Mathematical model of the role of degradation on matrix development in hydrogel scaffold

Despite tremendous advances in the field of tissue engineering, a number of obstacles are still hindering its successful translation to the clinic. One of these challenges has been to design cell-laden scaffolds that can provide an appropriate environment for cells to successfully synthesize new tissue while providing a mechanical support that can resist physiological loads at the early stage of in situ implementation. A solution to this problem has been to balance tissue growth and scaffold degradation by creating new hydrogel systems that possess both hydrolytic and enzymatic degradation behaviors. Very little is known, however, about the complex behavior of these systems, emphasizing the need for a rigorous mathematical approach that can eventually assist and guide experimental advances. This paper introduces a mathematical and numerical formulation based on mixture theory, to describe the degradation, swelling, and transport of extracellular matrix (ECM) molecules released by cartilage cells (chondrocytes) within a hydrogel scaffold. The model particularly investigates the relative roles of hydrolytic and enzymatic degradations on ECM diffusion and their impacts on two important outcomes: the extent of ECM transport (and deposition) and the evolution of the scaffold’s mechanical integrity. Numerical results based on finite element show that if properly tuned, enzymatic degradation differs from hydrolytic degradation in that it can create a degradation front that is key to maintaining scaffold stiffness while allowing ECM deposition. These results therefore suggest a hydrogel design that could enable successful in situ cartilage tissue engineering.     

Articular cartilage tensile integrity: modulation by matrix depletion is maturation-dependent

Articular cartilage function depends on the molecular composition and structure of its extracellular matrix (ECM). The collagen network (CN) provides cartilage with tensile integrity, but must also remodel during growth. Such remodeling may depend on matrix molecules interacting with the CN to modulate the tensile behavior of cartilage. The objective of this study was to determine the effects of increasingly selective matrix depletion on tensile properties of immature and mature articular cartilage, and thereby establish a framework for identifying molecules involved in CN remodeling. Depletion of immature cartilage with guanidine, chondroitinase ABC, chondroitinase AC, and Streptomyces hyaluronidase markedly increased tensile integrity, while the integrity of mature cartilage remained unaltered after depletion with guanidine. The enhanced tensile integrity after matrix depletion suggests that certain ECM components of immature matrix serve to inhibit CN interactions and may act as modulators of physiological alterations of cartilage geometry and tensile properties during growth/maturation.     

Effects of diacerein on biosynthesis activities of chondrocytes in culture

The maintenance of articular cartilage integrity requires a balance between anabolic and catabolic processes which are under the control of chondrocytes. These cells are living in an anaerobic environment and normally do not divide. They are responsible for the continuous maintenance of the cartilage extracellular matrix (ECM). Although multiple factors are involved in the dynamic homeostasis of cartilage, increases in cytokines such as interleukin-1 (IL-1) are associated with a decrease in synthesis and an increase in degradation of the proteoglycans and collagens. Conversely, growth factors such as transforming growth factor-beta (TGF-beta) stimulate chondrocyte synthesis of collagens and proteoglycans, and reduce the activity of IL-1 stimulated metalloproteases, thus opposing the inhibitory and catabolic effects of IL-1. By its capability to reduce IL-1 effects and to stimulate TGF-beta expression in cultured articular chondrocytes, diacerein could favour anabolic processes in the OA cartilage and, hence may contribute to delay the progression of the disease.     

Detection of cartilage matrix degradation by autofluorescence lifetime

Cartilage is a vital organ to maintain joint function. Upon arthritis, proteolytic enzymes initiate degradation of cartilage extracellular matrix (ECM) resulting in eventual loss of joint function. However, there are only limited ways of non-invasively monitoring early chemical changes in cartilage matrix. Here we report that the autofluorescence decay profiles of cartilage tissue are significantly affected by proteolytic degradation of cartilage ECM and can be characterised by measurements of the autofluorescence lifetime (AFL). A compact multidimensional fluorometer coupled to a fibre-optic probe was developed for single point measurements of AFL and applied to cartilage that was treated with different proteinases. Upon treating cartilage with bacterial collagenase, trypsin or matrix metalloproteinase 1, a significant dose and time dependent decrease of AFL was observed. Our data suggest that AFL of cartilage tissue is a potential non-invasive readout to monitor cartilage matrix integrity that may contribute to future diagnosis of cartilage defects as well as monitoring the efficacy of anti-joint therapeutic agents.     

Altered endochondral bone development in matrix metalloproteinase 13-deficient mice

The assembly and degradation of extracellular matrix (ECM) molecules are crucial processes during bone development. In this study, we show that ECM remodeling is a critical rate-limiting step in endochondral bone formation. Matrix metalloproteinase (MMP) 13 (collagenase 3) is poised to play a crucial role in bone formation and remodeling because of its expression both in terminal hypertrophic chondrocytes in the growth plate and in osteoblasts. Moreover, a mutation in the human MMP13 gene causes the Missouri variant of spondyloepimetaphyseal dysplasia. Inactivation of Mmp13 in mice through homologous recombination led to abnormal skeletal growth plate development. Chondrocytes differentiated normally but their exit from the growth plate was delayed. The severity of the Mmp13- null growth plate phenotype increased until about 5 weeks and completely resolved by 12 weeks of age. Mmp13-null mice had increased trabecular bone, which persisted for months. Conditional inactivation of Mmp13 in chondrocytes and osteoblasts showed that increases in trabecular bone occur independently of the improper cartilage ECM degradation caused by Mmp13 deficiency in late hypertrophic chondrocytes. Our studies identified the two major components of the cartilage ECM, collagen type II and aggrecan, as in vivo substrates for MMP13. We found that degradation of cartilage collagen and aggrecan is a coordinated process in which MMP13 works synergistically with MMP9. Mice lacking both MMP13 and MMP9 had severely impaired endochondral bone, characterized by diminished ECM remodeling, prolonged chondrocyte survival, delayed vascular recruitment and defective trabecular bone formation (resulting in drastically shortened bones). These data support the hypothesis that proper ECM remodeling is the dominant rate-limiting process for programmed cell death, angiogenesis and osteoblast recruitment during normal skeletal morphogenesis.     

A validated model of GAG deposition,cell distribution,and growth of tissue engineered cartilage cultured in a rotating bioreactor

In this work a new phenomenological model of growth of cartilage tissue cultured in a rotating bioreactor is developed. It represents an advancement of a previously derived model of deposition of glycosaminoglycan (GAG) in engineered cartilage by (i) introduction of physiological mechanisms of proteoglycan accumulation in the extracellular matrix (ECM) as well as by correlating (ii) local cell densities and (iii) tissue growth to the ECM composition. In particular, previously established predictions and correlations of local oxygen concentrations and GAG synthesis rates are extended to distinguish cell secreted proteoglycan monomers free to diffuse in cell surroundings and outside from the engineered construct, from large aggrecan molecules, which are constrained within the ECM and practically immovable. The model includes kinetics of aggregation, that is, transformation of mobile GAG species into immobile aggregates as well as maintenance of the normal ECM composition after the physiological GAG concentration is reached by incorporation of a product inhibition term. The model also includes mechanisms of the temporal evolution of cell density distributions and tissue growth under in vitro conditions. After a short initial proliferation phase the total cell number in the construct remains constant, but the local cell distribution is leveled out by GAG accumulation and repulsion due to negative molecular charges. Furthermore, strong repulsive forces result in expansion of the local tissue elements observed macroscopically as tissue growth (i.e., construct enlargement). The model is validated by comparison with experimental data of (i) GAG distribution and leakage, (ii) spatial‐temporal distributions of cells, and (iii) tissue growth reported in previous works. Validation of the model predictive capability—against a selection of measured data that were not used to construct the model—suggests that the model successfully describes the interplay of several simultaneous processes carried out during in vitro cartilage tissue regeneration and indicates that this approach could also be attractive for application in other tissue engineering systems. Biotechnol. Bioeng. 2010. 105: 842–853. © 2009 Wiley Periodicals, Inc.     

Extracellular matrix integrity affects the mechanical behaviour of in-situ chondrocytes under compression

Cartilage lesions change the microenvironment of cells and may accelerate cartilage degradation through catabolic responses from chondrocytes. In this study, we investigated the effects of structural integrity of the extracellular matrix (ECM) on chondrocytes by comparing the mechanics of cells surrounded by an intact ECM with cells close to a cartilage lesion using experimental and numerical methods. Experimentally, 15% nominal compression was applied to bovine cartilage tissues using a light-transmissible compression system. Target cells in the intact ECM and near lesions were imaged by dual-photon microscopy. Changes in cell morphology (N_cell=32 for both ECM conditions) were quantified. A two-scale (tissue level and cell level) Finite Element (FE) model was also developed. A 15% nominal compression was applied to a non-linear, biphasic tissue model with the corresponding cell level models studied at different radial locations from the centre of the sample in the transient phase and at steady state. We studied the Green-Lagrange strains in the tissue and cells. Experimental and theoretical results indicated that cells near lesions deform less axially than chondrocytes in the intact ECM at steady state. However, cells near lesions experienced large tensile strains in the principal height direction, which are likely associated with non-uniform tissue radial bulging. Previous experiments showed that tensile strains of high magnitude cause an up-regulation of digestive enzyme gene expressions. Therefore, we propose that cartilage degradation near tissue lesions may be due to the large tensile strains in the principal height direction applied to cells, thus leading to an up-regulation of catabolic factors.     

Tensile and compressive properties of healthy and osteoarthritic human articular cartilage

Osteoarthritis (OA) is a disease affecting articular cartilage and the underlying bone, resulting from many biological and mechanical interacting factors which change the extracellular matrix (ECM) and cells and lead to increasing levels of cartilage degeneration, like softening, fibrillation, ulceration and cartilage loss. The early diagnosis of the disease is fundamental to prevent pain, further tissue degeneration and reduce hospital costs. Although morphological modifications can be detected by modern non-invasive diagnostic techniques, they may not be evident in the early stages of OA. The mechanical properties of articular cartilage are related to its composition and structure and are sensitive to even small changes in the ECM that could occur in early OA. The aim of the present study was to compare the mechanical properties of healthy and OA cartilage using a combined experimental-numerical approach. Experimental assessments consisted of step wise confined and unconfined compression and tension stress relaxation tests on disks (for compression) or strips (for tension) of cartilage obtained from human femoral heads discarded from the operating room after total hip replacement. The numerical model was based on the biphasic theory and included the tension-compression non-linearity. Considering OA samples vs normal samples, the static compressive modulus was 55-68% lower, the permeability was 60-80% higher, the dynamic compressive modulus was 59-64% lower, the static tension modulus was 72-83% lower. The model successfully simulated the experimental tests performed on healthy and OA cartilage and was used in combination with the experimental tests to evaluate the role of different ECM components in the mechanical response of normal and OA cartilage.     

The Mechanical Behaviour of Chondrocytes Predicted with a Micro-structural Model of Articular Cartilage

The integrity of articular cartilage depends on the proper functioning and mechanical stimulation of chondrocytes, the cells that synthesize extracellular matrix and maintain tissue health. The biosynthetic activity of chondrocytes is influenced by genetic factors, environmental influences, extracellular matrix composition, and mechanical factors. The mechanical environment of chondrocytes is believed to be an important determinant for joint health, and chondrocyte deformation in response to mechanical loading is speculated to be an important regulator of metabolic activity. In previous studies of chondrocyte deformation, articular cartilage was described as a biphasic material consisting of a homogeneous, isotropic, linearly elastic solid phase, and an inviscid fluid phase. However, articular cartilage is known to be anisotropic and inhomogeneous across its depth. Therefore, isotropic and homogeneous models cannot make appropriate predictions for tissue and cell stresses and strains. Here, we modelled articular cartilage as a transversely isotropic, inhomogeneous (TI) material in which the anisotropy and inhomogeneity arose naturally from the microstructure of the depth-dependent collagen fibril orientation and volumetric fraction, as well as the chondrocyte shape and volumetric fraction. The purpose of this study was to analyse the deformation behaviour of chondrocytes using the TI model of articular cartilage. In order to evaluate our model against experimental results, we simulated indentation and unconfined compression tests for nominal compressions of 15%. Chondrocyte deformations were analysed as a function of location within the tissue. The TI model predicted a non-uniform behaviour across tissue depth: in indentation testing, cell height decreased by 43% in the superficial zone and between 11 and 29% in the deep zone. In unconfined compression testing, cell height decreased by 32% in the superficial zone, 25% in the middle, and 18% in the deep zones. This predicted non-uniformity is in agreement with experimental studies. The novelty of this study is the use of a cartilage material model accounting for the intrinsic inhomogeneity and anisotropy of cartilage caused by its microstructure.     

力学环境对软骨基质代谢的影响

正常关节软骨所受压力是由动态压力与静态压力交替完成。压力引起软骨一系列生理变化包括细胞及细胞外基质成分变形、组织内液体流动、水流电位和生理生化变化。这些变化直接调控细胞外基质代谢。体外构建有良好功能的组织工程化软骨是目前软骨病变、缺损理想的修复方法。研究力学环境对软骨基质代谢的影响,对构建组织工程化软骨有深远意义。     

Mechanical control of tissue morphogenesis during embryological development

Twenty years ago, we proposed a model of developmental control based on tensegrity architecture, in which tissue pattern formation in the embryo is controlled through mechanical interactions between cells and extracellular matrix (ECM) which place the tissue in a state of isometric tension (prestress). The model proposed that local changes in the mechanical compliance of the ECM, for example, due to regional variations in basement membrane degradation beneath growing epithelium, may result in local stretching of the ECM and associated adherent cells, much like a "run-in-a-stocking". Cell growth and function would be controlled locally though physical distortion of the associated cells, or changes in cytoskeletal tension. Importantly, experimental studies have demonstrated that cultured cells can be switched between different fates, including growth, differentiation, apoptosis, directional motility and different stem cell lineages, by modulating cell shape. Experiments in whole embryonic organ rudiments also have confirmed the tight correlation between basement membrane thinning, cell tension generation and new bud and branch formation during tissue morphogenesis and that this process can be inhibited or accelerated by dissipating or enhancing cytoskeletal tension, respectively. Taken together, this work confirms that mechanical forces generated in the cytoskeleton of individual cells and exerted on ECM scaffolds, play a critical role in the sculpting of the embryo.     

Mitochondrial respiratory chain function promotes extracellular matrix integrity in cartilage

Energy metabolism and extracellular matrix (ECM) function together orchestrate and maintain tissue organization, but crosstalk between these processes is poorly understood. Here, we used single-cell RNA-Seq (scRNA-Seq) analysis to uncover the importance of the mitochondrial respiratory chain for ECM homeostasis in mature cartilage. This tissue produces large amounts of a specialized ECM to promote skeletal growth during development and maintain mobility throughout life. A combined approach of high-resolution scRNA-Seq, mass spectrometry/matrisome analysis, and atomic force microscopy was applied to mutant mice with cartilage-specific inactivation of respiratory chain function. This genetic inhibition in cartilage results in the expansion of a central area of 1-month-old mouse femur head cartilage, showing disorganized chondrocytes and increased deposition of ECM material. scRNA-Seq analysis identified a cell cluster–specific decrease in mitochondrial DNA–encoded respiratory chain genes and a unique regulation of ECM-related genes in nonarticular chondrocytes. These changes were associated with alterations in ECM composition, a shift in collagen/noncollagen protein content, and an increase of collagen crosslinking and ECM stiffness. These results demonstrate that mitochondrial respiratory chain dysfunction is a key factor that can promote ECM integrity and mechanostability in cartilage and presumably also in many other tissues.     

Methods in studying ECM degradation

Almost all tissues in our body contain specific cells associated with the tissue itself, and an extracellular matrix (ECM) that consists of a variety of proteins of which the bulk is formed by different types of collagens, glycoproteins and proteoglycans. The ECM plays a pivotal role in numerous processes not only related to the mechanical properties of a tissue, but also in modulating cellular activity. For a proper functioning of a tissue remodeling of the ECM is essential. Some connective tissues are characterized by a very rapid turnover (e.g. periodontal ligament) whereas others hardly show signs of turnover (e.g. cartilage). In all situations degradation of the ECM constituents occur. Under certain conditions, especially during a pathological situation, a high level of degradation may take place. In other situations matrix synthesis and deposition outstrips breakdown, leading to a fibrosis. In order to obtain information on the level of degradation of the different ECM components, various methods have been employed. A number of these methods will be discussed in this article.     

An immunohistochemical study of localization of type I and type II collagens in mandibular condylar cartilage compared with tibial growth plate

Immunohistochemical localization of type I and type II collagens was examined in the rat mandibular condylar cartilage (as the secondary cartilage) and compared with that in the tibial growth plate (as the primary cartilage) using plastic embedded tissues. In the condylar cartilage, type I collagen was present not only in the extracellular matrix (ECM) of the fibrous, proliferative, and transitional cell layers, but also in the ECM of the maturative and hypertrophic cell layers. Type II collagen was present in the ECM of the maturative and hypertrophic cell layers. In the growth plate, type II collagen was present in the ECM of whole cartilaginous layers; type I collagen was not present in the cartilage but in the perichondrium and the bone matrices. These results indicate that differences exist in the components of the ECM between the primary and secondary cartilages. It is suggested that these two tissues differ in the developmental processes and/or in the reactions to their own local functional needs.     

A biomechanical role for perlecan in the pericellular matrix of articular cartilage

Chondrocytes are surrounded by a narrow pericellular matrix (PCM) that is biochemically, structurally, and biomechanically distinct from the bulk extracellular matrix (ECM) of articular cartilage. While the PCM is often defined by the presence of type VI collagen, other macromolecules such as perlecan, a heparan sulfate (HS) proteoglycan, are also exclusively localized to the PCM in normal cartilage and likely contribute to PCM structural integrity and biomechanical properties. Though perlecan is essential for normal cartilage development, its exact role in the PCM is unknown. The objective of this study was to determine the biomechanical role of perlecan in the articular cartilage PCM in situ and its potential as a defining factor of the PCM. To this end, atomic force microscopy (AFM) stiffness mapping was combined with dual immunofluorescence labeling of cryosectioned porcine cartilage samples for type VI collagen and perlecan. While there was no difference in overall PCM mechanical properties between type VI collagen- and perlecan-based definitions of the PCM, within the PCM, interior regions containing both type VI collagen and perlecan exhibited lower elastic moduli than more peripheral regions rich in type VI collagen alone. Enzymatic removal of HS chains from perlecan with heparinase III increased PCM elastic moduli both overall and locally in interior regions rich in both perlecan and type VI collagen. Heparinase III digestion had no effect on ECM elastic moduli. Our findings provide new evidence for perlecan as a defining factor in both the biochemical and biomechanical properties of the PCM.     

The interplay between tissue growth and scaffold degradation in engineered tissue constructs

In vitro tissue engineering is emerging as a potential tool to meet the high demand for replacement tissue, caused by the increased incidence of tissue degeneration and damage. A key challenge in this field is ensuring that the mechanical properties of the engineered tissue are appropriate for the in vivo environment. Achieving this goal will require detailed understanding of the interplay between cell proliferation, extracellular matrix (ECM) deposition and scaffold degradation. In this paper, we use a mathematical model (based upon a multiphase continuum framework) to investigate the interplay between tissue growth and scaffold degradation during tissue construct evolution in vitro. Our model accommodates a cell population and culture medium, modelled as viscous fluids, together with a porous scaffold and ECM deposited by the cells, represented as rigid porous materials. We focus on tissue growth within a perfusion bioreactor system, and investigate how the predicted tissue composition is altered under the influence of (1) differential interactions between cells and the supporting scaffold and their associated ECM, (2) scaffold degradation, and (3) mechanotransduction-regulated cell proliferation and ECM deposition. Numerical simulation of the model equations reveals that scaffold heterogeneity typical of that obtained from $\mu $ CT scans of tissue engineering scaffolds can lead to significant variation in the flow-induced mechanical stimuli experienced by cells seeded in the scaffold. This leads to strong heterogeneity in the deposition of ECM. Furthermore, preferential adherence of cells to the ECM in favour of the artificial scaffold appears to have no significant influence on the eventual construct composition; adherence of cells to these supporting structures does, however, lead to cell and ECM distributions which mimic and exaggerate the heterogeneity of the underlying scaffold. Such phenomena have important ramifications for the mechanical integrity of engineered tissue constructs and their suitability for implantation in vivo.     

A transversely isotropic, transversely homogeneous microstructural-statistical model of articular cartilage

Articular cartilage is a multi-phasic, composite, fibre-reinforced material. Therefore, its mechanical properties are determined by the tissue microstructure. The presence of cells (chondrocytes) and collagen fibres within the proteoglycan matrix influences, at a local and a global level, the material symmetries. The volumetric concentration and shape of chondrocytes, and the volumetric concentration and spatial arrangement of collagen fibres have been observed to change as a function of depth in articular cartilage. In particular, collagen fibres are perpendicular to the bone-cartilage interface in the deep zone, their orientation is almost random in the middle zone, and they are parallel to the surface in the superficial zone. The aim of this work is to develop a model of elastic properties of articular cartilage based on its microstructure. In previous work, we addressed this problem based on Piola's notation for fourth-order tensors. Here, mathematical tools initially developed for transversely isotropic composite materials comprised of a statistical orientation of spheroidal inclusions are extended to articular cartilage, while taking into account the dependence of the elastic properties on cartilage depth. The resulting model is transversely isotropic and transversely homogeneous (TITH), the transverse plane being parallel to the bone-cartilage interface and the articular surface. Our results demonstrate that the axial elastic modulus decreases from the deep zone to the articular surface, a result that is in good agreement with experimental findings. Finite element simulations were carried out, in order to explore the TITH model's behaviour in articular cartilage compression tests. The force response, fluid flow and displacement fields obtained with the TITH model were compared with the classical linear elastic, isotropic, homogeneous (IH) model, showing that the IH model is unable to predict the non-uniform behaviour of the tissue. Based on considerations that the mechanical stability of the tissue depends on its topological and microstructural properties, our long-term goal is to clearly understand the stability conditions in topological terms, and the relationship with the growth and remodelling mechanisms in the healthy and diseased tissue.     

An immunohistochemical study of localization of type I and type II collagens in mandibular condylar cartilage compared with tibial growth plate

Summary Immunohistochemical localization of type I and type II collagens was examined in the rat mandibular condylar cartilage (as the secondary cartilage) and compared with that in the tibial growth plate (as the primary cartilage) using plastic embedded tissues. In the condylar cartilage, type I collagen was present not only in the extracellular matrix (ECM) of the fibrous, proliferative, and transitional cell layers, but also in the ECM of the maturative and hypertrophic cell layers. Type II collagen was present in the ECM of the maturative and hypertrophic cell layers. In the growth plate, type II collagen was present in the ECM of whole cartilaginous layers; type I collagen was not present in the cartilage but in the perichondrium and the bone matrices. These results indicate that differences exist in the components of the ECM between the primary and secondary cartilages. It is suggested that these two tissues differ in the developmental processes and/or in the reactions to their own local functional needs.     

Fibril reinforced poroelastic model predicts specifically mechanical behavior of normal,proteoglycan depleted and collagen degraded articular cartilage

Degradation of collagen network and proteoglycan (PG) macromolecules are signs of articular cartilage degeneration. These changes impair cartilage mechanical function. Effects of collagen degradation and PG depletion on the time-dependent mechanical behavior of cartilage are different. In this study, numerical analyses, which take the compression-tension nonlinearity of the tissue into account, were carried out using a fibril reinforced poroelastic finite element model. The study aimed at improving our understanding of the stress-relaxation behavior of normal and degenerated cartilage in unconfined compression. PG and collagen degradations were simulated by decreasing the Young's modulus of the drained porous (nonfibrillar) matrix and the fibril network, respectively. Numerical analyses were compared to results from experimental tests with chondroitinase ABC (PG depletion) or collagenase (collagen degradation) digested samples. Fibril reinforced poroelastic model predicted the experimental behavior of cartilage after chondroitinase ABC digestion by a major decrease of the drained porous matrix modulus (-64+/-28%) and a minor decrease of the fibril network modulus (-11+/-9%). After collagenase digestion, in contrast, the numerical analyses predicted the experimental behavior of cartilage by a major decrease of the fibril network modulus (-69+/-5%) and a decrease of the drained porous matrix modulus (-44+/-18%). The reduction of the drained porous matrix modulus after collagenase digestion was consistent with the microscopically observed secondary PG loss from the tissue. The present results indicate that the fibril reinforced poroelastic model is able to predict specifically characteristic alterations in the stress-relaxation behavior of cartilage after enzymatic modifications of the tissue. We conclude that the compression-tension nonlinearity of the tissue is needed to capture realistically the mechanical behavior of normal and degenerated articular cartilage.     

Aging enhances a mechanically-induced reduction in tendon strength by an active process involving matrix metalloproteinase activity

Age-associated and degenerative loss of functional integrity in soft tissues develops from effects of cumulative and subtle changes in their extracellular matrix (ECM). The highly ordered tendon ECM provides the tissue with its tensile strength during loading. As age and exercise collide in the high incidence of tendinopathies, we hypothesized that aged tendons fail due to cumulative damage resulting from a combination of diminished matrix repair and fragmentation of ECM proteins induced by prolonged cyclical loading, and that this is an active cell-mediated process. We developed an equine tendon explant model to examine the effect of age on the influence of prolonged cyclical loading at physiologically relevant strain rates (5% strain, 1 Hz for 24 h) on tissue mechanical properties, loss of ECM protein and matrix metalloproteinase (MMP) expression. We show significantly diminished mechanical strength of cyclically loaded tissue compared to controls (39.7 +/- 12%, P 相似文献