Proteoglycan synthesis in organ cultures from regions of bovine tendon subjected to different mechanical forces

The role of SLRPs and large aggregating proteoglycans in collagen fibrillogenesis, extracellular matrix assembly, and mechanical function of fibrocartilage

PURPOSE

Proteoglycans, especially small leucine rich proteoglycans (SLRPs), play major roles in facilitating the development and regulation of collagen fibers and other extracellular matrix components. However, their roles in fibrocartilage have not been widely reviewed. Here, we discuss both SLRP and large aggregating proteoglycan's roles in collagen fibrillogenesis and extracellular matrix assembly in fibrocartilage tissues such as the meniscus, annulus fibrosus (AF), and TMJ disc. We also discuss their expression levels throughout development, aging and degeneration, as well as repair.

METHODS

A review of literature discussing proteoglycans and collagen fibrillogenesis in fibrocartilage was conducted and data from these manuscripts were analyzed and grouped to discuss trends throughout the tissue's architectural zones and developmental stage.

RESULTS

The spatial collagen architecture of these fibrocartilaginous tissues is reflected in the distribution of proteoglycans expressed, suggesting that each proteoglycan plays an important role in the type of architecture presented and associated mechanical function.

CONCLUSION

The unique structure-function relationship of fibrocartilage makes the varied architectures throughout the tissues imperative for their success and understanding the functions of these proteoglycans in developing and maintaining the fiber structure could inform future work in fibrocartilage replacement using tissue engineered constructs.

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.

Release of pro-inflammatory cytokines from muscle and bone causes tenocyte death in a novel rotator cuff in vitro explant culture model

PURPOSE

Tendinopathy is a significant clinical problem thought to be associated with altered mechanical loading. Explant culture models allow researchers to alter mechanical loading in a controlled in vitro environment while maintaining tenocytes in their native matrix. However, current models do not accurately represent commonly injured tendons, ignoring contributions of associated musculature and bone, as well as regional collagen structure. This study details the characterization of amouse rotator cuff explant culture model, including bone, tendon, and muscle (BTM).

MATERIALS AND METHODS

Following harvest, BTM explants were maintained in stress-deprived culture for one week and tendon was then assessed for changes in cell viability, metabolism, matrix structure and content.

RESULTS

Matrix turnover occurred throughout culture as manifested in both gene expression and biosynthesis, but this did not translate to net changes in total collagen or sulfated glycosaminoglycan content. Furthermore, tendon structure was not significantly altered throughout culture. However, we found significant cell death in BTM tendons after 3 days in culture, which we hypothesize is cytokine-induced. Using a targeted multiplex assay, we found high levels of pro-inflammatory cytokines released to the culture medium from muscle and bone, levels that did cause cell deathin tendon-alone controls.

CONCLUSIONS

Overall, this model presents an innovative approach to understandingrotator cuff injury and tenocyte mechanobiology in a clinically-relevant tendon structure. Our model can be a powerful tool to investigate how mechanical and biological stimuli can alter normal tendon health and lead to tendon degeneration, and may provide a testbed for therapeutics for tendon repair.

In vitro studies to evaluate the effect of varying culture conditions and IPL fluencies on tenocyte activities

Tendons are dense, fibrous connective tissues which carry out the essential physiological role of transmitting mechanical forces from skeletal muscle to bone. From a clinical perspective, tendinopathy is very common, both within the sporting arena and amongst the sedentary population. Studies have shown that light therapy may stimulate tendon healing, and more recently, intense pulsed light (IPL) has attracted attention as a potential treatment modality for tendinopathy; however, its mechanism of action and effect on the tendon cells (tenocytes) is poorly understood. The present study therefore investigates the influence of IPL on an in vitro bovine tendon model. Tenocytes were irradiated with IPL at different devise settings and under variable culture conditions (e.g. utilising cell culture media with or without the pH indicator dye phenol red), and changes in tenocyte viability and migration were subsequently investigated using Alamar blue and scratch assays, respectively. Our data demonstrated that IPL fluencies of up to 15.9 J/cm² proved harmless to the tenocyte cultures (this was the case using culture media with or without phenol red) and resulted in a significant increase in cell viability under certain culture conditions. Furthermore, IPL treatment of tenocytes did not affect the rate of cell migration. This study demonstrates that irradiation with IPL is not detrimental to the tenocytes and may increase their viability under certain conditions, thus validating our in vitro model. Further studies are required to elucidate the effects of IPL application in the clinical situation.

Experimental evaluation of multiscale tendon mechanics

Tendon's primary function is a mechanical link between muscle and bone. The hierarchical structure of tendon and specific compositional constituents are believed to be critical for proper mechanical function. With increased appreciation for tendon importance and the development of various technological advances, this review paper summarizes recent experimental approaches that have been used to study multiscale tendon mechanics, includes an overview of studies that have evaluated the role of specific tissue constituents, and also proposes challenges/opportunities facing tendon study. Tendon has been demonstrated to have specific structural characteristics (e.g., multi-level hierarchy, crimp pattern, helix) and complex mechanical properties (e.g., non-linearity, anisotropy, viscoelasticity). Physical mechanisms including uncrimping, fiber sliding, and collagen reorganization have been shown to govern tendon mechanical responses under both static and dynamic loading. Several tendon constituents with relatively small quantities have been suggested to play a role in its mechanics, although some results are conflicting. Further research should be performed to understand the interplay and communication of tendon mechanical properties across levels of the hierarchical structure, and further show how each of these components contribute to tendon mechanics. The studies summarized and discussed in this review have helped elucidate important aspects of multiscale tendon mechanics, which is a prerequisite for analyzing stress/strain transfer between multiple scales and identifying key principles of mechanotransduction. This information could further facilitate interpreting the functional diversity of tendons from different species, different locations, and even different developmental stages, and then better understand and identify fundamental concepts related to tendon degeneration, disease, and healing. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1353-1365, 2017.

Tissue-engineering strategies for the tendon/ligament-to-bone insertion

Injuries to connective tissues are painful and disabling and result in costly medical expenses. These injuries often require reattachment of an unmineralized connective tissue to bone. The uninjured tendon/ligament-to-bone insertion (enthesis) is a functionally graded material that exhibits a gradual transition from soft tissue (i.e., tendon or ligament) to hard tissue (i.e., mineralized bone) through a fibrocartilaginous transition region. This transition is believed to facilitate force transmission between the two dissimilar tissues by ameliorating potentially damaging interfacial stress concentrations. The transition region is impaired or lost upon tendon/ligament injury and is not regenerated following surgical repair or natural healing, exposing the tissue to risk of reinjury. The need to regenerate a robust tendon-to-bone insertion has led a number of tissue engineering repair strategies. This review treats the tendon-to-bone insertion site as a tissue structure whose primary role is mechanical and discusses current and emerging strategies for engineering the tendon/ligament-to-bone insertion in this context. The focus lies on strategies for producing mechanical structures that can guide and subsequently sustain a graded tissue structure and the associated cell populations.

Fibrocartilage tissue engineering: the role of the stress environment on cell morphology and matrix expression

Although much is known about the effects of uniaxial mechanical loading on fibrocartilage development, the stress fields to which fibrocartilaginous regions are subjected to during development are mutiaxial. That fibrocartilage develops at tendon-to-bone attachments and in compressive regions of tendons is well established. However, the three-dimensional (3D) nature of the stresses needed for the development of fibrocartilage is not known. Here, we developed and applied an in vitro system to determine whether fibrocartilage can develop under a state of periodic hydrostatic tension in which only a single principal component of stress is compressive. This question is vital to efforts to mechanically guide morphogenesis and matrix expression in engineered tissue replacements. Mesenchymal stromal cells in a 3D culture were exposed to compressive and tensile stresses as a result of an external tensile hydrostatic stress field. The stress field was characterized through mechanical modeling. Tensile cyclic stresses promoted spindle-shaped cells, upregulation of scleraxis and type one collagen, and cell alignment with the direction of tension. Cells experiencing a single compressive stress component exhibited rounded cell morphology and random cell orientation. No difference in mRNA expression of the genes Sox9 and aggrecan was observed when comparing tensile and compressive regions unless the medium was supplemented with the chondrogenic factor transforming growth factor beta3. In that case, Sox9 was upregulated under static loading conditions and aggrecan was upregulated under cyclic loading conditions. In conclusion, the fibrous component of fibrocartilage could be generated using only mechanical cues, but generation of the cartilaginous component of fibrocartilage required biologic factors in addition to mechanical cues. These studies support the hypothesis that the 3D stress environment influences cell activity and gene expression in fibrocartilage development.

Fell-Muir lecture: cartilage 2010 - the known unknowns

Over the past 40 years there have been giant steps forward in our understanding of cellular and molecular biology that have given us the framework by which to understand tissue organization and tissue function on a range of scales. However, although the progress has been great, the more we have discovered, the more we are aware of what we don't yet know. In this article, I would like to flag up some issues of cartilage biology, function and pathology where we still have significant ignorance. As scientists we all provide contributions to add to the greater understanding of science and progress is on a broad front, but gaps are left where particular difficulty is encountered and in life sciences it is no different. Progress is fast where new knowledge and techniques pave the way, but where study is complex and relevant techniques poorly developed the gaps are left behind. In cartilage research and matrix biology, the gaps can particularly be seen at interfaces between disciplines and where technology development has lagged behind and in the particular challenges of understanding how molecular properties can explain tissue macro properties.

Aggrecanolysis and in vitro matrix degradation in the immature bovine meniscus: mechanisms and functional implications

Introduction

Little is known about endogenous or cytokine-stimulated aggrecan catabolism in the meniscal fibrocartilage of the knee. The objectives of this study were to characterize the structure, distribution, and processing of aggrecan in menisci from immature bovines, and to identify mechanisms of extracellular matrix degradation that lead to changes in the mechanical properties of meniscal fibrocartilage.

Methods

Aggrecanase activity in the native immature bovine meniscus was examined by immunolocalization of the aggrecan NITEGE neoepitope. To investigate mechanisms of cytokine-induced aggrecan catabolism in this tissue, explants were treated with interleukin-1α (IL-1) in the absence or presence of selective or broad spectrum metalloproteinase inhibitors. The sulfated glycosaminoglycan (sGAG) and collagen contents of explants and culture media were quantified by biochemical methods, and aggrecan catabolism was examined by Western analysis of aggrecan fragments. The mechanical properties of explants were determined by dynamic compression and shear tests.

Results

The aggrecanase-generated NITEGE neoepitope was preferentially localized in the middle and outer regions of freshly isolated immature bovine menisci, where sGAG density was lowest and blood vessels were present. In vitro treatment of explants with IL-1 triggered the accumulation of NITEGE in the inner and middle regions. Middle region explants stimulated with IL-1 exhibited substantial decreases in sGAG content, collagen content, and mechanical properties. A broad spectrum metalloproteinase inhibitor significantly reduced sGAG loss, abrogated collagen degradation, and preserved tissue mechanical properties. In contrast, an inhibitor selective for ADAMTS-4 and ADAMTS-5 was least effective at blocking IL-1-induced matrix catabolism and loss of mechanical properties.

Conclusions

Aggrecanase-mediated aggrecanolysis, typical of degenerative articular cartilage, may play a physiologic role in the development of the immature bovine meniscus. IL-1-induced release of sGAG and loss of mechanical properties can be ascribed primarily to the activity of MMPs or aggrecanases other than ADAMTS-4 and ADAMTS-5. These results may have implications for the clinical management of osteoarthritis.

The effect of glucocorticoids on tendon cell viability in human tendon explants

BACKGROUND AND PURPOSE

Previous studies on the culture of human tenocytes have shown that dexamethasone and triamcinolone reduce cell viability, suppress cell proliferation, and reduce collagen synthesis. However, such cell cultures lack the extracellular matrix and three-dimensional structure of normal tendons, which affects their response to stimuli. We established a human tendon explant culture system and tested the effects of dexamethasone and triamcinolone on cell viability.

METHODS

Primary human tendon explant cultures were prepared from healthy hamstring tendons. Tendon strips were harvested from hamstring tendons and cultured in 24-well plates in Dulbecco's modification of Eagle's Medium (DMEM) supplemented with 2% fetal calf serum. The tendon explants were treated with 0 microM (control), 10 microM, or 100 microM dexamethasone sodium phosphate or 0 microM (control), 10 microM, or 100 microM triamcinolone acetonide in DMEM for 96 h. Cell viability was measured by Alamar blue assay before and after glucocorticoid treatment.

RESULTS

Incubation with 10 microM and 100 microM dexamethasone reduced cell viability in human tendon explants by 35% and 45%, respectively, as compared to a 6% increase in the controls (p = 0.01, mixed-effects ANOVA). Triamcinolone at 10 microM and 100 microM reduced cell viability by 33% and 36%, respectively, as compared to a 9% increase in the controls (p = 0.07, mixed-effects ANOVA).

INTERPRETATION

Human tendon explant cultures can be used to study the effects of glucocorticoids on human tendon. Dexamethasone and triamcinolone suppress the cell viability of human tendon in its natural 3-dimensional environment with matrix anchorage. Human tendon explant cultures provide a species-specific model for further investigation of the effects of glucocorticoids on the metabolism of the extracellular matrix of human tendon, and on its mechanical properties.

Structure and component alteration of rabbit Achilles tendon in tissue culture

The aim of this study was to investigate alterations of cultured tendon tissues to determine whether tissue culture is a useful method for biological analyses of the tendon. Tendon tissues for tissue culture were isolated from Achilles tendons of rabbits. The tendon segments were placed one segment per well and incubated in growth medium consisting of Dullbecco's modified Eagle's medium supplemented with 5% fetal bovine serum at 37 degrees C in a humidified atmosphere with 5% CO(2) for various periods. The alignment of collagen fibrils was preserved for 48 h, but tendon structure has disintegrated at 96 h. Alcian blue staining and gelatine zymography revealed that proteoglycan markedly diminished and that matrix metalloproteinase (MMPs) activity was upregulated sharply at 72 and 96 h. The ratio of collagen fibrils with large diameter had increased and the mean diameter and mass average diameter value had reached maximum at 48 h. The values then decreased and mean diameters at 72 and 96 h were significantly different from that at 48 h. At 96 h, the ratio of collagen fibrils with small diameters had increased and collagen fibrils with large diameters had disappeared. These findings indicate that structural alteration is possible to be induced by disintegration of collagen fibrils and disappearance of glycosaminoglycans from extracellular matrix (ECM), subsequent of upregulation of MMPs activity. Although the study period is limited, the tissue culture method is available for investigating cell-ECM interaction in tendons.

Effects of the Frequency and Duration of Cyclic Stress on the Mechanical Properties of Cultured Collagen Fascicles From the Rabbit Patellar Tendon

The effects of frequency or duration of cyclic stress on the mechanical properties of collagen fascicles were studied by means of in vitro tissue culture experiments. Collagen fascicles of approximately 300μm in diameter were obtained from rabbit patellar tendons. During culture, cyclic stress having the peak stress of approximately 2MPa was applied to the fascicles at 1Hz for 1hour∕day (1Hz-1h group), at 1Hz for 4hours∕day (1Hz-4h group), or at 4Hz for 1hour∕day (4Hz-1h group). The frequency of 4Hz and the duration of 1hour∕day are considered to be similar to those of the in vivo stress applied to fascicles in the intact rabbit patellar tendon. After culture for 1 or 2weeks, the mechanical properties of the fascicles were determined using a micro-tensile tester, and were compared to the properties of non-cultured, fresh fascicles (control group) and the fascicles cultured under no load condition (non-loaded group). The tangent modulus and tensile strength of fascicles in the 4Hz-1h group were similar to those in the control group; however, the fascicles of the 1Hz-1h and 1Hz-4h groups had significantly lower values than those of the control group. There was no significant difference in the tensile strength between the 1Hz-1h and non-loaded groups, although the strength in the 1Hz-4h group was significantly higher than that of the non-loaded group. It was concluded that the frequency and duration of cyclic stress significantly affect the mechanical properties of cultured collagen fascicles. If we apply cyclic stress having the frequency and duration which are experienced in vivo, the biomechanical properties are maintained at control, normal level. Lower frequencies or less cycles of applied force induce adverse effects.

The optic nerve head as a biomechanical structure: a new paradigm for understanding the role of IOP-related stress and strain in the pathophysiology of glaucomatous optic nerve head damage

We propose here a conceptual framework for understanding the optic nerve head (ONH) as a biomechanical structure. Basic principles of biomechanical engineering are used to propose a central role for intraocular pressure (IOP)-related stress and strain in the physiology of ONH aging and the pathophysiology of glaucomatous damage. Our paradigm suggests that IOP-related stress and strain (1) are substantial within the load-bearing connective tissues of the ONH even at low levels of IOP and (2) underlie both ONH aging and the two central pathophysiologies of glaucomatous damage--mechanical failure of the connective tissues of the lamina cribrosa, scleral canal wall, and peripapillary sclera, and axonal compromise within the lamina cribrosa by a variety of mechanisms. Modeling the ONH as a biomechanical structure generates a group of testable hypotheses regarding the central mechanisms of glaucomatous damage and provides a logic for classifying the principal components of the susceptibility of an individual ONH to a given level of IOP.

Large aggregating and small leucine-rich proteoglycans are degraded by different pathways and at different rates in tendon

This work investigated the kinetics of catabolism and the catabolic fate of the newly synthesized (35)S-labelled proteoglycans present in explant cultures of tendon. Tissue from the proximal region of bovine deep flexor tendon was incubated with [(35)S]sulfate for 6 h and then placed in explant cultures for periods of up to 15 days. The amount of radiolabel associated with proteoglycans and free [(35)S]sulfate lost to the medium and retained in the matrix was determined for each day in culture. It was shown that the rate of catabolism of radiolabelled small proteoglycans (decorin and biglycan) was significantly slower (T((1/2)) > 20 days) compared with the radiolabelled large proteoglycans (aggrecan and versican) that were rapidly lost from the tissue (T((1/2)) approximately 2 days). Both the small and large newly synthesized proteoglycans were lost from the matrix with either intact or proteolytically modified core proteins. When explant cultures of tendon were maintained either at 4 degrees C or in the presence of the lysosomotrophic agent ammonium chloride, inhibition of the cellular catabolic pathway for small proteoglycans was demonstrated indicating the involvement of cellular activity and lysosomes in the catabolism of small proteoglycans. It was estimated from these studies that approximately 60% of the radiolabelled small proteoglycans that were lost from the tissue were degraded by the intracellular pathway present in tendon cells. This work shows that the pathways of catabolism for large aggregating and small leucine-rich proteoglycans are different in tendon and this may reflect the roles that these two populations of proteoglycans play in the maintenance of the extracellular matrix of tendon.

Biology of Fibrocartilage Cells

Fibrocartilage is an avascular tissue that is best documented in menisci, intervertebral discs, tendons, ligaments, and the temporomandibular joint. Several of these sites are of particular interest to those in the emerging field of tissue engineering. Fibrocartilage cells frequently resemble chondrocytes in having prominent rough endoplasmic reticulum, many glycogen granules, and lipid droplets, and intermediate filaments together with and actin stress fibers that help to determine cell organization in the intervertebral disc. Fibrocartilage cells can synthesize a variety of matrix molecules including collagens, proteoglycans, and noncollagenous proteins. All the fibrillar collagens (types I, II, III, V, and XI) have been reported, together with FACIT (types IX and XII) and network-forming collagens (types VI and X). The proteoglycans include large, aggregating types (aggrecan and versican) and small, leucine-rich types (decorin, biglycan, lumican, and fibromodulin). Less attention has been paid to noncollagenous proteins, although tenascin-C expression may be modulated by mechanical strain. As in hyaline cartilage, matrix metalloproteinases are important in matrix turnover and fibrocartilage cells are capable of apoptosis.

A novel fibrocartilaginous tendon from an elasmobranch fish ( Rhinoptera bonasus)

Tendons of the jaw adductor muscles of a hard prey crushing stingray exhibit similar adaptations to compressive and shear loads as those seen in mammalian tendons. Ventral intermandibular tendon from the cownose ray, Rhinoptera bonasus, has a prominent fibrocartilaginous pad that lies between a fibrous region of the tendon and the mineralized tissue of the jaw. Histologically the pad is similar to the fibrocartilaginous meniscus of mammals, and these tissues also share some biochemical traits. Proteoglycan (PG) content in the fibrocartilaginous pad is nearly four times higher than in the linearly arrayed tendinous tissue. The predominant PGs appear to be an aggrecan-like molecule and a decorin-like molecule. The decorin-like molecule is quite small when compared to mammalian decorin (20-80 kDa vs. 100-200 kDa). This study is the first to document adaptations to compressive/shear loading in tendon from a cartilaginous fish, and the similarities to the mammalian condition argue for the early evolution of this reactive ability of tendinous tissue.

The influence of ageing and exercise on tendon growth and degeneration--hypotheses for the initiation and prevention of strain-induced tendinopathies

Strain-induced tendinopathy is a common injury in both human and equine athletes, with increasing incidence associated with greater involvement in sport and an increasingly aged population. This paper reviews our studies on the abundant non-collagenous protein, cartilage oligomeric matrix protein (COMP), in equine tendons. Its variation between tendon type and site, age and exercise has provided an insight into how age and exercise influence tendon growth and maturation. Tendons can be broadly divided into two types, reflecting their different matrix composition and function: the energy-storing tendons used for weight-bearing and locomotion, which suffer a high incidence of strain-induced tendinopathy, and positional tendons involved in limb placement or manipulative skills. It would appear that while energy-storing tendon can respond to the mechanical forces applied to it during growth, there is no evidence that it can do so after skeletal maturity. Instead, cumulative fatigue damage causes degeneration at the molecular level, potentially weakening it and increasing the risk of clinical injury. Appropriate exercise regimes early in life may help to improve the quality of growing tendon, thereby reducing the incidence of injury during ageing or subsequent athletic career.

Biomimetic approaches to tendon repair

The linear organization of collagen fibers in tendons results in optimal stiffness and strength at low strains under tensile load. However, this organization makes repairing ruptured or lacerated tendons extremely difficult. Current suturing techniques to join split ends of tendons, while providing sufficient mechanical strength to prevent gapping, are inadequate to carry normal loads. Immobilization protocols necessary to restore tendon congruity result in scar formation at the repair site and peripheral adhesions that limit excursion. These problems are reviewed to emphasize the need for novel approaches to tendon repair, one of which is the development of biomimetic tendons. The objective of the empirical work described here was to produce biologically-based, biocompatible tendon replacements with appropriate mechanical properties to enable immediate mobilization following surgical repair. Nor-dihydroguaiaretic acid (NDGA), a di-catechol from creosote bush, caused a dose dependent increase in the material properties of reconstituted collagen fibers, achieving a 100-fold increase in strength and stiffness over untreated fibers. The maximum tensile strength of the optimized NDGA treated fibers averaged 90 MPa; the elastic modulus of these fibers averaged 580 MPa. These properties were independent of strain rates ranging from 0.60 to 600 mm/min. Fatigue tests established that neither strength nor stiffness were affected after 80 k cycles at 5% strain. Treated fibers were not cytotoxic to tendon fibroblasts. Fibroblasts attached and proliferated on NDGA treated collagen normally. NDGA-fibers did not elicit a foreign body response nor did they stimulate an immune reaction during six weeks in vivo. The fibers survived 6 weeks with little evidence of fragmentation or degradation. The polymerization scheme described here produces a fiber-reinforced NDGA-polymer with mechanical properties approaching an elastic solid. The strength, stiffness and fatigue properties of the NDGA-treated fibers are comparable to those of tendon. These fibers are biocompatible with tendon fibroblasts and elicit little rejection or antigenic response in vivo. These results indicate that NDGA polymerization may provide a viable approach for producing collagenous materials that can be used to bridge gaps in ruptured or lacerated tendons. The tendon-like properties of the NDGA-fiber would allow early mobilization after surgical repair. We predict that timely loading of parted tendons joined by this novel biomaterial will enhance mechanically driven production of neo-tendon by the colonizing fibroblasts and result in superior repair and rapid return to normal properties.

Effects of static stress on the mechanical properties of cultured collagen fascicles from the rabbit patellar tendon

In-vitro tissue culture experiments were performed to study the effects of static stress on the mechanical properties of collagen fascicles obtained from the rabbit patellar tendon. After collagen fascicles having the diameter of approximately 300 microm were cultured for 1 and 2 wk under static stress between 0 and 3 MPa, their mechanical properties and crimp morphology were determined using a micro-tensile tester and a light microscope, respectively. The tensile strength and tangent modulus of the fascicles were significantly decreased by culture under no load compared to control fascicles. A statistically significant correlation, which was described by a quadratic curve, was observed between applied stress and tensile strength. The maximum tensile strength (16.7 MPa) was obtained at the applied stress of 1.2 MPa; the strength was within a range of control values. There was a similar correlation between applied stress and tangent modulus, and the modulus was maintained at control level under 1.3 MPa stress. The stress of 1.2 to 1.3 MPa is equivalent to approximately 50 percent of the peak stress developed in the intact rabbit patellar tendon by running. Strain at failure of cultured collagen fascicles was negatively correlated with applied stress, and that at 1.2 to 1.3 MPa stress was almost the same as the control value. Crimp morphology in the fascicles cultured under about 1.2 MPa stress was similar to that in control fascicles. These results indicate that cultured collagen fascicles change the mechanical properties and structure in response to static tensile stress. In addition, their mechanical properties and structure are maintained at control level if the static stress of 50 percent of in-vivo peak stress is applied.

Optical anisotropy of a pig tendon under compression

The proximal region of the superficial digital flexor tendon of pigs passes under the tibiotarsal joint, where it is subjected to compressional and tensional forces. This region was divided into a surface portion (sp), which is in direct contact with the bone and into a deep portion (dp), which is the layer opposite the articulating surface. The purpose of this work was to analyse the distribution and organisation of the collagen bundles and proteoglycans in the extracellular matrix in sp and dp. Toluidine-blue-stained sections were analysed under a polarising microscope. Strong basophilia and metachromasia were observed in sp, demonstrating accumulation of proteoglycan in a region bearing compression, but the intensity was reduced the further layers were from the bone. Linear dichroism confirmed that the glycosaminoglycan molecules were disposed predominantly parallel to the longest axis of the collagen fibrils. Birefringence analysis showed a higher molecular order and aggregation of the collagen bundles in areas where the tension was more prominent. The crimp pattern was more regular in dp than in sp, probably as a requirement for tendon stretching. The optical anisotropy exhibited by the collagen bundles also confirmed the helical organisation of the collagen bundles in the tendon. Hyaluronidase digestion caused a decrease in the basophilia, but this was not eliminated, supporting the idea that in the matrix, proteoglycans are not completely available to the enzyme action.

Differences in mesenchymal tissue repair

Variations in certain mesenchymal tissue healing processes are not widely recognized. The current review summarizes key differences in healing mechanisms and healing potential after injury to soft tissues having different healing outcomes.

Regional structural and viscoelastic properties of fibrocartilage upon dynamic nanoindentation of the articular condyle

Fibrocartilage,a tissue with macromaterial properties between dense fibrous tissue and hyaline cartilage, is not well understood in its ultrastructure and regional viscoelastic properties. Here nanoindentation with atomic force microscopy was performed on fresh fibrocartilage samples of rabbit jaw joint condyles. Each sample was divided into anteromedial, anterolateral, posteromedial, and posterolateral regions for probing and topographic imaging in 2 x 2 microm and 10 x 10 microm scan sizes. Young's moduli differed significantly among these regions in a descending gradient from the anteromedial (2.34 +/- 0.26 MPa) to the posterolateral (0.95 +/- 0.06 MPa). The Poisson ratio, defined as lateral strain over axial strain, had the same gradient distribution: highest for the anteromedial region (0.46 +/- 0.05) and lowest for the posterolateral region (0.31 +/- 0.05). The same four regions showed a descending gradient of surface roughness: highest for the anteromedial (321.6 +/- 13.8 nm) and lowest for the posterolateral (155.6 +/- 12.6 nm). Thus, the regional ultrastructural and viscoelastic properties of fibrocartilage appear to be coregulated. Based on these region-specific gradient distributions, fibrocartilage is constructed to withstand tissue-borne shear stresses, which likely propagate across its different regions. A model of shear gradient and concentric gradient is proposed to describe the region-specific capacity of fibrocartilage to sustain shear stresses in tendons, ligaments, joints, and the healing bone across species.

An in vitro comparison of human flexor and extensor tendon cells

This study examined the in vitro phenotype of cells cultured from both flexor and extensor tendons. Matrix proteins secreted by tendon cells were examined, together with their response to mechanical strain, using cells from the two types of tendon. Immunocytochemical staining using specific antibodies to matrix proteins demonstrated that flexor tendon cells synthesised only three of the five proteins stained for, whilst extensor tendon cells synthesised all five. Gel electrophoresis (used to separate mixtures of proteins) demonstrated that protein secretion was similar for both cell types. Both cell types showed similar patterns of synthesis for total collagen and total protein over time. No difference was found in the response to cyclical strain of cells from the two types of tendon; both cell proliferation and collagen synthesis were stimulated. The study demonstrated that cells obtained from two different types of tendon behave similarly when exposed to identical environmental conditions in vitro.

Tenascin-C in tendon regions subjected to compression

Tendon regions subjected almost exclusively to tension differ from regions subjected to high levels of compression as well as tension. Regions not exposed to compression consist primarily of spindle-shaped fibroblasts surrounded by densely packed longitudinally oriented collagen fibrils formed principally from type-I collagen. In contrast, regions subjected to compression have a fibrocartilagenous structure and composition: they consist of spherical cells surrounded by a matrix containing hyaline cartilage proteoglycans (aggrecan) and type-II collagen as well as type-I collagen. Reducing their adhesion to the matrix may help cells in the latter regions establish and maintain a spherical shape and minimize their deformation when the tissue is subjected to mechanical stress. We hypothesized that expression of tenascin-C, an anti-adhesive protein, is part of the adaptation of tendon cells to compression that helps establish and maintain fibrocartilagenous regions. To test this hypothesis, we compared segments of bovine flexor tendons subjected to repetitive compression (distal) with segments that are not subjected to compression (proximal) to determine whether they differed in tenascin-C content and expression. RNA and protein analyses showed that tenascin-C expression was elevated in the distal tendon. Tendon cells from the distal segment expressed more tenascin-C mRNA than did cells from the proximal segments for as long as 4 days in cell culture, indicating that increased tenascin-C expression is a relatively stable feature of the distal cells. Moreover, purified tenascin-C inhibited the attachment of cultured tendon cells to fibronectin. These observations support the hypothesis that tenascin-C expression is a cellular adaptation to compression that helps establish and maintain fibrocartilagenous regions of tendons by decreasing cell-matrix adhesion.

Effects of intraoral splint wear on proteoglycans in the temporomandibular joint disc

Intraoral splints are a common dental treatment for dysfunctions of the temporomandibular joint (TMJ), but their effects on the structures of the joint, specifically the disc, have not been well investigated. This study examined proteoglycans (PGs) of the TMJ disc of the miniature pig and tested for alterations resulting from intraoral splint wear. Sixteen female pigs were divided into three groups: control (C), control splint (CS), and protrusive splint (PS). Splinted groups received chrome-cobalt ramp splints which were worn continuously for 2 months. PG content within various disc locations was determined by colorimeteric assay. PG synthesis and type were examined by labeling with (35)S-sulfate and SDS-PAGE analysis. Average water content of the disc was 77.1%, which places it at the high end of the normal range for collagenous biomaterials (60-80%). PGs migrating to the positions typical of aggrecan, biglycan, and decorin on SDS-PAGE were present in all locations of all groups. The highest content and synthesis of PGs were always found in the intermediate band of the disc regardless of group (P < 0.05), supporting the notion that this band encounters heavy compressive loading during function. The joints of animals from both splinted groups showed a high frequency of gross pathology. Biglycan synthesis was increased in both splinted groups (P < 0.05). Newly synthesized biglycan had a shorter migration distance in the intermediate bands of the CS group, suggesting increased hydrodynamic size. These findings suggest that intraoral splint wear may cause disc damage or remodeling.

Hyaluronic acid modulates cell proliferation unequally in intrasynovial and extrasynovial rabbit tendons in vitro

As tendons differ in biochemical composition and cellular capacities, we have compared dose response effects of hyaluronic acid on cell proliferation and synthesis of matrix components in intermediate and proximal segments of intrasynovial deep flexor tendons and extrasynovial peroneus rabbit tendons in vitro. Compared with matched control tendons, hyaluronic acid inhibited cell proliferation in intermediate and proximal intrasynovial flexor tendon segments at the concentrations of 0.1-2.0 mg/ml and 0.5-2.0 mg/ml respectively, but in extrasynovial tendon segments only at the concentration of 0.5 mg/ml. Hyaluronic acid did not affect synthesis of proteoglycan, collagen and non-collagen protein in either type of tendon. These results show that hyaluronic acid modulates cell proliferation unequally in intra- and extrasynovial tendons without affecting the synthesis of matrix components in the two types of tendons, indicating differential hyaluronic acid sensitivity and a possible mechanism of action.

An in vivo model for load-modulated remodeling in the rabbit flexor tendon

This study tested the hypothesis that eliminating in vivo compression to the wrap-around, fibrocartilage-rich zone of the flexor digitorum profundus tendon results in rapid depletion of fibrocartilage and changes in its mechanical properties, microstructure, extracellular matrix composition, and cellularity. The right flexor digitorum profundus tendons of 2.5-3-year-old rabbits were translocated anteriorly to eliminate in vivo compression and shear to the fibrocartilage zone and, at 4 weeks after surgery, were compared with tendons that had sham surgery and with untreated tendons. The translocated tissue showed a significant increase in equilibrium strain under a compressive creep load (p < 0.05). The thickness and area of the fibrocartilage zone also decreased significantly (p < 0.05). The nuclear density decreased by 40% in the fibrocartilage zone (p < 0.005); however, nuclear shape and orientation were not significantly altered. Glycosaminoglycan content in the fibrocartilage zone was also depleted by 40% (p < 0.02). The tightly woven basket weave-like mesh of collagen fibers in the zone appeared more loosely organized, suggesting matrix reorganization due to translocation. Moreover, immunoreactive type-II collagen and link protein in the fibrocartilage zone also decreased. With use of this unique in vivo model, this research clearly elucidates how changing tissue function (by removing compressive forces) rapidly alters tissue form.

Proteoglycan expression in the rat temporomandibular joint in response to unilateral bite raise

The vertebrate articular tissue consists of collagen fibers embedded in a ground substance. Collagen resists tensile forces, while proteoglycans in the ground substance provide resilience and resistance to compression. It was hypothesized that unilateral bite raise would induce increasing expression of proteoglycans in TMJ articular tissues. As a test of this hypothesis, six- and nine-week-old Sprague-Dawley rats received unilateral bite-raising appliances bonded to their right upper molars for 4 wks. A group of nine-week-old rats was housed for an additional 4 wks after removal of the appliances they had worn for 4 wks. Proteoglycans that carry abundant chondroitin sulfate and keratan sulfate side-chains, most likely aggrecans, were detected by safranin O in the fibrocartilaginous zone of the condyle in parasagittal sections. A monoclonal antibody against a large chondroitin sulfate proteoglycan related to versican reacted strongly in the surface fibrous layer of the mandibular condyle and moderately in the discs of the treated specimens. Computer quantification for safranin O and anti-versican antibody staining revealed that the average intensities of the treated specimens were significantly higher than those of their corresponding sham-operated controls, and the average intensities of the treatment-reversal specimens had no significant differences from their corresponding sham-operated controls. Thus, unilateral bite raise appeared to have induced an increase in the expression of aggrecan in the condylar cartilage and a proteoglycan related to versican in the TMJ disc and the articular surface of the condyle. The elevated proteoglycan expression is interpreted to suggest that unilateral bite raise leads to an increase in the magnitude of compressive forces in the rat temporomandibular joint.

Alterations of glycosaminoglycans during patellar tendon autograft healing after posterior cruciate ligament replacement. A biochemical study in a sheep model

In each of 30 skeletally mature sheep, the posterior cruciate ligament was replaced in one knee by a free patellar tendon autograft using the central third of the ipsilateral patellar tendon. The healing autograft was compared with the contralateral posterior cruciate ligament and the patellar tendons and posterior cruciate ligaments of nonoperated animals. The content of glycosaminoglycans, chondroitin sulfate disaccharides, and dermatan sulfate disaccharides was assessed biochemically at six periods during the 2 years after surgery. The total glycosaminoglycans and chondroitin sulfate disaccharides in the native posterior cruciate ligament was threefold that in the native patellar tendon. In contrast, the amount of dermatan sulfate disaccharides was similar in both the native tendon and native ligament. In the autograft, glycosaminoglycans and chondroitin sulfate disaccharides increased significantly to about 144% and 172%, respectively, of the contralateral posterior cruciate ligament at Week 104. The dermatan sulfate disaccharides in the autograft also showed a significant increase up to Week 26, followed by a remarkable but not significant decrease until the end of the study. In the contralateral posterior cruciate ligament, the dermatan sulfate disaccharides increased significantly between Weeks 52 and 104. Thus, the amount of dermatan sulfate disaccharides was similar in both the autograft and the contralateral posterior cruciate ligament after 2 years. This study suggests that the patellar tendon autograft did not completely assume the biochemical properties of the posterior cruciate ligament.

Division of flexor tendons causes progressive degradation of tendon matrix in rabbits

Deep flexor tendons of 30 rabbits were divided at the ankle level. The effects of unloading on the synthesis and content of matrix components, the synthesis of DNA, and dry weight were investigated. The ability of the tendons to synthesize collagen during short-term culture and the contents of matrix components decreased inversely to the time of unloading. 12 weeks following division, the fibrocartilagnious segments had lost 2/5 of their dry weight, 2/3 of proteoglycan and 1/3 of collagen and non-collagen protein content. Less pronounced losses were observed in the non-fibrocartilaginous segments. A transient increase in cell proliferation in both types of segments was found. These findings indicate that divided flexor tendons undergo a progressive degradation, degradation, which may have implications for delayed suture of deep flexor tendon injuries.

Exposure to air during surgery inhibits cellular activity in flexor tendons

In order to investigate the cellular effects of exposure to air during surgery and to compare the effects of simultaneous irrigation with physiological saline, the deep flexor tendons of both forepaws of 12 rabbits were surgically exposed. In one experiment, the extent of surgical exposure and, in a second experiment, the time of exposure was evaluated. Treated segments of the flexor tendons were collected and labelled in vitro for determination of the ability to synthesize DNA, proteoglycan, collagen and non-collagen protein. With increasing surgical exposure in vivo, an increasing rate of cellular proliferation was observed in segments of the exposed deep flexor tendons examined in vitro. Synthesis of matrix components and the rate of cellular proliferation were reduced by 50% after 40 to 100 minutes of exposure to air and by nearly 100% after 120 minutes of exposure. In contrast, irrigated tendons retained their cellular capacity to proliferate.

Cellular activity in e-PTFE reconstructed pulleys and adjacent regions of deep flexor tendons. An experimental biochemical study in rabbits

The biological adaptation and integration of flexor tendon pulleys reconstructed with expanded polytetrafluorethylene (e-PTFE) were investigated 3 months after operation. E-PTFE pulleys showed no signs of failure, adverse tissue reactions or detrimental effects on the surface of underlying tendons. Cells in pulleys reconstructed with e-PTFE synthesized proteoglycan, collagen and non-collagen protein at a higher rate, and DNA at a lower rate, than cells in sham operated pulleys. Cells in pulleys reconstructed with e-PTFE synthesized proteoglycan at a lower rate, and protein at a higher rate, than cells in e-PTFE strips placed subcutaneously. Flexor tendon segments underlying reconstructed or sham operated pulleys synthesized matrix components and DNA at comparable rates.

Differential effects of insulin-like growth factor-I on matrix and DNA synthesis in various regions and types of rabbit tendons

Tendon healing and integration of tendon grafts may be site or donor specific. To determine if differences exist in sensitivity to growth factors that have the potential to influence tendon repair, we compared the effects of recombinant human insulin-like growth factor-I on various types of tendon segments. The dose response effects on proteoglycan, collagen, noncollagen protein, and DNA synthesis were investigated in short-term explant cultures of intrasynovial intermediate and proximal segments of deep flexor tendons extrasynovial segments of deep flexor tendons, and Achilles tendons of rabbits. The four different types of tendon segments cultured in media without recombinant human insulin-like growth factor-I synthesized similar amounts of each of the matrix components. Intrasynovial proximal segments synthesized 15 times less DNA than other tendon segments. Recombinant human insulin-like growth factor-I stimulated matrix and DNA synthesis of all tendon segments in a dose-dependent manner in intervals from 10 to 1,000 ng/ml. The potency (LogED50) of the stimulation did not differ between the segments. The estimated maximal stimulation (E(max)) of proteoglycan synthesis by recombinant human insulin-like growth factor-I was higher, and of collagen and noncollagen protein synthesis was lower, in intrasynovial proximal segments as compared with that of the other types of segments. In contrast, the estimated maximal stimulation of DNA synthesis by recombinant human insulin-like growth factor-I was 6-fold higher than controls in all types of tendons. These findings demonstrate differences in mitotic capacity between anatomical regions of tendons during culture without recombinant human insulin-like growth factor-I and in matrix synthesis after stimulation with it.

Topographic variation in biglycan and decorin synthesis by articular cartilage in the early stages of osteoarthritis: an experimental study in sheep

Articular cartilage contains large molecular weight proteoglycans that aggregate with hyaluronic acid (aggrecan) and small species, particularly biglycan (dermatan sulphate proteoglycan-1) and decorin (dermatan sulphate proteoglycan-2), that do not. Mechanical stresses have been shown to profoundly influence the metabolism of aggrecan by articular chondrocytes; however, there are limited corresponding data on the metabolism of dermatan sulphate proteoglycans 1 and 2. The objective of this study was to examine the metabolism of aggrecan, biglycan, and decorin in articular cartilage from different weight-bearing areas of normal ovine stifle joints and in joints 6 months after menisectomy, a procedure that has been shown to induce early osteorthritic changes. [35S]proteoglycans synthesised by cartilage explants from eight different weight-bearing regions of unoperated and meniscectomised ovine stifle joints during 48 hours of culture were separated by size-exclusion chromatography, hydrophobic chromatography, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and were quantitated by phosphor-screen autoradiography. The synthesis and degradation of the proteoglycans were expressed relative to the DNA content of the explants. In control joints, the cartilage exposed to high contact stress synthesised significantly less proteoglycan overall and more decorin than joint regions bearing less stress. Explants from high stress regions also released significantly greater amounts of resident proteoglycans (dimethylmethylene blue positive) into media during culture. After lateral meniscectomy, the lateral tibial and femoral cartilages showed elevated biosynthesis of both 35S-dermatan sulphate proteoglycans 1 and 2. This chondrocyte biosynthetic response was accompanied by increased catabolism of aggrecan and the release of its degradation products into culture media. These experiments revealed, in normal joints, a topographic variation in proteoglycan synthesis by articular cartilage that was related to the mechanical stress to which the tissues were subjected in vivo. This biosynthetic pattern changed when the load distribution of the joint was altered by unilateral meniscectomy. These data suggest that an altered chondrocyte phenotypic expression of proteoglycans in response to abnormal mechanical loading is an early event in osteoarthritis.

The effect of compressive loading on proteoglycan turnover in cultured fetal tendon

A fibrocartilaginous tissue develops in tendon at the point where the tendon wraps under bone and is subjected to transverse compressive loading in addition to tension. This tissue is characterized by a high level of large proteoglycan (aggrecan), which could accumulate because of increased synthesis, diminished turnover, or both. To examine the effect of loading on proteoglycan turnover segments of fetal tendon in sterile culture were subjected to cyclic, uniaxial compression loading to 30% of initial thickness once every 6 sec. for 72 h, and then allowed to incorporate 35S-sulfate for 12 h. The rate of loss of newly-synthesized 35S-proteoglycans from tissue was determined during subsequent culture for up to 12 days, with or without continued loading. Proteoglycan was lost from fetal tendon segments rapidly during the first 3 days of culture and slowly thereafter. Loss of newly-synthesized proteoglycan from adult tendon fibrocartilage was linear, with a half life of 12 d. Segments of fetal tendon subjected to cyclic compression before labeling synthesized more proteoglycan. These segments lost a greater percent of labeled proteoglycan to medium during a subsequent 12-day culture period than matched segments that had not experienced loading. Analysis of medium and tissue proteoglycans by SDS polyacrylamide gel electrophoresis and sieve chromatography indicated that small proteoglycans (decorin and biglycan) were retained in both loaded and non-loaded tissue whereas large proteoglycans (migrating in the Vo of a Sepharose CL-4B column) were readily lost. It is concluded that the 3-day loading regimen did not diminish turnover of large proteoglycan. To the contrary, although synthesis of large proteoglycan was enhanced by the loading regimen, these proteoglycans were still rapidly lost from the fetal tissue.

Effect of stress deprivation and cyclic tensile loading on the material and morphologic properties of canine flexor digitorum profundus tendon: an in vitro study

The effect of stress deprivation and cyclic tensile loading on the mechanical and histologic properties of the canine flexor digitorum profundus tendon was examined using an in vitro system. Stress deprivation resulted in a progressive and statistically significant decrease in the tensile modulus over an 8-week period. Histologically, stress-deprived tendons demonstrated quantitative changes in the morphology and number of cells and in the alignment of collagen. The change in tensile properties was not associated with an alteration in the water content of the tissue, but the change appeared to be dependent on the presence of a viable cell population. Dead (acellular) tendons did not undergo any alteration in tensile modulus in this in vitro system. In vitro cyclic tensile loading of tendons over a 4-week time period resulted in a significant increase in the tensile modulus (93% of the control) compared with that of the stress-deprived tendons (68% of the control). This loading regimen also maintained the normal histologic pattern of the tendons. The results of this study are similar to those previously reported for in vivo studies and suggest that this in vitro model may represent a valid system with which to test the effects of various stress conditions on the tensile properties of tissues.

Small proteoglycans from different regions of the fibrocartilaginous temporomandibular joint disc

Proteoglycans were isolated from two zones--the periphery and the inner zone--of bovine temporomandibular joint articular discs and separated into two pools by gel-filtration. Proteoglycans in the low molecular mass pool were further resolved by hydrophobic affinity chromatography into two groups identified by cyanogen bromide peptide analysis, amino acid analysis and amino-terminal sequence analysis as PGI (biglycan) and PGII (decorin). These two proteoglycans were isolated in approximately equal proportions from the 'inner' disc tissue but PGII predominated in the 'outer' tissue. Direct chemical analysis showed that the glycosaminoglycan chains on both PGI and PGII were high in iduronate (64-68% of total uronic acid). The dermatan sulfate chains on proteoglycans from the inner disc tissue were longer than those from the outer tissue. Comparison of the galactosamine contents of the intact proteoglycans with electrophoretic mobilities of the isolated dermatan sulfate chains showed that the PGI from the disc carries two dermatan sulfate chains. Inclusion of disc DS-PGI in a solution of soluble type I collagen lengthened the lag-phase, steepened the turbidity-time curve and increased the final opacity attained during fibril formation in vitro. The median fibril diameter and the range of diameters were both higher in the presence of DS-PGI. By contrast, disc DS-PGII reduced the slope of the turbidity-time curve but had little effect on the final turbidity or the fibril diameter.

Activity vs. rest in the treatment of bone, soft tissue and joint injuries

One of the most important advances in the treatment of musculoskeletal injuries has come from understanding that controlled early resumption of activity can promote restoration of function, and that treatment of injuries with prolonged rest may delay recovery and adversely affect normal tissues. In the last decade of the nineteenth century two widely respected orthopaedists with extensive clinical experience strongly advocated opposing treatments of musculoskeletal injuries. Hugh Owen Thomas in Liverpool believed that enforced, uninterrupted prolonged rest produced the best results. He noted that movement of injured tissues increased inflammation, and that, "It would indeed be as reasonable to attempt to cure a fever patient by kicking him out of bed, as to benefit joint disease by a wriggling at the articulation." Just Lucas-Championnier in Paris took the opposite position. He argued that early controlled active motion accelerated restoration of function, although he noted that mobility had to be given in limited doses. In general, Thomas' views met with greater acceptance in the early part of this century, but experimental studies of the last several decades generally support Lucas-Championneir. They confirm and help explain the deleterious effects of prolonged rest and the beneficial effects of activity on the musculoskeletal tissues. They have shown that maintenance of normal bone, tendon and ligament, articular cartilage and muscle structure and composition require repetitive use, and that changes in the patterns of tissue loading can strengthen or weaken normal tissues. Although all the musculoskeletal tissues can respond to repetitive loading, they vary in the magnitude and type of response to specific patterns of activity. Furthermore, their responsiveness may decline with increasing age. Skeletal muscle and bone demonstrate the most apparent response to changes in activity in individuals of any age. Cartilage and dense fibrous tissues also can respond to loading, but the responses are more difficult to measure. The effects of loading on injured tissues have been less extensively studied, but the available evidence indicates that repair tissues respond to loading and, like immature normal tissues, may be more sensitive to cyclic loading and motion than mature normal tissues. However, early motion and loading of injured tissues is not without risks. Premature or excessive loading and motion of repair tissue can inhibit or stop repair. Unfortunately, the optimal methods of facilitating healing by early application of loading and motion have not been defined.(ABSTRACT TRUNCATED AT 400 WORDS)

Variations in cellular proliferation and matrix synthesis in intrasynovial and extrasynovial tendons: an in vitro study in dogs

Intrasynovial and extrasynovial flexor tendon grafts recently have been shown to have dissimilar patterns of cellular survival and host integration within the digital sheath. In an effort to determine if fundamental differences exist between these two types of tendons, we investigated the biochemical composition and cellular activity of intrasynovial and extrasynovial tendon segments of 12 adult mongrel dogs in short-term explant culture in MCDB 105 and in DMEM media. Proteoglycan, collagen and noncollagen protein synthesis and content and DNA synthesis were determined following culture in both media. Intrasynovial tendon segments cultured in MCDB 105 medium synthesized significantly less collagen, noncollagen protein, and DNA and had similar amounts of proteoglycans compared to extrasynovial tendons. Comparison of intrasynovial and extrasynovial tendon segment responses in DMEM medium showed that intrasynovial tendons synthesized more proteoglycan, protein, and DNA than they did in MCDB 105. Extrasynovial tendons had similar rates of matrix component and DNA synthesis in both media. Findings that the synthesis of matrix components and DNA between intrasynovial flexor and extrasynovial peroneal tendon segments differ significantly indicate that intrasynovial flexor tendons may be specially adapted to the nutritional milieu provided by an intrasynovial environment. These data are supported by the selective successful stimulation of fibrocartilaginous segments within intrasynovial flexor tendons in media favored for the culture of cartilaginous tissue.

Aggrecan in bovine tendon

Large proteoglycans were purified by ion-exchange chromatography, gel filtration and CsCl gradient centrifugation from the compressed and tensional regions of adult bovine deep flexor tendon. Tryptic peptide maps of proteoglycan from the compressed region were very similar to maps of aggrecan from bovine articular cartilage, with evidence for the presence of all fifteen previously identified markers from the G1, G2 and G3 domains. The presence of aggrecan in these samples was confirmed by sequencing the G1 peptide YPIHTPR. The equivalent maps for large proteoglycan from tensional tendon were also consistent with the presence of aggrecan, and this was confirmed by sequencing three marker peptides from each of the G2 and G3 domains. However, G1 marker peptides were conspicuously absent from tensional samples. Northern blots for aggrecan mRNA showed high levels in cells from compressed tendon and articular cartilage. Extended exposure revealed a lower level of hybridization to RNA from tensional tendon as well. The results confirm that aggrecan, which is similar in core protein structure to articular cartilage aggrecan, is the predominant chondroitin sulfate-bearing large proteoglycan of compressed tendon. The results also indicate that aggrecan fragments lacking the G1 domain can account for the small amounts of chondroitin sulfate-bearing large proteoglycan in tensional regions of adult tendon.

Histochemical aspects of the proteoglycans of patellar tendon autografts used to replace the posterior cruciate ligament

In the female German black-faced sheep the posterior cruciate ligament was replaced by a free patellar tendon autograft and after 2, 6, 16, 26 and 52 weeks tissue samples of the graft's center (axial region far from bones) were removed for histochemistry and electron microscopy. To localize the proteoglycans Alcian Blue and 0.3 M MgCl2 were added to the fixative solution. The distribution of the proteoglycans in the graft was compared to that of a normal patellar tendon and of a normal posterior cruciate ligament. In the patellar tendon spindle-shaped cells predominated and proteoglycans appeared as short filaments at regular intervals between the collagen fibrils. In the posterior cruciate ligament chondroid cells and long filaments in a net-work-like arrangement were seen. In the patellar tendon autografts short interfibrillar filaments prevailed after 2, 6 and 16 weeks. After 26 weeks and particularly after 52 weeks long filaments also appeared. Digestion with Chondroitinase ABC, AC and Hyaluronidase suggested that the short filaments were PGs containing dermatan sulfate. In grafts, in the early phases the fibroblasts predominated, while in the late phases mainly chondroid cells were observed. The grafts showed aspects of the normal posterior cruciate ligament. However, differences remained, for example the thin collagen fibrils, which could represent one of the reasons for a secondary graft failure.

Proteoglycans in the intervertebral disc of young dogs following strenuous running exercise

The proteoglycans (PGs) of intervertebral disc were studied in ten beagles which ran on a treadmill for one year (up to 40 km/day) and in ten non-running control dogs. Nucleus pulposus and annulus fibrosus from cervical (C5) and thoracic (T6 and T12) discs were labeled in vitro with 35SO4. The extractability, concentration and synthesis of PGs, and the electrophoretic subpopulations, aggregation and glycosaminoglycan chain lengths of newly-synthesized and total PGs were measured. Sulfate incorporation was significantly elevated by running in the C5 disc and reduced in the annulus of T6 discs. In the annulus of the T6 discs the concentration of total PGs was significantly lower although that of dermatan sulfate PGs was actually higher than in the controls. The results show that enhanced loading of the spine exerts significant alterations in the intervertebral disc PGs in a spine-level specific manner. In the most strained area of the spine (upper thoracic), the alterations in the runners suggest compromised biomechanical properties of the disc.

Small proteoglycans

In this review the structure and functions of two non-related proteoglycan families are discussed. One family represents a group of extracellular matrix macromolecules characterized by core proteins with leucine-rich repeat motifs. Within this family special attention is given to those members which carry chondroitin or dermatan sulfate glycosaminoglycan chains. The second family is characterized by repeat sequences of serine and glycine. Their members are products of a single core protein gene and are characteristic constituents of secondary vesicles in cells of the haematopoietic lineage.

Small proteoglycans

In this review the structure and functions of two non-related proteoglycan families are discussed. One family represents a group of extracellular matrix macromolecules characterized by core proteins with leucine-rich repeat motifs. Within this family special attention is given to those members which carry chondroitin or dermatan sulfate glycosaminoglycan chains. The second family is characterized by repeat sequences of serine and glycine. Their members are products of a single core protein gene and are characteristic constituents of secretory vesicles in cells of the haematopoietic lineage.

Development of functional specializations within the maturing rabbit flexor digitorum profundus tendon

Along its length, the rabbit flexor digitorum profundus (FDP) tendon exhibits two functionally specialized regions: classical tendon (CT) and fibrocartilage (FC). We examined their development in rabbits, ranging in age from newborn to nine-months postnatal, using a combination of light microscopic, immunohistochemical and biochemical techniques. There is little histodifferentiation in newborn tendon. Both regions are composed of thin collagenous fibers, numerous fibroblast-like cells and a low molecular weight dermatan-sulfate proteoglycan. Regional specialization has begun by two-weeks postnatal and by three-months postnatal, FC regions have been transformed into a true fibrocartilage characterized by a complex collagenous and elastic fiber network, numerous chondrocytes and a matrix rich in a high molecular weight predominantly chondroitin-sulfate proteoglycan and type II collagen. These features are elaborated between three and nine-months postnatal. CT regions undergo little substantial change during growth and maturation. The rabbit is born altricial and incapable of adult patterns of locomotion. We propose that the developmental expression of functional specializations within the FDP tendon is closely linked with the onset of different physical demands arising from the adoption of adult patterns of locomotion.