Local Tissue Mechanics Research Articles

Unraveling the complexity of vascular tone regulation: a multiscale computational approach to integrating chemo-mechano-biological pathways with cardiovascular biomechanics

Vascular tone regulation is a crucial aspect of cardiovascular physiology, with significant implications for overall cardiovascular health. However, the precise physiological mechanisms governing smooth muscle cell contraction and relaxation remain uncertain. The complexity of vascular tone regulation stems from its multiscale and multifactorial nature, involving global hemodynamics, local flow conditions, tissue mechanics, and biochemical pathways. Bridging this knowledge gap and translating it into clinical practice presents a challenge. In this paper, a computational model is presented to integrate chemo-mechano-biological pathways with cardiovascular biomechanics, aiming to unravel the intricacies of vascular tone regulation. The computational framework combines an algebraic description of global hemodynamics with detailed finite element analyses at the scale of vascular segments for describing their passive and active mechanical response, as well as the molecular transport problem linked with chemo-biological pathways triggered by wall shear stresses. Their coupling is accounted for by considering a two-way interaction. Specifically, the focus is on the role of nitric oxide-related molecular pathways, which play a critical role in modulating smooth muscle contraction and relaxation to maintain vascular tone. The computational framework is employed to examine the interplay between localized alterations in the biomechanical response of a specific vessel segment—such as those induced by calcifications or endothelial dysfunction–and the broader global hemodynamic conditions—both under basal and altered states. The proposed approach aims to advance our understanding of vascular tone regulation and its impact on cardiovascular health. By incorporating chemo-mechano-biological mechanisms into in silico models, this study allows us to investigate cardiovascular responses to multifactorial stimuli and incorporate the role of adaptive homeostasis in computational biomechanics frameworks.

Brain tissue mechanics is governed by microscale relations of the tissue constituents

Local mechanical tissue properties are a critical regulator of cell function in the central nervous system (CNS) during development and disorder. However, we still don't fully understand how the mechanical properties of individual tissue constituents, such as cell nuclei or myelin, determine tissue mechanics. Here we developed a model predicting local tissue mechanics, which induces non-affine deformations of the tissue components. Using the mouse hippocampus and cerebellum as model systems, we show that considering individual tissue components alone, as identified by immunohistochemistry, is not sufficient to reproduce the local mechanical properties of CNS tissue. Our results suggest that brain tissue shows a universal response to applied forces that depends not only on the amount and stiffness of the individual tissue constituents but also on the way how they assemble. Our model may unify current incongruences between the mechanics of soft biological tissues and the underlying constituents and facilitate the design of better biomedical materials and engineered tissues. To this end, we provide a freely-available platform to predict local tissue elasticity upon providing immunohistochemistry images and stiffness values for the constituents of the tissue.

Local characterization of collagen architecture and mechanical failure properties of fibrous plaque tissue of atherosclerotic human carotid arteries.

Atherosclerotic plaque rupture in carotid arteries is a major cause of cerebrovascular events. Plaque rupture is the mechanical failure of the heterogeneous fibrous plaque tissue. Local characterization of the tissue's failure properties and the collagen architecture are of great importance to have insights in plaque rupture for clinical event prevention. Previous studies were limited to average rupture properties and global structural characterization, and did not provide the necessary local information. In this study, we assessed the local collagen architecture and failure properties of fibrous plaque tissue, by analyzing 30 tissue strips from 18 carotid plaques. Our study framework entailed second harmonic generation imaging for local collagen orientation and dispersion, and uniaxial tensile testing and digital image correlation for local tissue mechanics. The results showed that 87% of the imaged locations had collagen orientation close to the circumferential direction (0°) of the artery, and substantial dispersion locally. All regions combined, median [Q1:Q3] of the predominant angle measurements was -2° [-16°:16°]. The stretch ratio measurements clearly demonstrated a nonuniform stretch ratio distribution in the tissue under uniaxial loading. The rupture initiation regions had significantly higher stretch ratios (1.26 [1.15-1.40]) than the tissue average stretch ratio (1.11 [1.10-1.16]). No significant difference in collagen direction and dispersion was identified between the rupture regions and the rest of the tissue. The presented study forms an initial step towards gaining better insights into the characterization of local structural and mechanical fingerprints of fibrous plaque tissue in order to aid improved assessment of plaque rupture risk. STATEMENT OF SIGNIFICANCE: Plaque rupture risk assessment, critical to prevent cardiovascular events, requires knowledge on local failure properties and structure of collagenous plaque tissue. Our current knowledge is unfortunately limited to tissue's overall ultimate failure properties with scarce information on collagen architecture. In this study, local failure properties and collagen architecture of fibrous plaque tissue were obtained. We found predominant circumferential alignment of collagen fibers with substantial local dispersion. The tissue showed nonuniform stretch distribution under uniaxial tensile loading, with high stretches at rupture spots. This study highlights the significance of local mechanical and structural assessment for better insights into plaque rupture and the potential use of local stretches as risk marker for plaque rupture for patient-specific clinical applications.

Mild hypercholesterolemia impacts achilles sub-tendon mechanical properties in young rats

BackgroundHypercholesterolemia is associated with tendon pathology, but the reasons underpinning this relationship are not well understood. Cholesterol can accumulate in the tendon non-collagenous matrix which may affect both global and local tissue mechanics. Changes to the local strain environment within tendon may have significant implications for mechanosensitive tenocytes. Here, we investigated the association between elevated blood cholesterol and presence of tendon lipids in the Achilles tendon. We expected lipids to be localised in the proteoglycan-rich inter-sub-tendon matrix (ISTM), therefore we also sought to examine the impact of this on the biomechanical and viscoelastic properties of the ISTM.MethodsThe Achilles tendons of 32 young wild-type (SD) and 32 apolipoprotein E knock-out rats (ApoE−/−) were harvested at 15.6 ± 2.3 weeks of age. 32 specimens underwent histological examination to assess the distribution of lipids throughout sub-tendons and ISTM. The remaining specimens were prepared for biomechanical testing, where the ISTM between the gastrocnemius and soleus sub-tendons was subjected to shear load mechanical testing. A sub-set of tests were video recorded to enable a strain analysis.ResultsApoE−/− serum cholesterol was double that of SD rats (mean 2.25 vs. 1.10 mg/ml, p < 0.001) indicating a relatively mild hypercholesterolemia phenotype. Nonetheless, we found histological evidence of esterified lipids in the ISTM and unesterified lipids in the sub-tendons, although the location or intensity of staining was not appreciably different between rat strains. Despite a lack of observable histological differences in lipid content between groups, there were significant differences in the mechanical and viscoelastic behaviour of the Achilles sub-tendon matrix.ConclusionEven slightly elevated cholesterol may result in subtle changes to tendon biomechanical properties and hence injury risk. The young age of our cohort and the mild phenotype of our ApoE−/− rats are likely to have limited our findings and so we also conclude that the ApoE−/− rat model is not well suited for investigating the biomechanical impact of tendon xanthomas on Achilles sub-tendon function.

Local tissue mechanics control cardiac pacemaker cell embryonic patterning.

Cardiac pacemaker cells (CPCs) initiate the electric impulses that drive the rhythmic beating of the heart. CPCs reside in a heterogeneous, ECM-rich microenvironment termed the sinoatrial node (SAN). Surprisingly, little is known regarding the biochemical composition or mechanical properties of the SAN, and how the unique structural characteristics present in this region of the heart influence CPC function remains poorly understood. Here, we have identified that SAN development involves the construction of a "soft" macromolecular ECM that specifically encapsulates CPCs. In addition, we demonstrate that subjecting embryonic CPCs to substrate stiffnesses higher than those measured in vivo results in loss of coherent electrical oscillation and dysregulation of the HCN4 and NCX1 ion channels required for CPC automaticity. Collectively, these data indicate that local mechanics play a critical role in maintaining the embryonic CPC function while also quantitatively defining the range of material properties that are optimal for embryonic CPC maturation.

Direct estimation of regional lung volume change from paired and single CT images using residual regression neural network.

Chest computed tomography (CT) enables characterization of pulmonary diseases by producing high-resolution and high-contrast images of the intricate lung structures. Deformable image registration is used to align chest CT scans at different lung volumes, yielding estimates of local tissue expansion and contraction. We investigated the utility of deep generative models for directly predicting local tissue volume change from lung CT images, bypassing computationally expensive iterative image registration and providing a method that can be utilized in scenarios where either one or two CT scans areavailable. A residual regression convolutional neural network, called Reg3DNet+, is proposed for directly regressing high-resolution images of local tissue volume change (i.e., Jacobian) from CT images. Image registration was performed between lung volumes at total lung capacity (TLC) and functional residual capacity (FRC) using a tissue mass- and structure-preserving registration algorithm. The Jacobian image was calculated from the registration-derived displacement field and used as the ground truth for local tissue volume change. Four separate Reg3DNet+ models were trained to predict Jacobian images using a multifactorial study design to compare the effects of network input (i.e., single image vs. paired images) and output space (i.e., FRC vs. TLC). The models were trained and evaluated on image datasets from the COPDGene study. Models were evaluated against the registration-derived Jacobian images using local, regional, and global evaluationmetrics. Statistical analysis revealed that both factors - network input and output space - were significant determinants for change in evaluation metrics. Paired-input models performed better than single-input models, and model performance was better in the output space of FRC rather than TLC. Mean structural similarity index for paired-input models was 0.959 and 0.956 for FRC and TLC output spaces, respectively, and for single-input models was 0.951 and 0.937. Global evaluation metrics demonstrated correlation between registration-derived Jacobian mean and predicted Jacobian mean: coefficient of determination (r2 ) for paired-input models was 0.974 and 0.938 for FRC and TLC output spaces, respectively, and for single-input models was 0.598 and 0.346. After correcting for effort, registration-derived lobar volume change was strongly correlated with the predicted lobar volume change: for paired-input models r2 was 0.899 for both FRC and TLC output spaces, and for single-input models r2 was 0.803 and 0.862, respectively. Convolutional neural networks can be used to directly predict local tissue mechanics, eliminating the need for computationally expensive image registration. Networks that use paired CT images acquired at TLC and FRC allow for more accurate prediction of local tissue expansion compared to networks that use a single image. Networks that only require a single input image still show promising results, particularly after correcting for effort, and allow for local tissue expansion estimation in cases where multiple CT scans are not available. For single-input networks, the FRC image is more predictive of local tissue volume change compared to the TLCimage.

Preventing Engrailed-1 activation in fibroblasts yields wound regeneration without scarring.

Skin scarring, the end result of adult wound healing, is detrimental to tissue form and function. Engrailed-1 lineage-positive fibroblasts (EPFs) are known to function in scarring, but Engrailed-1 lineage-negative fibroblasts (ENFs) remain poorly characterized. Using cell transplantation and transgenic mouse models, we identified a dermal ENF subpopulation that gives rise to postnatally derived EPFs by activating Engrailed-1 expression during adult wound healing. By studying ENF responses to substrate mechanics, we found that mechanical tension drives Engrailed-1 activation via canonical mechanotransduction signaling. Finally, we showed that blocking mechanotransduction signaling with either verteporfin, an inhibitor of Yes-associated protein (YAP), or fibroblast-specific transgenic YAP knockout prevents Engrailed-1 activation and promotes wound regeneration by ENFs, with recovery of skin appendages, ultrastructure, and mechanical strength. This finding suggests that there are two possible outcomes to postnatal wound healing: a fibrotic response (EPF-mediated) and a regenerative response (ENF-mediated).

Regeneration without scarring

Wound Healing Wounds in adult mammals typically heal by forming fibrotic scars. Mascharak et al. found that a specific population of skin fibroblasts ( Engrailed-1 lineage–negative fibroblasts) activate expression of Engrailed-1 and turn on profibrotic cellular programs in response to local tissue mechanics in wounds (see the Perspective by Konieczny and Naik). When mechanical signaling was inhibited in these cells (using either genetic deletion or small-molecule inhibition), skin wounds in mice no longer formed scars but instead healed by regeneration, restoring skin with normal hair follicles and glands, extracellular matrix, and mechanical strength. Science , this issue p. [eaba2374][1]; see also p. [346][2] [1]: /lookup/doi/10.1126/science.aba2374 [2]: /lookup/doi/10.1126/science.abi5770

Stabilization of Damaged Articular Cartilage with Hydrogel-Mediated Reinforcement and Sealing.

Cartilage injuries and subsequent tissue deterioration impact millions of patients. Since the regeneration of functional hyaline cartilage remains elusive, methods to stabilize the remaining tissue, and prevent further deterioration, would be of significant clinical utility and prolong joint function. Finite element modeling shows that fortification of the degenerate cartilage (Reinforcement) and reestablishment of a superficial zone (Sealing) are both required to restore fluid pressurization within the tissue and restrict fluid flow and matrix loss from the defect surface. Here, a hyaluronic acid (HA) hydrogel system is designed to both interdigitate with and promote the sealing of the degenerated cartilage. Interdigitating fortification restores both bulk and local pericellular tissue mechanics, reestablishing the homeostatic mechanotransduction of endogenous chondrocytes within the tissue. This HA therapy is further functionalized to present chemo mechanical cues that improve the attachment and direct the response of mesenchymal stem/stromal cells at the defect site, guiding localized extracellular matrix deposition to "seal" the defect. Together, these results support the therapeutic potential, across cell and tissue length scales, of an innovative hydrogel therapy for the treatment of damaged cartilage.

Wound Healing Myofibroblasts Proliferate Clonally and in a Mechanoresponsive Manner

INTRODUCTION: Fibroblasts are integral in regulating tissue homeostasis. Notably, activated fibroblasts (myofibroblasts) play critical roles in wound healing including producing and depositing extracellular matrix and driving wound closure. Much research has been aimed at characterizing subpopulations of fibroblasts with specific activities in wound healing; however, information regarding the origins and heterogeneity of myofibroblasts remains incompletely explored. Our lab has previously shown that fibroblast activity in wound healing is dependent on tissue mechanics (specifically focal-adhesion kinase signaling). The aim of our research was to determine the origins of myofibroblasts that respond to dermal injury and to understand their proliferation, mechanoresponsiveness, and heterogeneity. METHODS: In order to study the characteristics of wound healing myofibroblasts, we used the rainbow reporter mouse (Rosa26VT2/GK3). The rainbow system has a 4-color reporter construct, which after induction and recombination, cells express 1 of 4 fluorescent proteins and all progeny cells are then marked with the same color as their parent cell permitting precise clonal analysis and lineage tracing. We used a wounding model that mimics human wound healing kinetics in which full-thickness wounds were created on the dorsal dermis of Rainbow mice and stented with silicone rings. Wound tissues were examined using confocal microscopy. In order to investigate the influence of local tissue mechanics in our wound healing model, a small molecule FAK-inhibitor (or vehicle control) was applied to the mouse wounds. Imaris software was used for imaging analysis. RESULTS: Imaging analysis suggests the presence of progenitor-type fibroblasts that proliferate clonally and radially during wound healing. Clonal expansion can be appreciated when comparing cross-sectional images of uninjured control dermis to wounded dermis. Using an activated fibroblast driver (aSMA-CreERT2) with the rainbow mouse model, radial proliferation of fibroblast clones is observed. Bulk RNA-seq data shows that the clonal proliferation of wound healing fibroblasts demonstrates an upregulation in mechanoresponsive gene pathways (ie, those involving expression of FAK). Imaging analysis reveals that FAK-inhibition results in disordered clonal proliferation when compared with normal wound controls. CONCLUSIONS: Fibroblast proliferation in response to dermal injury is clonal, suggesting the presence of progenitor-type fibroblasts. This clonal proliferation is FAK-dependent suggesting that this phenomenon is highly reliant on mechanical signaling pathways.

Mapping cellular-scale internal mechanics in 3D tissues with thermally responsive hydrogel probes

Local tissue mechanics play a critical role in cell function, but measuring these properties at cellular length scales in living 3D tissues can present considerable challenges. Here we present thermoresponsive, smart material microgels that can be dispersed or injected into tissues and optically assayed to measure residual tissue elasticity after creep over several weeks. We first develop and characterize the sensors, and demonstrate that internal mechanical profiles of live multicellular spheroids can be mapped at high resolutions to reveal broad ranges of rigidity within the tissues, which vary with subtle differences in spheroid aggregation method. We then show that small sites of unexpectedly high rigidity develop in invasive breast cancer spheroids, and in an in vivo mouse model of breast cancer progression. These focal sites of increased intratumoral rigidity suggest new possibilities for how early mechanical cues that drive cancer cells towards invasion might arise within the evolving tumor microenvironment.

Biologically structured materials

Biomimetics, biomechanics, and tissue engineering are three multidisciplinary fields that have been contemplated in this research to attain the objective of improving prosthetic implants reliability. Since testing and mathematical methods are closely interlaced, a promising approach seemed to be the combination of in vitro and in vivo experiments with computer simulations (in silico). An innovative biomimetics and biomechanics approach, and a new synthetic structure providing a microenvironment, which is mechanically coherent and nutrient conducive for tissue osteoblast cell cultures used in regenerative medicine, are presented. The novel hybrid ceramic-polymeric nanocomposites are mutually investigated by finite element analysis (FEA) biomimetic modeling, anatomic reconstruction, quantitative-computed-tomography characterization, computer design of tissue scaffold. The starting base materials are a class of innovative highly bioactive hybrid ceramic-polymeric materials set-up by the proponent research group that will be used as a bioactive matrix for the preparation of in situ bio-mineralized techno- structured porous nanocomposites. This study treats biomimetics, biomechanics and tissue engineering as strongly correlated multidisciplinary fields combined to design bone tissue scaffolds. The growth, maintenance, and ossification of bone are fundamental and are regulated by the mechanical cues that are imposed by physical activities: this biomimetic/biomechanical approach will be pursued in designing the experimental procedures for in vitro scaffold mineralization and ossification. Bio-tissue mathematical modeling serves as a central repository to interface design, simulation, and tissue fabrication. Finite element computer analyses will be used to study the role of local tissue mechanics on endochondral ossification patterns, skeletal morphology and mandible thickness distributions using single and multi-phase continuum material representations of clinical cases of patients implanted with the traditional protocols. New protocols will be hypothesized for the use of the new biologically techno-structured hybrid materials.

Finite element modeling of osteochondral explant impacts predicts locations of cell death

Purpose: Post-traumatic osteoarthritis (PTOA) is a subset of osteoarthritis initiated by an injury such as a blunt impact. However, the local tissue mechanics associated with the earliest stages of PTOA remain undefined. Computational modeling techniques such as finite element analysis provide a means to evaluate mechanical environments that are challenging to measure experimentally. The objective of this work was to compare specimen-specific stresses and strains computed using finite element analysis to confocal microscopy images of chondrocyte viability. Methods: Four viable osteochondral explants (25 mm x 25 mm x 15 mm) were harvested from the tibial plateau of healthy bovine stifle joints (knee). Once removed, the explants were washed twice in HBSS, cultured in normal media (45% DMEM, 45% F12, 10% fetal bovine serum, 100 U/mL penicillin, 100 μg/mL streptomycin, and 2.5 μm/mL amphotericin B) and incubated at 37°C, 5% CO2, and 5% O2. The following day a surface scan of the articular surface of each explant (EinScan Pro 2X Pro; Shining 3D) and ultrasound measurements (Olympus 35DL) of cartilage thickness were acquired. Next, the explants were subject to a 2 J/cm2 impact using a 6 mm flat, beveled stainless-steel platen. After impact, the samples were placed in fresh porcine media and returned to the incubator for 24 h. Cartilage explants were cut with an Isomet 1000 (Buehler, Lake Bluff, IL) slightly off center of the impact and faced by trimming 0.5 mm of the cartilage with a scalpel. These provided a coronal plane cross-section of the explant through the center of the impact site. The explants were washed in phenol red free DMEM/F12 media and stained for 30 min in the incubator with 1 mg/mL Calcein Green AM and 200 nM MitoTracker Deep Red (Life Technologies, Waltham, MA). The samples were then washed and imaged on an Olympus FV1000 (Shinjuku, Japan) confocal microscope. Images were acquired on either edge of the impact, 1 mm inside each edge of the impact site, and in the center of the impact (Figure 1) using a 10X objective lens. An additional top-down view of the articular surface was acquired. Specimen-specific finite element models were generated using the surface scan, cartilage thickness measurements, and geometric measurements of each explant. Cartilage was modeled as hyperelastic with a first-order Ogden strain energy potential. Acceleration of the impact platen was measured (PCB Piezotronics accelerometer), and double integration was used to calculate platen displacement at max tissue compression and impact force. von Mises stress and normal z strain values were compared from corresponding nodes within confocal images (Figure 2). Cell death in the superficial zone and deeper regions of cartilage was assessed. Results: Cell death was present in the superficial zone in 4/4 peripheral edge locations, 3/4 locations interior to the impactor edge and 3/4 of the middle sites. There was no superficial cell death on the central edge of the impact zone. Locations experiencing > 40% strain were associated with high levels of cell death in the superficial zone. When strains exceeded 60%, cell death extended into the deeper zones of the cartilage. It was noteworthy that the highest strains were found at the 1-mm interior to the impact platen edge on all specimens. Von Mises stresses were consistently &gt 30 MPa on the edge of the impact site; however, no obvious relationships were identified between von Mises values and cell death in the superficial, middle, or deep zones. Conclusions: We found strong associations between cartilage strain and chondrocyte death. The highest normal z strain occurs just interior to the beveled edge of the impact platen, and not on the peripheral-most edge of the impact platen. While we did find associations between locations of elevated stress, strain, and cell death, the magnitudes of those values remain to be validated. While important, cell death is a relatively broad measure of injury and associations between cellular health and function and these mechanical measures should be explored.View Large Image Figure ViewerDownload Hi-res image Download (PPT)

Presents some Biologically Structured Materials

In this paper bio-tissue, mathematical modeling serves as a central repository to interface design, simulation and tissue fabrication. Finite element computer analyses will be used to study the role of local tissue mechanics on endochondral ossification patterns, skeletal morphology and mandible thickness distributions using single and multi-phase continuum material representations of clinical cases of patients implanted with the traditional protocols. New protocols will be hypothesized for the use of the new biologically techno-structured hybrid materials.

Biologically structured materials

In this paper bio-tissue mathematical modeling serves as a central repository to interface design, simulation, and tissue fabrication. Finite element computer analyses will be used to study the role of local tissue mechanics on endochondral ossification patterns, skeletal morphology and mandible thickness distributions using single and multi-phase continuum material representations of clinical cases of patients implanted with the traditional protocols. New protocols will be hypothesized for the use of the new biologically techno-structured hybrid materials.

Planar Differential Growth Rates Initiate Precise Fold Positions in Complex Epithelia.

SummaryTissue folding is a fundamental process that shapes epithelia into complex 3D organs. The initial positioning of folds is the foundation for the emergence of correct tissue morphology. Mechanisms forming individual folds have been studied, but the precise positioning of folds in complex, multi-folded epithelia is less well-understood. We present a computational model of morphogenesis, encompassing local differential growth and tissue mechanics, to investigate tissue fold positioning. We use the Drosophila wing disc as our model system and show that there is spatial-temporal heterogeneity in its planar growth rates. This differential growth, especially at the early stages of development, is the main driver for fold positioning. Increased apical layer stiffness and confinement by the basement membrane drive fold formation but influence positioning to a lesser degree. The model successfully predicts the in vivo morphology of overgrowth clones and wingless mutants via perturbations solely on planar differential growth in silico.

Biologically Structured Materials

In this paper bio-tissue mathematical modeling serves as a central repository to interface design, simulation, and tissue fabrication. Finite element computer analyses will be used to study the role of local tissue mechanics on endochondral ossification patterns, skeletal morphology and mandible thickness distributions using single and multi-phase continuum material representations of clinical cases of patients implanted with the traditional protocols. New protocols will be hypothesized for the use of the new biologically techno-structured hybrid materials.

A review of techniques for visualising soft tissue microstructure deformation and quantifying strain Ex Vivo.

Many biological tissues have a complex hierarchical structure allowing them to function under demanding physiological loading conditions. Structural changes caused by ageing or disease can lead to loss of mechanical function. Therefore, it is necessary to characterise tissue structure to understand normal tissue function and the progression of disease. Ideally intact native tissues should be imaged in 3D and under physiological loading conditions. The current published in situ imaging methodologies demonstrate a compromise between imaging limitations and maintaining the samples native mechanical function. This review gives an overview of in situ imaging techniques used to visualise microstructural deformation of soft tissue, including three case studies of different tissues (tendon, intervertebral disc and artery). Some of the imaging techniques restricted analysis to observational mechanics or discrete strain measurement from invasive markers. Full-field local surface strain measurement has been achieved using digital image correlation. Volumetric strain fields have successfully been quantified from in situ X-ray microtomography (micro-CT) studies of bone using digital volume correlation but not in soft tissue due to low X-ray transmission contrast. With the latest developments in micro-CT showing in-line phase contrast capability to resolve native soft tissue microstructure, there is potential for future soft tissue mechanics research where 3D local strain can be quantified. These methods will provide information on the local 3D micromechanical environment experienced by cells in healthy, aged and diseased tissues. It is hoped that future applications of in situ imaging techniques will impact positively on the design and testing of potential tissue replacements or regenerative therapies. LAY DESCRIPTION: The soft tissues in our bodies, such as tendons, intervertebral discs and arteries, have evolved to have complicated structures which deform and bear load during normal function. Small changes in these structures can occur with age and disease which then leads to loss of function. Therefore, it is important to image tissue microstructure in 3D and under functional conditions. This paper gives an overview of imaging techniques used to record the deformation of soft tissue microstructures. Commonly there are compromises between obtaining the best imaging result and retaining the samples native structure and function. For example, invasive markers and dissecting samples damages the tissues natural structure, and staining or clearing (making the tissue more transparent) can distort tissue structure. Structural deformation has been quantified from 2D imaging techniques (digital image correlation) to create surface strain maps which help identify local tissue mechanics. When extended to 3D (digital volume correlation), deformation measurement has been limited to bone samples using X-ray micro-CT. Recently it has been possible to image the 3D structure of soft tissue using X-ray micro-CT meaning that there is potential for internal soft tissue mechanics to be mapped in 3D. Future application of micro-CT and digital volume correlation will be important for soft tissue mechanics studies particularly to understand normal function, progression of disease and in the design of tissue replacements.

Metastasis in context: modeling the tumor microenvironment with cancer-on-a-chip approaches.

ABSTRACTMost cancer deaths are not caused by the primary tumor, but by secondary tumors formed through metastasis, a complex and poorly understood process. Cues from the tumor microenvironment, such as the biochemical composition, cellular population, extracellular matrix, and tissue (fluid) mechanics, have been indicated to play a pivotal role in the onset of metastasis. Dissecting the role of these cues from the tumor microenvironment in a controlled manner is challenging, but essential to understanding metastasis. Recently, cancer-on-a-chip models have emerged as a tool to study the tumor microenvironment and its role in metastasis. These models are based on microfluidic chips and contain small chambers for cell culture, enabling control over local gradients, fluid flow, tissue mechanics, and composition of the local environment. Here, we review the recent contributions of cancer-on-a-chip models to our understanding of the role of the tumor microenvironment in the onset of metastasis, and provide an outlook for future applications of this emerging technology.

Fatigue loading of tendon results in collagen kinking and denaturation but does not change local tissue mechanics

Fatigue loading is a primary cause of tendon degeneration, which is characterized by the disruption of collagen fibers and the appearance of abnormal (e.g., cartilaginous, fatty, calcified) tissue deposits. The formation of such abnormal deposits, which further weakens the tissue, suggests that resident tendon cells acquire an aberrant phenotype in response to fatigue damage and the resulting altered mechanical microenvironment. While fatigue loading produces clear changes in collagen organization and molecular denaturation, no data exist regarding the effect of fatigue on the local tissue mechanical properties. Therefore, the objective of this study was to identify changes in the local tissue stiffness of tendons after fatigue loading. We hypothesized that fatigue damage would reduce local tissue stiffness, particularly in areas with significant structural damage (e.g., collagen denaturation). We tested this hypothesis by identifying regions of local fatigue damage (i.e., collagen fiber kinking and molecular denaturation) via histologic imaging and by measuring the local tissue modulus within these regions via atomic force microscopy (AFM). Counter to our initial hypothesis, we found no change in the local tissue modulus as a consequence of fatigue loading, despite widespread fiber kinking and collagen denaturation. These data suggest that immediate changes in topography and tissue structure – but not local tissue mechanics – initiate the early changes in tendon cell phenotype as a consequence of fatigue loading that ultimately culminate in tendon degeneration.