The Interfascicular Matrix of Energy Storing Tendons Houses Heterogenous Cell Populations Disproportionately Affected by Aging

Energy storing tendons such as the human Achilles and equine superficial digital flexor tendon (SDFT) are prone to injury, with incidence increasing with aging, peaking in the 5th decade of life in the human Achilles tendon. The interfascicular matrix (IFM), which binds tendon fascicles, plays a key role in energy storing tendon mechanics, and aging alterations to the IFM negatively impact tendon function. While the mechanical role of the IFM in tendon function is well-established, the biological role of IFM-resident cell populations remains to be elucidated. Therefore, the aim of this study was to identify IFM-resident cell populations and establish how these populations are affected by aging. Cells from young and old SDFTs were subjected to single cell RNA-sequencing, and immunolabelling for markers of each resulting population used to localise cell clusters. Eleven cell clusters were identified, including tenocytes, endothelial cells, mural cells, and immune cells. One tenocyte cluster localised to the fascicular matrix, whereas nine clusters localised to the IFM. Interfascicular tenocytes and mural cells were preferentially affected by aging, with differential expression of genes related to senescence, dysregulated proteostasis and inflammation. This is the first study to establish heterogeneity in IFM cell populations, and to identify age-related alterations specific to IFM-localised cells.

Human vascularised synovium-on-a-chip: a mechanically stimulated, microfluidic model to investigate synovial inflammation and monocyte recruitment

Healthy synovium is critical for joint homeostasis. Synovial inflammation (synovitis) is implicated in the onset, progression and symptomatic presentation of arthritic joint diseases such as rheumatoid arthritis and osteoarthritis. Thus, the synovium is a promising target for the development of novel, disease-modifying therapeutics. However, target exploration is hampered by a lack of good pre-clinical models that accurately replicate human physiology and that are developed in a way that allows for widespread uptake. The current study presents a multi-channel, microfluidic, organ-on-a-chip (OOAC) model, comprising a 3D configuration of the human synovium and its associated vasculature, with biomechanical and inflammatory stimulation, built upon a commercially available OOAC platform. Healthy human fibroblast-like synoviocytes (hFLS) were co-cultured with human umbilical vein endothelial cells (HUVECs) with appropriate matrix proteins, separated by a flexible, porous membrane. The model was developed within the Emulate organ-chip platform enabling the application of physiological biomechanical stimulation in the form of fluid shear and cyclic tensile strain. The hFLS exhibited characteristic morphology, cytoskeletal architecture and matrix protein deposition. Synovial inflammation was initiated through the addition of interleukin-1β(IL-1β) into the synovium channel resulting in the increased secretion of inflammatory and catabolic mediators, interleukin-6 (IL-6), prostaglandin E2 (PGE2), matrix metalloproteinase 1 (MMP-1), as well as the synovial fluid constituent protein, hyaluronan. Enhanced expression of the inflammatory marker, intercellular adhesion molecule-1 (ICAM-1), was observed in HUVECs in the vascular channel, accompanied by increased attachment of circulating monocytes. This vascularised human synovium-on-a-chip model recapitulates a number of the functional characteristics of both healthy and inflamed human synovium. Thus, this model offers the first human synovium organ-chip suitable for widespread adoption to understand synovial joint disease mechanisms, permit the identification of novel therapeutic targets and support pre-clinical testing of therapies.

Guidelines for ex vivo mechanical testing of tendon

Tendons are critical for the biomechanical function of joints. Tendons connect muscles to bones and allow for the transmission of muscle forces to facilitate joint motion. Therefore, characterizing the tensile mechanical properties of tendons is important for the assessment of functional tendon health and efficacy of treatments for acute and chronic injuries. In this guidelines paper, we review methodological considerations, testing protocols, and key outcome measures for mechanical testing of tendons. The goal of the paper is to present a simple set of guidelines to the nonexpert seeking to perform tendon mechanical tests. The suggested approaches provide rigorous and consistent methodologies for standardized biomechanical characterization of tendon and reporting requirements across laboratories.

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

BACKGROUND

Hypercholesterolemia is associated with tendon pathology, but the reasons underpinning this relationship are not well understood. Cholesterol can accumulate in the tendon non-collagenous matrix which may affect both global and local tissue mechanics. Changes to the local strain environment within tendon may have significant implications for mechanosensitive tenocytes. Here, we investigated the association between elevated blood cholesterol and presence of tendon lipids in the Achilles tendon. We expected lipids to be localised in the proteoglycan-rich inter-sub-tendon matrix (ISTM), therefore we also sought to examine the impact of this on the biomechanical and viscoelastic properties of the ISTM.

METHODS

The Achilles tendons of 32 young wild-type (SD) and 32 apolipoprotein E knock-out rats (ApoE^-/-) were harvested at 15.6 ± 2.3 weeks of age. 32 specimens underwent histological examination to assess the distribution of lipids throughout sub-tendons and ISTM. The remaining specimens were prepared for biomechanical testing, where the ISTM between the gastrocnemius and soleus sub-tendons was subjected to shear load mechanical testing. A sub-set of tests were video recorded to enable a strain analysis.

RESULTS

ApoE^-/- serum cholesterol was double that of SD rats (mean 2.25 vs. 1.10 mg/ml, p < 0.001) indicating a relatively mild hypercholesterolemia phenotype. Nonetheless, we found histological evidence of esterified lipids in the ISTM and unesterified lipids in the sub-tendons, although the location or intensity of staining was not appreciably different between rat strains. Despite a lack of observable histological differences in lipid content between groups, there were significant differences in the mechanical and viscoelastic behaviour of the Achilles sub-tendon matrix.

CONCLUSION

Even slightly elevated cholesterol may result in subtle changes to tendon biomechanical properties and hence injury risk. The young age of our cohort and the mild phenotype of our ApoE^-/- rats are likely to have limited our findings and so we also conclude that the ApoE^-/- rat model is not well suited for investigating the biomechanical impact of tendon xanthomas on Achilles sub-tendon function.

Development and application of a novel in vivo overload model of the Achilles tendon in rat

Repetitive overload is a primary factor in tendon injury, causing progressive accumulation of matrix damage concurrent with a cellular response. However, it remains unclear how these events occur at the initial stages of the disease, making it difficult to identify appropriate treatment approaches. Here, we describe the development of a new model to cyclically load the Achilles tendon (AT) of rats in vivo and investigate the initial structural and cellular responses. The model utilizes controlled dorsiflexion of the ankle joint applied near maximal dorsiflexion, for 10,000 cycles at 3 Hz. Animals were subjected to a single bout of in vivo loading under anaesthesia, and either culled immediately (without recovery from anaesthesia), or 48 h or 4-weeks post-loading. Macro strains were assessed in cadavers, whilst tendon specific microdamage was assessed through collagen-hybridizing peptide (CHP) immunohistochemistry which highlighted a significant rise in CHP staining in loaded ATs compared to contralateral controls, indicating an accumulation of overload-induced damage. Staining for pro-inflammatory mediators (IL-6 and COX-2) and matrix degradation markers (MMP-3 and -13) also suggests an initial cellular response to overload. Model validation confirmed our approach was able to explore early overload-induced damage within the AT, with microdamage present and no evidence of broader musculoskeletal damage. The new model may be implemented to map the progression of tendinopathy in the AT, and thus study potential therapeutic interventions.

The impact of mild hypercholesterolemia on injury repair in the rat patellar tendon

Hypercholesterolemia is associated with tendon pathology and injury prevalence. Lipids can accumulate in the tendon's extracellular spaces, which may disrupt its hierarchical structure and the tenocytes physicochemical environment. We hypothesized that the tendon's ability to repair after injury would be attenuated with elevated cholesterol levels, leading to inferior mechanical properties. Fifty wild-type (sSD) and 50 apolipoprotein E knock-out rats (ApoE^{-/ -} ) were given a unilateral patellar tendon (PT) injury at 12 weeks old; the uninjured limb served as a control. Animals were euthanized at 3-, 14,- or 42-days postinjury and PT healing was investigated. ApoE^{-/ -} serum cholesterol was double that of SD rats (mean: 2.12 vs. 0.99 mg/mL, p < 0.001) and cholesterol level was related to the expression of several genes after injury; notably rats with higher cholesterol demonstrated a blunted inflammatory response. There was little physical evidence of tendon lipid content or differences in injury repair between groups, therefore we were not surprised that tendon mechanical or material properties did not differ between strains. The young age and the mild phenotype of our ApoE^{-/ -} rats might explain these findings. Hydroxyproline content was positively related to total blood cholesterol, but this result did not translate to observable biomechanical differences, perhaps due to the narrow range of cholesterol levels observed. Tendon inflammatory and healing activity is modulated at the mRNA level even with a mild hypercholesterolemia. These important initial impacts need to be investigated as they may contribute to the known consequences of cholesterol on tendons in humans.

Organ-on-a-Chip and Microfluidic Platforms for Oncology in the UK

Organ-on-chip systems are capable of replicating complex tissue structures and physiological phenomena. The fine control of biochemical and biomechanical cues within these microphysiological systems provides opportunities for cancer researchers to build complex models of the tumour microenvironment. Interest in applying organ chips to investigate mechanisms such as metastatsis and to test therapeutics has grown rapidly, and this review draws together the published research using these microfluidic platforms to study cancer. We focus on both in-house systems and commercial platforms being used in the UK for fundamental discovery science and therapeutics testing. We cover the wide variety of cancers being investigated, ranging from common carcinomas to rare sarcomas, as well as secondary cancers. We also cover the broad sweep of different matrix microenvironments, physiological mechanical stimuli and immunological effects being replicated in these models. We examine microfluidic models specifically, rather than organoids or complex tissue or cell co-cultures, which have been reviewed elsewhere. However, there is increasing interest in incorporating organoids, spheroids and other tissue cultures into microfluidic organ chips and this overlap is included. Our review includes a commentary on cancer organ-chip models being developed and used in the UK, including work conducted by members of the UK Organ-on-a-Chip Technologies Network. We conclude with a reflection on the likely future of this rapidly expanding field of oncological research.

Force Transmission Between the Gastrocnemius and Soleus Sub-Tendons of the Achilles Tendon in Rat

The Achilles tendon (AT) is comprised of three distinct sub-tendons bound together by the inter-subtendon matrix (ISTM). The interactions between sub-tendons will have important implications for AT function. The aim of this study was to investigate the extent to which the ISTM facilitates relative sliding between sub-tendons, and serves as a pathway for force transmission between the gastrocnemius (GAS) and soleus (SOL) sub-tendons of the rat AT. In this study, ATs were harvested from Wistar rats, and the mechanical behavior and composition of the ISTM were explored. To determine force transmission between sub-tendons, the proximal and distal ends of the GAS and SOL sub-tendons were secured, and the forces at each of these locations were measured during proximal loading of the GAS. To determine the ISTM mechanical behavior, only the proximal GAS and distal SOL were secured, and the ISTM was loaded in shear. Finally, for compositional analysis, histological examination assessed the distribution of matrix proteins throughout sub-tendons and the ISTM. The results revealed distinct differences between the forces at the proximal and distal ends of both sub-tendons when proximal loading was applied to the GAS, indicating force transmission between GAS and SOL sub-tendons. Inter-subtendon matrix tests demonstrated an extended initial low stiffness toe region to enable some sub-tendon sliding, coupled with high stiffness linear region such that force transmission between sub-tendons is ensured. Histological data demonstrate an enrichment of collagen III, elastin, lubricin and hyaluronic acid in the ISTM. We conclude that ISTM composition and mechanical behavior are specialized to allow some independent sub-tendon movement, whilst still ensuring capacity for force transmission between the sub-tendons of the AT.

A Recruitment Model of Tendon Viscoelasticity That Incorporates Fibril Creep and Explains Strain-Dependent Relaxation

Soft tissues exhibit complex viscoelastic behavior, including strain-rate dependence, hysteresis, and strain-dependent relaxation. In this paper, a model for soft tissue viscoelasticity is developed that captures all of these features and is based upon collagen recruitment, whereby fibrils contribute to tissue stiffness only when taut. We build upon existing recruitment models by additionally accounting for fibril creep and by explicitly modeling the contribution of the matrix to the overall tissue viscoelasticity. The fibrils and matrix are modeled as linear viscoelastic and each fibril has an associated critical strain (corresponding to its length) at which it becomes taut. The model is used to fit relaxation tests on three rat tail tendon fascicles and predict their response to cyclic loading. It is shown that all of these mechanical tests can be reproduced accurately with a single set of constitutive parameters, the only difference between each fascicle being the distribution of their fibril crimp lengths. By accounting for fibril creep, we are able to predict how the fibril length distribution of a fascicle changes over time under a given deformation. Furthermore, the phenomenon of strain-dependent relaxation is explained as arising from the competition between the fibril and matrix relaxation functions.

An in vitro investigation into the effects of 10 Hz cyclic loading on tenocyte metabolism

Tendinopathy is a prevalent, highly debilitating condition, with poorly defined etiology. A wide range of clinical treatments has been proposed, with systematic reviews largely supporting shock wave therapy or eccentric exercise. Characterizing these treatments have demonstrated both generate perturbations within tendon at a frequency of approximately 8-12 Hz. Consequently, it is hypothesized that loading in this frequency range initiates increased anabolic tenocyte behavior, promoting tendon repair. The primary aim of this study was to investigate the effects of 10 Hz perturbations on tenocyte metabolism, comparing gene expression in response to a 10 Hz and 1 Hz loading profile. Tenocytes from healthy and tendinopathic human tendons were seeded into 3D collagen gels and subjected to 15 minutes cyclic strain at 10 Hz or 1 Hz. Tenocytes from healthy tendon showed increased expression of all analyzed genes in response to loading, with significantly increased expression of inflammatory and degradative genes with 10 Hz, relative to 1 Hz loading. By contrast, whilst the response of tenocytes from tendinopathy tendon also increased with 10 Hz loading, the overall response profile was more varied and less intense, possibly indicative of an altered healing response. Through inhibition of the pathway, IL1 was shown to be involved in the degradative and catabolic response of cells to high-frequency loading, abrogating the loading response. This study has demonstrated for the first time that loading at a frequency of 10 Hz may enhance the metabolic response of tenocytes by initiating an immediate degradatory and inflammatory cell response through the IL1 pathway, perhaps as an initial stage of tendon healing.

Effects of cell adhesion motif, fiber stiffness, and cyclic strain on tenocyte gene expression in a tendon mimetic fiber composite hydrogel

We recently developed a fiber composite consisting of tenocytes seeded onto discontinuous fibers embedded within a hydrogel, designed to mimic physiological tendon micromechanics of tension and shear. This study examined if cell adhesion peptide (DGEA or YRGDS), fiber modulus (50 or 1300 kPa) and/or cyclic strain (5% strain, 1 Hz) influenced bovine tenocyte gene expression. Ten genes were analyzed and none were sensitive to peptide or fiber modulus in the absence of cyclic tensile strain. Genes associated with tendon (SCX and TNMD), collagens (COL1A1, COL3A1, COL11A1), and matrix remodelling (MMP1, MMP2, and TIMP3) were insensitive to cyclic strain. Contrarily, cyclic strain up-regulated IL6 by 30-fold and MMP3 by 10-fold in soft YRGDS fibers. IL6 expression in soft YRGDS fibers was 5.7 and 3.3-fold greater than in soft DGEA fibers and stiff RGD fibers, respectively, under cyclic strain. Our findings suggest that changes in the surrounding matrix can influence catabolic genes in tenocytes when cultured in a complex strain environment mimicking that of tendon, while having minimal effects on tendon and homeostatic genes.

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A fiber composite material exhibits multimodal shear and tension micromechanics.

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Gene expression of tenocytes is insensitive to fiber stiffness or YRGDS vs DGEA.

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Tendon marker and collagen genes are insensitive to fiber environment under strain.

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MMP3 and IL6 genes are sensitive to fiber stiffness and peptide under cyclic strain.

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Healthy tenocytes rapidly respond to their environment by a catabolic response.

Structure and collagen crimp patterns of functionally distinct equine tendons, revealed by quantitative polarised light microscopy (qPLM)

Structure-function relationships in tendons are directly influenced by the arrangement of collagen fibres. However, the details of such arrangements in functionally distinct tendons remain obscure. This study demonstrates the use of quantitative polarised light microscopy (qPLM) to identify structural differences in two major tendon compartments at the mesoscale: fascicles and interfascicular matrix (IFM). It contrasts functionally distinct positional and energy storing tendons, and considers changes with age. Of particular note, the technique facilitates the analysis of crimp parameters, in which cutting direction artefact can be accounted for and eliminated, enabling the first detailed analysis of crimp parameters across functionally distinct tendons.

IFM shows lower birefringence (0.0013 ± 0.0001 [−]), as compared to fascicles (0.0044 ± 0.0005 [−]), indicating that the volume fraction of fibres must be substantially lower in the IFM. Interestingly, no evidence of distinct fibre directional dispersions between equine energy storing superficial digital flexor tendons (SDFTs) and positional common digital extensor tendons (CDETs) were noted, suggesting either more subtle structural differences between tendon types or changes focused in the non-collagenous components.

By contrast, collagen crimp characteristics are strongly tendon type specific, indicating crimp specialisation is crucial in the respective mechanical function. SDFTs showed much finer crimp (21.1 ± 5.5 µm) than positional CDETs (135.4 ± 20.1 µm). Further, tendon crimp was finer in injured tendon, as compared to its healthy equivalents. Crimp angle differed strongly between tendon types as well, with average of 6.5 ± 1.4° in SDFTs and 13.1 ± 2.0° in CDETs, highlighting a substantially tighter crimp in the SDFT, likely contributing to its effective recoil capacity.

Statement of Significance

This is the first study to quantify birefringence in fascicles and interfascicular matrix of functionally distinct energy storing and positional tendons. It adopts a novel method – quantitative polarised light microscopy (qPLM) to measure collagen crimp angle, avoiding artefacts related to the direction of histological sectioning, and provides the first direct comparison of crimp characteristics of functionally distinct tendons of various ages.

A comparison of matched picrosirius red stained and unstained tendons sections identified non-homogenous staining effects, and leads us to recommend that only unstained sections are analysed in the quantitative manner.

qPLM is successfully used to assess birefringence in soft tissue sections, offering a promising tool for investigating the structural arrangements of fibres in (soft) tissues and other composite materials.

Elastin is Localised to the Interfascicular Matrix of Energy Storing Tendons and Becomes Increasingly Disorganised With Ageing

Tendon is composed of fascicles bound together by the interfascicular matrix (IFM). Energy storing tendons are more elastic and extensible than positional tendons; behaviour provided by specialisation of the IFM to enable repeated interfascicular sliding and recoil. With ageing, the IFM becomes stiffer and less fatigue resistant, potentially explaining why older tendons become more injury-prone. Recent data indicates enrichment of elastin within the IFM, but this has yet to be quantified. We hypothesised that elastin is more prevalent in energy storing than positional tendons, and is mainly localised to the IFM. Further, we hypothesised that elastin becomes disorganised and fragmented, and decreases in amount with ageing, especially in energy storing tendons. Biochemical analyses and immunohistochemical techniques were used to determine elastin content and organisation, in young and old equine energy storing and positional tendons. Supporting the hypothesis, elastin localises to the IFM of energy storing tendons, reducing in quantity and becoming more disorganised with ageing. These changes may contribute to the increased injury risk in aged energy storing tendons. Full understanding of the processes leading to loss of elastin and its disorganisation with ageing may aid in the development of treatments to prevent age related tendinopathy.

The relative compliance of energy-storing tendons may be due to the helical fibril arrangement of their fascicles

A nonlinear elastic microstructural model is used to investigate the relationship between structure and function in energy-storing and positional tendons. The model is used to fit mechanical tension test data from the equine common digital extensor tendon (CDET) and superficial digital flexor tendon (SDFT), which are used as archetypes of positional and energy-storing tendons, respectively. The fibril crimp and fascicle helix angles of the two tendon types are used as fitting parameters in the mathematical model to predict their values. The outer fibril crimp angles were predicted to be 15.1° ± 2.3° in the CDET and 15.8° ± 4.1° in the SDFT, and the average crimp angles were predicted to be 10.0° ± 1.5° in the CDET and 10.5° ± 2.7° in the SDFT. The crimp angles were not found to be statistically significantly different between the two tendon types (p = 0.572). By contrast, the fascicle helix angles were predicted to be 7.9° ± 9.3° in the CDET and 29.1° ± 10.3° in the SDFT and were found to be statistically highly significantly different between the two tendon types (p < 0.001). This supports previous qualitative observations that helical substructures are more likely to be found in energy-storing tendons than in positional tendons and suggests that the relative compliance of energy-storing tendons may be directly caused by these helical substructures.

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 transverse isotropic viscoelastic constitutive model for aortic valve tissue

A new anisotropic viscoelastic model is developed for application to the aortic valve (AV). The directional dependency in the mechanical properties of the valve, arising from the predominantly circumferential alignment of collagen fibres, is accounted for in the form of transverse isotropy. The rate dependency of the valve's mechanical behaviour is considered to stem from the viscous (η) dissipative effects of the AV matrix, and is incorporated as an explicit function of the deformation rate ([Formula: see text]). Model (material) parameters were determined from uniaxial tensile deformation tests of porcine AV specimens at various deformation rates, by fitting the model to each experimental dataset. It is shown that the model provides an excellent fit to the experimental data across all different rates and satisfies the condition of strict local convexity. Based on the fitting results, a nonlinear relationship between η and [Formula: see text] is established, highlighting a 'shear-thinning' behaviour for the AV with increase in the deformation rate. Using the model and these outcomes, the stress-deformation curves of the AV tissue under physiological deformation rates in both the circumferential and radial directions are predicted and presented. To verify the predictive capabilities of the model, the stress-deformation curves of AV specimens at an intermediate deformation rate were estimated and validated against the experimental data at that rate, showing an excellent agreement. While the model is primarily developed for application to the AV, it may be applied without the loss of generality to other collagenous soft tissues possessing a similar structure, with a single preferred direction of embedded collagen fibres.

Distribution of proteins within different compartments of tendon varies according to tendon type

Although the predominant function of all tendons is to transfer force from muscle to bone and position the limbs, some tendons additionally function as energy stores, reducing the energetic cost of locomotion. To maximise energy storage and return, energy‐storing tendons need to be more extensible and elastic than tendons with a purely positional function. These properties are conferred in part by a specialisation of a specific compartment of the tendon, the interfascicular matrix, which enables sliding and recoil between adjacent fascicles. However, the composition of the interfascicular matrix is poorly characterised and we therefore tested the hypothesis that the distribution of elastin and proteoglycans differs between energy‐storing and positional tendons, and that protein distribution varies between the fascicular matrix and the interfascicular matrix, with localisation of elastin and lubricin to the interfascicular matrix. Protein distribution in the energy‐storing equine superficial digital flexor tendon and positional common digital extensor tendon was assessed using histology and immunohistochemistry. The results support the hypothesis, demonstrating enrichment of lubricin in the interfascicular matrix in both tendon types, where it is likely to facilitate interfascicular sliding. Elastin was also localised to the interfascicular matrix, specifically in the energy‐storing superficial digital flexor tendon, which may account for the greater elasticity of the interfascicular matrix in this tendon. A differential distribution of proteoglycans was identified between tendon types and regions, which may indicate a distinct role for each of these proteins in tendon. These data provide important advances into fully characterising structure–function relationships within tendon.

Tendon Structure and Composition

Tendons are soft, fibrous tissues that connect muscle to bone. Their main function is to transfer muscle generated force to the bony skeleton, facilitating movement around a joint, and as such they are relatively passive, inelastic structures, able to resist high forces. Tendons are predominantly composed of collagen, which is arranged in a hierarchical manner parallel to the long axis of the tendon, resulting in high tensile strength. Tendon also contains a range of non-collagenous proteins, present in low amounts, which nevertheless have important functional roles. In this chapter, we describe general tendon composition and structure, and discuss how variations in composition and structure at different levels of the tendon hierarchy confer specific mechanical properties, which are related to tendon function.

In vivo biological response to extracorporeal shockwave therapy in human tendinopathy

Extracorporeal shock wave therapy (ESWT) is a non-invasive treatment for chronic tendinopathies, however little is known about the in-vivo biological mechanisms of ESWT. Using microdialysis, we examined the real-time biological response of healthy and pathological tendons to ESWT. A single session of ESWT was administered to the mid-portion of the Achilles tendon in thirteen healthy individuals (aged 25.7 ± 7.0 years) and patellar or Achilles tendon of six patients with tendinopathies (aged 39.0 ± 14.9 years). Dialysate samples from the surrounding peri-tendon were collected before and immediately after ESWT. Interleukins (IL)-1β, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12p70, IL-17A, vascular endothelial growth factor and interferon-γ were quantified using a cytometric bead array while gelatinase activity (MMP-2 and -9) was examined using zymography. There were no statistical differences between the biological tissue response to ESWT in healthy and pathological tendons. IL-1β, IL-2, IL-6 and IL-8 were the cytokines predominantly detected in the tendon dialysate. IL-1β and IL-2 did not change significantly with ESWT. IL-6 and IL-8 concentrations were elevated immediately after ESWT and remained significantly elevated for four hours post-ESWT (p < 0.001). Pro-forms of MMP-2 and -9 also increased after ESWT (p < 0.003), whereas there were no significant changes in active MMP forms. In addition, the biological response to ESWT treatment could be differentiated between possible responders and non-responders based on a minimum 5-fold increase in any inflammatory marker or MMP from pre- to post-ESWT. Our findings provide novel evidence of the biological mechanisms underpinning ESWT in humans in vivo. They suggest that the mechanical stimulus provided by ESWT might aid tendon remodelling in tendinopathy by promoting the inflammatory and catabolic processes that are associated with removing damaged matrix constituents. The non-response of some individuals may help to explain why ESWT does not improve symptoms in all patients and provides a potential focus for future research.

The interfascicular matrix enables fascicle sliding and recovery in tendon, and behaves more elastically in energy storing tendons

While the predominant function of all tendons is to transfer force from muscle to bone and position the limbs, some tendons additionally function as energy stores, reducing the cost of locomotion. Energy storing tendons experience extremely high strains and need to be able to recoil efficiently for maximum energy storage and return. In the equine forelimb, the energy storing superficial digital flexor tendon (SDFT) has much higher failure strains than the positional common digital extensor tendon (CDET). However, we have previously shown that this is not due to differences in the properties of the SDFT and CDET fascicles (the largest tendon subunits). Instead, there is a greater capacity for interfascicular sliding in the SDFT which facilitates the greater extensions in this particular tendon (Thorpe et al., 2012). In the current study, we exposed fascicles and interfascicular matrix (IFM) from the SDFT and CDET to cyclic loading followed by a test to failure. The results show that IFM mechanical behaviour is not a result of irreversible deformation, but the IFM is able to withstand cyclic loading, and is more elastic in the SDFT than in the CDET. We also assessed the effect of ageing on IFM properties, demonstrating that the IFM is less able to resist repetitive loading as it ages, becoming stiffer with increasing age in the SDFT. These results provide further indications that the IFM is important for efficient function in energy storing tendons, and age-related alterations to the IFM may compromise function and predispose older tendons to injury.

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Fascicle sliding enables high levels of extension in energy storing tendons.

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Sliding mechanics are governed by the interfascicular matrix (IFM).

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We assessed IFM extension and recovery.

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IFM elasticity and recovery are greater in energy storing tendons.

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The IFM plays an important role in the function of energy storing tendons.

Proteomic analysis reveals age-related changes in tendon matrix composition, with age- and injury-specific matrix fragmentation

Energy storing tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT), are highly prone to injury, the incidence of which increases with aging. The cellular and molecular mechanisms that result in increased injury in aged tendons are not well established but are thought to result in altered matrix turnover. However, little attempt has been made to fully characterize the tendon proteome nor determine how the abundance of specific tendon proteins changes with aging and/or injury. The aim of this study was, therefore, to assess the protein profile of normal SDFTs from young and old horses using label-free relative quantification to identify differentially abundant proteins and peptide fragments between age groups. The protein profile of injured SDFTs from young and old horses was also assessed. The results demonstrate distinct proteomic profiles in young and old tendon, with alterations in the levels of proteins involved in matrix organization and regulation of cell tension. Furthermore, we identified several new peptide fragments (neopeptides) present in aged tendons, suggesting that there are age-specific cleavage patterns within the SDFT. Proteomic profile also differed between young and old injured tendon, with a greater number of neopeptides identified in young injured tendon. This study has increased the knowledge of molecular events associated with tendon aging and injury, suggesting that maintenance and repair of tendon tissue may be reduced in aged individuals and may help to explain why the risk of injury increases with aging.

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

Many tendon injuries are believed to result from repetitive motion or overuse, leading to the accumulation of micro-damage over time. In vitro fatigue loading can be used to characterise damage during repeated use and investigate how this may relate to the aetiology of tendinopathy.

This study considered the effect of fatigue loading on fascicles from two functionally distinct bovine tendons: the digital extensor and deep digital flexor. Micro-scale extension mechanisms were investigated in fascicles before or after a period of cyclic creep loading, comparing two different measurement techniques – the displacement of a photo-bleached grid and the use of nuclei as fiducial markers.

Whilst visual damage was clearly identified after only 300 cycles of creep loading, these visual changes did not affect either gross fascicle mechanics or fascicle microstructural extension mechanisms over the 900 fatigue cycles investigated. However, significantly greater fibre sliding was measured when observing grid deformation rather than the analysis of nuclei movement. Measurement of microstructural extension with both techniques was localised and this may explain the absence of change in microstructural deformation in response to fatigue loading. Alternatively, the data may demonstrate that fascicles can withstand a degree of matrix disruption with no impact on mechanics. Whilst use of a photo-bleached grid to directly measure the collagen is the best indicator of matrix deformation, nuclei tracking may provide a better measure of the strain perceived directly by the cells.

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Tendon fascicle gross mechanics and micro-scale deformation investigated after fatigue loading.

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Fascicles can withstand a degree of matrix disruption without impact on mechanics.

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More fibre sliding was observed measuring grid deformation than tracking nuclei.

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Nuclei tracking may better represent the strains experienced by cells than grid deformation.

Fascicles from energy-storing tendons show an age-specific response to cyclic fatigue loading

Some tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT), act as energy stores, stretching and recoiling to increase efficiency during locomotion. Our previous observations of rotation in response to applied strain in SDFT fascicles suggest a helical structure, which may provide energy-storing tendons with a greater ability to extend and recoil efficiently. Despite this specialization, energy-storing tendons are prone to age-related tendinopathy. The aim of this study was to assess the effect of cyclic fatigue loading (FL) on the microstructural strain response of SDFT fascicles from young and old horses. The data demonstrate two independent age-related mechanisms of fatigue failure; in young horses, FL caused low levels of matrix damage and decreased rotation. This suggests that loading causes alterations to the helix substructure, which may reduce their ability to recoil and recover. By contrast, fascicles from old horses, in which the helix is already compromised, showed greater evidence of matrix damage and suffer increased fibre sliding after FL, which may partially explain the age-related increase in tendinopathy. Elucidation of helix structure and the precise alterations occurring owing to both ageing and FL will help to develop appropriate preventative and repair strategies for tendinopathy.

Functionally distinct tendon fascicles exhibit different creep and stress relaxation behaviour

Most overuse tendinopathies are thought to be associated with repeated microstrain below the failure threshold, analogous to the fatigue failure that affects materials placed under repetitive loading. Investigating the progression of fatigue damage within tendons is therefore of critical importance. There are obvious challenges associated with the sourcing of human tendon samples for in vitro analysis so animal models are regularly adopted. However, data indicates that fatigue life varies significantly between tendons of different species and with different stresses in life. Positional tendons such as rat tail tendon or the bovine digital extensor are commonly applied in in vitro studies of tendon overuse, but there is no evidence to suggest their behaviour is indicative of the types of human tendon particularly prone to overuse injuries. In this study, the fatigue response of the largely positional digital extensor and the more energy storing deep digital flexor tendon of the bovine hoof were compared to the semitendinosus tendon of the human hamstring. Fascicles from each tendon type were subjected to either stress or strain controlled fatigue loading (cyclic creep or cyclic stress relaxation respectively). Gross fascicle mechanics were monitored after cyclic stress relaxation and the mean number of cycles to failure investigated with creep loading. Bovine extensor fascicles demonstrated the poorest fatigue response, while the energy storing human semitendinosus was the most fatigue resistant. Despite the superior fatigue response of the energy storing tendons, confocal imaging suggested a similar degree of damage in all three tendon types; it appears the more energy storing tendons are better able to withstand damage without detriment to mechanics.

Fatigue loading of tendon

Tendon injuries, often called tendinopathies, are debilitating and painful conditions, generally considered to develop as a result of tendon overuse. The aetiology of tendinopathy remains poorly understood, and whilst tendon biopsies have provided some information concerning tendon appearance in late-stage disease, there is still little information concerning the mechanical and cellular events associated with disease initiation and progression. Investigating this in situ is challenging, and numerous models have been developed to investigate how overuse may generate tendon fatigue damage and how this may relate to tendinopathy conditions. This article aims to review these models and our current understanding of tendon fatigue damage. We review the strengths and limitations of different methodologies for characterizing tendon fatigue, considering in vitro methods that adopt both viable and non-viable samples, as well as the range of different in vivo approaches. By comparing data across model systems, we review the current understanding of fatigue damage development. Additionally, we compare these findings with data from tendinopathic tissue biopsies to provide some insights into how these models may relate to the aetiology of tendinopathy. Fatigue-induced damage consistently highlights the same microstructural, biological and mechanical changes to the tendon across all model systems and also correlates well with the findings from tendinopathic biopsy tissue. The multiple testing routes support matrix damage as an important contributor to tendinopathic conditions, but cellular responses to fatigue appear complex and often contradictory.

The role of the non-collagenous matrix in tendon function

Tendon consists of highly ordered type I collagen molecules that are grouped together to form subunits of increasing diameter. At each hierarchical level, the type I collagen is interspersed with a predominantly non-collagenous matrix (NCM) (Connect. Tissue Res., 6, 1978, 11). Whilst many studies have investigated the structure, organization and function of the collagenous matrix within tendon, relatively few have studied the non-collagenous components. However, there is a growing body of research suggesting the NCM plays an important role within tendon; adaptations to this matrix may confer the specific properties required by tendons with different functions. Furthermore, age-related alterations to non-collagenous proteins have been identified, which may affect tendon resistance to injury. This review focuses on the NCM within the tensional region of developing and mature tendon, discussing the current knowledge and identifying areas that require further study to fully understand structure-function relationships within tendon. This information will aid in the development of appropriate techniques for tendon injury prevention and treatment.

Capacity for sliding between tendon fascicles decreases with ageing in injury prone equine tendons: a possible mechanism for age-related tendinopathy?

Age-related tendinopathy is common in both humans and horses; the initiation and progression of which is similar between species. The majority of tendon injuries occur to high-strain energy storing tendons, such as the human Achilles tendon and equine superficial digital flexor (SDFT). By contrast, the low-strain positional human anterior tibialis tendon and equine common digital extensor (CDET) are rarely injured. It has previously been established that greater extension occurs at the fascicular interface in the SDFT than in the CDET; this may facilitate the large strains experienced during locomotion in the SDFT without damage occurring to the fascicles. This study investigated the alterations in whole tendon, fascicle and interfascicular mechanical properties in the SDFT and CDET with increasing age. It was hypothesised that the amount of sliding at the fascicular interface in the SDFT would decrease with increasing horse age, whereas the properties of the interface in the CDET would remain unchanged with ageing. Data support the hypothesis; there were no alterations in the mechanical properties of the whole SDFT or its constituent fascicles with increasing age. However, there was significantly less sliding at the fascicular interface at physiological loads in samples from aged tendons. There was no relationship between fascicle sliding and age in the CDET. The increase in stiffness of the interfascicular matrix in aged SDFT may result in the fascicles being loaded at an earlier point in the stress strain curve, increasing the risk of damage. This may predispose aged tendons to tendinopathy.

Specialization of tendon mechanical properties results from interfascicular differences

Tendons transfer force from muscle to bone. Specific tendons, including the equine superficial digital flexor tendon (SDFT), also store and return energy. For efficient function, energy-storing tendons need to be more extensible than positional tendons such as the common digital extensor tendon (CDET), and when tested in vitro have a lower modulus and failure stress, but a higher failure strain. It is not known how differences in matrix organization contribute to distinct mechanical properties in functionally different tendons. We investigated the properties of whole tendons, tendon fascicles and the fascicular interface in the high-strain energy-storing SDFT and low-strain positional CDET. Fascicles failed at lower stresses and strains than tendons. The SDFT was more extensible than the CDET, but SDFT fascicles failed at lower strains than CDET fascicles, resulting in large differences between tendon and fascicle failure strain in the SDFT. At physiological loads, the stiffness at the fascicular interface was lower in the SDFT samples, enabling a greater fascicle sliding that could account for differences in tendon and fascicle failure strain. Sliding between fascicles prior to fascicle extension in the SDFT may allow the large extensions required in energy-storing tendons while protecting fascicles from damage.

Strain Transfer Through the Aortic Valve

The complex structural organization of the aortic valve (AV) extracellular matrix (ECM) enables large and highly nonlinear tissue level deformations. The collagen and elastin (elastic) fibers within the ECM form an interconnected fibrous network (FN) and are known to be the main load-bearing elements of the AV matrix. The role of the FN in enabling deformation has been investigated and documented. However, there is little data on the correlation between tissue level and FN-level strains. Investigating this correlation will help establish the mode of strain transfer (affine or nonaffine) through the AV tissue as a key feature in microstructural modeling and will also help characterize the local FN deformation across the AV sample in response to applied tissue level strains. In this study, the correlation between applied strains at tissue level, macrostrains across the tissue surface, and local FN strains were investigated. Results showed that the FN strain distribution across AV samples was inhomogeneous and nonuniform, as well as anisotropic. There was no direct transfer of the deformation applied at tissue level to the fibrous network. Loading modes induced in the FN are different than those applied at the tissue as a result of different local strains in the valve layers. This nonuniformity of local strains induced internal shearing within the FN of the AV, possibly exposing the aortic valve interstitial cells (AVICs) to shear strains and stresses.

Microstructural stress relaxation mechanics in functionally different tendons

Tendons experience widely varying loading conditions in vivo. They may be categorised by their function as either positional tendons, which are used for intricate movements and experience lower stress, or as energy storage tendons which act as highly stressed springs during locomotion. Structural and compositional differences between tendons are thought to enable an optimisation of their properties to suit their functional environment. However, little is known about structure-function relationships in tendon. This study adopts porcine flexor and extensor tendon fascicles as examples of high stress and low stress tendons, comparing their mechanical behaviour at the micro-level in order to understand their stress relaxation response. Stress-relaxation was shown to occur predominantly through sliding between collagen fibres. However, in the more highly stressed flexor tendon fascicles, more fibre reorganisation was evident when the tissue was exposed to low strains. By contrast, the low load extensor tendon fascicles appears to have less capacity for fibre reorganisation or shearing than the energy storage tendon, relying more heavily on fibril level relaxation. The extensor fascicles were also unable to sustain loads without rapid and complete stress relaxation. These findings highlight the need to optimise tendon repair solutions for specific tendons, and match tendon properties when using grafts in tendon repairs.

Increased expression of IL-6 family members in tendon pathology

OBJECTIVES

Histological examination of pathological tendon generally does not reveal signs of inflammation. However, the inflammatory cytokine IL-6 has been shown to be expressed in ruptured rotator cuff tendon. The aim of this study was to investigate the expression of IL-6 family members in painful posterior tibialis tendon (PTT) and in painful and ruptured Achilles tendon (AT) compared with normal tendon.

METHODS

AT samples were obtained from cadavers (normal) or from patients undergoing surgical procedures to treat chronic painful tendinopathy or ruptured tendon. PTT samples were obtained from patients undergoing surgery for other reasons (normal) and from patients with PTT dysfunction (painful). Total RNA was extracted and mRNA expression was analysed by quantitative real-time PCR.

RESULTS

Collagen type I α-chain I (COL1A1) expression was increased in both painful PTT and AT compared with normal. Ciliary neurotrophic factor levels were increased in painful PTT only. In the painful AT, cyclooxygenase-2 (COX2) and IL-6 expression increased compared with normal. In the ruptured AT, levels of VEGF A, COX2, oncostatin-M, leukaemia inhibitory factor and IL-6 expression were higher compared with both normal and painful AT. IL-6R expression decreased in both painful and ruptured AT compared with normal.

CONCLUSION

Painful AT and PTT show different expression patterns, indicating a substantial difference between those two tendinopathies. Inflammatory markers are up-regulated in painful and particularly in ruptured AT, pointing towards a role of inflammation not only in rupture healing, but also in Achilles tendinopathy.

Anisotropic strain transfer through the aortic valve and its relevance to the cellular mechanical environment

Aortic valve interstitial cells are responsible for maintaining the valve in response to their local mechanical environment. However, the complex organization of the extracellular matrix means cell strains cannot be directly derived from gross strains, and knowledge of tissue structure-function correlations is fundamental towards understanding mechanotransduction. This study investigates strain transfer through the valve, hypothesizing that organization of the valve matrix leads to non-homogenous local strains. Radial and circumferential samples were cut from aortic valve leaflets and subjected to quasi-static mechanical characterization. Further samples were imaged using confocal microscopy, to determine local strains in the matrix. Mechanical data demonstrated that the valve was significantly stronger and stiffer when loaded circumferentially, comparable with previous studies. Micromechanical studies demonstrated that strain transfer through the matrix is anisotropic and indirect, with local strains consistently smaller than applied strains in both orientations. Under radial loading, strains were transferred linearly to cells. However, under circumferential loading, strains were only one-third of applied values, with a less direct relationship between applied and local strains. This may result from matrix reorganization, and be important for preventing cellular damage during normal valve function. These findings should be taken into account when investigating interstitial cell behaviours, such as cell metabolism and mechanotransduction.

Cyclic loading of tendon fascicles using a novel fatigue loading system increases interleukin-6 expression by tenocytes

Repetitive strain or 'overuse' is thought to be a major factor contributing to the development of tendinopathy. The aims of our study were to develop a novel cyclic loading system, and use it to investigate the effect of defined loading conditions on the mechanical properties and gene expression of isolated tendon fascicles. Tendon fascicles were dissected from bovine-foot extensors and subjected to cyclic tensile strain (1 Hz) at 30% or 60% of the strain at failure, for 0 h (control), 15 min, 30 min, 1 h, or 5 h. Post loading, a quasi-static test to failure assessed damage. Gene expression at a selected loading regime (1 h at 30% failure strain) was analyzed 6 h post loading by quantitative real-time polymerase chain reaction. Compared with unloaded controls, loading at 30% failure strain took 5 h to lead to a significant decrease in failure stress, whereas loading to 60% led to a significant reduction after 15 min. Loading for 1 h at 30% failure strain did not create significant structural damage, but increased Collagen-1-alpha-chain-1 and interleukin-6 (IL6) expression, suggesting a role of IL6 in tendon adaptation to exercise. Correlating failure properties with fatigue damage provides a method by which changes in gene expression can be associated with different degrees of fatigue damage.

A combined experimental and modelling approach to aortic valve viscoelasticity in tensile deformation

The quasi-static mechanical behaviour of the aortic valve (AV) is highly non-linear and anisotropic in nature and reflects the complex collagen fibre kinematics in response to applied loading. However, little is known about the viscoelastic behaviour of the AV. The aim of this study was to investigate porcine AV tissue under uniaxial tensile deformation, in order to establish the directional dependence of its viscoelastic behaviour. Rate dependency associated with different mechanical properties was investigated, and a new viscoelastic model incorporating rate effects developed, based on the Kelvin-Voigt model. Even at low applied loads, experimental results showed rate dependency in the stress-strain response, and also hysteresis and dissipation effects. Furthermore, corresponding values of each parameter depended on the loading direction. The model successfully predicted the experimental data and indicated a 'shear-thinning' behaviour. By extrapolating the experimental data to that at physiological strain rates, the model predicts viscous damping coefficients of 8.3 MPa s and 3.9 MPa s, in circumferential and radial directions, respectively. This implies that the native AV offers minimal resistance to internal shear forces induced by blood flow, a potentially critical design feature for substitute implants. These data suggest that the mechanical behaviour of the AV cannot be thoroughly characterised by elastic deformation and fibre recruitment assumptions alone.

Specimen dimensions influence the measurement of material properties in tendon fascicles

Stress, strain and modulus are regularly used to characterize material properties of tissue samples. However, when comparing results from different studies it is evident the reported material properties, particularly failure strains, vary hugely. The aim of our study was to characterize how and why specimen length and cross-sectional area (CSA) appear to influence failure stress, strain and modulus in fascicles from two functionally different tendons. Fascicles were dissected from five rat tails and five bovine foot extensors, their diameters determined by a laser micrometer, and loaded to failure at a range of grip-to-grip lengths. Strain to failure significantly decreased with increasing in specimen length in both rat and bovine fascicles, while modulus increased. Specimen length did not influence failure stress in rat tail fascicles, although in bovine fascicles it was significantly lower in the longer 40 mm specimens compared to 5 and 10 mm specimens. The variations in failure strain and modulus with sample length could be predominantly explained by end-effects. However, it was also evident that strain fields along the sample length were highly variable and notably larger towards the ends of the sample than the mid-section even at distances in excess of 5 mm from the gripping points. Failure strain, stress and modulus correlated significantly with CSA at certain specimen lengths. Our findings have implications for the mechanical testing of tendon tissue: while it is not always possible to control for fascicle length and/or CSA, these parameters have to be taken into account when comparing samples of different dimensions.

In situ multi-level analysis of viscoelastic deformation mechanisms in tendon collagen

Tendon is a hydrated multi-level fibre composite, in which time-dependent behaviour is well established. Studies indicate significant stress relaxation, considered important for optimising tissue stiffness. However, whilst this behaviour is well documented, the mechanisms associated with the response are largely unknown. This study investigates the sub-structural mechanisms occurring during stress relaxation at both the macro (fibre) and nano (fibril) levels of the tendon hierarchy. Stress relaxation followed a two-stage exponential behaviour, during which structural changes were visible at the fibre and fibril levels. Fibril relaxation and fibre sliding showed a double exponential response, while fibre sliding was clearly the largest contributor to relaxation. The amount of stress relaxation and sub-structural reorganisation increased with increasing load increments, but fibre sliding was consistently the largest contributor to stress relaxation. A simple model of tendon viscoelasticity at the fibril and fibre levels has been developed, capturing this behaviour by serially coupling a Voigt element (collagen fibril), with two Maxwell elements (non-collagenous matrix between fibrils and fibres). This multi-level analysis provides a first step towards understanding how sub-structural interactions contribute to viscoelastic behaviour. It indicates that nano- and micro-scale shearing are significant dissipative mechanisms, and the kinetics of relaxation follows a two-stage exponential decay, well fitted by serially coupled viscoelastic elements.

The influence of swelling and matrix degradation on the microstructural integrity of tendon

Tendon is multi-level fibre composite material, responsible for the transmission of forces from muscles to the skeleton. It is composed of a hierarchical arrangement of collagenous units surrounded by a proteoglycan-rich matrix, arranged to support strain transfer, and thus contribute to the mechanical behaviour of tendon. This study examines the effect of swelling and enzymatic degradation on structural integrity at different levels of the tendon hierarchy. Biochemical and microstructural analysis are used to examine the effects of incubation on the composition and swelling of the matrix, prior to a mechanical characterisation of sample integrity. Results indicated significant swelling of tendon fibrils and interfibrillar matrix after incubation in phosphate buffered saline, leading to a reduction in ultimate tensile load, with failure initiated between fibrils and sub-fibrils. In contrast, incubation with the enzyme chondroitinase ABC resulted in a total removal of glycosaminoglycan from the samples, and a subsequent reduction in the extent of swelling. These fascicles also demonstrated an increase in failure loads, with failure predominating between fibres. The findings from this work confirm the importance of the non-collagenous matrix components in controlling strain transfer within tendon structures. It also highlights the necessity to maintain samples within a suitable and controlled environment prior to testing.

Cyclic tensile strain upregulates collagen synthesis in isolated tendon fascicles

Mechanical stimulation has been implicated as an important regulatory factor in tendon homeostasis. In this study, a custom-designed tensile loading system was used to apply controlled mechanical stimulation to isolated tendon fascicles, in order to examine the effects of 5% cyclic tensile strain at 1 Hz on cell proliferation and matrix synthesis. Sample viability and gross structural composition were maintained over a 24 h loading period. Data demonstrated no statistically significant differences in cell proliferation or glycosaminoglycan production, however, collagen synthesis was upregulated with the application of cyclic tensile strain over the 24 h period. Moreover, a greater proportion of the newly synthesised matrix was retained within the sample after loading. These data provide evidence of altered anabolic activity within tendon in response to mechanical stimuli, and suggest the importance of cyclic tensile loading for the maintenance of the collagen hierarchy within tendon.

The Influence of Noncollagenous Matrix Components on the Micromechanical Environment of Tendon Fascicles

Tendon is composed of type I collagen fibers, interspersed with proteoglycan matrix and cells. Glycosaminoglycans may play a role in maintaining the structural integrity of tendon, preventing excessive shearing between collagen components. This study tests the hypothesis that tendon extension mechanisms can be altered by modifying the composition of noncollagenous matrix. Tendon explants were treated with phosphate buffered saline (PBS) or PBS + 0.5 U ml(-1) chondroitinase ABC. Structural changes were examined using TEM and biochemical analysis, while strain response was examined using confocal microscopy and gross mechanical characterization. Chondroitinase ABC removed 90% of glycosaminoglycans from the matrix. Results demonstrated significant swelling of fibrils and surrounding matrix when incubated in either solution. In response to applied strain, PBS incubated samples demonstrated significantly less sliding between adjacent fibers than nonincubated, and a 33% reduction in maximum force. By contrast, fascicles incubated in chondroitinase ABC demonstrated a similar strain response to nonincubated. Data indicate that collagen-proteoglycan binding characteristics can be influenced by incubation and this, in turn, can influence the preferred extension mechanisms adopted by fascicles. This highlights the importance of maintaining fascicles within their natural environment to prevent structural or mechanical changes prior to subsequent analysis.

An investigation into the effects of the hierarchical structure of tendon fascicles on micromechanical properties

During physiological loading, a tendon is subjected to tensile strains in the region of up to 6 per cent. These strains are reportedly transmitted to cells, potentially initiating specific mechanotransduction pathways. The present study examines the local strain fields within tendon fascicles subjected to tensile strain in order to determine the mechanisms responsible for fascicle extension. A hierarchical approach to the analysis was adopted, involving micro and macro examination. Micro examination was carried out using a custom-designed rig, to enable the analysis of local tissue strains in isolated fascicles, using the cell nuclei as strain markers. In macro examination, a video camera was used to record images of the fascicles during mechanical testing, highlighting the point of crimp straightening and macro failure. Results revealed that local tensile strains within a collagen fibre were consistently smaller than the applied strain and showed no further increase once fibres were aligned. By contrast, between-group displacements, a measure of fibre sliding, continued to increase beyond crimp straightening, reaching a mean value of 3.9 per cent of the applied displacement at 8 per cent strain. Macro analysis displayed crimp straightening at a mean load of 1 N and sample failure occurred through the slow unravelling of the collagen fibres. Fibre sliding appears to provide the major mechanism enabling tendon fascicle extension within the rat-tail tendon. This process will necessarily affect local and cellular strains and consequently mechanotransduction pathways.

Development of a technique to determine strains in tendons using the cell nuclei

Tenocytes detect mechanical stimuli in vivo, and respond through mechanotransduction pathways to initiate matrix remodelling in tendons. Due to the crimped nature of tendon fascicles, the strain field throughout is non-homogeneous. The present study has developed a means to quantify the local strain fields within a fascicle by monitoring the relative movement and deformation of fluorescently labelled tenocyte nuclei. A stage mounted test rig was designed to apply tensile strain to fascicles. Rat tail and bovine extensor tendons were harvested for analysis, and the cell nuclei stained and visualised using an inverted confocal microscope. As the fascicles were subjected to gross strains of up to 5%, the movement of selected tenocyte nuclei were recorded. Results from a series of cell nuclei from both tendon sources revealed that local strains were significantly less than the applied strain. The nuclei length to width ratio, an indicator of cell deformation, also increased with applied strain, most significantly between 2 and 3% applied strain.