Early stage fatigue damage occurs in bovine tendon fascicles in the absence of changes in mechanics at either the gross or micro-structural level

Multimodal and multiscale characterization reveals how tendon structure and mechanical response are altered by reduced loading

Tendons are collagen-based connective tissues where the composition, structure and mechanics respond and adapt to the local mechanical environment. Adaptation to prolonged inactivity can result in stiffer tendons that are more prone to injury. However, the complex relation between reduced loading, structure, and mechanical performance is still not fully understood. This study combines mechanical testing with high-resolution synchrotron X-ray imaging, scattering techniques and histology to elucidate how reduced loading affects the structural properties and mechanical response of rat Achilles tendons on multiple length scales. The results show that reduced in vivo loading leads to more crimped and less organized fibers and this structural inhomogeneity could be the reason for the altered mechanical response. Unloading also seems to change the fibril response, possibly by altering the strain partitioning between hierarchical levels, and to reduce cell density. This study elucidates the relation between in vivo loading, the Achilles tendon nano-, meso‑structure and mechanical response. The results provide fundamental insights into the mechanoregulatory mechanisms guiding the intricate biomechanics, tissue structural organization, and performance of complex collagen-based tissues. STATEMENT OF SIGNIFICANCE: Achilles tendon properties allow a dynamic interaction between muscles and tendon and influence force transmission during locomotion. Lack of physiological loading can have dramatic effects on tendon structure and mechanical properties. We have combined the use of cutting-edge high-resolution synchrotron techniques with mechanical testing to show how reduced loading affects the tendon on multiple hierarchical levels (from nanoscale up to whole organ) clarifying the relation between structural changes and mechanical performance. Our findings set the first step to address a significant healthcare challenge, such as the design of tailored rehabilitations that take into consideration structural changes after tendon immobilization.

Review of human supraspinatus tendon mechanics. Part I: fatigue damage accumulation and failure

Repetitive stress injuries to the rotator cuff, and particularly the supraspinatus tendon (SST), are highly prevalent and debilitating. These injuries typically occur through the application of cyclic load below the threshold necessary to cause acute tears, leading to accumulation of incremental damage that exceeds the body's ability to heal, resulting in decreased mechanical strength and increased risk of frank rupture at lower loads. Consistent progression of fatigue damage across multiple model systems suggests a generalized tendon response to overuse. This finding may allow for interventions before gross injury of the SST occurs. Further research into the human SST response to fatigue loading is necessary to characterize the fatigue life of the tendon, which will help determine the frequency, duration, and magnitude of load spectra the SST may experience before injury. Future studies may allow in vivo SST strain analysis during specific activities, generation of a human SST stress-cycle curve, and characterization of damage and repair related to repetitive tasks.

Review of human supraspinatus tendon mechanics. Part II: tendon healing response and characterization of tendon health

Overuse injuries of the rotator cuff, particularly of the supraspinatus tendon (SST), are highly prevalent and debilitating in work, sport, and daily activities. Despite the clinical significance of these injuries, there remains a large degree of uncertainty regarding the pathophysiology of injury, optimal methods of nonoperative and operative repair, and how to adequately assess tendon injury and healing. The tendon response to fatigue damage resulting from overuse is different from that of acute rupture and results in either an adaptive (healing) or a maladaptive (degenerative) response. Factors associated with the degenerative response include increasing age, smoking, hypercholesterolemia, biological sex (variable by tendon), diabetes mellitus, and excessive load post fatigue damage. After injury, the average healing rate of tendon is approximately 1% per day and may be significantly influenced by biologic sex (females have lower collagen synthesis rates) and excessive load after damage. Although magnetic resonance imaging (MRI) is considered the gold standard in assessing acute tears as well as tendinopathic change in the SST, ultrasonography has proven to be a valuable tool to measure tendinopathic change in real time. Ultrasonography can determine multiple mechanical and structural parameters of the SST that are altered in fatigue loading. Thus, ultrasonography may be utilized to understand how these parameters change in response to SST overuse, and may aid in determining the activity level that places the SST at greater risk of rupture.

Dynamic Load Model Systems of Tendon Inflammation and Mechanobiology

Dynamic loading is a shared feature of tendon tissue homeostasis and pathology. Tendon cells have the inherent ability to sense mechanical loads that initiate molecular-level mechanotransduction pathways. While mature tendons require physiological mechanical loading in order to maintain and fine tune their extracellular matrix architecture, pathological loading initiates an inflammatory-mediated tissue repair pathway that may ultimately result in extracellular matrix dysregulation and tendon degeneration. The exact loading and inflammatory mechanisms involved in tendon healing and pathology is unclear although a precise understanding is imperative to improving therapeutic outcomes of tendon pathologies. Thus, various model systems have been designed to help elucidate the underlying mechanisms of tendon mechanobiology via mimicry of the in vivo tendon architecture and biomechanics. Recent development of model systems has focused on identifying mechanoresponses to various mechanical loading platforms. Less effort has been placed on identifying inflammatory pathways involved in tendon pathology etiology, though inflammation has been implicated in the onset of such chronic injuries. The focus of this work is to highlight the latest discoveries in tendon mechanobiology platforms and specifically identify the gaps for future work. An interdisciplinary approach is necessary to reveal the complex molecular interplay that leads to tendon pathologies and will ultimately identify potential regenerative therapeutic targets.

The Role of the Non-Collagenous Extracellular Matrix in Tendon and Ligament Mechanical Behavior: A Review

Tendon is a connective tissue that transmits loads from muscle to bone, while ligament is a similar tissue that stabilizes joint articulation by connecting bone to bone. The 70-90% of tendon and ligament's extracellular matrix (ECM) is composed of a hierarchical collagen structure that provides resistance to deformation primarily in the fiber direction, and the remaining fraction consists of a variety of non-collagenous proteins, proteoglycans, and glycosaminoglycans (GAGs) whose mechanical roles are not well characterized. ECM constituents such as elastin, the proteoglycans decorin, biglycan, lumican, fibromodulin, lubricin, and aggrecan and their associated GAGs, and cartilage oligomeric matrix protein (COMP) have been suggested to contribute to tendon and ligament's characteristic quasi-static and viscoelastic mechanical behavior in tension, shear, and compression. The purpose of this review is to summarize existing literature regarding the contribution of the non-collagenous ECM to tendon and ligament mechanics, and to highlight key gaps in knowledge that future studies may address. Using insights from theoretical mechanics and biology, we discuss the role of the non-collagenous ECM in quasi-static and viscoelastic tensile, compressive, and shear behavior in the fiber direction and orthogonal to the fiber direction. We also address the efficacy of tools that are commonly used to assess these relationships, including enzymatic degradation, mouse knockout models, and computational models. Further work in this field will foster a better understanding of tendon and ligament damage and healing as well as inform strategies for tissue repair and regeneration.

A Novel, Open Source, Low-Cost Bioreactor for Load-Controlled Cyclic Loading of Tendon Explants

A major risk factor for tendinopathy is tendon overuse (i.e., fatigue loading). Fatigue loading of tendon damages the extracellular matrix and induces tissue degeneration. However, the specific mechanisms linking tendon fatigue damage with tissue degeneration are unclear. While explant models of tendon fatigue loading have been used to address this knowledge gap, they predominantly employ bioreactors that apply cyclic displacements/strains rather than loads/stresses, which are more physiologically relevant. This is because of the technical complexity and cost of building a load-controlled bioreactor, which requires multiple motors, load cells, and computationally intensive feedback loops. Here, we present a novel, low-cost, load-controlled bioreactor that applies cyclic loading to multiple tendon explants by offloading weights from a single motorized stage. Using an optional load cell, we validated that the bioreactor can effectively provide load-controlled fatigue testing of mouse and rat tendon explants while maintaining tissue viability. Furthermore, all the design files, bill of materials, and operating software are available "open source" (https://github.com/Szczesnytendon/Bioreactor) so that anyone can easily manufacture and use the bioreactor for their own research. Therefore, this novel load-controlled bioreactor will enable researchers to study the mechanisms driving fatigue-induced tendon degeneration in a more physiologically relevant and cost-effective manner.

Quantitative Assessment of Tendon Hierarchical Structure by Combined Second Harmonic Generation and Immunofluorescence Microscopy

Histological evaluation of healing tendons is primarily focused on monitoring restoration of longitudinal collagen alignment, although the elastic property of energy-storing flexor tendons is largely attributed to interfascicular sliding facilitated by the interfascicular matrix (IFM). The objectives of this study were to explore the utility of second harmonic generation (SHG) imaging to objectively assess cross-sectional tendon fascicle architecture, to combine SHG microscopy with elastin immunofluorescence to assess the ultrastructure of collagen and elastin in longitudinal and transverse sections, and lastly, to quantify changes in IFM elastin and fascicle collagen alignment of normal and collagenase-injured flexor tendons. Paraffin-embedded transverse and longitudinal histological sections (10-μm thickness) derived from normal and collagenase-injured (6- and 16-week time-points) equine superficial digital flexor tendons were de-paraffinized, treated with Tris EDTA at 80°C for epitope retrieval, and incubated with mouse monoclonal anti-elastin antibody (1:100 dilution) overnight. Anti-mouse IgG Alexa Flour 546 secondary antibody was applied, and sections were mounted with ProLong Gold reagent with 4',6-diamidino-2-phenylindole (DAPI). Nuclei (DAPI) and elastin (Alexa Fluor 546) signals were captured by using standard confocal imaging with 405 and 543 nm excitation wavelengths, respectively. The SHG signal was captured by using a tunable Ti:Sapphire laser tuned to 950 nm to visualize type I collagen. Quantitative measurements of fascicle cross-sectional area (CSA), IFM thickness in transverse SHG-DAPI merged z-stacks, fascicle/IFM elastin area fraction (%), and elastin-collagen alignment in longitudinal SHG-elastin merged z-stacks were conducted by using ImageJ software. Using this methodology, fascicle CSA, IFM thickness, and IFM elastin area fraction (%) at 6 weeks (∼2.25-fold; ∼2.8-fold; 60% decrease; p < 0.001) and 16 weeks (∼2-fold; ∼1.5-fold; 70% decrease; p < 0.001) after collagenase injection, respectively, were found to be significantly different from normal tendon. IFM elastin and fascicle collagen alignment characterized via fast Fourier transform (FFT) frequency plots at 16 weeks demonstrated that collagen re-alignment was more advanced than that of elastin. The integration of SHG-derived quantitative measurements in transverse and longitudinal tendon sections supports comprehensive assessment of tendon structure. Our findings demonstrate the importance of including IFM and non-collagenous proteins in tendon histological evaluations, tasks that can be effectively carried out by using SHG and immunofluorescence microscopy. Impact statement This work demonstrated that second harmonic generation microscopy in conjunction with elastin immunofluorescence provided a comprehensive assessment of multiscale structural re-organization in healing tendon than when restricted to longitudinal collagen fiber alignment alone. Utilizing this approach for tendon histomorphometry is ideal not only to improve our understanding of hierarchical structural changes that occur after tendon injury and during remodeling but also to monitor the efficacy of therapeutic approaches.

Tendon explant models for physiologically relevant invitro study of tissue biology - a perspective

Background: Tendon disorders increasingly afflict our aging society but we lack the scientific understanding to clinically address them. Clinically relevant models of tendon disease are urgently needed as established small animal models of tendinopathy fail to capture essential aspects of the disease. Two-dimensional and three-dimensional cell and tissue culture models are similarly limited, lacking many physiological extracellular matrix cues required to maintain tissue homeostasis or guide matrix remodeling. These cues reflect the biochemical and biomechanical status of the tissue, and encode information regarding the mechanical and metabolic competence of the tissue. Tendon explants overcome some of these limitations and have thus emerged as a valuable tool for the discovery and study of mechanisms associated with tendon homeostasis and pathophysiology. Tendon explants retain native cell-cell and cell-matrix connections, while allowing highly reproducible experimental control over extrinsic factors like mechanical loading and nutritional availability. In this sense tendon explant models can deliver insights that are otherwise impossible to obtain from in vivo animal or in vitro cell culture models. Purpose: In this review, we aimed to provide an overview of tissue explant models used in tendon research, with a specific focus on the value of explant culture systems for the controlled study of the tendon core tissue. We discuss their advantages, limitations and potential future utility. We include suggestions and technical recommendations for the successful use of tendon explant cultures and conclude with an outlook on how explant models may be leveraged with state-of-the-art biotechnologies to propel our understanding of tendon physiology and pathology.

Tendon tissue microdamage and the limits of intrinsic repair

The transmission of mechanical muscle force to bone for musculoskeletal stability and movement is one of the most important functions of tendon. The load-bearing tendon core is composed of highly aligned collagen-rich fascicles interspersed with stromal cells (tenocytes). Despite being built to bear very high mechanical stresses, supra-physiological/repetitive mechanical overloading leads to tendon microdamage in fascicles, and potentially to tendon disease and rupture. To date, it is unclear to what extent intrinsic healing mechanisms of the tendon core compartment can repair microdamage. In the present study, we investigated the healing capacity of the tendon core compartment in an ex vivo tissue explant model. To do so, we isolated rat tail tendon fascicles, damaged them by applying a single stretch to various degrees of sub-rupture damage and longitudinally assessed downstream functional and structural changes over a period of several days. Functional damage was assessed by changes in the elastic modulus of the material stress-strain curves, and biological viability of the resident tenocytes. Structural damage was quantified using a fluorescent collagen hybridizing peptide (CHP) to label mechanically disrupted collagen structures. While we observed functional mechanical damage for strains above 2% of the initial fascicle length, structural collagen damage was only detectable for 6% strain and beyond. Minimally loaded/damaged fascicles (2-4% strain) progressively lost elastic modulus over the course of tissue culture, despite their collagen structures remaining intact with high degree of maintained cell viability. In contrast, more severely overloaded fascicles (6-8% strain) with damage at the molecular/collagen level showed no further loss of the elastic modulus but markedly decreased cell viability. Surprisingly, in these heavily damaged fascicles the elastic modulus partially recovered, an effect also seen in further experiments on devitalized fascicles, implying the possibility of a non-cellular but matrix-driven mechanism of molecular repair. Overall, our findings indicate that the tendon core has very little capacity for self-repair of microdamage. We conclude that stromal tenocytes likely do not play a major role in anabolic repair of tendon matrix microdamage, but rather mediate catabolic matrix breakdown and communication with extrinsic cells that are able to effect tissue repair.

Load transfer, damage, and failure in ligaments and tendons

The function of ligaments and tendons is to support and transmit loads applied to the musculoskeletal system. These tissues are often able to perform their function for many decades; however, connective tissue disease and injury can compromise ligament and tendon integrity. A range of protein and non-protein constituents, combined in a complex structural hierarchy from the collagen molecule to the tissue and covering nanometer to centimeter length scales, govern tissue function, and impart characteristic non-linear material behavior. This review summarizes the structure of ligaments and tendons, the roles of their constituent components for load transfer across the hierarchy of structure, and the current understanding of how damage occurs in these tissues. Disease and injury can alter the constituent make-up and structural organization of ligaments and tendons, affecting tissue function, while also providing insight to the role and interactions of individual constituents. The studies and techniques presented here have helped to understand the relationship between tissue constituents and the physical mechanisms (e.g., stretching, sliding) that govern material behavior at and between length scales. In recent years, new techniques have been developed to probe ever smaller length scales and may help to elucidate mechanisms of load transfer and damage and the molecular constituents involved in the in the earliest stages of ligament and tendon damage. A detailed understanding of load transfer and damage from the molecular to the tissue level may elucidate targets for the treatment of connective tissue diseases and inform practice to prevent and rehabilitate ligament and tendon injuries. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3093-3104, 2018.

Ultrastructure of tendon rupture depends on strain rate and tendon type

Previous research has shown that both the mechanics and elongation mechanisms of tendon and ligament vary with strain rate during tensile loading. In this study, we sought to determine if the ultrastructural damage created during tendon rupture also varies with strain rate. A bovine forelimb model was used, allowing two anatomically proximate but physiologically distinct tendons to be studies: the positional common digital extensor tendon, and the energy storing superficial digital flexor tendon. Samples from the two tendon types were ruptured at rates of either 1%/s or 10%/s. Relative to unruptured control samples, changes to collagen fibril structure were assessed using scanning electron microscopy (SEM), and changes to collagen molecule packing were studied using differential scanning calorimetry (DSC). Rupture at 1%/s caused discrete plasticity damage that extended along the length of collagen fibrils in both the extensor and flexor tendons. Consistent with this, DSC showed molecular packing disruption relative to control samples. Both SEM and DSC showed that extensor tendon fibrils sustained more severe damage than the more highly crosslinked flexor tendon fibrils. Increasing strain rate during rupture decreased the level of longitudinal disruption experienced by the collagen fibrils of both tendon types. Disruption to D-banding was no longer seen in the extensor tendon fibrils, and discrete plasticity damage was completely eliminated in the flexor tendon fibrils, indicating a transition to localized point failure. Ultrastructural damage resulting from tendon rupture depends on both strain rate and tendon type. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2842-2850, 2018.

Fatigue of soft fibrous tissues: Multi-scale mechanics and constitutive modeling

In recent experimental studies a possible damage mechanism of collagenous tissues mainly caused by fatigue was disclosed. In this contribution, a multi-scale constitutive model ranging from the tropocollagen (TC) molecule level up to bundles of collagen fibers is proposed and utilized to predict the elastic and inelastic long-term tissue response. Material failure of collagen fibrils is elucidated by a permanent opening of the triple helical collagen molecule conformation, triggered either by overstretching or reaction kinetics of non-covalent bonds. This kinetics is described within a probabilistic framework of adhesive detachments of molecular linkages providing collagen fiber integrity. Both intramolecular and interfibrillar linkages are considered. The final constitutive equations are validated against recent experimental data available in literature for both uniaxial tension to failure and the evolution of fatigue in subsequent loading cycles. All material parameters of the proposed model have a clear physical interpretation.

STATEMENT OF SIGNIFICANCE

Irreversible changes take place at different length scales of soft fibrous tissues under supra-physiological loading and alter their macroscopic mechanical properties. Understanding the evolution of those histologic pathologies under loading and incorporating them into a continuum mechanical framework appears to be crucial in order to predict long-term evolution of various diseases and to support the development of tissue engineering.

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.

Development of overuse tendinopathy: A new descriptive model for the initiation of tendon damage during cyclic loading

Tendinopathic tissue has long been characterized by changes to collagen microstructure. However, initial tendon damage from excessive mechanical loading-a hallmark of tendinopathy development-could occur at the nanoscale level of collagen fibrils. Indeed, it is on this scale that tenocytes interact directly with tendon matrix, and excessive collagen fibril damage not visible at the microscale could trigger a degenerative cascade. In this study, we explored whether initiation of tendon damage during cyclic loading occurs via a longitudinal compression-induced buckling mechanism of collagen fibrils leading to nanoscale kinkband development. Two groups of tendons were cyclically loaded to equivalent peak stresses. In each loading cycle, tendons in one group were unloaded to the zero displacement mark, while those in the other group were unloaded to a nominal level of tension, minimizing the potential for fibril buckling. Tendons that were unloaded to the zero displacement mark ruptured significantly sooner during cyclic loading (1,446 ± 737 vs. 4,069 ± 1,129 cycles), indicating that significant fatigue damage is accrued in the low stress, toe region of the load-deformation response. Ultrastructural analysis using scanning electron microscopy of tendons stopped after 1,000 cycles showed that maintaining a nominal tension slowed the accumulation of kinkbands, supporting a longitudinal compression-induced buckling mechanism as the basis for kinkband development. Based on our results, we present a new descriptive model for the initiation of tendon damage during cyclic loading. The so-called Compression of Unrecovered Elongation or CUE Model may provide useful insight into the development of tendinopathy. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:467-476, 2018.

Sex differences in tendon structure and function

Tendons play a critical role in the transmission of forces between muscles and bones, and chronic tendon injuries and diseases are among the leading causes of musculoskeletal disability. Little is known about sex-based differences in tendon structure and function. Our objective was to evaluate the mechanical properties, biochemical composition, transcriptome, and cellular activity of plantarflexor tendons from 4 month old male and female C57BL/6 mice using in vitro biomechanics, mass spectrometry-based proteomics, genome-wide expression profiling, and cell culture techniques. While the Achilles tendons of male mice were approximately 6% larger than female mice (p < 0.05), the cell density of female mice was around 19% greater than males (p < 0.05). No significant differences in mechanical properties (p > 0.05) of plantaris tendons were observed. Mass spectrometry proteomics analysis revealed no significant difference between sexes in the abundance of major extracellular matrix (ECM) proteins such as collagen types I (p = 0.30) and III (p = 0.68), but female mice had approximately twofold elevations (p < 0.05) in less abundant ECM proteins such as fibronectin, periostin, and tenascin C. The transcriptome of male and female tendons differed by only 1%. In vitro, neither the sex of the serum that fibroblasts were cultured in, nor the sex of the ECM in which they were embedded, had profound effects on the expression of collagen and cell proliferation genes. Our results indicate that while male mice expectedly had larger tendons, male and female tendons have very similar mechanical properties and biochemical composition, with small increases in some ECM proteins and proteoglycans evident in female tendons. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2117-2126, 2017.

Molecular level detection and localization of mechanical damage in collagen enabled by collagen hybridizing peptides

Mechanical injury to connective tissue causes changes in collagen structure and material behaviour, but the role and mechanisms of molecular damage have not been established. In the case of mechanical subfailure damage, no apparent macroscale damage can be detected, yet this damage initiates and potentiates in pathological processes. Here, we utilize collagen hybridizing peptide (CHP), which binds unfolded collagen by triple helix formation, to detect molecular level subfailure damage to collagen in mechanically stretched rat tail tendon fascicle. Our results directly reveal that collagen triple helix unfolding occurs during tensile loading of collagenous tissues and thus is an important damage mechanism. Steered molecular dynamics simulations suggest that a likely mechanism for triple helix unfolding is intermolecular shearing of collagen α-chains. Our results elucidate a probable molecular failure mechanism associated with subfailure injuries, and demonstrate the potential of CHP targeting for diagnosis, treatment and monitoring of tissue disease and injury.

Collagen denaturation is thought to occur during tissue mechanical damage, but its role in damage initiation is still unclear. Here, the authors use a collagen hybridizing peptide to provide insights into the molecular mechanisms leading to collagen unfolding during tendon mechanical stretch.

Recapitulating the Micromechanical Behavior of Tension and Shear in a Biomimetic Hydrogel for Controlling Tenocyte Response

A fiber composite system is presented which recapitulates the fiber-composite-like nature of tissues and generates similar modes of shear and tension. The shear/tension ratio can be customized during composite manufacture and incorporates viable cells. The system is a valuable tool for mechanotransduction research, providing a platform with physiologically relevant conditions for investigating cell behavior in different tissue types.

A Novel Organ Culture Model to Quantify Collagen Remodeling in Tree Shrew Sclera

Increasing evidence suggests that unknown collagen remodeling mechanisms in the sclera underlie myopia development. We are proposing a novel organ culture system in combination with two-photon fluorescence imaging to quantify collagen remodeling at the tissue- and lamella-level. Tree shrew scleral shells were cultured up to 7 days in serum-free media and cellular viability was investigated under: (i) minimal tissue manipulations; (ii) removal of intraocular tissues; gluing the eye to a washer using (iii) 50 μL and (iv) 200 μL of cyanoacrylate adhesive; (v) supplementing media with Ham's F-12 Nutrient Mixture; and (vi) culturing eyes subjected to 15 mmHg intraocular pressure in our new bioreactor. Two scleral shells of normal juvenile tree shrews were fluorescently labeled using a collagen specific protein and cultured in our bioreactor. Using two-photon microscopy, grid patterns were photobleached into and across multiple scleral lamellae. These patterns were imaged daily for 3 days, and tissue-/lamella-level strains were calculated from the deformed patterns. No significant reduction in cell viability was observed under conditions (i) and (v). Compared to condition (i), cell viability was significantly reduced starting at day 0 (condition (ii)) and day 3 (conditions (iii, iv, vi)). Tissue-level strain and intralamellar shear angel increased significantly during the culture period. Some scleral lamellae elongated while others shortened. Findings suggest that tree shrew sclera can be cultured in serum-free media for 7 days with no significant reduction in cell viability. Scleral fibroblasts are sensitive to tissue manipulations and tissue gluing. However, Ham's F-12 Nutrient Mixture has a protective effect on cell viability and can offset the cytotoxic effect of cyanoacrylate adhesive. This is the first study to quantify collagen micro-deformations over a prolonged period in organ culture providing a new methodology to study scleral remodeling in myopia.

Fascicles and the interfascicular matrix show adaptation for fatigue resistance in energy storing tendons

Tendon is composed of rope-like fascicles, bound together by interfascicular matrix (IFM). Our previous work shows that the IFM is critical for tendon function, facilitating sliding between fascicles to allow tendons to stretch. This function is particularly important in energy storing tendons, which experience extremely high strains during exercise, and therefore require the capacity for considerable inter-fascicular sliding and recoil. This capacity is not required in positional tendons. Whilst we have previously described the quasi-static properties of the IFM, the fatigue resistance of the IFM in functionally distinct tendons remains unknown. We therefore tested the hypothesis that fascicles and IFM in the energy storing equine superficial digital flexor tendon (SDFT) are more fatigue resistant than those in the positional common digital extensor tendon (CDET). Fascicles and IFM from both tendon types were subjected to cyclic fatigue testing until failure, and mechanical properties were calculated. The results demonstrated that both fascicles and IFM from the energy storing SDFT were able to resist a greater number of cycles before failure than those from the positional CDET. Further, SDFT fascicles and IFM exhibited less hysteresis over the course of testing than their counterparts in the CDET. This is the first study to assess the fatigue resistance of the IFM, demonstrating that IFM has a functional role within tendon and contributes significantly to tendon mechanical properties. These data provide important advances into fully characterising tendon structure-function relationships.

Statement of Significance

Understanding tendon-structure function relationships is crucial for the development of effective preventative measures and treatments for tendon injury. In this study, we demonstrate for the first time that the interfascicular matrix is able to withstand a high degree of cyclic loading, and is specialised for improved fatigue resistance in energy storing tendons. These findings highlight the importance of the interfascicular matrix in the function of energy storing tendons, and potentially provide new avenues for the development of treatments for tendon injury which specifically target the interfascicular matrix.

Revisiting the continuum model of tendon pathology: what is its merit in clinical practice and research?

The pathogenesis of tendinopathy and the primary biological change in the tendon that precipitates pathology have generated several pathoaetiological models in the literature. The continuum model of tendon pathology, proposed in 2009, synthesised clinical and laboratory-based research to guide treatment choices for the clinical presentations of tendinopathy. While the continuum has been cited extensively in the literature, its clinical utility has yet to be fully elucidated. The continuum model proposed a model for staging tendinopathy based on the changes and distribution of disorganisation within the tendon. However, classifying tendinopathy based on structure in what is primarily a pain condition has been challenged. The interplay between structure, pain and function is not yet fully understood, which has partly contributed to the complex clinical picture of tendinopathy. Here we revisit and assess the merit of the continuum model in the context of new evidence. We (1) summarise new evidence in tendinopathy research in the context of the continuum, (2) discuss tendon pain and the relevance of a model based on structure and (3) describe relevant clinical elements (pain, function and structure) to begin to build a better understanding of the condition. Our goal is that the continuum model may help guide targeted treatments and improved patient outcomes.

Collagen V-heterozygous and -null supraspinatus tendons exhibit altered dynamic mechanical behaviour at multiple hierarchical scales

Tendons function using a unique set of mechanical properties governed by the extracellular matrix and its ability to respond to varied multi-axial loads. Reduction of collagen V expression, such as in classic Ehlers-Danlos syndrome, results in altered fibril morphology and altered macroscale mechanical function in both clinical and animal studies, yet the mechanism by which changes at the fibril level lead to macroscale functional changes has not yet been investigated. This study addresses this by defining the multiscale mechanical response of wild-type, collagen V-heterozygous and -null supraspinatus tendons. Tendons were subjected to mechanical testing and analysed for macroscale properties, as well as microscale (fibre re-alignment) and nanoscale (fibril deformation and sliding) responses. In many macroscale parameters, results showed a dose-dependent response with severely decreased properties in the null group. In addition, both heterozygous and null groups responded to load faster than in wild-type tendons via earlier fibre re-alignment and fibril stretch. However, the heterozygous group exhibited increased fibril sliding, while the null group exhibited no fibril sliding. These studies demonstrate that dynamic responses play an important role in determining overall function and that collagen V is a critical regulator in the development of these relationships.

Multiscale strain analysis of tendon subjected to shear and compression demonstrates strain attenuation, fiber sliding, and reorganization

The manner in which strains are passed down the hierarchical length scales of tendons dictates how cells within the collagen network regulate the tissue response to loading. How tendons deform in different hierarchical levels under shear and compression is unknown. The aims of this study were: (i) to evaluate whether specific regions of bovine deep digital flexor tendons exhibited different strain attenuation from macro to micro length scales, and (ii) to elucidate mechanisms responsible for tendon deformation under shear and compression. Samples from distal and proximal regions of flexor tendons were subjected to three-step incremental stress-relaxation tests. Images of tissue markers, photobleached lines on collagen fibers, and nuclei locations were collected before and after loading. Results showed that strain transfer was attenuated from tissue to local matrix under both shear and compression. Nuclear aspect ratios exhibited smaller changes for distal samples, suggesting that cells are more shielded from deformation in the distal region. Collagen fiber sliding was observed to contribute significantly in response to shear, while uncrimping and fiber reorganization were the predominant mechanisms under compression. This study provides insight into microscale mechanisms responsible for multiscale strain attenuation of tendons under non-tensile macroscale loading.

Use of biomimetic microtissue spheroids and specific growth factor supplementation to improve tenocyte differentiation and adaptation to a collagen-based scaffold in vitro

Tenocytes represent a valuable source of cells for the purposes of tendon tissue engineering and regenerative medicine and as such, should possess a high degree of tenogenic differentiation prior to their use in vivo in order to achieve maximal efficacy. In the current report, we identify an efficient means by which to maintain differentiated tenocytes in vitro by employing the hanging drop technique in combination with defined growth media supplements. Equine tenocytes retained a more differentiated state when cultured as scaffold-free microtissue spheroids in low serum-containing medium supplemented with L-ascorbic acid 2-phosphate, insulin and transforming growth factor (TGF)-β1. This was made evident by significant increases in the expression levels of pro-tenogenic markers collagen type I (COL1A2), collagen type III (COL3A1), scleraxis (SCX) and tenomodulin (TNMD), as well as by enhanced levels of collagen type I and tenomodulin protein. Furthermore, tenocytes cultured under these conditions demonstrated a typical spindle-like morphology and when embedded in collagen gels, became highly aligned with respect to the orientation of the collagen structure following their migration out from the microtissue spheroids. Our findings therefore provide evidence to support the use of a biomimetic microtissue approach to culturing tenocytes and that in combination with the defined growth media described, can improve their differentiation status and functional repopulation of collagen matrix.

Tendon functional extracellular matrix

This article is one of a series, summarizing views expressed at the Orthopaedic Research Society New Frontiers in Tendon Research Conference. This particular article reviews the three workshops held under the "Functional Extracellular Matrix" stream. The workshops focused on the roles of the tendon extracellular matrix, such as performing the mechanical functions of tendon, creating the local cell environment, and providing cellular cues. Tendon is a complex network of matrix and cells, and its biological functions are influenced by widely varying extrinsic and intrinsic factors such as age, nutrition, exercise levels, and biomechanics. Consequently, tendon adapts dynamically during development, aging, and injury. The workshop discussions identified research directions associated with understanding cell-matrix interactions to be of prime importance for developing novel strategies to target tendon healing or repair.

Tendon extracellular matrix damage, degradation and inflammation in response to in vitro overload exercise

The role of inflammation in tendon injury is uncertain and a topic of current interest. In vitro studies of tendon accelerated overload damage can serve as a valuable source of information on the early stages of tendinopathy. Viable fascicle bundles from bovine flexor tendons were subjected to cyclic uniaxial loading from 1-10% strain. Immuno-staining for inflammatory markers and matrix degradation markers was performed on the samples after mechanical testing. Loaded samples exhibited visible extracellular matrix damage, with disrupted collagen fibers and fiber kinks, and notable damage to the interfascicular matrix. Inflammatory markers COX-2 and IL-6 were only expressed in the cyclically loaded samples. Collagen degradation markers MMP-1 and C1,2C were colocalized in many areas, with staining occurring in the interfascicular matrix or the fascicular tenocytes. These markers were present in control samples, but staining became increasingly intense with loading. Little MMP-3 or MMP-13 was evident in control sections. In loaded samples, some sections showed intense staining of these markers, again localized to interfascicular regions. This study suggests that inflammatory markers may be expressed rapidly after tendon overload exercise. Interestingly, both inflammation and damage-induced matrix remodeling seem to be concentrated in, or in the vicinity of, the highly cellular interfascicular matrix.