Anatomical heterogeneity of tendon: Fascicular and interfascicular tendon compartments have distinct proteomic composition

Structural Variations of Tendons: A Systematic Search and Narrative Review of Histological Differences Between Tendons, Tendon Regions, Sex, and Age

Tendons are force-transmitting structures which facilitate musculoskeletal functioning. Characterizing variations between different anatomical tendons, regions within tendons, as well as between the sexes and with age can improve understanding of tendon physiology and pathology. A systematic search of the literature was conducted to identify and summarize microscopic structural (histological) variations in normal/healthy tendons in relation to these variables (Tendon, Region, Age, Sex, and Other). Regional differences within individual tendons have been investigated in numerous studies, however investigations comparing histological variations between a range of different tendons are sparse, with most focusing on a few select tendons. When injured, ageing tendons typically have a greater degree of pathological changes than younger tendons, but few studies have documented variations in tendon histology throughout typical (uninjured) ageing or across large age spans. Similarly, sex-related observations of tendon structure are underreported. This narrative review summarizes studies on these topics and explores interactions between these variables, as well as the implications of these in the context of selecting control samples for studies of tendon pathology. Future studies should endeavour to improve knowledge of tendon structural variations-specifically focusing on normal tendons-to facilitate understanding of tendon structure-function relationships, physiological mechanisms involved in tendon damage/healing, and to aid clinical research and practice.

Inflammation and response to bacterial infection as potential drivers of equine odontoclastic tooth resorption and hypercementosis: A proteomics insight

BACKGROUND

Equine dental diseases significantly impact a horse's overall health, performance and quality of life. They can result in secondary infections and digestive disturbances, potentially leading to colic. A recently described disease affecting the incisors of horses is equine odontoclastic tooth resorption and hypercementosis (EOTRH). Understanding EOTRH is crucial for early diagnosis, effective management and prevention of its severe consequences.

OBJECTIVES

To determine proteomic differences in incisor cementum in horses with and without clinical EOTRH.

STUDY DESIGN

Comparative and observational clinical study.

METHODS

Teeth were extracted (N = 5) and cementum was isolated using a diamond wire. Proteins were extracted using an optimised sequential workflow, and trypsin was digested for mass spectrometry. Protein identification and label-free quantification were undertaken.

RESULTS

In total 1149 unique proteins were detected in cementum across all samples. We identified four proteins exclusively in EOTRH-affected cementum. EOTRH samples showed a higher heterogeneity than healthy samples. In total, 54 proteins were increased in EOTRH, and 64 proteins were reduced (adjusted p-value <0.05). Inflammatory proteins, such as cathepsin G (p = 0.004), neutrophil elastase (p = 0.003), bactericidal permeability-increasing protein (p = 0.002), azurocidin (p = 0.003) and lactotransferrin (p = 0.002) were all increased in EOTRH. Pathway analysis revealed that antimicrobial peptides (Z score 2.65, p = 1.93E-09) and neutrophil degranulation (Z-score 1.89, p = 1.7E-04) were commonly up-regulated canonical pathways.

MAIN LIMITATIONS

The sample size was limited. Lack of age-matched healthy controls.

CONCLUSION

EOTRH leads to biochemical changes within the cementum proteome, which are important in explaining the physiological changes occurring in disease. Differentially abundant proteins may represent promising biomarkers for earlier disease detection and the establishment of a cell-based model could provide further insight into the role these proteins play in hypercementosis and resorption.

The aging Achilles tendon: model-predicted changes in calf muscle neuromechanics

Forward propulsion depends on the forces generated by the triceps surae muscles and transmitted through the muscles' subtendons, which merge and twist to form the Achilles tendon (AT). As people age, the AT may undergo structural changes that could alter the subtendons' ability to transmit forces or function with some independence; prominent changes include increased tendon compliance and a proliferation of interfascicular adhesions compared to younger tendon. However, the effects of age-related changes on the subtendons are difficult to isolate in vivo. Here, we used a Hill-type musculoskeletal model of the triceps surae muscle-subtendon units to simulate the effects of age-related changes on gastrocnemius (GAS) and soleus (SOL) muscle contractile dynamics across a range of physiological force levels during fixed-end contractions. We simulated individual and dual muscle excitations with altered tendon compliance (εo = 3 %, 6 %, 9 %) and inclusion of a shared tendon. Consistent with fundamental muscle mechanics, compared to stiffer tendons, increased tendon compliance elicited more than three times the GAS and SOL fiber shortening and greater muscle excitation - effects that increased with requisite force demand. However, our model results also suggest combinatory effects of increased tendon compliance and interfascicle adhesions in the aging AT that deleteriously amplify redistribution from the GAS to the SOL which may be functionally detrimental during gait.

Recent developments in Achilles tendon risk-analyzing rupture factors for enhanced injury prevention and clinical guidance: Current implications of regenerative medicine

Background

In recent years, many countries have actively implemented programs and strategies to promote physical education and sports. Despite these efforts, the increase in physical activity has been accompanied by a significant rise in muscle and tendon-ligament injuries, with Achilles tendon rupture being the most prevalent, accounting for 47 % of such injuries. This review aims to summarize all significant factors determining the predisposition of the Achilles tendon to rupture, to develop effective personalized prevention measures.

Objective

To identify and evaluate the risk factors contributing to Achilles tendon rupture and to develop strategies for personalized prevention.

Methods

This review utilized data from several databases, including Elsevier, Global Health, PubMed-NCBI, Embase, Medline, Scopus, ResearchGate, RSCI, Cochrane Library, Google Scholar, eLibrary.ru, and CyberLeninka. Both non-modifiable and modifiable risk factors for Achilles tendon injuries and ruptures were analyzed.

Results

The analysis identified several non-modifiable risk factors, such as genetic predisposition, anatomical and functional features of the Achilles tendon, sex, and age. These factors should be considered when selecting sports activities and designing training programs. Modifiable risk factors included imbalanced nutrition, improper exercise regimens, and inadequate monitoring of Achilles tendon conditions in athletes. Early treatment of musculoskeletal injuries, Achilles tendon diseases, foot deformities, and metabolic disorders is crucial. Long-term drug use and its risk assessment were also highlighted as important considerations. Furthermore, recent clinical advancements in both conventional and surgical methods to treat Achilles tendon injuries were described. The efficacy of these therapies in enhancing functional outcomes in individuals with Achilles injuries was compared. Advancements in cell-based and scaffold-based therapies aimed at enhancing cell regeneration and repairing Achilles injuries were also discussed.

Discussion

The combination of several established factors significantly increases the risk of Achilles tendon rupture. Addressing these factors through personalized prevention strategies can effectively reduce the incidence of these injuries. Proper nutrition, regular monitoring, timely treatment, and the correction of metabolic disorders are essential components of a comprehensive prevention plan.

Conclusion

Early identification of Achilles tendon risk factors allows for the timely development of effective personalized prevention strategies. These measures can contribute significantly to public health preservation by reducing the incidence of Achilles tendon ruptures associated with physical activity and sports. Continued research and clinical advancements in treatment methods will further enhance the ability to prevent and manage Achilles tendon injuries.

The translational potential of this article

This study identifies key modifiable and non-modifiable risk factors for Achilles tendon injuries, paving the way for personalized prevention strategies. Emphasizing nutrition, exercise, and early treatment of musculoskeletal issues, along with advancements in cell-based therapies, offers promising avenues for improving recovery and outcomes. These findings can guide clinical practices in prevention and rehabilitation, ultimately reducing Achilles injuries and enhancing public health.

Leveraging in vivo animal models of tendon loading to inform tissue engineering approaches

Tendon injuries disrupt successful transmission of force between muscle and bone, resulting in reduced mobility, increased pain, and significantly reduced quality of life for affected patients. There are currently no targeted treatments to improve tendon healing beyond conservative methods such as rest and physical therapy. Tissue engineering approaches hold great promise for designing instructive biomaterials that could improve tendon healing or for generating replacement graft tissue. More recently, engineered microphysiological systems to model tendon injuries have been used to identify therapeutic targets. Despite these advances, current tissue engineering efforts that aim to regenerate, replace, or model injured tendons have largely failed due in large part to a lack of understanding of how the mechanical environment of the tendon influences tissue homeostasis and how altered mechanical loading can promote or prevent disease progression. This review article draws inspiration from what is known about tendon loading from in vivo animal models and identifies key metrics that can be used to benchmark success in tissue engineering applications. Finally, we highlight important challenges and opportunities for the field of tendon tissue engineering that should be taken into consideration in designing engineered platforms to understand or improve tendon healing.

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.

Earlier proteoglycan turnover promotes higher efficiency matrix remodeling in MRL/MpJ tendons

While most mammalian tissue regeneration is limited, the Murphy Roths Large (MRL/MpJ) mouse has been identified to regenerate several tissues, including tendon. Recent studies have indicated that this regenerative response is innate to the tendon tissue and not reliant on a systemic inflammatory response. Therefore, we hypothesized that MRL/MpJ mice may also exhibit a more robust homeostatic regulation of tendon structure in response to mechanical loading. To assess this, MRL/MpJ and C57BL/6J flexor digitorum longus tendon explants were subjected to stress-deprived conditions in vitro for up to 14 days. Explant tendon health (metabolism, biosynthesis, and composition), matrix metalloproteinase (MMP) activity, gene expression, and tendon biomechanics were assessed periodically. We found a more robust response to the loss of mechanical stimulus in the MRL/MpJ tendon explants, exhibiting an increase in collagen production and MMP activity consistent with previous in vivo studies. This greater collagen turnover was preceded by an early expression of small leucine-rich proteoglycans and proteoglycan-degrading MMP-3, promoting efficient regulation and organization of newly synthesized collagen and allowing for more efficient overall turnover in MRL/MpJ tendons. Therefore, mechanisms of MRL/MpJ matrix homeostasis may be fundamentally different from that of B6 tendons and may indicate better recovery from mechanical microdamage in MRL/MpJ tendons. We demonstrate here the utility of the MRL/MpJ model in elucidating mechanisms of efficient matrix turnover and its potential to shed light on new targets for more effective treatments for degenerative matrix changes brought about by injury, disease, or aging.

Equine tendon mechanical behaviour: Prospects for repair and regeneration applications

Tendons are dense connective tissues that play an important role in the biomechanical function of the musculoskeletal system. The mechanical forces have been implicated in every aspect of tendon biology. Tendon injuries are frequently occurring and their response to treatments is often unsatisfactory. A better understanding of tendon biomechanics and mechanobiology can help develop treatment options to improve clinical outcomes. Recently, tendon tissue engineering has gained more attention as an alternative treatment due to its potential to overcome the limitations of current treatments. This review first provides a summary of tendon mechanical properties, focusing on recent findings of tendon mechanobiological responses. In the next step, we highlight the biomechanical parameters of equine energy-storing and positional tendons. The final section is devoted to how mechanical loading contributes to tenogenic differentiation using bioreactor systems. This study may help develop novel strategies for tendon injury prevention or accelerate and improve tendon healing.

The tendon interfascicular basement membrane provides a vascular niche for CD146+ cell subpopulations

Introduction: The interfascicular matrix (IFM; also known as the endotenon) is critical to the mechanical adaptations and response to load in energy-storing tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT). We hypothesized that the IFM is a tendon progenitor cell niche housing an exclusive cell subpopulation. Methods: Immunolabelling of equine superficial digital flexor tendon was used to identify the interfascicular matrix niche, localising expression patterns of CD31 (endothelial cells), Desmin (smooth muscle cells and pericytes), CD146 (interfascicular matrix cells) and LAMA4 (interfascicular matrix basement membrane marker). Magnetic-activated cell sorting was employed to isolate and compare in vitro properties of CD146+ and CD146- subpopulations. Results: Labelling for CD146 using standard histological and 3D imaging of large intact 3D segments revealed an exclusive interfascicular cell subpopulation that resides in proximity to a basal lamina which forms extensive, interconnected vascular networks. Isolated CD146+ cells exhibited limited mineralisation (osteogenesis) and lipid production (adipogenesis). Discussion: This study demonstrates that the interfascicular matrix is a unique tendon cell niche, containing a vascular-rich network of basement membrane, CD31+ endothelial cells, Desmin+ mural cells, and CD146+ cell populations that are likely essential to tendon structure and/or function. Contrary to our hypothesis, interfascicular CD146+ subpopulations did not exhibit stem cell-like phenotypes. Instead, our results indicate CD146 as a pan-vascular marker within the tendon interfascicular matrix. Together with previous work demonstrating that endogenous tendon CD146+ cells migrate to sites of injury, our data suggest that their mobilisation to promote intrinsic repair involves changes in their relationships with local interfascicular matrix vascular and basement membrane constituents.

Tendon Aging

Tendons are mechanosensitive connective tissues responsible for the connection between muscles and bones by transmitting forces that allow the movement of the body, yet, with advancing age, tendons become more prone to degeneration followed by injuries. Tendon diseases are one of the main causes of incapacity worldwide, leading to changes in tendon composition, structure, and biomechanical properties, as well as a decline in regenerative potential. There is still a great lack of knowledge regarding tendon cellular and molecular biology, interplay between biochemistry and biomechanics, and the complex pathomechanisms involved in tendon diseases. Consequently, this reflects a huge need for basic and clinical research to better elucidate the nature of healthy tendon tissue and also tendon aging process and associated diseases. This chapter concisely describes the effects that the aging process has on tendons at the tissue, cellular, and molecular levels and briefly reviews potential biological predictors of tendon aging. Recent research findings that are herein reviewed and discussed might contribute to the development of precision tendon therapies targeting the elderly population.

Development of tropoelastin-functionalized anisotropic PCL scaffolds for musculoskeletal tissue engineering

The highly organized extracellular matrix (ECM) of musculoskeletal tissues, encompassing tendons, ligaments and muscles, is structurally anisotropic, hierarchical and multi-compartmental. These features collectively contribute to their unique function. Previous studies have investigated the effect of tissue-engineered scaffold anisotropy on cell morphology and organization for musculoskeletal tissue repair and regeneration, but the hierarchical arrangement of ECM and compartmentalization are not typically replicated. Here, we present a method for multi-compartmental scaffold design that allows for physical mimicry of the spatial architecture of musculoskeletal tissue in regenerative medicine. This design is based on an ECM-inspired macromolecule scaffold. Polycaprolactone (PCL) scaffolds were fabricated with aligned fibers by electrospinning and mechanical stretching, and then surface-functionalized with the cell-supporting ECM protein molecule, tropoelastin (TE). TE was attached using two alternative methods that allowed for either physisorption or covalent attachment, where the latter was achieved by plasma ion immersion implantation (PIII). Aligned fibers stimulated cell elongation and improved cell alignment, in contrast to randomly oriented fibers. TE coatings bound by physisorption or covalently following 200 s PIII treatment promoted fibroblast proliferation. This represents the first cytocompatibility assessment of novel PIII-treated TE-coated PCL scaffolds. To demonstrate their versatility, these 2D anisotropic PCL scaffolds were assembled into 3D hierarchical constructs with an internally compartmentalized structure to mimic the structure of musculoskeletal tissue.

The role of extracellular vesicle miRNAs and tRNAs in synovial fibroblast senescence

Extracellular vesicles are mediators of intercellular communication with critical roles in cellular senescence and ageing. In arthritis, senescence is linked to the activation of a pro-inflammatory phenotype contributing to chronic arthritis pathogenesis. We hypothesised that senescent osteoarthritic synovial fibroblasts induce senescence and a pro-inflammatory phenotype in non-senescent osteoarthritic fibroblasts, mediated through extracellular vesicle cargo. Small RNA-sequencing and mass spectrometry proteomics were performed on extracellular vesicles isolated from the secretome of non-senescent and irradiation-induced senescent synovial fibroblasts. β-galactosidase staining confirmed senescence in SFs. RNA sequencing identified 17 differentially expressed miRNAs, 11 lncRNAs, 14 tRNAs and one snoRNA and, 21 differentially abundant proteins were identified by mass spectrometry. Bioinformatics analysis of miRNAs identified fibrosis, cell proliferation, autophagy, and cell cycle as significant pathways, tRNA analysis was enriched for signaling pathways including FGF, PI3K/AKT and MAPK, whilst protein analysis identified PAX3-FOXO1, MYC and TFGB1 as enriched upstream regulators involved in senescence and cell cycle arrest. Finally, treatment of non-senescent synovial fibroblasts with senescent extracellular vesicles confirmed the bystander effect, inducing senescence in non-senescent cells potentially through down regulation of NF-κβ and cAMP response element signaling pathways thus supporting our hypothesis. Understanding the exact composition of EV-derived small RNAs of senescent cells in this way will inform our understanding of their roles in inflammation, intercellular communication, and as active molecules in the senescence bystander effect.

Bioactive extracellular matrix fragments in tendon repair

Tendinopathy is a common tendon disorder that causes pain, loss of strength and function, and local inflammation mainly characterized by hypoxia, collagen degradation, and extracellular matrix (ECM) disorganization. Generally, ECM degradation and remodeling is tightly regulated; however, hyperactivation of matrix metalloproteases (MMPs) contributes to excessive collagenolysis under pathologic conditions resulting in tendon ECM degradation. This review article focuses on the production, function, and signaling of matrikines for tendon regeneration following injury with insights into the expression, tissue compliance, and cell proliferation exhibited by various matrikines. Furthermore, the regenerative properties suggest translational significance of matrikines to improve the outcomes post-injury by assisting with tendon healing.

Tendon healing: a concise review on cellular and molecular mechanisms with a particular focus on the Achilles tendon

Tendon is a bradytrophic and hypovascular tissue, hence, healing remains a major challenge. The molecular key events involved in successful repair have to be unravelled to develop novel strategies that reduce the risk of unfavourable outcomes such as non-healing, adhesion formation, and scarring. This review will consider the diverse pathophysiological features of tendon-derived cells that lead to failed healing, including misrouted differentiation (e.g. de- or transdifferentiation) and premature cell senescence, as well as the loss of functional progenitors. Many of these features can be attributed to disturbed cell-extracellular matrix (ECM) or unbalanced soluble mediators involving not only resident tendon cells, but also the cross-talk with immigrating immune cell populations. Unrestrained post-traumatic inflammation could hinder successful healing. Pro-angiogenic mediators trigger hypervascularization and lead to persistence of an immature repair tissue, which does not provide sufficient mechano-competence. Tendon repair tissue needs to achieve an ECM composition, structure, strength, and stiffness that resembles the undamaged highly hierarchically ordered tendon ECM. Adequate mechano-sensation and -transduction by tendon cells orchestrate ECM synthesis, stabilization by cross-linking, and remodelling as a prerequisite for the adaptation to the increased mechanical challenges during healing. Lastly, this review will discuss, from the cell biological point of view, possible optimization strategies for augmenting Achilles tendon (AT) healing outcomes, including adapted mechanostimulation and novel approaches by restraining neoangiogenesis, modifying stem cell niche parameters, tissue engineering, the modulation of the inflammatory cells, and the application of stimulatory factors.Cite this article: Bone Joint Res 2022;11(8):561-574.

Tenogenic induction of human adipose-derived stem cells by soluble tendon extracellular matrix: composition and transcriptomic analyses

Background

Tendon healing is clinically challenging largely due to its inferior regenerative capacity. We have previously prepared a soluble, DNA-free, urea-extracted bovine tendon-derived extracellular matrix (tECM) that exhibits strong pro-tenogenic bioactivity on human adipose-derived stem cells (hASCs). In this study, we aimed to elucidate the mechanism of tECM bioactivity via characterization of tECM protein composition and comparison of transcriptomic profiles of hASC cultures treated with tECM versus collagen type I (Col1) as a control ECM component.

Methods

The protein composition of tECM was characterized by SDS-PAGE, hydroxyproline assay, and proteomics analysis. To investigate tECM pro-tenogenic bioactivity and mechanism of action, differentiation of tECM-treated hASC cultures was compared to serum control medium or Col1-treated groups, as assessed via immunofluorescence for tenogenic markers and RNA Sequencing (RNA-Seq).

Results

Urea-extracted tECM yielded consistent protein composition, including collagens (20% w/w) and at least 17 non-collagenous proteins (< 100 kDa) based on MS analysis. Compared to current literature, tECM included key tendon ECM components that are functionally involved in tendon regeneration, as well as those that are involved in similar principal Gene Ontology (GO) functions (ECM-receptor interaction and collagen formation) and signaling pathways (ECM-receptor interaction and focal adhesion). When used as a cell culture supplement, tECM enhanced hASC proliferation and tenogenic differentiation compared to the Col1 and FBS treatment groups based on immunostaining of tenogenesis-associated markers. Furthermore, RNA-Seq analysis revealed a total of 584 genes differentially expressed among the three culture groups. Specifically, Col1-treated hASCs predominantly exhibited expression of genes and pathways related to ECM-associated processes, while tECM-treated hASCs expressed a mixture of ECM- and cell activity-associated processes, which may explain in part the enhanced proliferation and tenogenic differentiation of tECM-treated hASCs.

Conclusions

Our findings showed that urea-extracted tECM contained 20% w/w collagens and is significantly enriched with other non-collagenous tendon ECM components. Compared to Col1 treatment, tECM supplementation enhanced hASC proliferation and tenogenic differentiation as well as induced distinct gene expression profiles. These findings provide insights into the potential mechanism of the pro-tenogenic bioactivity of tECM and support the development of future tECM-based approaches for tendon repair.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03038-0.

The tendon microenvironment: Engineered in vitro models to study cellular crosstalk

Tendinopathy is a multi-faceted pathology characterized by alterations in tendon microstructure, cellularity and collagen composition. Challenged by the possibility of regenerating pathological or ruptured tendons, the healing mechanisms of this tissue have been widely researched over the past decades. However, so far, most of the cellular players and processes influencing tendon repair remain unknown, which emphasizes the need for developing relevant in vitro models enabling to study the complex multicellular crosstalk occurring in tendon microenvironments. In this review, we critically discuss the insights on the interaction between tenocytes and the other tendon resident cells that have been devised through different types of existing in vitro models. Building on the generated knowledge, we stress the need for advanced models able to mimic the hierarchical architecture, cellularity and physiological signaling of tendon niche under dynamic culture conditions, along with the recreation of the integrated gradients of its tissue interfaces. In a forward-looking vision of the field, we discuss how the convergence of multiple bioengineering technologies can be leveraged as potential platforms to develop the next generation of relevant in vitro models that can contribute for a deeper fundamental knowledge to develop more effective treatments.

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.

Treatment of partial injury of the calcaneus tendon with heterologous fibrin biopolymer and/or photobiomodulation in rats

The present study aimed to evaluate the new heterologous fibrin biopolymer associated, or not, with photobiomodulation therapy for application in tendon injuries, considered a serious and common orthopedic problem. Thus, 84 Rattus norvegicus had partial transection of the calcaneus tendon (PTCT) and were randomly divided into: control (CG); heterologous fibrin biopolymer (HFB); photobiomodulation (PBM); heterologous fibrin biopolymer + photobiomodulation (HFB + PBM). The animals received HFB immediately after PTCT, while PBM (660 nm, 40 mW, 0.23 J) started 24 h post injury and followed every 24 h for 7, 14, and 21 days. The results of the edema volume showed that after 24 h of PTCT, there was no statistical difference among the groups. After 7, 14, and 21 days, it was observed that the treatment groups were effective in reducing edema when compared to the control. The HFB had the highest edema volume reduction after 21 days of treatment. The treatment groups did not induce tissue necrosis or infections on the histopathological analysis. Tenocyte proliferation, granulation tissue, and collagen formation were observed in the PTCT area in the HFB and HFB + PBM groups, which culminated a better repair process when compared to the CG in the 3 experimental periods. Interestingly, the PBM group revealed, in histological analysis, major tendon injury after 7 days; however, in the periods of 14 and 21 days, the PBM had a better repair process compared to the CG. In the quantification of collagen, there was no statistical difference between the groups in the 3 experimental periods. The findings suggest that the HFB and PBM treatments, isolated or associated, were effective in reducing the volume of the edema, stimulating the repair process. However, the use of HFB alone was more effective in promoting the tendon repair process. Thus, the present study consolidates previous studies of tendon repair with this new HFB. Future clinical trials will be needed to validate this proposal.

The dispersion of viscoelastic properties of fascicle bundles within the tendon results from the presence of interfascicular matrix and flow of body fluids

In this work, we investigate differences in the mechanical and structural properties of tendon fascicle bundles dissected from different areas of bovine tendons. The properties of tendon fascicle bundles were investigated by means of uniaxial tests with relaxation periods and hysteresis, dynamic mechanical analysis (DMA), as well as magnetic resonance imaging (MRI). Uniaxial tests with relaxation periods revealed greater elastic modulus, hysteresis, as well as stress drop during the relaxation of samples dissected from the posterior side of the tendon. However, the normalized stress relaxation curves did not show a statistically significant difference in the stress drop between specimens cut from different zones or between different strain levels. Using dynamic mechanical analysis, we found that fascicle bundles dissected from the anterior side of the tendon had lower storage and loss moduli, which could result from altered fluid flow within the interfascicular matrix (IFM). The lower water content, diffusivity, and higher fractional anisotropy of the posterior part of the tendon, as observed using MRI, indicates a different structure of the IFM, which controls the flow of fluids within the tendon. Our results show that the viscoelastic response to dynamic loading is correlated with fluid flow within the IFM, which was confirmed during analysis of the MRI results. In contrast to this, the long-term relaxation of tendon fascicle bundles is controlled by viscoplasticity of the IFM and depends on the spatial distribution of the matrix within the tendon. Comparison of results from tensile tests, DMA, and MRI gives new insight into tendon mechanics and the role of the IFM. These findings may be useful in improving the diagnosis of tendon injury and effectiveness of medical treatments for tendinopathies.

Extrinsic Macrophages Protect While Tendon Progenitors Degrade: Insights from a Tissue Engineered Model of Tendon Compartmental Crosstalk

Tendons are among the most mechanically stressed tissues of the body, with a functional core of type-I collagen fibers maintained by embedded stromal fibroblasts known as tenocytes. The intrinsic load-bearing core compartment of tendon is surrounded, nourished, and repaired by the extrinsic peritendon, a synovial-like tissue compartment with access to tendon stem/progenitor cells as well as blood monocytes. In vitro tendon model systems generally lack this important feature of tissue compartmentalization, while in vivo models are cumbersome when isolating multicellular mechanisms. To bridge this gap, an improved in vitro model of explanted tendon core stromal tissue (mouse tail tendon fascicles) surrounded by cell-laden collagen hydrogels that mimic extrinsic tissue compartments is suggested. Using this model, CD146+ tendon stem/progenitor cell and CD45+ F4/80+ bone-marrow derived macrophage activity within a tendon injury-like niche are recapitulated. It is found that extrinsic stromal progenitors recruit to the damaged core, contribute to an overall increase in catabolic ECM gene expression, and accelerate the decrease in mechanical properties. Conversely, it is found that extrinsic bone-marrow derived macrophages in these conditions adopt a proresolution phenotype that mitigates rapid tissue breakdown by outwardly migrated tenocytes and F4/80+ "tenophages" from the intrinsic tissue core.

Tendon and multiomics: advantages, advances, and opportunities

Tendons heal by fibrosis, which hinders function and increases re-injury risk. Yet the biology that leads to degeneration and regeneration of tendons is not completely understood. Improved understanding of the metabolic nuances that cause diverse outcomes in tendinopathies is required to solve these problems. 'Omics methods are increasingly used to characterize phenotypes in tissues. Multiomics integrates 'omic datasets to identify coherent relationships and provide insight into differences in molecular and metabolic pathways between anatomic locations, and disease stages. This work reviews the current literature pertaining to multiomics in tendon and the potential of these platforms to improve tendon regeneration. We assessed the literature and identified areas where 'omics platforms contribute to the field: (1) Tendon biology where their hierarchical complexity and demographic factors are studied. (2) Tendon degeneration and healing, where comparisons across tendon pathologies are analyzed. (3) The in vitro engineered tendon phenotype, where we compare the engineered phenotype to relevant native tissues. (4) Finally, we review regenerative and therapeutic approaches. We identified gaps in current knowledge and opportunities for future study: (1) The need to increase the diversity of human subjects and cell sources. (2) Opportunities to improve understanding of tendon heterogeneity. (3) The need to use these improvements to inform new engineered and regenerative therapeutic approaches. (4) The need to increase understanding of the development of tendon pathology. Together, the expanding use of various 'omics platforms and data analysis resulting from these platforms could substantially contribute to major advances in the tendon tissue engineering and regenerative medicine field.

Hierarchical ultrastructure: An overview of what is known about tendons and future perspective for tendon engineering

Abnormal tendons are rarely ever repaired to the natural structure and morphology of normal tendons. To better guide the repair and regeneration of injured tendons through a tissue engineering method, it is necessary to have insights into the internal morphology, organization, and composition of natural tendons. This review summarized recent researches on the structure and function of the extracellular matrix (ECM) components of tendons and highlight the application of multiple detection methodologies concerning the structure of ECMs. In addition, we look forward to the future of multi-dimensional biomaterial design methods and the potential of structural repair for tendon ECM components. In addition, focus is placed on the macro to micro detection methods for tendons, and current techniques for evaluating the extracellular matrix of tendons at the micro level are introduced in detail. Finally, emphasis is given to future extracellular matrix detection methods, as well as to how future efforts could concentrate on fabricating the biomimetic tendons.

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Summarize recent research on the structure and function of the extracellular matrix (ECM) components of tendons.

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Comments on current research methods concerning the structure of ECMs.

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Perspective on the future of multi-dimensional detection techniques and structural repair of tendon ECM components.

CD146 Delineates an Interfascicular Cell Sub-Population in Tendon That Is Recruited during Injury through Its Ligand Laminin-α4

The interfascicular matrix (IFM) binds tendon fascicles and contains a population of morphologically distinct cells. However, the role of IFM-localised cell populations in tendon repair remains to be determined. The basement membrane protein laminin-α4 also localises to the IFM. Laminin-α4 is a ligand for several cell surface receptors, including CD146, a marker of pericyte and progenitor cells. We used a needle injury model in the rat Achilles tendon to test the hypothesis that the IFM is a niche for CD146+ cells that are mobilised in response to tendon damage. We also aimed to establish how expression patterns of circulating non-coding RNAs alter with tendon injury and identify potential RNA-based markers of tendon disease. The results demonstrate the formation of a focal lesion at the injury site, which increased in size and cellularity for up to 21 days post injury. In healthy tendon, CD146+ cells localised to the IFM, compared with injury, where CD146+ cells migrated towards the lesion at days 4 and 7, and populated the lesion 21 days post injury. This was accompanied by increased laminin-α4, suggesting that laminin-α4 facilitates CD146+ cell recruitment at injury sites. We also identified a panel of circulating microRNAs that are dysregulated with tendon injury. We propose that the IFM cell niche mediates the intrinsic response to injury, whereby an injury stimulus induces CD146+ cell migration. Further work is required to fully characterise CD146+ subpopulations within the IFM and establish their precise roles during tendon healing.

Structure-function specialisation of the interfascicular matrix in the human achilles tendon

Tendon consists of highly aligned collagen-rich fascicles surrounded by interfascicular matrix (IFM). Some tendons act as energy stores to improve locomotion efficiency, but such tendons commonly obtain debilitating injuries. In equine tendons, energy storing is achieved primarily through specialisation of the IFM. However, no studies have investigated IFM structure-function specialisation in human tendons. Here, we compare the human positional anterior tibial tendon and energy storing Achilles tendons, testing the hypothesis that the Achilles tendon IFM has specialised composition and mechanical properties, which are lost with ageing. Data demonstrate IFM specialisation in the energy storing Achilles, with greater elasticity and fatigue resistance than in the positional anterior tibial tendon. With ageing, alterations occur predominantly to the proteome of the Achilles IFM, which are likely responsible for the observed trends towards decreased fatigue resistance. Knowledge of these key energy storing specialisations and their changes with ageing offers crucial insight towards developing treatments for tendinopathy.

Statement of significance

Developing effective therapeutics or preventative measures for tendon injury necessitates the understanding of healthy tendon function and mechanics. By establishing structure-function relationships in human tendon and determining how these are affected by ageing, potential targets for therapeutics can be identified. In this study, we have used a combination of mechanical testing, immunolabelling and proteomics analysis to study structure-function specialisations in human tendon. We demonstrate that the interfascicular matrix is specialised for energy storing in the Achilles tendon, and that its proteome is altered with ageing, which is likely responsible for the observed trends towards decreased fatigue resistance. Knowledge of these key energy storing specialisations and their changes with ageing offers crucial insight towards developing treatments and preventative approaches for tendinopathy.

"One Health" in tendinopathy research: Current concepts

Tendinopathy remains one of the most common musculoskeletal disorders affecting both human and equine athletes and presents a considerable therapeutic challenge. The following workshop report comes from the third Dorothy Havemeyer Symposium of Tendinopathy which provided a unique overview of our current understanding of both the basic science and the clinical challenges for diagnosing and treating tendinopathy in both species. Pathologically, tendon demonstrates alterations in both cellular, molecular, structural, and biomechanical features, leading to a spectrum of pathological endotypes. To develop novel interventions to manage, treat or prevent tendinopathies it is vital to understand the underlying mechanisms that lead to both tendon failure, and also regeneration and resolution of inflammation. The horse shows analogous pathology with both human Achilles tendinopathy (superficial digital flexor tendon) and intrathecal rotator cuff tears (deep digital flexor tendon tears) enabling scientists and clinicians from both medical and veterinary fields to work jointly on matching naturally occurring disease models. The experience in human medicine on the design, conduct, and impact of clinical trials has much to inform clinical trials in horses. There is a need to design appropriate studies to address clear questions, socialize the study to achieve good enrollment, and consider the significance and impact of the clinical question as well as the cost of addressing it. Because economics is often a limitation in equine medicine the use of observational studies, and specifically registries, should be given careful consideration.

The effect of exercise on the protein profile of rat knee joint intra- and extra-articular ligaments

Injuries to the intra-articular anterior cruciate ligament (ACL) and the extra-articular medial collateral ligament (MCL) result in significant knee joint instability, pain, and immobility. Moderate endurance-type exercise can increase ligament strength but little is known on the effect of short-term regular bouts of high-intensity exercise on the extracellular matrix (ECM) structure of knee ligaments. Therefore, this study aimed to identify the effect of short-term regular bouts high exercise on the proteome of the rat ACL and MCL using mass spectrometry. Sprague-Dawley male rats (n = 6) were split into control and exercise groups, and subjected to high-intensity training for four 4 weeks followed by proteomic analyses of the ACL and MCL. Knee joint health status was assessed using OARSI and a validated histological scoring system. Histopathological analyses demonstrated no significant changes in either in cruciate, collateral ligaments, or cartilage between the control and exercised knee joints. However, significant proteins were found to be more abundant in the exercised ACL compared to ACL control group but not between the exercised MCL and control MCL groups. The significant abundant proteins in ACL exercise groups were mostly cytoskeletal, ribosomal and enzymes with several abundant matrisomal proteins such as collagen proteins and proteoglycans being found in this group. In conclusion, our results indicate that short-term regular bouts of high-intensity exercise have an impact on the intra-articular ACL but not extra-articular MCL ECM protein expression.

Microdamage in the equine superficial digital flexor tendon

The forelimb superficial digital flexor tendon (SDFT) is an energy-storing tendon that is highly susceptible to injury during activities such as galloping and jumping, such that it is one of the most commonly reported causes of lameness in the performance horse. This review outlines the biomechanical and biothermal effects of strain on the SDFT and how these contribute to the accumulation of microdamage. The effect of age-related alterations on strain response and subsequent injury risk is also considered. Given that tendon is a slowly healing and poorly regenerative tissue, prompt detection of early stages of pathology in vivo and timely adaptations to training protocols are likely to have a greater outcome than advances in treatment. Early screening tools and detection protocols could subsequently be of benefit in identifying subclinical signs of degeneration during the training programme. This provides an opportunity for preventative strategies to be implemented to minimise incidences of SDFT injury and reduce recovery periods in elite performance horses. Therefore, this review will focus on the modalities available to implement early screening and prevention protocols as opposed to methods to diagnose and treat injuries.

Elastase treatment of tendon specifically impacts the mechanical properties of the interfascicular matrix

The tendon interfascicular matrix (IFM) binds tendon fascicles together. As a result of its low stiffness behaviour under small loads, it enables non-uniform loading and increased overall extensibility of tendon by facilitating fascicle sliding. This function is particularly important in energy storing tendons, with previous studies demonstrating enhanced extensibility, recovery and fatigue resistance in the IFM of energy storing compared to positional tendons. However, the compositional specialisations within the IFM that confer this behaviour remain to be elucidated. It is well established that the IFM is rich in elastin, therefore we sought to test the hypothesis that elastin depletion (following elastase treatment) will significantly impact IFM, but not fascicle, mechanical properties, reducing IFM resilience in all samples, but to a greater extent in younger tendons, which have a higher elastin content. Using a combination of quasi-static and fatigue testing, and optical imaging, we confirmed our hypothesis, demonstrating that elastin depletion resulted in significant decreases in IFM viscoelasticity, fatigue resistance and recoverability compared to untreated samples, with no significant changes to fascicle mechanics. Ageing had little effect on fascicle or IFM response to elastase treatment.

This study offers a first insight into the functional importance of elastin in regional specific tendon mechanics. It highlights the important contribution of elastin to IFM mechanical properties, demonstrating that maintenance of a functional elastin network within the IFM is essential to maintain IFM and thus tendon integrity.

Statement of significance

Developing effective treatments or preventative measures for musculoskeletal tissue injuries necessitates the understanding of healthy tissue function and mechanics. By establishing the contribution of specific proteins to tissue mechanical behaviour, key targets for therapeutics can be identified. Tendon injury is increasingly prevalent and chronically debilitating, with no effective treatments available.

Here, we investigate how elastin modulates tendon mechanical behaviour, using enzymatic digestion combined with local mechanical characterisation, and demonstrate for the first time that removing elastin from tendon affects the mechanical properties of the interfascicular matrix specifically, resulting in decreased recoverability and fatigue resistance. These findings provide a new level of insight into tendon hierarchical mechanics, important for directing development of novel therapeutics for tendon injury.

Characterization of the structure, vascularity, and stem/progenitor cell populations in porcine Achilles tendon (PAT)

This study aimed to characterize porcine Achilles tendon (PAT) in terms of its structural components, vascularity, and resident tendon cells. We found that PAT is composed of a paratenon sheath, a core of fascicles, and an endotenon/interfascicular matrix (IFM) that encases the fascicle bundles. We analyzed each of these three tendon components structurally using tissue sections and by isolating cells from each component and analyzing in vitro. Many blood vessel-like tissues were present in the paratenon and IFM but not in fascicles, and the vessels in the paratenon and IFM appeared to be inter-connected. Cells isolated from the paratenon and IFM displayed characteristics of vascular stem/progenitor cells expressing the markers CD105, CD31, with α-smooth muscle actin (α-SMA) localized surrounding blood vessels. The isolated cells from paratenon and IFM also harbored abundant stem/progenitor cells as evidenced by their ability to form colonies and express stem cell markers including CD73 and CD146. Furthermore, we demonstrate that both paratenon and IFM-isolated cells were capable of undergoing multi-differentiation. In addition, both paratenon and IFM cells expressed elastin, osteocalcin, tubulin polymerization promoting protein (TPPP), and collagen IV, whereas fascicle cells expressed none of these markers, except collagen I. The neurotransmitter substance P (SP) was also found in the paratenon and IFM-localized surrounding blood vessels. The findings of this study will help us to better understand the vascular and cellular mechanisms of tendon homeostasis, injury, healing, and regeneration.

Extracellular matrix at the muscle - tendon interface: functional roles, techniques to explore and implications for regenerative medicine

The muscle-tendon interface is an anatomically specialized region that is involved in the efficient transmission of force from muscle to tendon. Due to constant exposure to loading, the interface is susceptible to injury. Current treatment methods do not meet the socioeconomic demands of reduced recovery time without compromising the risk of reinjury, requiring the need for developing alternative strategies. The extracellular matrix (ECM) present in muscle, tendon, and at the interface of these tissues consists of unique molecules that play significant roles in homeostasis and repair. Better, understanding the function of the ECM during development, injury, and aging has the potential to unearth critical missing information that is essential for accelerating the repair at the muscle-tendon interface. Recently, advanced techniques have emerged to explore the ECM for identifying specific roles in musculoskeletal biology. Simultaneously, there is a tremendous increase in the scope for regenerative medicine strategies to address the current clinical deficiencies. Advancements in ECM research can be coupled with the latest regenerative medicine techniques to develop next generation therapies that harness ECM for treating defects at the muscle-tendon interface. The current work provides a comprehensive review on the role of muscle and tendon ECM to provide insights about the role of ECM in the muscle-tendon interface and discusses the latest research techniques to explore the ECM to gathered information for developing regenerative medicine strategies.

Tendon Extracellular Matrix Assembly, Maintenance and Dysregulation Throughout Life

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.

Achilles Subtendon Structure and Behavior as Evidenced From Tendon Imaging and Computational Modeling

The Achilles tendon is the largest and strongest tendon in the human body and is essential for storing elastic energy and positioning the foot for walking and running. Recent research into Achilles tendon anatomy and mechanics has revealed the importance of the Achilles subtendons, which are unique and semi-independent structures arising from each of the three muscular heads of the triceps surae. Of particular importance is the ability for the subtendons to slide, the role that this has in healthy tendons, and the alteration of this property in aging and disease. In this work, we discuss technical approaches that have led to the current understanding of Achilles subtendons, particularly imaging and computational modeling. We introduce a 3D geometrical model of the Achilles subtendons, built from dual-echo UTE MRI. We revisit and discuss computational models of Achilles subtendon twisting suggesting that optimal twist reduces both rupture loads and stress concentrations by distributing stresses. Second harmonic generation imaging shows collagenous subtendons within a rabbit Achilles tendon; a clear absence of signal between the subtendons indicates an inter-subtendon region on the order of 30 μm in our rabbit animal model. Entry of wheat germ agglutinin in both the inter-fascicular and the inter-subtendon regions suggests a glycoprotein-containing inter-subtendon matrix which may facilitate low friction sliding of the subtendons in healthy mammals. Lastly, we present a new computational model coupled with human exercise trials to demonstrate the magnitude of Achilles subtendon sliding which occurs during rehabilitation exercises for Achilles tendinopathy, and shows that specific exercise can maximize subtendon sliding and interface strains, without maximizing subtendon strains. This work demonstrates the value of imaging and computational modeling for probing tendon structure-function relationships and may serve to inform and develop treatments for Achilles tendinopathy.

Postnatal mechanical loading drives adaptation of tissues primarily through modulation of the non-collagenous matrix

Mature connective tissues demonstrate highly specialised properties, remarkably adapted to meet their functional requirements. Tissue adaptation to environmental cues can occur throughout life and poor adaptation commonly results in injury. However, the temporal nature and drivers of functional adaptation remain undefined. Here, we explore functional adaptation and specialisation of mechanically loaded tissues using tendon; a simple aligned biological composite, in which the collagen (fascicle) and surrounding predominantly non-collagenous matrix (interfascicular matrix) can be interrogated independently. Using an equine model of late development, we report the first phase-specific analysis of biomechanical, structural, and compositional changes seen in functional adaptation, demonstrating adaptation occurs postnatally, following mechanical loading, and is almost exclusively localised to the non-collagenous interfascicular matrix. These novel data redefine adaptation in connective tissue, highlighting the fundamental importance of non-collagenous matrix and suggesting that regenerative medicine strategies should change focus from the fibrous to the non-collagenous matrix of tissue.

Epigenetic mechanisms in Tendon Ageing

INTRODUCTION

Tendon is a composite material with a well-ordered hierarchical structure exhibiting viscoelastic properties designed to transfer force. It is recognized that the incidence of tendon injury increases with age, suggesting a deterioration in homeostatic mechanisms or reparative processes. This review summarizes epigenetic mechanisms identified in ageing healthy tendon.

SOURCES OF DATA

We searched multiple databases to produce a systematic review on the role of epigenetic mechanisms in tendon ageing.

AREAS OF AGREEMENT

Epigenetic mechanisms are important in predisposing ageing tendon to injury.

AREAS OF CONTROVERSY

The relative importance of epigenetic mechanisms are unknown in terms of promoting healthy ageing. It is also unknown whether these changes represent protective mechanisms to function or predispose to pathology.

GROWING POINT

Epigenetic markers in ageing tendon, which are under-researched including genome-wide chromatin accessibility, should be investigated.

AREAS TIMELY FOR DEVELOPING RESEARCH

Metanalysis through integration of multiple datasets and platforms will enable a holistic understanding of the epigenome in ageing and its relevance to disease.

Comparative Analysis of the Extracellular Matrix Proteome across the Myotendinous Junction

The myotendinous junction is a highly interdigitated interface designed to transfer muscle-generated force to tendon. Understanding how this interface is formed and organized, as well as identifying tendon- and muscle-specific extracellular matrix (ECM), is critical for designing effective regenerative therapies to restore functionality to damaged muscle-tendon units. However, a comparative analysis of the ECM proteome across this interface has not been conducted. The goal of this study was to resolve the distribution of ECM proteins that are uniformly expressed as well as those specific to each of the muscle, tendon, and junction tissues. The soleus muscles from 5-month-old wild-type C57BL/6 mice were harvested and dissected into the central muscle (M) away from tendon, the junction between muscle and tendon (J) and the tendon (T). Tissues were processed by either homogenizing in guanidine hydrochloride or fractionating to isolate the ECM from more soluble intracellular components and then analyzed using liquid chromatography-tandem mass spectrometry. Overall, we found that both tissue processing methods generated similar ECM profiles. Many ECM were found across the muscle-tendon unit, including type I collagen and associated fibril-regulating proteins. The ECM identified exclusively in M were primarily related to the basal lamina, whereas those specific to T and J tissue included thrombospondins and other matricellular ECM. Type XXII collagen (COL22A1) was restricted to J, and we identified COL5A3 as a potential marker of the muscle-tendon interface. Immunohistochemical analysis of key proteins confirmed the restriction of some basal lamina proteins to M, tenascin-C to T, and COL22A1 to J. COL5A3, PRELP, and POSTN were visualized in the tissue surrounding the junction, suggesting that these proteins play a role in stabilizing the interface. This comparative map provides a guide for tissue-specific ECM that can facilitate the spatial visualization of M, J, and T tissues and inform musculoskeletal regenerative therapies.

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.

Bimodal Whole-Mount Imaging of Tendon Using Confocal Microscopy and X-ray Micro-Computed Tomography

Background

Three-dimensional imaging modalities for optically dense connective tissues such as tendons are limited and typically have a single imaging methodological endpoint. Here, we have developed a bimodal procedure utilising fluorescence-based confocal microscopy and x-ray micro-computed tomography for the imaging of adult tendons to visualise and analyse extracellular sub-structure and cellular composition in small and large animal species.

Results

Using fluorescent immunolabelling and optical clearing, we visualised the expression of the novel cross-species marker of tendon basement membrane, laminin-α4 in 3D throughout whole rat Achilles tendons and equine superficial digital flexor tendon 5 mm segments. This revealed a complex network of laminin-α4 within the tendon core that predominantly localises to the interfascicular matrix compartment. Furthermore, we implemented a chemical drying process capable of creating contrast densities enabling visualisation and quantification of both fascicular and interfascicular matrix volume and thickness by x-ray micro-computed tomography. We also demonstrated that both modalities can be combined using reverse clarification of fluorescently labelled tissues prior to chemical drying to enable bimodal imaging of a single sample.

Conclusions

Whole-mount imaging of tendon allowed us to identify the presence of an extensive network of laminin-α4 within tendon, the complexity of which cannot be appreciated using traditional 2D imaging techniques. Creating contrast for x-ray micro-computed tomography imaging of tendon using chemical drying is not only simple and rapid, but also markedly improves on previously published methods. Combining these methods provides the ability to gain spatio-temporal information and quantify tendon substructures to elucidate the relationship between morphology and function.

Plant Tissues as 3D Natural Scaffolds for Adipose, Bone and Tendon Tissue Regeneration

Decellularized tissues are a valid alternative as tissue engineering scaffolds, thanks to the three-dimensional structure that mimics native tissues to be regenerated and the biomimetic microenvironment for cells and tissues growth. Despite decellularized animal tissues have long been used, plant tissue decellularized scaffolds might overcome availability issues, high costs and ethical concerns related to the use of animal sources. The wide range of features covered by different plants offers a unique opportunity for the development of tissue-specific scaffolds, depending on the morphological, physical and mechanical peculiarities of each plant. Herein, three different plant tissues (i.e., apple, carrot, and celery) were decellularized and, according to their peculiar properties (i.e., porosity, mechanical properties), addressed to regeneration of adipose tissue, bone tissue and tendons, respectively. Decellularized apple, carrot and celery maintained their porous structure, with pores ranging from 70 to 420 μm, depending on the plant source, and were stable in PBS at 37°C up to 7 weeks. Different mechanical properties (i.e., Eapple = 4 kPa, Ecarrot = 43 kPa, Ecelery = 590 kPa) were measured and no indirect cytotoxic effects were demonstrated in vitro after plants decellularization. After coating with poly-L-lysine, apples supported 3T3-L1 preadipocytes adhesion, proliferation and adipogenic differentiation; carrots supported MC3T3-E1 pre-osteoblasts adhesion, proliferation and osteogenic differentiation; celery supported L929 cells adhesion, proliferation and guided anisotropic cells orientation. The versatile features of decellularized plant tissues and their potential for the regeneration of different tissues are proved in this work.

Heterogeneity of proteome dynamics between connective tissue phases of adult tendon

Maintenance of connective tissue integrity is fundamental to sustain function, requiring protein turnover to repair damaged tissue. However, connective tissue proteome dynamics remain largely undefined, as do differences in turnover rates of individual proteins in the collagen and glycoprotein phases of connective tissue extracellular matrix (ECM). Here, we investigate proteome dynamics in the collagen and glycoprotein phases of connective tissues by exploiting the spatially distinct fascicular (collagen-rich) and interfascicular (glycoprotein-rich) ECM phases of tendon. Using isotope labelling, mass spectrometry and bioinformatics, we calculate turnover rates of individual proteins within rat Achilles tendon and its ECM phases. Our results demonstrate complex proteome dynamics in tendon, with ~1000 fold differences in protein turnover rates, and overall faster protein turnover within the glycoprotein-rich interfascicular matrix compared to the collagen-rich fascicular matrix. These data provide insights into the complexity of proteome dynamics in tendon, likely required to maintain tissue homeostasis.

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.

Detection of Age-Related Changes in Tendon Molecular Composition by Raman Spectroscopy-Potential for Rapid, Non-Invasive Assessment of Susceptibility to Injury

The lack of clinical detection tools at the molecular level hinders our progression in preventing age-related tendon pathologies. Raman spectroscopy can rapidly and non-invasively detect tissue molecular compositions and has great potential for in vivo applications. In biological tissues, a highly fluorescent background masks the Raman spectral features and is usually removed during data processing, but including this background could help age differentiation since fluorescence level in tendons increases with age. Therefore, we conducted a stepwise analysis of fluorescence and Raman combined spectra for better understanding of the chemical differences between young and old tendons. Spectra were collected from random locations of vacuum-dried young and old equine tendon samples (superficial digital flexor tendon (SDFT) and deep digital flexor tendon (DDFT), total n = 15) under identical instrumental settings. The fluorescence-Raman spectra showed an increase in old tendons as expected. Normalising the fluorescence-Raman spectra further indicated a potential change in intra-tendinous fluorophores as tendon ages. After fluorescence removal, the pure Raman spectra demonstrated between-group differences in CH2 bending (1450 cm-1) and various ring-structure and carbohydrate-associated bands (1000-1100 cm-1), possibly relating to a decline in cellular numbers and an accumulation of advanced glycation end products in old tendons. These results demonstrated that Raman spectroscopy can successfully detect age-related tendon molecular differences.

Spectrum of Tendon Pathologies: Triggers, Trails and End-State

The biggest compartment of the musculoskeletal system is the tendons and ligaments. In particular, tendons are dense tissues connecting muscle to bone that are critical for the integrity, function and locomotion of this system. Due to the increasing age of our society and the overall rise in engagement in extreme and overuse sports, there is a growing prevalence of tendinopathies. Despite the recent advances in tendon research and due to difficult early diagnosis, a multitude of risk factors and vague understanding of the underlying biological mechanisms involved in the progression of tendon injuries, the toolbox of treatment strategies remains limited and non-satisfactory. This review is designed to summarize the current knowledge of triggers, trails and end state of tendinopathies.

The "other" 15-40%: The Role of Non-Collagenous Extracellular Matrix Proteins and Minor Collagens in Tendon

Extracellular matrix (ECM) determines the physiological function of all tissues, including musculoskeletal tissues. In tendon, ECM provides overall tissue architecture, which is tailored to match the biomechanical requirements of their physiological function, that is, force transmission from muscle to bone. Tendon ECM also constitutes the microenvironment that allows tendon-resident cells to maintain their phenotype and that transmits biomechanical forces from the macro-level to the micro-level. The structure and function of adult tendons is largely determined by the hierarchical organization of collagen type I fibrils. However, non-collagenous ECM proteins such as small leucine-rich proteoglycans (SLRPs), ADAMTS proteases, and cross-linking enzymes play critical roles in collagen fibrillogenesis and guide the hierarchical bundling of collagen fibrils into tendon fascicles. Other non-collagenous ECM proteins such as the less abundant collagens, fibrillins, or elastin, contribute to tendon formation or determine some of their biomechanical properties. The interfascicular matrix or endotenon and the outer layer of tendons, the epi- and paratenon, includes collagens and non-collagenous ECM proteins, but their function is less well understood. The ECM proteins in the epi- and paratenon may provide the appropriate microenvironment to maintain the identity of distinct tendon cell populations that are thought to play a role during repair processes after injury. The aim of this review is to provide an overview of the role of non-collagenous ECM proteins and less abundant collagens in tendon development and homeostasis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:23-35, 2020.

Tendon Biomechanics and Crimp Properties Following Fatigue Loading Are Influenced by Tendon Type and Age in Mice

In tendon, type-I collagen assembles together into fibrils, fibers, and fascicles that exhibit a wavy or crimped pattern that uncrimps with applied tensile loading. This structural property has been observed across multiple tendons throughout aging and may play an important role in tendon viscoelasticity, response to fatigue loading, healing, and development. Previous work has shown that crimp is permanently altered with the application of fatigue loading. This opens the possibility of evaluating tendon crimp as a clinical surrogate of tissue damage. The purpose of this study was to determine how fatigue loading in tendon affects crimp and mechanical properties throughout aging and between tendon types. Mouse patellar tendons (PT) and flexor digitorum longus (FDL) tendons were fatigue loaded while an integrated plane polariscope simultaneously assessed crimp properties at P150 and P570 days of age to model mature and aged tendon phenotypes (N = 10-11/group). Tendon type, fatigue loading, and aging were found to differentially affect tendon mechanical and crimp properties. FDL tendons had higher modulus and hysteresis, whereas the PT showed more laxity and toe region strain throughout aging. Crimp frequency was consistently higher in FDL compared with PT throughout fatigue loading, whereas the crimp amplitude was cycle dependent. This differential response based on tendon type and age further suggests that the FDL and the PT respond differently to fatigue loading and that this response is age-dependent. Together, our findings suggest that the mechanical and structural effects of fatigue loading are specific to tendon type and age in mice. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:36-42, 2020.

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.

The antiviral protein viperin regulates chondrogenic differentiation via CXCL10 protein secretion

Viperin (also known as radical SAM domain–containing 2 (RSAD2)) is an interferon-inducible and evolutionary conserved protein that participates in the cell's innate immune response against a number of viruses. Viperin mRNA is a substrate for endoribonucleolytic cleavage by RNase mitochondrial RNA processing (MRP) and mutations in the RNase MRP small nucleolar RNA (snoRNA) subunit of the RNase MRP complex cause cartilage-hair hypoplasia (CHH), a human developmental condition characterized by metaphyseal chondrodysplasia and severe dwarfism. It is unknown how CHH-pathogenic mutations in RNase MRP snoRNA interfere with skeletal development, and aberrant processing of RNase MRP substrate RNAs is thought to be involved. We hypothesized that viperin plays a role in chondrogenic differentiation. Using immunohistochemistry, real-time quantitative PCR, immunoblotting, ELISA, siRNA-mediated gene silencing, plasmid-mediated gene overexpression, label-free MS proteomics, and promoter reporter bioluminescence assays, we discovered here that viperin is expressed in differentiating chondrocytic cells and regulates their protein secretion and the outcome of chondrogenic differentiation by influencing transforming growth factor β (TGF-β)/SMAD family 2/3 (SMAD2/3) activity via C-X-C motif chemokine ligand 10 (CXCL10). Of note, we observed disturbances in this viperin–CXCL10–TGF-β/SMAD2/3 axis in CHH chondrocytic cells. Our results indicate that the antiviral protein viperin controls chondrogenic differentiation by influencing secretion of soluble proteins and identify a molecular route that may explain impaired chondrogenic differentiation of cells from individuals with CHH.

Tendon Remodeling in Response to Resistance Training, Anabolic Androgenic Steroids and Aging

Exercise training (ET), anabolic androgenic steroids (AAS), and aging are potential factors that affect tendon homeostasis, particularly extracellular matrix (ECM) remodeling. The goal of this review is to aggregate findings regarding the effects of resistance training (RT), AAS, and aging on tendon homeostasis. Data were gathered from our studies regarding the impact of RT, AAS, and aging on the calcaneal tendon (CT) of rats. We demonstrated a series of detrimental effects of AAS and aging on functional and biomechanical parameters, including the volume density of blood vessel cells, adipose tissue cells, tendon calcification, collagen content, the regulation of the major proteins related to the metabolic/development processes of tendons, and ECM remodeling. Conversely, RT seems to mitigate age-related tendon dysfunction. Our results suggest that AAS combined with high-intensity RT exert harmful effects on ECM remodeling, and also instigate molecular and biomechanical adaptations in the CT. Moreover, we provide further information regarding the harmful effects of AAS on tendons at a transcriptional level, and demonstrate the beneficial effects of RT against the age-induced tendon adaptations of rats. Our studies might contribute in terms of clinical approaches in favor of the benefits of ET against tendinopathy conditions, and provide a warning on the harmful effects of the misuse of AAS on tendon development.