Regulation of tissue fibrosis by the biomechanical environment

Pathophysiology and clinical relevance of atrial myopathy

Atrial myopathy is a condition that consists of electrical, structural, contractile, and autonomic remodeling of the atria and is the substrate for development of atrial fibrillation, the most common arrhythmia. Pathophysiologic mechanisms driving atrial myopathy are inflammation, oxidative stress, atrial stretch, and neurohormonal signals, e.g., angiotensin-II and aldosterone. These mechanisms initiate the structural and functional remodeling of the atrial myocardium. Novel therapeutic strategies are being developed that target the pathophysiologic mechanisms of atrial myopathy. In this review, we will discuss the pathophysiology of atrial myopathy, as well as diagnostic and therapeutic strategies.

A review of the impact, pathophysiology, and management of atrial fibrillation in patients with heart failure with preserved ejection fraction

Patients with heart failure with preserved ejection fraction (HFpEF) and atrial fibrillation (AF) have increased mortality and increased risk of stroke. Due to the heterogeneous nature of both disease processes, it is difficult to ascertain whether the diagnosis and progression of AF is the cause of deterioration or if it is a symptom of worsening heart failure. This presents physicians with a clinical conundrum of whether optimizing their heart failure will decrease the overall AF burden or if restoration of sinus rhythm is necessary to optimize patients with HFpEF. In this paper, we will review the impact of AF in patients with HFpEF, the pathophysiology and heterogeneity of HFpEF and AF, and the management of these patients. As HFpEF and AF become more prevalent, managing these disease processes needs standardization to improve outcomes. Further research is needed to understand the complex interplay between AF and HFpEF to help determine the best management strategy.

Atrial Cardiomyopathy in Valvular Heart Disease: From Molecular Biology to Clinical Perspectives

This review discusses the evolving topic of atrial cardiomyopathy concerning valvular heart disease. The pathogenesis of atrial cardiomyopathy involves multiple factors, such as valvular disease leading to atrial structural and functional remodeling due to pressure and volume overload. Atrial enlargement and dysfunction can trigger atrial tachyarrhythmia. The complex interaction between valvular disease and atrial cardiomyopathy creates a vicious cycle of aggravating atrial enlargement, dysfunction, and valvular disease severity. Furthermore, atrial remodeling and arrhythmia can predispose to atrial thrombus formation and stroke. The underlying pathomechanism of atrial myopathy involves molecular, cellular, and subcellular alterations resulting in chronic inflammation, atrial fibrosis, and electrophysiological changes. Atrial dysfunction has emerged as an essential determinant of outcomes in valvular disease and heart failure. Despite its predictive value, the detection of atrial fibrosis and dysfunction is challenging and is not included in the clinical routine. Transthoracic echocardiography and cardiac magnetic resonance imaging are the main diagnostic tools for atrial cardiomyopathy. Recently published data have revealed that both left atrial volumes and functional parameters are independent predictors of cardiovascular events in valvular disease. The integration of atrial function assessment in clinical practice might help in early cardiovascular risk estimation, promoting early therapeutic intervention in valvular disease.

Electrospun nanofiber mesh with connective tissue growth factor and mesenchymal stem cells for pelvic floor repair: Long-term study

Pelvic organ prolapse (POP) affects many women, with an estimated lifetime risk of surgical intervention of 18.7%. There is a need for alternative approaches as the use of synthetic nondegradable mesh was stopped due to severe adverse events, and as current methods for pelvic floor repair have high POP recurrence rates. Thus, we hypothesized that electrospun degradable meshes with stem cells and growth factor were safe and durable for the long term in elderly rats. In an abdominal repair model, electrospun polycaprolactone (PCL) meshes coated with connective tissue growth factor (CTGF)/PEG-fibrinogen (PF) and rat mesenchymal stem cells were implanted in elderly female rats and removed after in average 53 weeks (53-week group). Collagen amount and production were quantified by qPCR and Western blotting. Moreover, histological appearance and biomechanical properties were evaluated. Results were compared with previous results of young rats with identical mesh implanted for 24 weeks (24-week group). The 53-week group differed from the 24-week group in terms of (1) reduced collagen III, (2) strong reduction in foreign body response, and (3) altered histological appearance. We found comparable biomechanical properties, aside from higher, not significant, mean tissue stiffness in the 53-week group. Lastly, we identified mesh components 53 weeks after implantation. This study provides new insights into future POP repair in postmenopausal women by showing how CTGF/PF-coated electrospun PCL meshes with stem cells exhibit sufficient support, biocompatibility, and no mesh-related complications long term in an abdominal repair model in elderly rats.

Composition, host responses and clinical applications of bioadhesives

Bioadhesives are medical devices used to join or seal tissues that have been injured or incised. They have been classified into tissue adhesives, sealants, and hemostatic agents. Bioadhesives such as FloSeal®, CoSeal®, BioGlue®, Evicel®, Tisseel®, Progel™ PALS, and TissuGlu® have been commercialized and used in clinical setting. They can be formulated with natural or synthetic components or a combination of both including albumin, glutaraldehyde, chitosan, cyanoacrylate, fibrin and thrombin, gelatin, polyethylene glycol (PEG), along with urethanes. Each formulation has intrinsic properties and has been developed and validated for a specific application. This review article briefs the mechanisms by which bioadhesives forms adhesion to tissues and highlights the correlation between bioadhesives composition and their potential host responses. Furthermore, clinical applications of bioadhesives and their application-driven requirements are outlined.

Quantitative assessment of transmural fibrosis profile in the human atrium: evidence for a three-dimensional arrhythmic substrate by slice-to-slice histology

AIMS

Intramural fibrosis represents a crucial factor in the formation of a three-dimensional (3D) substrate for atrial fibrillation (AF). However, the transmural distribution of fibrosis and its relationship with atrial overload remain largely unknown. The aim of this study is to quantify the transmural profile of atrial fibrosis in patients with different degrees of atrial dilatation and arrhythmic profiles by a high-resolution 3D histology method.

METHODS AND RESULTS

Serial microtome-cut tissue slices, sampling the entire atrial wall thickness at 5 µm spatial resolution, were obtained from right atrial appendage specimens in 23 cardiac surgery patients. Atrial slices were picrosirius red stained, imaged by polarized light microscopy, and analysed by a custom-made segmentation algorithm. In all patients, the intramural fibrosis content displayed a progressive decrease alongside tissue depth, passing from 68.6 ± 11.6% in the subepicardium to 10-13% in the subendocardium. Distinct transmural fibrotic profiles were observed in patients with atrial dilatation with respect to control patients, where the first showed a slower decrease of fibrosis along tissue depth (exponential decay constant: 171.2 ± 54.5 vs. 80.9 ± 24.4 µm, P < 0.005). Similar slow fibrotic profiles were observed in patients with AF (142.8 ± 41.7 µm). Subepicardial and midwall levels of fibrosis correlated with the degree of atrial dilatation (ρ = 0.72, P < 0.001), while no correlation was found in subendocardial layers.

CONCLUSIONS

Quantification of fibrosis transmural profile at high resolution is feasible by slice-to-slice histology. Deeper penetration of fibrosis in subepicardial and midwall layers in dilated atria may concur to the formation of a 3D arrhythmic substrate.

Mechanoimmunology: Are inflammatory epigenetic states of macrophages tuned by biophysical factors?

Many inflammatory diseases that are responsible for a majority of deaths are still uncurable, in part as the underpinning pathomechanisms and how to combat them is still poorly understood. Tissue-resident macrophages play pivotal roles in the maintenance of tissue homeostasis, but if they gradually convert to proinflammatory phenotypes, or if blood-born proinflammatory macrophages persist long-term after activation, they contribute to chronic inflammation and fibrosis. While biochemical factors and how they regulate the inflammatory transcriptional response of macrophages have been at the forefront of research to identify targets for therapeutic interventions, evidence is increasing that physical factors also tune the macrophage phenotype. Recently, several mechanisms have emerged as to how physical factors impact the mechanobiology of macrophages, from the nuclear translocation of transcription factors to epigenetic modifications, perhaps even DNA methylation. Insight into the mechanobiology of macrophages and associated epigenetic modifications will deliver novel therapeutic options going forward, particularly in the context of increased inflammation with advancing age and age-related diseases. We review here how biophysical factors can co-regulate pro-inflammatory gene expression and epigenetic modifications and identify knowledge gaps that require urgent attention if this therapeutic potential is to be realized.

Dynamic actuation enhances transport and extends therapeutic lifespan in an implantable drug delivery platform

Fibrous capsule (FC) formation, secondary to the foreign body response (FBR), impedes molecular transport and is detrimental to the long-term efficacy of implantable drug delivery devices, especially when tunable, temporal control is necessary. We report the development of an implantable mechanotherapeutic drug delivery platform to mitigate and overcome this host immune response using two distinct, yet synergistic soft robotic strategies. Firstly, daily intermittent actuation (cycling at 1 Hz for 5 minutes every 12 hours) preserves long-term, rapid delivery of a model drug (insulin) over 8 weeks of implantation, by mediating local immunomodulation of the cellular FBR and inducing multiphasic temporal FC changes. Secondly, actuation-mediated rapid release of therapy can enhance mass transport and therapeutic effect with tunable, temporal control. In a step towards clinical translation, we utilise a minimally invasive percutaneous approach to implant a scaled-up device in a human cadaveric model. Our soft actuatable platform has potential clinical utility for a variety of indications where transport is affected by fibrosis, such as the management of type 1 diabetes.

Drug delivery implants suffer from diminished release profiles due to fibrous capsule formation over time. Here, the authors use soft robotic actuation to modulate the immune response of the host to maintain drug delivery over the longer-term and to perform controlled release in vivo.

Biomaterials to enhance stem cell transplantation

The successful transplantation of stem cells has the potential to transform regenerative medicine approaches and open promising avenues to repair, replace, and regenerate diseased, damaged, or aged tissues. However, pre-/post-transplantation issues of poor cell survival, retention, cell fate regulation, and insufficient integration with host tissues constitute significant challenges. The success of stem cell transplantation depends upon the coordinated sequence of stem cell renewal, specific lineage differentiation, assembly, and maintenance of long-term function. Advances in biomaterials can improve pre-/post-transplantation outcomes by integrating biophysiochemical cues and emulating tissue microenvironments. This review highlights leading biomaterials-based approaches for enhancing stem cell transplantation.

Angiotensin receptor blocker alleviates liver fibrosis by altering the mechanotransduction properties of hepatic stellate cells

Angiotensin receptor blockers have been reported to be beneficial to liver fibrosis, but the relevant molecular and cellular mechanisms remain unclear. We herein investigated whether low-dose angiotensin receptor blocker alleviated liver fibrosis through mechanotransduction regulation. Hydrostatic pressure-induced liver fibrosis model was established in mice by ligating partially the inferior vena cava, and then randomly received a very low dose of losartan (0.5 mg/kg) or placebo treatment for 8 weeks. We found that losartan administration interfered the expression of several mechanotransductive molecules, and effectively alleviated liver fibrosis. Using a commercial device, we further confirmed that ex vivo loading of hepatic stellate cells to 50 mmHg hydrostatic pressure for 24 h significantly upregulated RhoA, ROCK, AT1R, and p-MLC2, which was effectively attenuated by adding 10 nM losartan in medium. Our in vivo and ex vivo experimental data suggest that low-dose angiotensin receptor blockers may alleviate hydrostatic pressure-induced liver fibrosis by altering the mechanotransduction properties of hepatic stellate cells.NEW & NOTEWORTHY Our ex vivo and in vivo experiments clearly indicated that low-dose losartan alleviated liver fibrosis, likely by modulating the mechanotransduction properties of HSCs. Uncovering the biomechanical signaling pathway of ARB treatment on liver fibrosis will be helpful to develop novel molecular targeting therapy for liver diseases.

Is the future scarless? - Fibroblasts as targets for scarless wound healing: a narrative review

Introduction: Scarless healing is the ideal outcome of wound healing and is exhibited in some species. This narrative review assembles the current understanding of fibroblast heterogenicity along with the latest fibroblast-related targets for scar reduction therapies. Human regenerative wound healing is deemed possible due to the wound regeneration already seen in the early gestation foetus. Methods: This literature narrative review was undertaken by searching PubMed and Web of Science databases and Google Scholar to find articles concerning the fibroblast involvement in wound healing. We evaluated and collated these articles to form a consensus of the current understanding of the field. Discussion: This article describes current understanding of fibroblast heterogenicity and involvement in wound healing, focusing on the role of fibroblasts during physiological scarring. We also present the current most promising targets involving fibroblasts in the reduction of scarring and how we can manipulate the behaviour of fibroblasts to mimic the wound regeneration models in the human foetus. These targets include the pro-fibrotic EN1 positive fibroblast lineage, TGFβ1 inhibition, and genetic therapies utilising miRNAs and siRNAs. Conclusion: No therapies are currently available to eradicate scarring; however, treatment options are available to reduce the appearance of scarring. Further research into the heterogenicity and interactions of fibroblasts in both the foetus and adult is needed, and this may lead to the development of novel treatments against scarring.

Epidermal-Derived Hedgehog Signaling Drives Mesenchymal Proliferation during Digit Tip Regeneration

Hand injuries often result in significant functional impairments and are rarely completely restored. The spontaneous regeneration of injured appendages, which occurs in salamanders and newts, for example, has been reported in human fingertips after distal amputation, but this type of regeneration is rare in mammals and is incompletely understood. Here, we study fingertip regeneration by amputating murine digit tips, either distally to initiate regeneration, or proximally, causing fibrosis. Using an unbiased microarray analysis, we found that digit tip regeneration is significantly associated with hair follicle differentiation, Wnt, and sonic hedgehog (SHH) signaling pathways. Viral over-expression and genetic knockouts showed the functional significance of these pathways during regeneration. Using transgenic reporter mice, we demonstrated that, while both canonical Wnt and HH signaling were limited to epidermal tissues, downstream hedgehog signaling (through Gli) occurred in mesenchymal tissues. These findings reveal a mechanism for epidermal/mesenchyme interactions, governed by canonical hedgehog signaling, during digit regeneration. Further research into these pathways could lead to improved therapeutic outcomes after hand injuries in humans.

Exercising immune cells: The immunomodulatory role of exercise on atrial fibrillation

Exercise training is generally beneficial for cardiovascular health, improving stroke volume, cardiac output, and aerobic capacity. Despite these benefits, some evidence indicates that endurance training may increase the risk of atrial fibrillation (AF), particularly in highly trained individuals. Among multiple mechanisms, autonomic tone changes and atrial remodeling have been proposed as main contributors for exercise-induced AF. However, the contribution of local and systemic immunity is poorly understood in the development of atrial arrhythmogenic substrates. Here we aim to update the field of immunomodulation in the context of exercise and AF by compiling and reconciling the most recent evidence from preclinical and human studies and rationalize the applicability of "lone" AF terminology in athletes.

Modulating the foreign body response of implants for diabetes treatment

Diabetes Mellitus is a group of diseases characterized by high blood glucose levels due to patients' inability to produce sufficient insulin. Current interventions often require implants that can detect and correct high blood glucose levels with minimal patient intervention. However, these implantable technologies have not reached their full potential in vivo due to the foreign body response and subsequent development of fibrosis. Therefore, for long-term function of implants, modulating the initial immune response is crucial in preventing the activation and progression of the immune cascade. This review discusses the different molecular mechanisms and cellular interactions involved in the activation and progression of foreign body response (FBR) and fibrosis, specifically for implants used in diabetes. We also highlight the various strategies and techniques that have been used for immunomodulation and prevention of fibrosis. We investigate how these general strategies have been applied to implants used for the treatment of diabetes, offering insights on how these devices can be further modified to circumvent FBR and fibrosis.

Engineered microenvironment for the study of myofibroblast mechanobiology

Myofibroblasts are mechanosensitive cells and a variety of their behaviours including differentiation, migration, force production and biosynthesis are regulated by the surrounding microenvironment. Engineered cell culture models have been developed to examine the effect of microenvironmental factors such as the substrate stiffness, the topography and strain of the extracellular matrix (ECM) and the shear stress on myofibroblast biology. These engineered models provide well-mimicked, pathophysiologically relevant experimental conditions that are superior to those enabled by the conventional two-dimensional (2D) culture models. In this perspective, we will review the recent advances in the development of engineered cell culture models for myofibroblasts and outline the findings on the myofibroblast mechanobiology under various microenvironmental conditions. These studies have demonstrated the power and utility of the engineered models for the study of microenvironment-regulated cellular behaviours. The findings derived using these models contribute to a greater understanding of how myofibroblast behaviour is regulated in tissue repair and pathological scar formation.

Cell orientation under stretch: Stability of a linear viscoelastic model

The sensitivity of cells to alterations in the microenvironment and in particular to external mechanical stimuli is significant in many biological and physiological circumstances. In this regard, experimental assays demonstrated that, when a monolayer of cells cultured on an elastic substrate is subject to an external cyclic stretch with a sufficiently high frequency, a reorganization of actin stress fibres and focal adhesions happens in order to reach a stable equilibrium orientation, characterized by a precise angle between the cell major axis and the largest strain direction. To examine the frequency effect on the orientation dynamics, we propose a linear viscoelastic model that describes the coupled evolution of the cellular stress and the orientation angle. We find that cell orientation oscillates tending to an angle that is predicted by the minimization of a very general orthotropic elastic energy, as confirmed by a bifurcation analysis. Moreover, simulations show that the speed of convergence towards the predicted equilibrium orientation presents a changeover related to the viscous-elastic transition for viscoelastic materials. In particular, when the imposed oscillation period is lower than the characteristic turnover rate of the cytoskeleton and of adhesion molecules such as integrins, reorientation is significantly faster.

Biomaterials for protein delivery for complex tissue healing responses

Tissue repair requires a complex cascade of events mediated by a variety of cells, proteins, and matrix molecules; however, the healing cascade can be easily disrupted by numerous factors, resulting in impaired tissue regeneration. Recent advances in biomaterials for tissue regeneration have increased the ability to tailor the delivery of proteins and other biomolecules to injury sites to restore normal healing cascades and stimulate robust tissue repair. In this review, we discuss the evolution of the field toward creating biomaterials that precisely control protein delivery to stimulate tissue regeneration, with a focus on addressing complex and dynamic injury environments. We highlight biomaterials that leverage different mechanisms to deliver and present proteins involved in healing cascades, tissue targeting and mimicking strategies, materials that can be triggered by environmental cues, and integrated strategies that combine multiple biomaterial properties to improve protein delivery. Improvements in biomaterial design to address complex injury environments will expand our understanding of both normal and aberrant tissue repair processes and ultimately provide a better standard of patient care.

Atrial fibrosis and substrate based characterization in atrial fibrillation: Time to move forwards

Atrial fibrillation (AF) is the most commonly encountered cardiac arrhythmia in clinical practice. However, current therapeutic interventions for atrial fibrillation have limited clinical efficacy as a consequence of major knowledge gaps in the mechanisms sustaining atrial fibrillation. From a mechanistic perspective, there is increasing evidence that atrial fibrosis plays a central role in the maintenance and perpetuation of atrial fibrillation. Electrophysiologically, atrial fibrosis results in alterations in conduction velocity, cellular refractoriness, and produces conduction block promoting meandering, unstable wavelets and micro-reentrant circuits. Clinically, atrial fibrosis has also linked to poor clinical outcomes including AF-related thromboembolic complications and arrhythmia recurrences post catheter ablation. In this article, we review the pathophysiology behind the formation of fibrosis as AF progresses, the role of fibrosis in arrhythmogenesis, surrogate markers for detection of fibrosis using cardiac magnetic resonance imaging, echocardiography and electroanatomic mapping, along with their respective limitations. We then proceed to review the current evidence behind therapeutic interventions targeting atrial fibrosis, including drugs and substrate-based catheter ablation therapies followed by the potential future use of electro phenotyping for AF characterization to overcome the limitations of contemporary substrate-based methodologies.

Molecular Mechanisms of Obesity-Linked Cardiac Dysfunction: An Up-Date on Current Knowledge

Obesity is defined as excessive body fat accumulation, and worldwide obesity has nearly tripled since 1975. Excess of free fatty acids (FFAs) and triglycerides in obese individuals promote ectopic lipid accumulation in the liver, skeletal muscle tissue, and heart, among others, inducing insulin resistance, hypertension, metabolic syndrome, type 2 diabetes (T2D), atherosclerosis, and cardiovascular disease (CVD). These diseases are promoted by visceral white adipocyte tissue (WAT) dysfunction through an increase in pro-inflammatory adipokines, oxidative stress, activation of the renin-angiotensin-aldosterone system (RAAS), and adverse changes in the gut microbiome. In the heart, obesity and T2D induce changes in substrate utilization, tissue metabolism, oxidative stress, and inflammation, leading to myocardial fibrosis and ultimately cardiac dysfunction. Peroxisome proliferator-activated receptors (PPARs) are involved in the regulation of carbohydrate and lipid metabolism, also improve insulin sensitivity, triglyceride levels, inflammation, and oxidative stress. The purpose of this review is to provide an update on the molecular mechanisms involved in obesity-linked CVD pathophysiology, considering pro-inflammatory cytokines, adipokines, and hormones, as well as the role of oxidative stress, inflammation, and PPARs. In addition, cell lines and animal models, biomarkers, gut microbiota dysbiosis, epigenetic modifications, and current therapeutic treatments in CVD associated with obesity are outlined in this paper.

The Epithelial-to-Mesenchymal Transition as a Possible Therapeutic Target in Fibrotic Disorders

Fibrosis is a chronic and progressive disorder characterized by excessive deposition of extracellular matrix, which leads to scarring and loss of function of the affected organ or tissue. Indeed, the fibrotic process affects a variety of organs and tissues, with specific molecular background. However, two common hallmarks are shared: the crucial role of the transforming growth factor-beta (TGF-β) and the involvement of the inflammation process, that is essential for initiating the fibrotic degeneration. TGF-β in particular but also other cytokines regulate the most common molecular mechanism at the basis of fibrosis, the Epithelial-to-Mesenchymal Transition (EMT). EMT has been extensively studied, but not yet fully explored as a possible therapeutic target for fibrosis. A deeper understanding of the crosstalk between fibrosis and EMT may represent an opportunity for the development of a broadly effective anti-fibrotic therapy. Here we report the evidences of the relationship between EMT and multi-organ fibrosis, and the possible therapeutic approaches that may be developed by exploiting this relationship.

Advanced Pulmonary Sarcoidosis

At least 5% of sarcoidosis patients die from their disease, usually from advanced pulmonary sarcoidosis. The three major problems encountered in advanced pulmonary sarcoidosis are pulmonary fibrosis, pulmonary hypertension, and respiratory infections. Pulmonary fibrosis is the result of chronic inflammation, but other factors including abnormal wound healing may be important. Sarcoidosis-associated pulmonary hypertension (SAPH) is multifactorial including parenchymal fibrosis, vascular granulomas, and hypoxia. Respiratory infections can be cause by structural changes in the lung and impaired immunity due to sarcoidosis or therapy. Anti-inflammatory therapy alone is not effective in most forms of advanced pulmonary sarcoidosis. New techniques, including high-resolution computer tomography and 18F-fluorodeoxyglucose positron emission tomography (PET) have proved helpful in identifying the cause of advanced disease and directing specific therapy.

Mechanobiology of Macrophages: How Physical Factors Coregulate Macrophage Plasticity and Phagocytosis

In addition to their early-recognized functions in host defense and the clearance of apoptotic cell debris, macrophages play vital roles in tissue development, homeostasis, and repair. If misregulated, they steer the progression of many inflammatory diseases. Much progress has been made in understanding the mechanisms underlying macrophage signaling, transcriptomics, and proteomics, under physiological and pathological conditions. Yet, the detailed mechanisms that tune circulating monocytes/macrophages and tissue-resident macrophage polarization, differentiation, specification, and their functional plasticity remain elusive. We review how physical factors affect macrophage phenotype and function, including how they hunt for particles and pathogens, as well as the implications for phagocytosis, autophagy, and polarization from proinflammatory to prohealing phenotype. We further discuss how this knowledge can be harnessed in regenerative medicine and for the design of new drugs and immune-modulatory drug delivery systems, biomaterials, and tissue scaffolds.

Functionalized Enzyme-Responsive Biomaterials to Model Tissue Stiffening in vitro

The mechanical properties of the cellular microenvironment play a crucial role in modulating cell function, and many pathophysiological processes are accompanied by variations in extracellular matrix (ECM) stiffness. Lysyl oxidase (LOx) is one of the enzymes involved in several ECM-stiffening processes. Here, we engineered poly(ethylene glycol) (PEG)-based hydrogels with controlled mechanical properties in the range typical of soft tissues. These hydrogels were functionalized featuring free primary amines, which allows an additional chemical LOx-responsive behavior with increase in crosslinks and hydrogel elastic modulus, mimicking biological ECM-stiffening mechanisms. Hydrogels with elastic moduli in the range of 0.5-4 kPa were obtained after a first photopolymerization step. The increase in elastic modulus of the functionalized and enzyme-responsive hydrogels was also characterized after the second-step enzymatic reaction, recording an increase in hydrogel stiffness up to 0.5 kPa after incubation with LOx. Finally, hydrogel precursors containing HepG2 (bioinks) were used to form three-dimensional (3D) in vitro models to mimic hepatic tissue and test PEG-based hydrogel biocompatibility. Hepatic functional markers were measured up to 7 days of culture, suggesting further use of such 3D models to study cell mechanobiology and response to dynamic variation of hydrogels stiffness. The results show that the functionalized hydrogels presented in this work match the mechanical properties of soft tissues, allow dynamic variations of hydrogel stiffness, and can be used to mimic changes in the microenvironment properties of soft tissues typical of inflammation and pathological changes at early stages (e.g., fibrosis, cancer).

A multilayer micromechanical elastic modulus measuring method in ex vivo human aneurysmal abdominal aortas

Abdominal aortic aneurysms (AAA) are common and potentially life-threatening aortic dilatations, due to the effect of hemodynamic changes on the aortic wall. Previous research has shown a potential pathophysiological role for increased macroscopic aneurysmal wall stiffness; however, not investigating micromechanical stiffness. We aimed to compile a new protocol to examine micromechanical live aortic stiffness (elastic moduli), correlated to histological findings with quantitative immunofluorescence (QIF). Live AAA biopsies (n = 7) and non-dilated aortas (controls; n = 3) were sectioned. Local elastic moduli of aortic intima, media and adventitia were analysed in the direction towards the lumen and vice versa with nanoindentation. Smooth muscle cells (SMC), collagen and fibroblasts were examined using QIF. Nanoindentation of AAA vs. controls demonstrated a 4-fold decrease in elastic moduli (p = 0.022) for layers combined and a 26-fold decrease (p = 0.017) for media-to-intima direction. QIF of AAA vs. controls revealed a 4-, 3- and 6-fold decrease of SMC, collagen and fibroblasts, respectively (p = 0.036). Correlations were found between bidirectional intima and media measurements (ρ = 0.661, p = 0.038) and all QIF analyses (ρ = 0.857-0.905, p = 0.002-0.007). We present a novel protocol to analyse microscopic elastic moduli in live aortic tissues using nanoindentation. Hence, our preliminary findings of decreased elastic moduli and altered wall composition warrant further microscopic stiffness investigation to potentially clarify AAA pathophysiology and to explore potential treatment by wall strengthening. STATEMENT OF SIGNIFICANCE: Although extensive research on the pathophysiology of dilated abdominal aortas (aneurysms) has been performed, the exact underlying pathways are still largely unclear. Previously, the macroscopic stiffness of the pathologic and healthy aortic wall has been studied. This study however, for the first time, studied the microscopic stiffness changes in live tissue of dilated and non-dilated abdominal aortas. This new protocol provides a device to analyse the alterations on cellular level within their microenvironment, whereas previous studies studied the aorta as a whole. Outcomes of these measurements might help to better understand the underlying origin of the incidence and progression of aneurysms and other aortic diseases.

Unraveling the Mechanobiology of Extracellular Matrix

Cells need to be anchored to extracellular matrix (ECM) to survive, yet the role of ECM in guiding developmental processes, tissue homeostasis, and aging has long been underestimated. How ECM orchestrates the deterioration of healthy to pathological tissues, including fibrosis and cancer, also remains poorly understood. Inquiring how alterations in ECM fiber tension might drive these processes is timely, as mechanobiology is a rapidly growing field, and many novel mechanisms behind the mechanical forces that can regulate protein, cell, and tissue functions have recently been deciphered. The goal of this article is to review how forces can switch protein functions, and thus cell signaling, and thereby inspire new approaches to exploit the mechanobiology of ECM in regenerative medicine as well as for diagnostic and therapeutic applications. Some of the mechanochemical switching concepts described here for ECM proteins are more general and apply to intracellular proteins as well.

A model for positive feedback control of the transformation of fibroblasts to myofibroblasts

The phenotypic conversion of normal fibroblasts to myofibroblasts is central to normal wound healing and to pathological fibrosis that can occur in the heart and many other tissues. The transformation occurs in two stages. The first stage is driven mainly by mechanical changes such as increased stiffness of the heart due to hypertension and cellular contractility. The second stage requires both increasing stiffness and biochemical factors such as the growth factor, TGFβ. As more and more cells convert from weakly contractile fibroblasts to strongly contractile myofibroblasts, the stiffness of the ventricular muscle increases. We propose a simple model for the establishment of non-equilibrium steady states with different compositions of fibroblasts and myofibroblasts. Under some conditions a positive feedback loop resulting from the increasing stiffness caused by increasing numbers of myofibroblasts can produce a bifurcation between steady states with low and high myofibroblast content. We illustrate the large mechanical differences between normal fibroblasts and myofibroblasts with measurements in engineered tissue constructs.

Regeneration of Dermis: Scarring and Cells Involved

There are many studies on certain skin cell specifications and their contribution to wound healing. In this review, we provide an overview of dermal cell heterogeneity and their participation in skin repair, scar formation, and in the composition of skin substitutes. The papillary, reticular, and hair follicle associated fibroblasts differ not only topographically, but also functionally. Human skin has a number of particular characteristics that are different from murine skin. This should be taken into account in experimental procedures. Dermal cells react differently to skin wounding, remodel the extracellular matrix in their own manner, and convert to myofibroblasts to different extents. Recent studies indicate a special role of papillary fibroblasts in the favorable outcome of wound healing and epithelial-mesenchyme interactions. Neofolliculogenesis can substantially reduce scarring. The role of hair follicle mesenchyme cells in skin repair and possible therapeutic applications is discussed. Participation of dermal cell types in wound healing is described, with the addition of possible mechanisms underlying different outcomes in embryonic and adult tissues in the context of cell population characteristics and extracellular matrix composition and properties. Dermal white adipose tissue involvement in wound healing is also overviewed. Characteristics of myofibroblasts and their activity in scar formation is extensively discussed. Cellular mechanisms of scarring and possible ways for its prevention are highlighted. Data on keloid cells are provided with emphasis on their specific characteristics. We also discuss the contribution of tissue tension to the scar formation as well as the criteria and effectiveness of skin substitutes in skin reconstruction. Special attention is given to the properties of skin substitutes in terms of cell composition and the ability to prevent scarring.

Fibrosis in tissue engineering and regenerative medicine: treat or trigger?

Fibrosis is a life-threatening pathological condition resulting from a dysfunctional tissue repair process. There is no efficient treatment and organ transplantation is in many cases the only therapeutic option. Here we review tissue engineering and regenerative medicine (TERM) approaches to address fibrosis in the cardiovascular system, the kidney, the lung and the liver. These strategies have great potential to achieve repair or replacement of diseased organs by cell- and material-based therapies. However, paradoxically, they might also trigger fibrosis. Cases of TERM interventions with adverse outcome are also included in this review. Furthermore, we emphasize the fact that, although organ engineering is still in its infancy, the advances in the field are leading to biomedically relevant in vitro models with tremendous potential for disease recapitulation and development of therapies. These human tissue models might have increased predictive power for human drug responses thereby reducing the need for animal testing.

Is there any advantage of using stand-alone cages? A numerical approach

Background

Segment fusion using interbody cages supplemented with pedicle screw fixation is the most common surgery for the treatment of low back pain. However, there is still much controversy regarding the use of cages in a stand-alone fashion. The goal of this work is to numerically compare the influence that each surgery has on lumbar biomechanics.

Methods

A non-linear FE model of the whole lumbar spine was developed to compare between two types of cages (OLYS and NEOLIF) with and without supplementary fixation. The motion of the whole spine was analysed and the biomechanical environment of the adjacent segments to the operated one was studied. Moreover, the risk of subsidence of the cages was qualitatively evaluated.

Results

A great ROM reduction occurred when supplementary fixation was used. This stiffening increased the stresses at the adjacent levels. It might be hypothesised that the overloading of these segments could be related with the clinically observed adjacent disc degeneration. Meanwhile, the stand-alone cages allowed for a wider movement, and therefore, the influence of the surgery on adjacent discs was much lower. Regarding the risk of subsidence, the contact pressure magnitude was similar for both intervertebral cage designs and near the value of the maximum tolerable pressure of the endplates.

Conclusions

A minimally invasive posterior insertion of an intervertebral cage (OLYS or NEOLIF) was compared using a stand-alone design or adding supplementary fixation. The outcomes of these two techniques were compared, and although stand-alone cage may diminish the risk of disease progression to the adjacent discs, the spinal movement in this case could compromise the vertebral fusion and might present a higher risk of cage subsidence.

Electronic supplementary material

The online version of this article (10.1186/s12938-019-0684-8) contains supplementary material, which is available to authorized users.

Acute and chronic demyelinated CNS lesions exhibit opposite elastic properties

Increased deposition of extracellular matrix (ECM) is a known inhibitor of axonal regrowth and remyelination. Recent in vitro studies have demonstrated that oligodendrocyte differentiation is impacted by the physical properties of the ECM. However, characterization of the mechanical properties of the healthy and injured CNS myelin is challenging, and has largely relied on non-invasive, low-resolution methods. To address this, we have employed atomic force microscopy to perform micro-indentation measurements of demyelinated tissue at cellular scale. Analysis of mouse and human demyelinated brains indicate that acute demyelination results in decreased tissue stiffness that recovers with remyelination; while chronic demyelination is characterized by increased tissue stiffness, which correlates with augmented ECM deposition. Thus, changes in the mechanical properties of the acutely (softer) or chronically (stiffer) demyelinated brain might contribute to differences in their regenerative capacity. Our findings are relevant to the optimization of cell-based therapies aimed at promoting CNS regeneration and remyelination.

Skin Changes During Ageing

The skin provides the primary protection for the body against external injuries and is essential in the maintenance of general homeostasis. During ageing, resident cells become senescent and the extracellular matrix, mainly in the dermis, is progressively damaged affecting the normal organization of the skin and its capacity for repair. In parallel, extrinsic factors such as ultraviolet irradiation, pollution, and intrinsic factors such as diabetes or vascular disease can further accelerate this phenomenon. Indeed, numerous mechanisms are involved in age-induced degradation of the skin and these also relate to non-healing or chronic wounds in the elderly. In particular, the generation of reactive oxygen species seems to play a major role in age-related skin modifications. Certainly, targeting both the hormonal status of the skin or its surface nutrition can slow down age-induced degradation of the skin and improve healing of skin damage in the elderly. Skin care regimens that prevent radiation and pollution damage, and reinforce the skin surface and its microbiota are among the different approaches able to minimize the effects of ageing on the skin.

Effect of N-(2-aminoethyl) ethanolamine on hypertrophic scarring changes in vitro: Finding novel anti-fibrotic therapies

Hypertrophic scars (HS) limit movement, decrease quality of life, and remain a major impediment to rehabilitation from burns. However, no effective pharmacologic therapies for HS exist. Here we tested the in vitro anti-fibrotic effects of the novel chemical N-(2-aminoethyl) ethanolamine (AEEA) at non-toxic concentrations. Scanning electron microscopy showed that AEEA markedly altered the structure of the extracellular matrix (ECM) produced by primary dermal fibroblasts isolated from a HS of a burn patient (HTS). Compression atomic force microscopy revealed that AEEA stiffened the 3D nanostructure of ECM formed by HTS fibroblasts. Western blot analysis in three separate types of primary human dermal fibroblasts (including HTS) showed that AEEA exposure increased the extractability of type I collagen in a dose- and time-dependent fashion, while not increasing collagen synthesis. A comparison of the electrophoretic behavior of the same set of samples under native and denaturing conditions suggested that AEEA alters the 3D structure of type I collagen. The antagonization effect of AEEA to TGF-β1 on ECM formation was also observed. Furthermore, analyses of the anti-fibrotic effects of analogs of AEEA (with modified pharmacophores) suggest the existence of a chemical structure-activity relationship. Thus, AEEA and its analogs may inhibit HS development; further study and optimization of analogs may be a promising strategy for the discovery for effective HS therapies.

Bioinspired liver scaffold design criteria

Maintaining hepatic functional characteristics in-vitro is considered one of the main challenges in engineering liver tissue. As hepatocytes cultured ex-vivo are deprived of their native extracellular matrix (ECM) milieu, developing scaffolds that mimic the biomechanical and physicochemical properties of the native ECM is thought to be a promising approach for successful tissue engineering and regenerative medicine applications. On the basis that the decellularized liver matrix represents the ideal design template for engineering bioinspired hepatic scaffolds, to derive quantitative descriptors of liver ECM architecture, we characterised decellularised liver matrices in terms of their biochemical, viscoelastic and structural features along with porosity, permeability and wettability. Together, these data provide a unique set of quantitative design criteria which can be used to generate guidelines for fabricating biomaterial scaffolds for liver tissue engineering. As proof-of-concept, we investigated hepatic cell response to substrate viscoelasticity. On collagen hydrogels mimicking decellularised liver mechanics, cells showed superior morphology, higher viability and albumin secretion than on stiffer and less viscous substrates. Although scaffold properties are generally inspired by those of native tissues, our results indicate significant differences between the mechano-structural characteristics of untreated and decellularised hepatic tissue. Therefore, we suggest that design rules - such as mechanical properties and swelling behaviour - for engineering biomimetic scaffolds be re-examined through further studies on substrates matching the features of decellularized liver matrices.

Stem Cell Differentiation is Regulated by Extracellular Matrix Mechanics

Stem cells mechanosense the stiffness of their microenvironment, which impacts differentiation. Although tissue hydration anti-correlates with stiffness, extracellular matrix (ECM) stiffness is clearly transduced into gene expression via adhesion and cytoskeleton proteins that tune fates. Cytoskeletal reorganization of ECM can create heterogeneity and influence fates, with fibrosis being one extreme.

Manual Mobilization of Subcutaneous Fibrosis in Mice

OBJECTIVE

The aim of this study was to induce the remodeling of subcutaneous fibrosis in mice by the manual mobilization of skin and subcutaneous tissue.

METHODS

Seven days after the induction of subcutaneous fibrosis, mice were divided into 3 groups: control, stretch, and manual mobilization. Stretch was achieved by elongating the trunk, and manual mobilization was achieved by using the indicator fingertip of both hands, side by side, touching the back and performing a brief stretch. Stretch or manual mobilization was performed once a day for 7 days.

RESULTS

Fibrosis was present in the subcutaneous tissue of control animals, whereas brief stretch and manual mobilization were found to reduce fibrosis.

CONCLUSIONS

Mechanical stimulation through manual mobilization, or brief stretching, reduced subcutaneous fibrosis after tissue injury.

In wound repair vimentin mediates the transition of mesenchymal leader cells to a myofibroblast phenotype

Following injury, mesenchymal repair cells are activated to function as leader cells that modulate wound healing. These cells have the potential to differentiate to myofibroblasts, resulting in fibrosis and scarring. The signals underlying these differing pathways are complex and incompletely understood. The ex vivo mock cataract surgery cultures are an attractive model with which to address this question. With this model we study, concurrently, the mechanisms that control mesenchymal leader cell function in injury repair within their native microenvironment and the signals that induce this same cell population to acquire a myofibroblast phenotype when these cells encounter the environment of the adjacent tissue culture platform. Here we show that on injury, the cytoskeletal protein vimentin is released into the extracellular space, binds to the cell surface of the mesenchymal leader cells located at the wound edge in the native matrix environment, and supports wound closure. In profibrotic environments, the extracellular vimentin pool also links specifically to the mesenchymal leader cells and has an essential role in signaling their fate change to a myofibroblast. These findings suggest a novel role for extracellular, cell-surface–associated vimentin in mediating repair-cell function in wound repair and in transitioning these cells to a myofibroblast phenotype.

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.

Mechanoregulation of Myofibroblast Fate and Cardiac Fibrosis

During myocardial infarction, myocytes die and are replaced by a specialized fibrotic extracellular matrix, otherwise known as scarring. Fibrotic scarring presents a tremendous hemodynamic burden on the heart, as it creates a stiff substrate, which resists diastolic filling. Fibrotic mechanisms result in permanent scarring which often leads to hypertrophy, arrhythmias, and a rapid progression to failure. Despite the deep understanding of fibrosis in other tissues, acquired through previous investigations, the mechanisms of cardiac fibrosis remain unclear. Recent studies suggest that biochemical cues as well as mechanical cues regulate cells in myocardium. However, the steps in myofibroblast transdifferentiation, as well as the molecular mechanisms of such transdifferentiation in vivo, are poorly understood. This review is focused on defining myofibroblast physiology, scar mechanics, and examining current findings of myofibroblast regulation by mechanical stress, stiffness, and topography for understanding fibrotic disease dynamics.

The chemokines CXCL12 and CXCL14 differentially regulate connective tissue markers during limb development

Connective tissues (CT) support and connect organs together. Understanding the formation of CT is important, as CT deregulation leads to fibrosis. The identification of CT specific markers has contributed to a better understanding of CT function during development. In developing limbs, Osr1 transcription factor is involved in the differentiation of irregular CT while the transcription factor Scx labels tendon. In this study, we show that the CXCL12 and CXCL14 chemokines display distinct expression pattern in limb CT during chick development. CXCL12 positively regulates the expression of OSR1 and COL3A1, a collagen subtype of irregular CT, while CXCL14 activates the expression of the tendon marker SCX. We provide evidence that the CXCL12 effect on irregular CT involves CXCR4 receptor and vessels. In addition, the expression of CXCL12, CXCL14 and OSR genes is suppressed by the anti-fibrotic BMP signal. Finally, mechanical forces, known to be involved in adult fibrosis, control the expression of chemokines, CT-associated transcription factors and collagens during limb development. Such unexpected roles of CXCL12 and CXCL14 chemokines during CT differentiation can contribute to a better understanding of the fibrosis mechanisms in adult pathological conditions.

Engineering a highly elastic human protein-based sealant for surgical applications

Surgical sealants have been used for sealing or reconnecting ruptured tissues but often have low adhesion, inappropriate mechanical strength, cytotoxicity concerns, and poor performance in biological environments. To address these challenges, we engineered a biocompatible and highly elastic hydrogel sealant with tunable adhesion properties by photocrosslinking the recombinant human protein tropoelastin. The subcutaneous implantation of the methacryloyl-substituted tropoelastin (MeTro) sealant in rodents demonstrated low toxicity and controlled degradation. All animals survived surgical procedures with adequate blood circulation by using MeTro in an incisional model of artery sealing in rats, and animals showed normal breathing and lung function in a model of surgically induced rat lung leakage. In vivo experiments in a porcine model demonstrated complete sealing of severely leaking lung tissue in the absence of sutures or staples, with no clinical or sonographic signs of pneumothorax during 14 days of follow-up. The engineered MeTro sealant has high potential for clinical applications because of superior adhesion and mechanical properties compared to commercially available sealants, as well as opportunity for further optimization of the degradation rate to fit desired surgical applications on different tissues.

Matrix Mechanosensing: From Scaling Concepts in 'Omics Data to Mechanisms in the Nucleus, Regeneration, and Cancer

Many of the most important molecules of life are polymers. In animals, the most abundant of the proteinaceous polymers are the collagens, which constitute the fibrous matrix outside cells and which can also self-assemble into gels. The physically measurable stiffness of gels, as well as tissues, increases with the amount of collagen, and cells seem to sense this stiffness. An understanding of this mechanosensing process in complex tissues, including fibrotic disease states with high collagen, is now utilizing 'omics data sets and is revealing polymer physics-type, nonlinear scaling relationships between concentrations of seemingly unrelated biopolymers. The nuclear structure protein lamin A provides one example, with protein and transcript levels increasing with collagen 1 and tissue stiffness, and with mechanisms rooted in protein stabilization induced by cytoskeletal stress. Physics-based models of fibrous matrix, cytoskeletal force dipoles, and the lamin A gene circuit illustrate the wide range of testable predictions emerging for tissues, cell cultures, and even stem cell-based tissue regeneration. Beyond the epigenetics of mechanosensing, the scaling in cancer of chromosome copy number variations and other mutations with tissue stiffness suggests that genomic changes are occurring by mechanogenomic processes that now require elucidation.

A highly adhesive and naturally derived sealant

Conventional surgical techniques to seal and repair defects in highly stressed elastic tissues are insufficient. Therefore, this study aimed to engineer an inexpensive, highly adhesive, biocompatible, and biodegradable sealant based on a modified and naturally derived biopolymer, gelatin methacryloyl (GelMA). We tuned the degree of gelatin modification, prepolymer concentration, photoinitiator concentration, and crosslinking conditions to optimize the physical properties and adhesion of the photocrosslinked GelMA sealants. Following ASTM standard tests that target wound closure strength, shear resistance, and burst pressure, GelMA sealant was shown to exhibit adhesive properties that were superior to clinically used fibrin- and poly(ethylene glycol)-based glues. Chronic in vivo experiments in small as well as translational large animal models proved GelMA to effectively seal large lung leakages without the need for sutures or staples, presenting improved performance as compared to fibrin glue, poly(ethylene glycol) glue and sutures only. Furthermore, high biocompatibility of GelMA sealant was observed, as evidenced by a low inflammatory host response and fast in vivo degradation while allowing for adequate wound healing at the same time. Combining these results with the low costs, ease of synthesis and application of the material, GelMA sealant is envisioned to be commercialized not only as a sealant to stop air leakages, but also as a biocompatible and biodegradable hydrogel to support lung tissue regeneration.

Micro-Mechanical Viscoelastic Properties of Crosslinked Hydrogels Using the Nano-Epsilon Dot Method

Engineering materials that recapitulate pathophysiological mechanical properties of native tissues in vitro is of interest for the development of biomimetic organ models. To date, the majority of studies have focused on designing hydrogels for cell cultures which mimic native tissue stiffness or quasi-static elastic moduli through a variety of crosslinking strategies, while their viscoelastic (time-dependent) behavior has been largely ignored. To provide a more complete description of the biomechanical environment felt by cells, we focused on characterizing the micro-mechanical viscoelastic properties of crosslinked hydrogels at typical cell length scales. In particular, gelatin hydrogels crosslinked with different glutaraldehyde (GTA) concentrations were analyzed via nano-indentation tests using the nano-epsilon dot method. The experimental data were fitted to a Maxwell Standard Linear Solid model, showing that increasing GTA concentration results in increased instantaneous and equilibrium elastic moduli and in a higher characteristic relaxation time. Therefore, not only do gelatin hydrogels become stiffer with increasing crosslinker concentration (as reported in the literature), but there is also a concomitant change in their viscoelastic behavior towards a more elastic one. As the degree of crosslinking alters both the elastic and viscous behavior of hydrogels, caution should be taken when attributing cell response merely to substrate stiffness, as the two effects cannot be decoupled.

Cartilage canals in the distal intermediate ridge of the tibia of fetuses and foals are surrounded by different types of collagen

Some epiphyseal growth cartilage canals are surrounded by a ring of hypereosinophilic matrix consisting of collagen type I. Absence of the collagen type I ring may predispose canal vessels to failure and osteochondrosis, which can lead to fragments in joints (osteochondrosis dissecans). It is not known whether the ring develops in response to programming or biomechanical force. The distribution that may reveal the function of the ring has only been described in the distal femur of a limited number of foals. It is also not known which cells are responsible for producing the collagen ring. The aims of the current study were to examine fetuses and foals to infer whether the ring forms in response to biomechanical force or programming, to describe distribution and to investigate which cell type produces the ring. The material consisted of 46 fetuses and foals from 293 days of gestation to 142 days old, of both sexes and different breeds, divided into three groups, designated the naïve group up to and including the day of birth, the adapting group from 2 days up to and including 14 days old, and the loaded group from 15 days and older. The distal tibia was sawn into parasagittal slabs and the cranial half of the central slab from the intermediate ridge was examined by light microscopy and immunohistochemical staining for collagen type I. Presence, completeness and location of the collagen ring was compared, as was the quantity of perivascular mesenchymal cells. An eosinophilic ring present on HE-stained sections was seen in every single fetus and foal examined, which corresponded to collagen type I in immunostained sections. A higher proportion of cartilage canals were surrounded by an eosinophilic ring in the naïve and adapting groups at 73 and 76%, respectively, compared with the loaded group at 51%. When considering only patent canals, the proportion of canals with an eosinophilic ring was higher in the adapting and loaded than the naïve group of foals. The ring was present around 90 and 81% of patent canals in the deep and middle layers, respectively, compared with 58% in the superficial layer, and the ring was more often complete around deep compared with superficial canals. The ring was absent or partial around chondrifying canals. When an eosinophilic ring was present around patent canals, it was more common for the canal to contain one or more layers of perivascular mesenchymal cells rather than few to no layers. It was also more common for the collagen ring to be more complete around canals that contained many as opposed to few mesenchymal cells. In conclusion, the proportion of cartilage canals that had an eosinophilic ring was similar in all three groups of fetuses and foals, indicating that the presence of the collagen ring was mostly programmed, although some adaptation was evident. The ring was more often present around deep, compared with superficial canals, indicating a role in preparation for ossification. The collagen ring appeared to be produced by perivascular mesenchymal cells.

Molecular and Cellular Mechanisms of Atrial Fibrosis in Atrial Fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. Atrial fibrosis has emerged as an important pathophysiological contributor and has been linked to AF recurrences, resistance to therapy and complications. Here, the author reviews the molecular and cellular mechanisms that control atrial fibrosis. It is important to note that not all tissue fibrosis is identical. For example, reactive (interstitial) fibrosis increases the amount of collagen between cardiac muscle bundles without fundamentally altering muscle bundle architecture. Replacement (reparative) fibrosis replaces dead cardiomyocytes with extracellular matrix tissue and fibroblasts, preserving tissue integrity at the expense of muscle bundle continuity. Replacement fibrosis may be much more disruptive to electric conduction and more difficult to reverse than reactive fibrosis. The author reviews the complex signaling systems that cause fibrosis, including those connected to connective tissue growth factor, angiotensin-II, platelet-derived growth factor, and transforming growth factor-β. The author then considers the molecular constitution of fibrous tissue, including the production and maturation of collagen and the roles of important extracellular matrix proteins such as fibronectin, tenascin-C, and thrombospodin-1. The author then discusses the evolving evidence for an important role of Ca²⁺ entry in the profibrotic activation of fibroblasts, along with evidence that dysregulation of Ca²⁺-transporting transient potential receptor channels and inward rectifier K⁺ channels in AF fibroblasts is profibrotic. Finally, the author reviews the evidence for micro-ribonucleic acid involvement in atrial fibrotic signaling and AF promotion. It is hoped that an improved understanding of the mechanisms controlling atrial fibrosis will open up new opportunities for AF prevention and management.

Review: bioreactor design towards generation of relevant engineered tissues: focus on clinical translation

In tissue engineering and regenerative medicine, studies that utilize 3D scaffolds for generating voluminous tissues are mostly confined in the realm of in vitro research and preclinical animal model testing. Bioreactors offer an excellent platform to grow and develop 3D tissues by providing conditions that mimic their native microenvironment. Aligning the bioreactor development process with a focus on patient care will aid in the faster translation of the bioreactor technology to clinics. In this review, we discuss the various factors involved in the design of clinically relevant bioreactors in relation to their respective applications. We explore the functional relevance of tissue grafts generated by bioreactors that have been designed to provide physiologically relevant mechanical cues on the growing tissue. The review discusses the recent trends in non-invasive sensing of the bioreactor culture conditions. It provides an insight to the current technological advancements that enable in situ, non-invasive, qualitative and quantitative evaluation of the tissue grafts grown in a bioreactor system. We summarize the emerging trends in commercial bioreactor design followed by a short discussion on the aspects that hamper the 'push' of bioreactor systems into the commercial market as well as 'pull' factors for stakeholders to embrace and adopt widespread utility of bioreactors in the clinical setting. Copyright © 2017 John Wiley & Sons, Ltd.

Stiffness-Controlled Thermoresponsive Hydrogels for Cell Harvesting with Sustained Mechanical Memory

Most mechanobiological investigations focused on in situ mechanical regulation of cells on stiffness-controlled substrates with few downstream applications, as it is still challenging to harvest and expand mechanically primed cells by enzymatic digestion (e.g., trypsin) without interrupting cellular mechanical memory between passages. This study develops thermoresponsive hydrogels with controllable stiffness to generate mechanically primed cells with intact mechanical memory for augmented wound healing. No significant cellular property alteration of the fibroblasts primed on thermoresponsive hydrogels with varied stiffness has been observed through thermoresponsive harvesting. When reseeding the harvested cells for further evaluation, softer hydrogels are proven to better sustain the mechanical priming effects compared to rigid tissue culture plate, which indicates that both the stiffness-controlled substrate and thermoresponsive harvesting are required to sustain cellular mechanical memory between passages. Moreover, epigenetics analysis reveals that thermoresponsive harvesting could reduce the rearrangement and loss of chromatin proteins compared to that of trypsinization. In vivo wound healing using mechanically primed fibroblasts shows featured epithelium and sebaceous glands, which indicates augmented skin recovery compared with trypsinized fibroblasts. Thus, the thermoresponsive hydrogel-based cell harvesting system offers a powerful tool to investigate mechanobiology between cell passages and produces abundant cells with tailored mechanical priming properties for cell-based applications.

Sample, testing and analysis variables affecting liver mechanical properties: A review

Given the critical role of liver mechanics in regulating cell response and directing the development of tissue fibrosis, accurately characterising its mechanical behaviour is of relevance for both diagnostic purposes as well as for tissue engineering and for the development of in-vitro models. Determining and quantifying the mechanical behaviour of soft biological tissues is, however, highly challenging due to their intrinsic labile nature. Indeed, a unique set of values of liver mechanical properties is still lacking to date; testing conditions can significantly affect sample status and hence the measured behaviour and reported results are strongly dependent on the adopted testing method and configuration as well as sample type and status. This review aims at summarising the bulk mechanical properties of liver described in the literature, discussing the possible sources of variation and their implications on the reported results. We distinguish between the intrinsic mechanical behaviour of hepatic tissue, which depends on sample variables, and the measured mechanical properties which also depend on the testing and analysis methods. Finally, the review provides guidelines on tissue preparation and testing conditions for generating reproducible data which can be meaningfully compared across laboratories.

STATEMENT OF SIGNIFICANCE

Soft tissue mechanics is widely investigated, but poorly understood. This review identifies and discusses sample and testing variables which can influence the mechanical behaviour of hepatic tissue and consequently the measured mechanical properties. To encourage the biomaterial community towards more standardized testing of soft tissues and enable comparisons between data from different laboratories, we have established new testing methods and experimental recommendations for sample preparation and testing. The review could be of wide interest to scientists involved in biomaterials research because it addresses and proposes guidelines for several issues related to the mechanical testing of soft tissues whose implications have not been considered together before.