loads that regulate many cell behaviors. In this study, we developed an experimental system to produce complex strain patterns for the study of strain magnitude, anisotropy, and gradient effects on cells in culture. A standard equibiaxial cell stretching system was modified by affixing glass coverslips (5, 10, or 15 mm diameter) to the center of 35 mm diameter flexible-bottomed culture wells. Ring inserts were utilized to limit applied strain to different levels in each individual well at a given vacuum pressure thus enabling parallel experiments at different strain levels. Deformation fields were measured using high-density mapping for up to 6% applied strain. The addition of the rigid inclusion creates strong circumferential and radial strain gradients, with a continuous range of stretch anisotropy ranging from strip biaxial to equibiaxial strain and radial strains up to 24% near the inclusion. Dermal fibroblasts seeded within our 2D system (5 mm inclusions; 2% applied strain for 2 days at 0.2 Hz) demonstrated the characteristic orientation perpendicular to the direction of principal strain. Dermal fibroblasts seeded within fibrin gels (5 mm inclusions; 6% applied strain for 8 days at 0.2 Hz) oriented themselves similarly and compacted their surrounding matrix to an increasing extent with local strain magnitude. This study verifies how inhomogeneous strain fields can be produced in a tunable and simply constructed system and demonstrates the potential utility for studying gradients with a continuous spectrum of strain magnitudes and anisotropies.

Stem cells in drug discovery, tissue engineering, and regenerative medicine: emerging opportunities and challenges

Stem cells, irrespective of their origin, have emerged as valuable reagents or tools in human health in the past 2 decades. Initially, a research tool to study fundamental aspects of developmental biology is now the central focus of generating transgenic animals, drug discovery, and regenerative medicine to address degenerative diseases of multiple organ systems. This is because stem cells are pluripotent or multipotent cells that can recapitulate developmental paths to repair damaged tissues. However, it is becoming clear that stem cell therapy alone may not be adequate to reverse tissue and organ damage in degenerative diseases. Existing small-molecule drugs and biologicals may be needed as "molecular adjuvants" or enhancers of stem cells administered in therapy or adult stem cells in the diseased tissues. Hence, a combination of stem cell-based, high-throughput screening and 3D tissue engineering approaches is necessary to advance the next wave of tools in preclinical drug discovery. In this review, the authors have attempted to provide a basic account of various stem cells types, as well as their biology and signaling, in the context of research in regenerative medicine. An attempt is made to link stem cells as reagents, pharmacology, and tissue engineering as converging fields of research for the next decade.

Cellular and molecular effects of mechanical stretch on vascular cells and cardiac myocytes

Cells in the cardiovascular system are permanently subjected to mechanical forces due to the pulsatile nature of blood flow and shear stress, created by the beating heart. These haemodynamic forces play an important role in the regulation of vascular development, remodelling, wound healing and atherosclerotic lesion formation. Mechanical stretch can modulate several different cellular functions in VSMCs (vascular smooth muscle cells). These functions include, but are not limited to, cell alignment and differentiation, migration, survival or apoptosis, vascular remodelling, and autocrine and paracrine functions. Laminar shear stress exerts anti-apoptotic, anti-atherosclerotic and antithrombotic effects on ECs (endothelial cells). Mechanical stretch of cardiac myocytes can modulate growth, apoptosis, electric remodelling, alterations in gene expression, and autocrine and paracrine effects. The aim of the present review is primarily to summarize the cellular and molecular effects of mechanical stretch on vascular cells and cardiac myocytes, emphasizing the molecular mechanisms underlying the regulation. Knowledge of the impact of mechanical stretch on the cardiovascular system is vital to the understanding of the pathogenesis of cardiovascular diseases, and is also crucial to provide new insights into the prevention and therapy of cardiovascular diseases.

Cell mechanics at multiple scales

The response of cells to mechanical stresses is a field of growing inquiry. It is well known that both the morphologic and molecular expression of cells depend, in part, on the local mechanical environment, especially for cells such as endothelial cells that experience shear stress, stretch, and pressures. To systematically study the large variety of responses of cells to physical forces (e.g., signaling, adhesion, or stiffness changes), a number of techniques have been developed and used. Here we present methods for three types of cell mechanical studies, from the multicellular to the subcellular scales, and describe the basic principle and main use of each technique along with some design and setup considerations.

Dynamical theory of active cellular response to external stress

We present a comprehensive, theoretical treatment of the orientational response to external stress of active, contractile cells embedded in a gel-like elastic medium. The theory includes both the forces that arise from the deformation of the matrix as well as forces due to the internal regulation of the stress fibers and focal adhesions of the cell. We calculate the time-dependent response of both the magnitude and the direction of the elastic dipole that characterizes the active forces exerted by the cell, for various situations. For static or quasistatic external stress, cells orient parallel to the stress while for high frequency dynamic external stress, cells orient nearly perpendicular. Both numerical and analytical calculations of these effects are presented. In addition we predict the relaxation time for the cellular response for both slowly and rapidly varying external stresses; several characteristic scaling regimes for the relaxation time as a function of applied frequency are predicted. We also treat the case of cells for which the regulation of the stress fibers and focal adhesions is controlled by strain (instead of stress) and show that the predicted dependence of the cellular orientation on the Poisson ratio of the matrix can differentiate strain vs stress regulation of cellular response.

Two characteristic regimes in frequency-dependent dynamic reorientation of fibroblasts on cyclically stretched substrates

Cells adherent on a cyclically stretched substrate with a periodically varying uniaxial strain are known to dynamically reorient nearly perpendicular to the strain direction. We investigate the dynamic reorientation of rat embryonic and human fibroblast cells over a range of stretching frequency from 0.0001 to 20 s(-1) and strain amplitude from 1% to 15%. We report quantitative measurements that show that the mean cell orientation changes exponentially with a frequency-dependent characteristic time from 1 to 5 h. At subconfluent cell densities, this characteristic time for reorientation shows two characteristic regimes as a function of frequency. For frequencies below 1 s(-1), the characteristic time decreases with a power law as the frequency increases. For frequencies above 1 s(-1), it saturates at a constant value. In addition, a minimum threshold frequency is found below that no significant cell reorientation occurs. Our results are consistent for the two different fibroblast types and indicate a saturation of molecular mechanisms of mechanotransduction or response machinery for subconfluent cells within the frequency regime under investigation. For confluent cell layers, we observe similar behaviors of reorientation under cyclic stretch but no saturation in the characteristic time with frequency, suggesting that cell-cell contacts can play an important role in the response machinery of cells under mechanical strain.

Role of cyclic strain frequency in regulating the alignment of vascular smooth muscle cells in vitro

The arterial system is subjected to cyclic strain because of periodic alterations in blood pressure, but the effects of frequency of cyclic strain on arterial smooth muscle cells (SMCs) remain unclear. Here, we investigated the potential role of the cyclic strain frequency in regulating SMC alignment using an in vitro model. Aortic SMCs were subject to cyclic strain at one elongation but at various frequencies using a Flexercell Tension Plus system. It was found that the angle information entropy, the activation of integrin-beta1, p38 MAPK, and F/G actin ratio of filaments were all changed in a frequency-dependent manner, which was consistent with SMC alignment under cyclic strain with various frequencies. A treatment with anti-integrin-beta1 antibody, SB202190, or cytochalasin D inhibited the cyclic strain frequency-dependent SMC alignment. These observations suggested that the frequency of cyclic strain plays a role in regulating the alignment of vascular SMCs in an intact actin filament-dependent manner, and cyclic strain at 1.25 Hz was the most effective frequency influencing SMC alignment. Furthermore, integrin-beta1 and p38 MAPK possibly mediated cyclic strain frequency-dependent SMC alignment.

Dynamic fibroblast cultures: response to mechanical stretching

Mechanical forces play an important role in the organization, growth and function of tissues. Dynamic extracellular environment affects cellular behavior modifying their orientation and their cytoskeleton. In this work, human fibroblasts have been subjected for three hours to increasing substrate deformations (1-25%) applied as cyclic uniaxial stretching at different frequencies (from 0.25 Hz to 3 Hz). Our objective was to identify whether and in which ranges the different deformations magnitude and rate were the factors responsible of the cell alignment and if actin cytoskeleton modification was involved in these responses. After three hours of cyclically stretched substrate, results evidenced that fibroblasts aligned perpendicularly to the stretch direction at 1% substrate deformation and reached statistically higher orientation at 2% substrate deformation with unmodified values at 5-20%, while 25% substrate deformation induced cellular death. It was also shown that a percentage of cells oriented perpendicularly to the deformation were not influenced by increased frequency of cyclical three hours deformations (0.25%3 Hz). Cyclic substrate deformation was shown also to involve actin fibers which orient perpendicularly to the stress direction as well. Thus, we argue that a substrate deformation induces a dynamic change in cytoskeleton able to modify the entire morphology of the cells.

An introductory review of cell mechanobiology

Mechanical loads induce changes in the structure, composition, and function of living tissues. Cells in tissues are responsible for these changes, which cause physiological or pathological alterations in the extracellular matrix (ECM). This article provides an introductory review of the mechanobiology of load-sensitive cells in vivo, which include fibroblasts, chondrocytes, osteoblasts, endothelial cells, and smooth muscle cells. Many studies have shown that mechanical loads affect diverse cellular functions, such as cell proliferation, ECM gene and protein expression, and the production of soluble factors. Major cellular components involved in the mechanotransduction mechanisms include the cytoskeleton, integrins, G proteins, receptor tyrosine kinases, mitogen-activated protein kinases, and stretch-activated ion channels. Future research in the area of cell mechanobiology will require novel experimental and theoretical methodologies to determine the type and magnitude of the forces experienced at the cellular and sub-cellular levels and to identify the force sensors/receptors that initiate the cascade of cellular and molecular events.

Small-Diameter Artificial Arteries Engineered In Vitro

Although the need for a functional arterial replacement is clear, the lower blood flow velocities of small-diameter arteries like the coronary artery have led to the failure of synthetic materials that are successful for large-diameter grafts. Although autologous vessels remain the standard for small diameter grafts, many patients do not have a vessel suitable for use because of vascular disease, amputation, or previous harvest. As a result, tissue engineering has emerged as a promising approach to address the shortcomings of current therapies. Investigators have explored the use of arterial tissue cells or differentiated stem cells combined with various types of natural and synthetic scaffolds to make tubular constructs and subject them to chemical and/or mechanical stimulation in an attempt to develop a functional small-diameter arterial replacement graft with varying degrees of success. Here, we review the progress in all these major facets of the field.

Physical determinants of cell organization in soft media

Cell adhesion is an integral part of many physiological processes in tissues, including development, tissue maintenance, angiogenesis, and wound healing. Recent advances in materials science (including microcontact printing, soft lithography, microfluidics, and nanotechnology) have led to strongly improved control of extracellular ligand distribution and of the properties of the micromechanical environment. As a result, the investigation of cellular response to the physical properties of adhesive surfaces has become a very active area of research. Sophisticated use of elastic substrates has revealed that cell organization in soft media is determined by active mechanosensing at cell-matrix adhesions. In order to determine the underlying mechanisms, quantification and biophysical modelling are essential. In tissue engineering, theory might help to design new environments for cells.

Functional efficacy of tendon repair processes

Despite various attempts to repair and replace injured tendon, an understanding of the repair processes and a systematic approach to achieving functional efficacy remain elusive. In this review the epidemiology of tendon injury and repair is first examined. Using a traditional paradigm for repair assessment, the biology and biomechanics of normal tendon, natural healing, and repair are then explored. New treatment strategies such as functional tissue engineering are discussed, including a functional approach to treatment that involves the development of in vivo functional design parameters to judge the acceptability of a repair outcome. The paper concludes with future directions.

Cell stiffness and receptors: evidence for cytoskeletal subnetworks

Viscoelastic models of cells often treat cells as homogeneous objects. However, studies have demonstrated that cellular properties are local and can change dramatically on the basis of the location probed. Because membrane receptors are linked in various ways to the intracellular space, with some receptors linking to the cytoskeleton and others diffusing freely without apparent linkages, the cellular physical response to mechanical stresses is expected to depend on the receptor engaged. In this study, we tested the hypothesis that cellular mechanical stiffness as measured via cytoskeletally linked receptors is greater than stiffness measured via receptors that are not cytoskeletally linked. We used a magnetic micromanipulator to apply linear stresses to magnetic beads attached to living cells via selected receptors. One of the receptor classes probed, the dystroglycan receptors, is linked to the cytoskeleton, while the other, the transferrin receptors, is not. Fibronectin-coated beads were used to test cellular mechanical properties of the cytoskeleton without membrane dependence by allowing the beads to endocytose. For epithelial cells, transferrin-dependent stiffness and endocytosed bead-dependent stiffness were similar, while dystroglycan-dependent stiffness was significantly lower. For smooth muscle cells, dystroglycan-dependent stiffness was similar to the endocytosed bead-dependent stiffness, while the transferrin-dependent stiffness was lower. The conclusion of this study is that the measured cellular stiffness is critically influenced by specific receptor linkage and by cell type and raises the intriguing possibility of the existence of separate cytoskeletal networks with distinct mechanical properties that link different classes of receptors.

Cellular engineering of conduits for coronary and lower limb bypass surgery: role of cell attachment peptides and pre-conditioning in optimising smooth muscle cells (SMC) adherence to compliant poly(carbonate-urea)urethane (MyoLink) scaffolds

OBJECTIVE

We are developing a hybrid arterial bypass graft of compliant poly(carbonate-urea)urethane (MyoLink), endothelial and smooth muscle cells (SMCs). To enhance adhesion of SMCs we assessed various attachment factors and the effect of pre-conditioning on cell retention.

METHODS

MyoLink segments were coated with either RGD, superfibronectin, fibronectin, fibronectin-like engineered polymer protein (FEPP), FEPP plus or type 1 collagen overnight. (111)Indium-radiolabelled SMCs were placed onto MyoLink segments for 48 h before being aspirated, then lavaged off. All grafts, aspirates and lavages were counted in a gamma counter. SMC viability on the MyoLink segments was also assessed for viability using the Alamar blue redox assay. Separately, MyoLink grafts lined with radiolabelled SMCs were divided into a pre-conditioned group, exposed to subarterial pulsatile flow whilst another group were held in static culture. After 1-week, grafts were exposed to arterial pulsatile flow whilst radioactivity was assessed using a gamma camera.

RESULTS

Only FEPP plus significantly enhanced SMC attachment: mean of 32+/-6% cell attachment compared to 21+/-5% for uncoated control. Cell viability was enhanced by all attachment factors except fibronectin. Pre-conditioning was shown to significantly enhance the retention of SMCs onto the MyoLink once exposed to pulsatile arterial flow: the final attachment was 57+/-7% for the static and 76+/-7% for the pre-conditioned group.

CONCLUSIONS

FEPP plus enhances SMC attachment to MyoLink. We believe this is because of its repeating sequences of RGD and its positive charge. Pre-conditioning enhances the retention of SMCs to MyoLink once exposed to pulsatile arterial flow.

Fibroblast orientation to stretch begins within three hours

Most connective tissue cells align in response to stretch. Previous studies have shown these responses occur within 12-14 h of initiation of stretch, but do not identify the time at which this orientation occurs, nor whether the orientation continues after cessation of stretch. To ascertain the earliest times at which fibroblast orientation occurs, we cultured primary human fibroblasts on deformable culture dishes and stretched (1 Hz, 8% uniaxial strain) them for up to 24 h. We photographed the cells at 0.5, 1-6, 8, 10, 12, 14, 16, and 24 h. Similarly cells were photographed at 1-3, or 4 h after cessation of stretch for stretch durations of 1, 2, and 3 h. Orientation of cells were ascertained by an interactive computer program. The fibroblasts began to orient by 2-3 h and orientation appeared nearly complete by 24 h. Cultures stretched for 2 or 3 h continued to exhibit greater degrees of orientation (compared to controls) for 2 or 3 h respectively after cessation of stretch. We conclude fibroblasts begin to orient within 3 h of initiation of stretch, and that they continue to orient for several hours after cessation of stretch.