The location of three collagen types in skeletal muscle

Three-Dimensional Microfibrous Scaffold with Aligned Topography Produced via a Combination of Melt-Extrusion Additive Manufacturing and Porogen Leaching for In Vitro Skeletal Muscle Modeling

Skeletal muscle tissue (SMT) has a highly hierarchical and anisotropic morphology, featuring aligned and parallel structures at multiple levels. Various factors, including trauma and disease conditions, can compromise the functionality of skeletal muscle. The in vitro modeling of SMT represents a useful tool for testing novel drugs and therapies. The successful replication of SMT native morphology demands scaffolds with an aligned anisotropic 3D architecture. In this work, a 3D PCL fibrous scaffold with aligned morphology was developed through the synergistic combination of Melt-Extrusion Additive Manufacturing (MEAM) and porogen leaching, utilizing PCL as the bulk material and PEG as the porogen. PCL/PEG blends with different polymer ratios (60/40, 50/50, 40/60) were produced and characterized through a DSC analysis. The MEAM process parameters and porogen leaching in bi-distilled water allowed for the development of a micrometric anisotropic fibrous structure with fiber diameters ranging from 10 to 100 µm, depending on PCL/PEG blend ratios. The fibrous scaffolds were coated with Gelatin type A to achieve a biomimetic coating for an in vitro cell culture and mechanically characterized via AFM. The 40/60 PCL/PEG scaffolds yielded the most homogeneous and smallest fibers and the greatest physiological stiffness. In vitro cell culture studies were performed by seeding C2C12 cells onto a selected scaffold, enabling their attachment, alignment, and myotube formation along the PCL fibers during a 14-day culture period. The resultant anisotropic scaffold morphology promoted SMT-like cell conformation, establishing a versatile platform for developing in vitro models of tissues with anisotropic morphology.

Effects of Aging on Collagen in the Skeletal Muscle of Mice

Aging affects several tissues in the body, including skeletal muscle. Multiple types of collagens are localized in the skeletal muscle and contribute to the maintenance of normal muscle structure and function. Since the effects of aging on muscle fibers vary by muscle fiber type, it is expected that the effects of aging on intramuscular collagen might be influenced by muscle fiber type. In this study, we examined the effect of aging on collagen levels in the soleus (slow-twitch muscle) and gastrocnemius (fast-twitch muscle) muscles of 3-, 10-, 24-, and 28-month-old male C57BL/6J mice using molecular and morphological analysis. It was found that aging increased collagen I, III, and VI gene expression and immunoreactivity in both slow- and fast-twitch muscles and collagen IV expression in slow-twitch muscles. However, collagen IV gene expression and immunoreactivity in fast-twitch muscle were unaffected by aging. In contrast, the expression of the collagen synthesis marker heat shock protein 47 in both slow- and fast-twitch muscles decreased with aging, while the expression of collagen degradation markers increased with aging. Overall, these results suggest that collagen gene expression and immunoreactivity are influenced by muscle fiber type and collagen type and that the balance between collagen synthesis and degradation tends to tilt toward degradation with aging.

A Comprehensive Analysis of Fibrillar Collagens in Lamprey Suggests a Conserved Role in Vertebrate Musculoskeletal Evolution

Vertebrates have distinct tissues which are not present in invertebrate chordates nor other metazoans. The rise of these tissues also coincided with at least one round of whole-genome duplication as well as a suite of lineage-specific segmental duplications. Understanding whether novel genes lead to the origin and diversification of novel cell types, therefore, is of great importance in vertebrate evolution. Here we were particularly interested in the evolution of the vertebrate musculoskeletal system, the muscles and connective tissues that support a diversity of body plans. A major component of the musculoskeletal extracellular matrix (ECM) is fibrillar collagens, a gene family which has been greatly expanded upon in vertebrates. We thus asked whether the repertoire of fibrillar collagens in vertebrates reflects differences in the musculoskeletal system. To test this, we explored the diversity of fibrillar collagens in lamprey, a jawless vertebrate which diverged from jawed vertebrates (gnathostomes) more than five hundred million years ago and has undergone its own gene duplications. Some of the principal components of vertebrate hyaline cartilage are the fibrillar collagens type II and XI, but their presence in cartilage development across all vertebrate taxa has been disputed. We particularly emphasized the characterization of genes in the lamprey hyaline cartilage, testing if its collagen repertoire was similar to that in gnathostomes. Overall, we discovered thirteen fibrillar collagens from all known gene subfamilies in lamprey and were able to identify several lineage-specific duplications. We found that, while the collagen loci have undergone rearrangement, the Clade A genes have remained linked with the hox clusters, a phenomenon also seen in gnathostomes. While the lamprey muscular tissue was largely similar to that seen in gnathostomes, we saw considerable differences in the larval lamprey skeletal tissue, with distinct collagen combinations pertaining to different cartilage types. Our gene expression analyses were unable to identify type II collagen in the sea lamprey hyaline cartilage nor any other fibrillar collagen during chondrogenesis at the stages observed, meaning that sea lamprey likely no longer require these genes during early cartilage development. Our findings suggest that fibrillar collagens were multifunctional across the musculoskeletal system in the last common ancestor of vertebrates and have been largely conserved, but these genes alone cannot explain the origin of novel cell types.

Traction and attraction: haptotaxis substrates collagen and fibronectin interact with chemotaxis by HGF to regulate myoblast migration in a microfluidic device

Cell migration is central to development, wound healing, tissue regeneration, and immunity. Despite extensive knowledge of muscle regeneration, myoblast migration during regeneration is not well understood. C2C12 mouse myoblast migration and morphology were investigated using a triple-docking polydimethylsiloxane-based microfluidic device in which cells moved under gravity-driven laminar flow on uniform (=) collagen (CN=), fibronectin (FN=), or opposing gradients (CN-FN or FN-CN). In haptotaxis experiments, migration was faster on FN= than on CN=. At 10 h, cells were more elongated on FN-CN and migration was faster than on the CN-FN substrate. Net migration distance on FN-CN at 10 h was greater than on CN-FN, as cells rapidly entered the channel as a larger population (bulk-cell movement, wave 1). Hepatocyte growth factor (HGF) stimulated rapid chemotaxis on FN= but not CN=, increasing migration speed at 10 h early in the channel at low HGF in a steep HGF gradient. HGF accelerated migration on FN= and bulk-cell movement on both uniform substrates. An HGF gradient also slowed cells in wave 2 moving on FN-CN, not CN-FN. Both opposing-gradient substrates affected the shape, speed, and net distance of migrating cells. Gradient and uniform configurations of HGF and substrate differentially influenced migration behavior. Therefore, haptotaxis substrate configuration potently modifies myoblast chemotaxis by HGF. Innovative microfluidic experiments advance our understanding of intricate complexities of myoblast migration. Findings can be leveraged to engineer muscle-tissue volumes for transplantation after serious injury. New analytical approaches may generate broader insights into cell migration.

Skeletal Muscle Extracellular Matrix - What Do We Know About Its Composition, Regulation, and Physiological Roles? A Narrative Review

Skeletal muscle represents the largest body-composition component in humans. In addition to its primary function in the maintenance of upright posture and the production of movement, it also plays important roles in many other physiological processes, including thermogenesis, metabolism and the secretion of peptides for communication with other tissues. Research attempting to unveil these processes has traditionally focused on muscle fibers, i.e., the contractile muscle cells. However, it is a frequently overlooked fact that muscle fibers reside in a three-dimensional scaffolding that consists of various collagens, glycoproteins, proteoglycans, and elastin, and is commonly referred to as extracellular matrix (ECM). While initially believed to be relatively inert, current research reveals the involvement of ECM cells in numerous important physiological processes. In interaction with other cells, such as fibroblasts or cells of the immune system, the ECM regulates muscle development, growth and repair and is essential for effective muscle contraction and force transmission. Since muscle ECM is highly malleable, its texture and, consequently, physiological roles may be affected by physical training and disuse, aging or various diseases, such as diabetes. With the aim to stimulate increased efforts to study this still poorly understood tissue, this narrative review summarizes the current body of knowledge on (i) the composition and structure of the ECM, (ii) molecular pathways involved in ECM remodeling, (iii) the physiological roles of muscle ECM, (iv) dysregulations of ECM with aging and disease as well as (v) the adaptations of muscle ECM to training and disuse.

The Effect of the Wooden Breast Fibrotic Myopathy in Broilers on Fibrillar Collagen Organization and Decorin-Collagen Binding

The wooden breast myopathy is identified by the palpation of a rigid pectoralis major muscle and results in myofiber necrosis and fibrosis in fast-growing, meat-type broilers. The fibrosis in wooden breast-affected muscle is characterized by the replacement of myofibers with extracellular matrix proteins, especially fibril-forming collagens. Studies have shown differences in collagen organization in fast-growing broiler lines, with tightly packed and highly aligned collagen organizations having a higher phenotypic incidence of wooden breast. The objective of the current study was to analyze collagen fibril organization further in two fast-growing broiler lines (Lines A and B) with incidence of wooden breast compared with a slower growing broiler Line C with no phenotypically detectable wooden breast. The small leucine-rich proteoglycan decorin was also studied for its interaction with collagen by immunogold detection. Decorin binds to fibrillar collagens and organizes their alignment and crosslinking, both of which will affect collagen functional properties. Key findings from the study showed that collagen shifts to larger diameter collagen fibril bundles with the wooden breast myopathy. Specifically, broilers affected with wooden breast from Line A had a more dramatic shift toward larger collagen fibril bundles compared with those affected from Line B. Wooden breast-affected Line A had collagen fibril bundles up to 8.4 µm, whereas Line B maximum size was 5.1 µm. Although decorin-collagen binding was not different overall in the wooden breast myopathy or broiler line, for small-diameter collagen fibril bundles, wooden breast-affected Line A had more decorin-collagen binding than wooden breast-affected Line B. Taken together, these data provide further evidence that multiple fibrotic myopathies are likely in fast-growing meat-type broilers.

A study of chewing muscles: Age-related changes in type I collagen and matrix metalloproteinase-2 expression

OBJECTIVE

In this study, the aim was to investigate the biochemical, physiological and histological changes that occur in masticatory muscles of the masticatory system with aging.

DESIGN

In this study, 14 BALB/c mice were used. Animals were divided into two equal groups of seven. Group I was organized as the group of young animals (n = 7) and Group II as the group of adult animals (n = 7). After routine histological follow-up was performed, the tissues were embedded in paraffin. 4-5 μm thick cross-sections were taken from paraffin-embedded tissues and they were stained with Haemotoxylin and Eosin Type I collagen and Matrix metalloproteinase-2 (MMP-2) immunohistochemically.

RESULTS

It was observed that there was a decrease and shrinking in blood vessels due to aging. In young mice, Type I collagen and MMP-2 immunoreactivity in the masseter muscle tissue showed low staining, while Type I collagen and MMP-2 immunoreactivity in the temporal muscle tissue showed moderate staining. Type I collagen and MMP-2 immunoreactivity were significantly higher in the masseter and temporal muscles of elderly mice (p = 0.001). In the H-score evaluation, MMP-2 immune reactivity was significantly lower in young mice than in older mice (p = 0.001).

CONCLUSION

It was determined that severe pain complications and functional losses are likely to occur with the increase of degeneration due to aging of masticator muscles.

Fibrillar Collagen Organization Associated with Broiler Wooden Breast Fibrotic Myopathy

Wooden breast (WB) is a fibrotic myopathy affecting the pectoralis major (p. major) muscle in fast-growing commercial broiler lines. Birds with WB are phenotypically detected by the palpation of a hard p. major muscle. A primary feature of WB is the fibrosis of muscle with the replacement of muscle fibers with extracellular matrix proteins, such as collagen. The ability of a tissue to be pliable and stretch is associated with the organization of collagen fibrils in the connective tissue areas surrounding muscle fiber bundles (perimysium) and around individual muscle fibers (endomysium). The objective of this study was to compare the structure and organization of fibrillar collagen by using transmission electron microscopy in two fast-growing broiler lines (Lines A and B) with incidence of WB to a slower growing broiler Line C with no phenotypically detectable WB. In Line A, the collagen fibrils were tightly packed in a parallel organization, whereas in Line B, the collagen fibrils were randomly aligned. Tightly packed collagen fibrils arranged in parallel are associated with nonpliable collagen that is highly cross-linked. This will lead to a phenotypically hard p. major muscle. In Line C, the fibrillar collagen was sparse in its distribution. Furthermore, the average collagen fibril diameter and banding D-period length were altered in Line A p. major muscles affected with WB. Taken together, these data are suggestive of different fibrotic myopathies beyond just what is classified as WB in fast-growing broiler lines.

Effect of aging on the tendon structure and tendon-associated gene expression in mouse foot flexor tendon

To evaluate the biological changes in tendons during the aging process, the present study examined the effect of aging on the tendon structure, distribution of collagen types I and III, and expression of tendon-associated genes, using flexor tendons in a mouse model. Histological assessment of the tendon structure and distribution of collagen types I and III were performed, and the expression of tendon-associated genes was evaluated in flexor digitorium longus tendons of young (8 weeks) and aged (78 weeks) female C57BL/6 mice. The results indicated that the Soslowsky score, based on the analysis of cellularity, fibroblastic changes, and collagen fiber orientation and disruption, was significantly increased, or worsened, in the tendons of the aged group compared with those in the young group. Furthermore, in the aged group, the distribution of type I collagen was decreased and the distribution of type III collagen was relatively increased compared with the young group. Finally, the mRNA expression levels of collagen (type I and type III) and tenogenic markers (Mohawk homeobox, tenomodulin and scleraxis BHLH transcription factor) were significantly decreased in the aged group compared with the young group. The present observations demonstrated that the structure of the tendons, distribution of types I and III collagen and the expression of tendon-associated genes were modulated by aging in the flexor tendon, and that these changes may contribute to the degeneration of tendons in tendinopathy.

Engineering Anisotropic Muscle Tissue using Acoustic Cell Patterning

Tissue engineering has offered unique opportunities for disease modeling and regenerative medicine; however, the success of these strategies is dependent on faithful reproduction of native cellular organization. Here, it is reported that ultrasound standing waves can be used to organize myoblast populations in material systems for the engineering of aligned muscle tissue constructs. Patterned muscle engineered using type I collagen hydrogels exhibits significant anisotropy in tensile strength, and under mechanical constraint, produced microscale alignment on a cell and fiber level. Moreover, acoustic patterning of myoblasts in gelatin methacryloyl hydrogels significantly enhances myofibrillogenesis and promotes the formation of muscle fibers containing aligned bundles of myotubes, with a width of 120-150 µm and a spacing of 180-220 µm. The ability to remotely pattern fibers of aligned myotubes without any material cues or complex fabrication procedures represents a significant advance in the field of muscle tissue engineering. In general, these results are the first instance of engineered cell fibers formed from the differentiation of acoustically patterned cells. It is anticipated that this versatile methodology can be applied to many complex tissue morphologies, with broader relevance for spatially organized cell cultures, organoid development, and bioelectronics.

Cancer invasion into musculature: Mechanics, molecules and implications

Tumor invasion along structural interphases of surrounding tumor-free tissue represents a key process during tumor progression. Much attention has been devoted to mechanisms of tumor cell migration within extracellular matrix (ECM)-rich connective tissue, however a comprehensive understanding of tumor invasion into tissue of higher structural complexity, such as muscle tissue, is lacking. Muscle invasion in cancer patients is often associated with destructive growth and worsened prognosis. Here, we review biochemical, geometrical and mechanical cues of smooth and skeletal muscle tissues and their relevance for guided invasion of cancer cells. As integrating concept, muscle-organizing ECM-rich surfaces of the epi-, peri- and endomysium provide cleft-like confined spaces along interfaces between dynamic muscle cells, which provide molecular and physical cues that guide migrating cancer cells, forming a possible contribution to cancer progression.

Composition, Architecture, and Functional Implications of the Connective Tissue Network of the Extraocular Muscles

Purpose

We examined the pattern and extent of connective tissue distribution in the extraocular muscles (EOMs) and determined the ability of the interconnected connective tissues to disseminate force laterally.

Methods

Human EOMs were examined for collagens I, III, IV, and VI; fibronectin; laminin; and elastin using immunohistochemistry. Connective tissue distribution was examined with scanning electron microscopy. Rabbit EOMs were examined for levels of force transmission longitudinally and transversely using in vitro force assessment.

Results

Collagens I, III, and VI localized to the endomysium, perimysium, and epimysium. Collagen IV, fibronectin, and laminin localized to the basal lamina surrounding all myofibers. All collagens localized similarly in the orbital and global layers throughout the muscle length. Elastin had the most irregular pattern and ran longitudinally and circumferentially throughout the length of all EOMs. Scanning electron microscopy showed these elements to be extensively interconnected, from endomysium through the perimysium to the epimysium surrounding the whole muscle. In vitro physiology demonstrated force generation in the lateral dimension, presumably through myofascial transmission, which was always proportional to the force generated in the longitudinally oriented muscles.

Conclusions

A striking connective tissue matrix interconnects all the myofibers and extends, via perimysial connections, to the epimysium. These interconnections are significant and allow measurable force transmission laterally as well as longitudinally, suggesting that they may contribute to the nonlinear force summation seen in motor unit recording studies. This provides strong evidence that separate compartmental movements are unlikely as no region is independent of the rest of the muscle.

Pulmonary and diaphragmatic pathology in collagen type I α1 mutant mice with osteogenesis imperfecta

BackgroundOsteogenesis imperfecta (OI) is most often caused by mutations in type I collagen genes. Respiratory complications have been largely attributed to spine and ribcage deformities. We hypothesized that direct involvement of the pulmonary parenchyma and/or diaphragm by the disease may occur.MethodsIn Col1a1^Jrt/+ mice, a model of severe dominant OI, mean linear intercept length (Lm) was used to assess the distal airspace size. Cross-sectional area (CSA) and myosin heavy chain (MyHC) phenotype of the diaphragm muscle fibers, as well as contractile properties, were determined. OI mice were also treated with neutralizing antibodies against transforming growth factor-β (TGF-β).ResultsDistal airspace enlargement occurred in OI mice (Lm +27%). Diaphragmatic thickness and fiber number were reduced, with increases in fast-twitch type IIx/IIb MyHC fibers. Ex vivo force generation (normalized for CSA) of the diaphragm was also significantly reduced. The increased Lm values found in OI mice were not prevented by anti-TGF-β antibody treatment.ConclusionsThe Col1a1^Jrt/+ mouse model of OI demonstrates: (1) pulmonary airspace enlargement not driven by TGF-β; and (2) reduced muscle mass and intrinsic contractile weakness of the diaphragm. These results suggest a complex and multifaceted basis for respiratory complications in OI that cannot be solely attributed to bone manifestations.

Rescue plan for Achilles: Therapeutics steering the fate and functions of stem cells in tendon wound healing

Due to the increasing age of our society and a rise in engagement of young people in extreme and/or competitive sports, both tendinopathies and tendon ruptures present a clinical and financial challenge. Tendon has limited natural healing capacity and often responds poorly to treatments, hence it requires prolonged rehabilitation in most cases. Till today, none of the therapeutic options has provided successful long-term solutions, meaning that repaired tendons do not recover their complete strength and functionality. Our understanding of tendon biology and healing increases only slowly and the development of new treatment options is insufficient. In this review, following discussion on tendon structure, healing and the clinical relevance of tendon injury, we aim to elucidate the role of stem cells in tendon healing and discuss new possibilities to enhance stem cell treatment of injured tendon. To date, studies mainly apply stem cells, often in combination with scaffolds or growth factors, to surgically created tendon defects. Deeper understanding of how stem cells and vasculature in the healing tendon react to growth factors, common drugs used to treat injured tendons and promising cellular boosters could help to develop new and more efficient ways to manage tendon injuries.

Variations in internal structure, composition and protein distribution between intra- and extra-articular knee ligaments and tendons

Tendons and ligaments play key roles in the musculoskeletal system in both man and animals. Both tissues can undergo traumatic injury, age‐related degeneration and chronic disease, causing discomfort, pain and increased susceptibility to wider degenerative joint disease. To date, tendon and ligament ultrastructural biology is relatively under‐studied in healthy, non‐diseased tissues. This information is essential to understand the pathology of these tissues with regard to function‐related injury and to assist with the future development of tissue‐engineered tendon and ligament structures. This study investigated the morphological, compositional and extracellular matrix protein distribution differences between tendons and ligaments around the non‐diseased canine stifle joint. The morphological, structural characteristics of different regions of the periarticular tendons and ligaments (the intra‐articular anterior cruciate ligament, the extra‐articular medial collateral ligament, the positional long digital extensor tendon and energy‐storing superficial digital flexor tendons) were identified using a novel semi‐objective histological scoring analysis and by determining their biochemical composition. Protein distribution of extracellular matrix collagens, proteoglycans and elastic fibre proteins in anterior cruciate ligament and long digital extensor tendon were also determined using immunostaining techniques. The anterior cruciate ligament was found to have significant morphological differences in comparison with the other three tissues, including less compact collagen architecture, differences in cell nuclei phenotype and increased glycosaminoglycan and elastin content. Intra‐ and interobserver differences of histology scoring resulted in an average score 0.7, indicative of good agreement between observers. Statistically significant differences were also found in the extracellular matrix composition in terms of glycosaminoglycan and elastin content, being more prominent in the anterior cruciate ligament than in the other three tissues. A different distribution of several extracellular matrix proteins was also found between long digital extensor tendon and anterior cruciate ligament, with a significantly increased immunostaining of aggrecan and versican in the anterior cruciate ligament. These findings directly relate to the different functions of tendon and ligament and indicate that the intra‐articular anterior cruciate ligament is subjected to more compressive forces, reflecting an adaptive response to normal or increased loads and resulting in different extracellular matrix composition and arrangement to protect the tissue from damage.

On The Origin of Shear Stress Induced Myogenesis Using PMMA Based Lab-on-Chip

One of the central themes in cell and tissue engineering is to develop an understanding as to how biophysical cues can influence cell functionality changes. The flow induced shear stress is regarded as one such biophysical cue to influence physiological changes in shear-sensitive tissues, in vivo. The origin of such phenomena is, however, poorly understood. While addressing such an issue, the present work demonstrates the intriguing synergistic effect of shear stress and spatial constraints in inducing aligned growth and differentiation of myoblast cells to myotubes. In a planned set of in vitro experiments, the regulation of laminar flow regime within a narrow window was obtained in a PMMA-based Lab-on-Chip (LOC) device, wherein the murine muscle cells (C2C12), chosen for their phenotypical differentiation stages, were cultured under graded shear conditions. The two factors of shear stress and spatial allowance were decoupled by another two sets of experiments. This aspect has been conclusively established using a PMMA device having a fixed width microchannel with varying shear and an identical amount of shear with different width of channels. On the basis of the extensive analysis of biochemical assays (WST-1, picogreen) together with gene expression using qRT-PCR and cell morphological changes (fluorescence/confocal microscopy), extensive differentiation of the myoblasts into myotubes is found to be dependent on both shear stress and spatial allocation with a maximum at an optimal shear of ca. 16 mPa. Quantitatively, the mRNA expression of myogenic biomarkers, i.e., myogenin, MyoD, and neogenin, exhibited 10- to 50-fold changes at ca. 16 mPa shear flow, compared to that under static conditions. Also, myotube aspect ratio and myotube density are modulated with shear stress and are in commensurate with gene expression changes. The flow cytometry analysis further confirmed that the cell cycle arrest at the G1/G0 phase triggers the onset of myogenesis. Taken together, the present study unambiguously establishes qualitative and quantitative biophysical basis for the origin of myogenesis toward the critical shear stress of murine myoblasts in a microfludic device, in vitro.

Correlation between change in muscle excursion and collagen content after tendon rupture and delayed repair

BACKGROUND

The objectives of the present study were to compare changes in muscle excursion, total collagen, and collagen subtypes after tenotomy over time and after delayed tendon repair.

METHODS

Tenotomy on the extensor digitorum tendon of the right second toes of 48 New Zealand White rabbits was performed; toes on the left leg were used as controls. Passive muscle excursion, total collagen content, and type I, III, and IV collagen contents were measured at 1, 2, 4, and 6 weeks after tenotomy. Next, passive muscle excursion and total collagen content were measured at 8 weeks after delayed tendon repair at 1, 2, 4, and 6 weeks after a tenotomy.

RESULTS

Passive muscle excursion decreased sequentially over time after tenotomy. Meanwhile, total collagen increased over time. These changes were significant after 4 weeks of injury. Type I collagen significantly increased, type III collagen significantly decreased, and type IV collagen had no significant change over time. Passive muscle excursion was negatively correlated with total collagen and type I collagen after tenotomy at each time point after tenotomy (p < 0.05). After tendon repair, increases in total collagen content after tenotomy were not reversed, despite early repairs at 1 and 2 weeks after tenotomy.

CONCLUSIONS

Increases in type I collagen were found to be associated with decreased excursion after tendon rupture. The increase in collagen that was observed after tenotomy was not reversed by repair within 8 weeks.

Genetic variation and exercise-induced muscle damage: implications for athletic performance, injury and ageing

Prolonged unaccustomed exercise involving muscle lengthening (eccentric) actions can result in ultrastructural muscle disruption, impaired excitation-contraction coupling, inflammation and muscle protein degradation. This process is associated with delayed onset muscle soreness and is referred to as exercise-induced muscle damage. Although a certain amount of muscle damage may be necessary for adaptation to occur, excessive damage or inadequate recovery from exercise-induced muscle damage can increase injury risk, particularly in older individuals, who experience more damage and require longer to recover from muscle damaging exercise than younger adults. Furthermore, it is apparent that inter-individual variation exists in the response to exercise-induced muscle damage, and there is evidence that genetic variability may play a key role. Although this area of research is in its infancy, certain gene variations, or polymorphisms have been associated with exercise-induced muscle damage (i.e. individuals with certain genotypes experience greater muscle damage, and require longer recovery, following strenuous exercise). These polymorphisms include ACTN3 (R577X, rs1815739), TNF (-308 G>A, rs1800629), IL6 (-174 G>C, rs1800795), and IGF2 (ApaI, 17200 G>A, rs680). Knowing how someone is likely to respond to a particular type of exercise could help coaches/practitioners individualise the exercise training of their athletes/patients, thus maximising recovery and adaptation, while reducing overload-associated injury risk. The purpose of this review is to provide a critical analysis of the literature concerning gene polymorphisms associated with exercise-induced muscle damage, both in young and older individuals, and to highlight the potential mechanisms underpinning these associations, thus providing a better understanding of exercise-induced muscle damage.

Three days of intermittent stretching after muscle disuse alters the proteins involved in force transmission in muscle fibers in weanling rats

The aim of this study was to determine the effects of intermittent passive manual stretching on various proteins involved in force transmission in skeletal muscle. Female Wistar weanling rats were randomly assigned to 5 groups: 2 control groups containing 21- and 30-day-old rats that received neither immobilization nor stretching, and 3 test groups that received 1) passive stretching over 3 days, 2) immobilization for 7 days and then passive stretching over 3 days, or 3) immobilization for 7 days. Maximal plantar flexion in the right hind limb was imposed, and the stretching protocol of 10 repetitions of 30 s stretches was applied. The soleus muscles were harvested and processed for HE and picrosirius staining; immunohistochemical analysis of collagen types I, III, IV, desmin, and vimentin; and immunofluorescence labeling of dystrophin and CD68. The numbers of desmin- and vimentin-positive cells were significantly decreased compared with those in the control following immobilization, regardless of whether stretching was applied (P<0.05). In addition, the semi-quantitative analysis showed that collagen type I was increased and type IV was decreased in the immobilized animals, regardless of whether the stretching protocol was applied. In conclusion, the largest changes in response to stretching were observed in muscles that had been previously immobilized, and the stretching protocol applied here did not mitigate the immobilization-induced muscle changes. Muscle disuse adversely affected several proteins involved in the transmission of forces between the intracellular and extracellular compartments. Thus, the 3-day rehabilitation period tested here did not provide sufficient time for the muscles to recover from the disuse maladaptations in animals undergoing postnatal development.

Transcriptional Profiling Identifies Location-Specific and Breed-Specific Differentially Expressed Genes in Embryonic Myogenesis in Anas Platyrhynchos

Skeletal muscle growth and development are highly orchestrated processes involving significant changes in gene expressions. Differences in the location-specific and breed-specific genes and pathways involved have important implications for meat productions and meat quality. Here, RNA-Seq was performed to identify differences in the muscle deposition between two muscle locations and two duck breeds for functional genomics studies. To achieve those goals, skeletal muscle samples were collected from the leg muscle (LM) and the pectoral muscle (PM) of two genetically different duck breeds, Heiwu duck (H) and Peking duck (P), at embryonic 15 days. Functional genomics studies were performed in two experiments: Experiment 1 directly compared the location-specific genes between PM and LM, and Experiment 2 compared the two breeds (H and P) at the same developmental stage (embryonic 15 days). Almost 13 million clean reads were generated using Illumina technology (Novogene, Beijing, China) on each library, and more than 70% of the reads mapped to the Peking duck (Anas platyrhynchos) genome. A total of 168 genes were differentially expressed between the two locations analyzed in Experiment 1, whereas only 8 genes were differentially expressed when comparing the same location between two breeds in Experiment 2. Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes pathways (KEGG) were used to functionally annotate DEGs (differentially expression genes). The DEGs identified in Experiment 1 were mainly involved in focal adhesion, the PI3K-Akt signaling pathway and ECM-receptor interaction pathways (corrected P-value<0.05). In Experiment 2, the DEGs were associated with only the ribosome signaling pathway (corrected P-value<0.05). In addition, quantitative real-time PCR was used to confirm 15 of the differentially expressed genes originally detected by RNA-Seq. A comparative transcript analysis of the leg and pectoral muscles of two duck breeds not only improves our understanding of the location-specific and breed-specific genes and pathways but also provides some candidate molecular targets for increasing muscle products and meat quality by genetic control.

Decellularized tissue and cell-derived extracellular matrices as scaffolds for orthopaedic tissue engineering

The reconstruction of musculoskeletal defects is a constant challenge for orthopaedic surgeons. Musculoskeletal injuries such as fractures, chondral lesions, infections and tumor debulking can often lead to large tissue voids requiring reconstruction with tissue grafts. Autografts are currently the gold standard in orthopaedic tissue reconstruction; however, there is a limit to the amount of tissue that can be harvested before compromising the donor site. Tissue engineering strategies using allogeneic or xenogeneic decellularized bone, cartilage, skeletal muscle, tendon and ligament have emerged as promising potential alternative treatment. The extracellular matrix provides a natural scaffold for cell attachment, proliferation and differentiation. Decellularization of in vitro cell-derived matrices can also enable the generation of autologous constructs from tissue specific cells or progenitor cells. Although decellularized bone tissue is widely used clinically in orthopaedic applications, the exciting potential of decellularized cartilage, skeletal muscle, tendon and ligament cell-derived matrices has only recently begun to be explored for ultimate translation to the orthopaedic clinic.

Profiling of differentially expressed microRNA and the bioinformatic target gene analyses in bovine fast- and slow-type muscles by massively parallel sequencing

MicroRNA (miRNA) are highly conserved, noncoding small RNA involved in post-transcriptional gene regulation in a variety of biological processes. To elucidate roles of miRNA in bovine muscle type specification and maintenance, we sought to determine differentially expressed miRNA between semitendinosus (STD) and masseter (MS) muscles from 3 Japanese black cattle by massively parallel sequencing. Differential gene expression of myosin heavy chain (MyHC) isoforms confirmed that STD and MS were MyHC-2x- and MyHC-1-abundant muscles, respectively. In total, 192 known miRNA and 20 potential new bovine miRNA were obtained from the sequencing. The differentially expressed miRNA with more than 2-fold difference in each muscle were identified. In particular, miR-196a and miR-885 were exclusively expressed in STD muscle, which was validated by quantitative reverse transcription-PCR (P=0.045 and P<0.001, respectively), whereas a slow type-directing miR-208b was highly expressed in MS compared with STD (false discovery rate<0.05). In addition, 16 potential novel miRNA were mapped and confirmed for their precursor structures by computational analyses. The results of functional annotation combined with in silico target analysis showed that the predicted target genes of miR-196a/b and miR-885 enriched gene ontology (GO) terms related to skeletal system development and regulation of transcription, respectively. Moreover, GO terms enriched from predicted targets miRNA suggested that STD-abundant- and MS-abundant-miRNA were associated with embryonic body planning and organ/tissue pattern formation, respectively. The present results revealed that the differentially expressed miRNA between the STD and MS muscles may play key roles to determine muscle type-specific tissue formation and maintenance in cattle thorough attenuating putative target genes involved in different developmental events.

The effect of aging on migration, proliferation, and collagen expression of tenocytes in response to ciprofloxacin

Quinolone-induced tendinopathy or tendon rupture tends to be age-related. However, the synergistic effects of quinolone and aging on tenocytes remained to be explored. Tenocytes intrinsic to rat Achilles tendon from two age groups (young: 2 months; and near senescent (old): 24 months) were treated with ciprofloxacin. Tenocyte migration and proliferation were assessed by transwell filter migration assay and MTT (3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay, respectively. Messenger RNA and protein expressions of types I and III collagen were determined by reverse transcription-polymerase chain reaction (RT/PCR) and Western blot analysis, respectively. Transwell filter migration assay revealed that ciprofloxacin inhibited tenocytes migration, which became more significant in old tenocytes (p < 0.05). The results of MTT assay revealed that tenocytes proliferation decreased after ciprofloxacin treatment (p < 0.05), which also became more significant in old tenocytes. The results of RT-PCR and Western blot analysis revealed that mRNA and protein expressions of type I collagen remained unchanged in either young or old tenocytes with ciprofloxacin treatment, whereas the expressions of type III collagen were down-regulated by ciprofloxacin, which was more significant in old tenocytes. In conclusion, aging potentiated the ciprofloxacin-mediated inhibition of migration, proliferation, and expression of type III collagen of tenocytes.

Tendon is covered by a basement membrane epithelium that is required for cell retention and the prevention of adhesion formation

The ability of tendons to glide smoothly during muscle contraction is impaired after injury by fibrous adhesions that form between the damaged tendon surface and surrounding tissues. To understand how adhesions form we incubated excised tendons in fibrin gels (to mimic the homeostatic environment at the injury site) and assessed cell migration. We noticed cells exiting the tendon from only the cut ends. Furthermore, treatment of the tendon with trypsin resulted in cell extravagation from the shaft of the tendons. Electron microscopy and immunolocalisation studies showed that the tendons are covered by a novel cell layer in which a collagen type IV/laminin basement membrane (BM) overlies a keratinised epithelium. PCR and western blot analyses confirmed the expression of laminin β1 in surface cells, only. To evaluate the cell retentive properties of the BM in vivo we examined the tendons of the Col4a1^+/Svc mouse that is heterozygous for a G-to-A transition in the Col4a1 gene that produces a G1064D substitution in the α1(IV) chain of collagen IV. The flexor tendons had a discontinuous BM, developed fibrous adhesions with overlying tissues, and were acellular at sites of adhesion formation. In further experiments, tenotomy of wild-type mice resulted in expression of laminin throughout the adhesion. In conclusion, we show the existence of a novel tendon BM-epithelium that is required to prevent adhesion formation. The Col4a1^+/Svc mouse is an effective animal model for studying adhesion formation because of the presence of a structurally-defective collagen type IV-containing BM.

Ciprofloxacin up-regulates tendon cells to express matrix metalloproteinase-2 with degradation of type I collagen

Ciprofloxacin-induced tendinopathy and tendon rupture have been previously described, principally affecting the Achilles tendon. This study was designed to investigate the effect of ciprofloxacin on expressions of matrix metalloproteinases (MMP)-2 and -9, tissue inhibitors of metalloproteinase (TIMP)-1 and -2 as well as type I collagen in tendon cells. Tendon cells intrinsic to rat Achilles tendon were treated with ciprofloxacin and then underwent MTT (tetrazolium) assay. Real-time reverse-transcription polymerase chain reaction (RT-PCR) and Western blot analysis were used, respectively, to evaluate the gene and protein expressions of type I collagen, and MMP-2. Gelatin zymography was used to evaluate the enzymatic activities of MMP-2 and -9. Reverse zymography was used to evaluate TIMP-1 and -2. Immunohistochemical staining for MMP-2 in ciprofloxacin-treated tendon explants was performed. Collagen degradation was evaluated by incubation of conditioned medium with collagen. The results revealed that ciprofloxacin up-regulated the expression of MMP-2 in tendon cells at the mRNA and protein levels. Immunohistochemistry also confirmed the increased expressions of MMP-2 in ciprofloxacin-treated tendon explants. The enzymatic activity of MMP-2 was up-regulated whereas that of MMP-9, TIMP-1 or TIMP-2 was unchanged. The amount of secreted type I collagen in the conditioned medium decreased and type I collagen was degraded after ciprofloxacin treatment. In conclusion, ciprofloxacin up-regulates the expressions of MMP-2 in tendon cells and thus degraded type I collagen. These findings suggest a possible mechanism of ciprofloxacin-associated tendinopathy.

Ibuprofen upregulates expressions of matrix metalloproteinase-1, -8, -9, and -13 without affecting expressions of types I and III collagen in tendon cells

Nonsteroidal antiinflammatory drugs are widely used to treat sports-related tendon injuries or tendinopathy. This study was designed to investigate the effect of ibuprofen on expressions of types I and III collagen, as well as collagen-degrading enzymes including matrix metalloproteinase (MMP)-1, -2, -8, -9, and -13. Rat Achilles tendon cells were treated with ibuprofen and then underwent MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. Reverse transcription-polymerase chain reaction was used to evaluate mRNA expressions of types I and III collagen, MMP-1, -2, -8, -9, and -13. Protein expressions of types I and III collagen, MMP-1, -8, and -13 were determined by Western blot analysis. Gelatin zymography was used to evaluate the enzymatic activities of MMP-2 and MMP-9. The results revealed that ibuprofen upregulated expressions of MMP-1, -8, -9, and -13, both at mRNA and protein levels. There was no effect of ibuprofen on mRNA and protein expressions of types I and III collagen. Gelatin zymography revealed that the enzymatic activity of MMP-9 was upregulated after ibuprofen treatment. In conclusion, ibuprofen upregulates the expressions of collagenases including MMP-1, -8, -9, and -13 without affecting the expressions of types I and III collagen. These findings suggest a molecular mechanism potentially accounting for the inhibition of tendon healing by ibuprofen.

Effects of celecoxib on migration, proliferation and collagen expression of tendon cells

Sports-related tendinopathy is commonly treated with nonsteroidal anti-inflammatory drugs including cyclooxygenase-2 inhibitor. Tendon healing requires migration of tendon cells to the repair site, followed by proliferation and synthesis of collagen. This study was designed to determine the effects of COX-2 inhibitor (celecoxib) on the migration, proliferation, and types I and III collagen expression of tendon cells intrinsic to rat Achilles tendon. Using cultured tendon cells, cell migration and proliferation were evaluated by transwell filter migration assay and by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay, respectively. The mRNA expression of alpha1(I) procollagen and alpha1(III) procollagen were determined by reverse transcription-polymerase chain reaction (RT-PCR). The protein expression of types I and III collagen were determined by immunocytochemistry. Dose-dependent celecoxib inhibition was demonstrated for migration of tendon cells through the transwell filter migration assay (p=0.002). Dose-dependent celecoxib inhibition of tendon cell proliferation also was demonstrated by MTT assay (p=0.004). However, both RT-PCR and immunocytochemical staining revealed that mRNA and protein expression of types I and III collagen remained constant after celecoxib treatment. In conclusion, celecoxib inhibits tendon cell migration and proliferation. However, the expression of types I and III collagen remained unchanged.

Distribution of collagen types I, III, and V in pregnant mouse endometrium

Decidualization in mice comprises a deep remodeling of extracellular matrix (ECM) components of the endometrium. In a previous biochemical study we showed that collagen types I and III are present in both pregnant and nonpregnant mouse endometrium, whereas collagen type V is expressed exclusively after the onset of decidualization. The distribution of collagen types in the pregnant mouse endometrium and possible changes of these molecular types in the different regions of the decidua is, however, not known. Using immunofluorescence and confocal microscopy we showed the presence of collagen types I, III, and V in the endometrial stroma of implantation and interimplantation sites from days 5 to 8 of pregnancy in the mouse. Collagen type III was chiefly expressed in the implantation sites and was the only collagen type to be present in the materno-fetal interface on the day of the embryo implantation. However, collagen type I was the predominant collagen in the interimplantation sites. Collagen type V was weakly expressed in the nondecidualized stroma during all periods but was expressed in larger amounts in the decidualized areas on day 7 of pregnancy, simultaneously with the accumulation of thick collagen fibrils in the same region. The highest immunofluorescence labeling for the three types of collagen was observed on day 7 when the antimesometrial decidual tissue achieved its greatest development. These data support previous studies that showed an intense ECM remodeling of the mouse endometrial stroma during the beginning of pregnancy. This outstanding remodeling may be important to stabilize placental anchorage.

Ultrasound stimulation of types I and III collagen expression of tendon cell and upregulation of transforming growth factor beta

Traumatic tendon injuries are commonly treated with ultrasound. However, previous research has not examined the molecular mechanism of this therapeutic effect on collagen synthesis of tendon cells. This study was designed to determine the effect of ultrasound on the expression of type I and type III collagen of tendon cells intrinsic to rat Achilles tendon. Whether a correlation exits between this effect and the expression of transforming growth factor beta (TGF-beta), which enhances collagen synthesis, was also investigated. Tendon cells after ultrasound treatment and protein expression of types I and III collagen were determined by immunocytochemistry. The mRNA expressions of alpha1(I) procollagen, alpha1(III) procollagen, and TGF-beta were determined by reverse transcription-polymerase chain reaction (RT-PCR). Furthermore, the concentration of TGF-beta in conditioned medium was evaluated by enzyme-linked immunosorbent assay (ELISA). Immunocytochemical staining revealed that ultrasound-treated tendon cells were stained more strongly for types I and III collagen than were control cells. Upregulation of procollagen alpha1(I) gene, procollagen alpha1(III) gene, and TGF-beta at the mRNA level was confirmed by RT-PCR. A dose-dependent increase in the concentration of TGF-beta in conditioned medium obtained from cells treated with ultrasound was demonstrated by ELISA assay (p = 0.043). In conclusion, ultrasound stimulates the expression of type I and type III collagen in a process that is likely mediated by the upregulation of TGF-beta.

Enzymic and non-enzymic cross-linking mechanisms in relation to turnover of collagen: relevance to aging and exercise

The molecular mechanisms involved in the aging of collagen and consequent increase in mechanical strength and stiffness occur in a series of enzymic and non-enzymic intermolecular cross-links. The enzymic mechanism involves divalent aldimine intermolecular cross-links derived from the reaction of aldehydes which then mature to trivalent cross-links and further stabilize the collagen fiber and is now well known. Recent studies have demonstrated that the rate of turnover and level of telopeptide lysyl hydroxylation modifies the nature of the cross-link and hence the mechanical strength of the fiber. The slow turnover of mature collagen subsequently allows accumulation of the products of the adventitious non-enzymic reaction of glucose with the lysines in the triple helix to form glucosyl lysine and its Amadori product, that is, the Maillard reaction. These products are subsequently oxidized to a complex series of advanced glycation end-products, some of which are intermolecular cross-links between the triple helices rendering the fiber too stiff for optimal functioning of the collagen fibers, and consequently of the particular tissue involved. The glycation reactions following maturation are true aging processes, and attempts at their specific inhibition involve competitive inhibition of the Maillard reaction and chemical cleavage of the glycation cross-links. It is clear that the nature of the age-related cross-links and hence tissue strength depends on the rate of turnover of the collagen. An examination of the particular effect of strenuous exercise on the rate of turnover of collagen and hence cross-linking in different tissues could lead to a better understanding of optimal sports training regimes.

Mechanobiology of tendon

Tendons are able to respond to mechanical forces by altering their structure, composition, and mechanical properties--a process called tissue mechanical adaptation. The fact that mechanical adaptation is effected by cells in tendons is clearly understood; however, how cells sense mechanical forces and convert them into biochemical signals that ultimately lead to tendon adaptive physiological or pathological changes is not well understood. Mechanobiology is an interdisciplinary study that can enhance our understanding of mechanotransduction mechanisms at the tissue, cellular, and molecular levels. The purpose of this article is to provide an overview of tendon mechanobiology. The discussion begins with the mechanical forces acting on tendons in vivo, tendon structure and composition, and its mechanical properties. Then the tendon's response to exercise, disuse, and overuse are presented, followed by a discussion of tendon healing and the role of mechanical loading and fibroblast contraction in tissue healing. Next, mechanobiological responses of tendon fibroblasts to repetitive mechanical loading conditions are presented, and major cellular mechanotransduction mechanisms are briefly reviewed. Finally, future research directions in tendon mechanobiology research are discussed.

Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading

The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.

Effects of jump training on passive mechanical stress and stiffness in rabbit skeletal muscle: role of collagen

AIMS

This study evaluated the effect of a 15-week jump training period on mechanical parameters and collagen concentration of different muscle types in rabbits, at 50, 90 and 140 days of age.

METHODS

Trained (T) animals were made to jump over obstacles in order to get food and water. The height of the obstacle was increased according to the animal's age. Control (C) animals were sedentary. Mechanical parameters (force, stress, stiffness) and collagen concentration were measured in Extensor Digitorum Longus (EDL), Rectus Femoris (RF), Semimembranosus Proprius (SMP) and Psoas Major (PSOAS).

RESULTS

Both EDL and RF collagen concentrations and passive mechanical parameters increased between 50 and 140 days of age (P<0.001), whereas SMP and PSOAS collagen concentrations decreased (P<0.001). Soluble collagen concentration decreased similarly with age in all muscles and groups. Exercise training at 140 days induced a significant increase in force (EDL 20.4% and RF 15.3%, 0.01<P<0.05), stress (EDL 26.8% and RF 22.6%, P<0.001), and stiffness (EDL 13.2% and RF 16%, 0.001<P<0.05) with a greater collagen concentration (EDL 16.1% and RF 19.1%, 0.001<P<0.05). Finally, stiffness and stress are well correlated with collagen concentration in EDL and RF muscles at 140 days (0.74<r<0.84, P<0.05).

CONCLUSION

These results suggest that jump training can increase muscle capabilities of strength and stiffness in fast-twitch mixed muscles with pennate architecture (EDL, RF). This increase is essentially mediated by a rise in cross-linked collagen fraction, coupled with a relative decrease in soluble collagen. The collagen covalent cross-links improve resistance and stability in force transmission processes during stretching.

Biochemical analysis of collagen in adhesive tissues formed after digital flexor tendon injuries

Restrictive adhesion is a major problem following flexor tendon surgery. We investigated crosslinks and types of collagen in two adhesive tissue models in chickens, characterized by loose adhesion and dense adhesion. The flexor digitorum profundus tendon was cut and sutured in two ways: (1) the severed tendon was sutured outside the tendon sheath (loose adhesion model), and (2) the tendon sheath was excised and the severed tendon was sutured on the damaged bony floor (dense adhesion model). Biochemical analysis of collagen was done for each group after tenorrhaphy. In the outside-sheath group, the ratio of the collagen crosslinks, dihydroxylysinonorleucine/hydroxylysinonorleucine, was lower than that in the sheath-excised group at 12 weeks, while the collagen crosslinks ratio remained considerably high in the sheath-excised group. The ratio of type III collagen to type I collagen in the outside-sheath group was higher than that in the sheath-excised group at 12 weeks and approached an almost normal value at 24 weeks. The collagen type III/I ratio remained low in the sheath-excised group. The current study shows that biochemical properties are different between loose and dense adhesions. In dense adhesive scars, pathological collagen turnover occurs even at 24 weeks after tenorrhaphy.

The hip joint: the fibrillar collagens associated with development and ageing in the rabbit

The fibrillar collagens associated with the articular cartilages, joint capsule and ligamentum teres of the rabbit hip joint were characterised from the 17 d fetus to the 2-y-old adult by immunohistochemical methods. Initially the putative articular cartilage contains types I, III and V collagens, but when cavitation is complete in the 25 d fetus, type II collagen appears. In the 17 d fetus, the cells of the chondrogenous layers express type I collagen mRNA, but not that of type II collagen. Types III and V collagens are present throughout life, particularly pericellularly. Type I collagen is lost. In all respects, the articular cartilage of the hip joint is similar to that of the knee. The joint capsule contains types I, III and V collagens. In the fetus the ligamentum teres contains types I and V collagens and the cells express type I collagen mRNA; type III collagen is confined mainly to its surface and insertions. After birth, the same distribution remains, but there is more type III collagen in the ligament, proper. The attachment to the cartilage of the head of the femur is marked only by fibres of type I collagen traversing the cartilage; the attachment cannot be distinguished in preparations localising types III and V collagens. The attachment to the bone at the lip of the acetabulum is via fibres of types I and V collagens and little type III is present. The ligament is covered by a sheath of types III and V collagens. Type II collagen was not located in any part of the ligamentum teres. The distribution of collagens in the ligamentum teres is similar to that in the collateral ligaments of the knee. Its insertions are unusual because no fibrocartilage was detected.

Characteristics of skeletal muscle in mdx mutant mice

We review the extensive research conducted on the mdx mouse since 1987, when demonstration of the absence of dystrophin in mdx muscle led to X-chromosome-linked muscular dystrophy (mdx) being considered as a homolog of Duchenne muscular dystrophy. Certain results are contradictory. We consider most aspects of mdx skeletal muscle: (i) the distribution and roles of dystrophin, utrophin, and associated proteins; (ii) morphological characteristics of the skeletal muscle and hypotheses put forward to explain the regeneration characteristic of the mdx mouse; (iii) special features of the diaphragm; (iv) changes in basic fibroblast growth factor, ion flux, innervation, cytoskeleton, adhesive proteins, mastocytes, and metabolism; and (v) different lines of therapeutic research.

Indirect evidence of human skeletal muscle damage and collagen breakdown after eccentric muscle actions

Indirect markers of muscle damage and collagen breakdown were recorded for up to 9 days after a bout of concentric, followed by a bout of eccentric, muscle actions. Nine untrained participants performed two bouts of 50 maximum effort repetitions on an isokinetic dynamometer (angular velocity 1.05 rad x s(-1), range of motion 1.75 rad). An initial concentric bout of muscle actions was followed by an eccentric bout 21 days later, using the same knee extensors. Concentric actions induced no changes in maximum voluntary isometric contraction force (MVC), nor induced any changes in the serum enzyme activities of creatine kinase, a lactate dehydrogenase isoenzyme (LDH-1), or alkaline phosphatase. Similarly, concentric actions induced no change in markers of collagen breakdown, namely plasma hydroxyproline and serum type 1 collagen concentration. In contrast, eccentric actions induced a 23.5+/-19.0% (mean+/-s) decrease in MVC immediately post-exercise (P < 0.05), and increased the serum enzyme activities of creatine kinase and LDH-1 to 486+/-792 and 90+/-11 IU.l(-1) respectively on day 3 post-exercise, and to 189+/-159 and 96+/-13 IU x l(-1) respectively on day 7 post-exercise (all P< 0.05). Eccentric actions induced no significant changes in plasma hydroxyproline, but increased collagen concentration on days 1 and 9 post-exercise (48.6% and 44.3% increases above pre-exercise on days 1 and 9 respectively; both P < 0.05). We conclude that eccentric but not concentric actions may result in temporary muscle damage, and that collagen breakdown may also be affected by eccentric actions. With caution, indices of collagen breakdown may be used to identify exercise-induced damage to connective tissue.

Age-related changes and location of type I, III and IV collagens during skeletal muscle development of double-muscled and normal bovine foetuses

The aim of this study was to investigate age-related changes in collagen content in muscles of normal and double-muscled (DM) bovine foetuses. Psoas major (PM) and triceps brachii (TB) muscles were collected from foetuses at 110 to 260 days post-conception (p.c.), frozen and powdered. Cyanogen bromide (CNBr) digestion and hydroxyproline measurements were carried out on the powder. CNBr peptides underwent SDS-polyacrylamide gel electrophoresis, and type I and III collagen relative variations were measured by densitometric analysis. Type I and III procollagen mRNA were located by in situ hybridization and types I, III and IV collagen located by indirect immunofluorescence. Although there was no significant difference between normal and DM animals in the amounts of collagen (except in PM muscle at 180 and 230 days p.c.), there was nevertheless a tendency to lower collagen content in muscles of DM animals. Amounts of hydroxyproline in PM and TB muscles from 110 to 230 days p.c. increased two- and threefold to 8.5 micrograms per mg of dry matter (d.m.) and 12 micrograms per mg d.m., respectively, and then decreased up to 260 days p.c., when they were twice as high in TB muscle (9.1 micrograms per mg d.m.) as in PM (4.5 micrograms per mg d.m.). The same difference in hydroxyproline levels was observed between normal adult PM and TB muscles. These variations were explained, mainly, by those of type I. In foetal muscle, in both genotypes and as in adult muscle, perimysium was types I and III, and endomysium type I, III and IV. Procollagen type I and III mRNA were located in perimysium. In conclusion, this study emphasizes that at the end of foetal life, collagen content is representative of what it will be in adult in muscle of both normal and DM animals.

Characterisation of human deltoid muscle in patients with impingement syndrome

Muscle biopsies from the anterior and medial parts of the deltoid of 11 male patients and six male controls were analysed with morphological and immunohistochemical methods. The distribution, area, and capillarization of the muscle fibres were determined, and the amount of connective tissue was measured with staining for type-III collagen. Compared with the controls, the patients with impingement syndrome had more type-I than type-II fibres, but the areas of the different types were almost the same. There was no difference in capillarization per fibre type between patients and controls, but the patients had more connective tissue. The results indicate that patients with impingement syndrome have morphological changes in the deltoid muscle, probably due to immobilisation and pain. They support the hypothesis that the deltoid muscle, the medial part in particular, is affected in patients with impingement syndrome.

Fetal and postnatal development of the patella, patellar tendon and suprapatella in the rabbit; changes in the distribution of the fibrillar collagens

The development of the patella, its associated tendons, and suprapatella of the rabbit knee joint is described from the 17 d fetus to the mature adult. The patellar tendon (ligament) with the patella on its posterior surface is seen in the 17 d fetus and is fully developed by 1 postnatal wk. It is composed of bundles of types I and V collagens separated by endotenons of types III and V collagens. Anteriorly there is an epitenon of types III and V collagens while synovium and a fat pad cover its posterior surface. In the 25 d fetus, the patella is cartilaginous and is separated from the femoral condyles. The cartilage contains type II collagen, but types I, III and V collagens are found along the articular surface. Ossification starts 1 postnatal wk and at 6 wk only the articular cartilage remains. In addition to type II, types III and V collagens are located around the chondrocyte lacunae. The long anterior junction between the patella and its tendon is fibrocartilaginous at 1 wk, but as ossification proceeds this is replaced by bone. Types I and V collagens are found in this region. The suprapatella on the posterior surface of the quadriceps tendon is first seen 1 wk postnatally as an area of irregularly organised fibres and chondrocyte-like cells. Types I, II, III and V collagens are present from 3 wk onwards. It is compared with the fibrocartilage of other tendons that are under compression. The arrangement of the collagens in the patellar tendon is discussed in relation to its use as a replacement for injured anterior cruciate ligaments. It is suggested that the structural differences between the patellar tendon and anterior cruciate ligament preclude the translocated tendon acquiring mechanical strength similar to that of a normal cruciate ligament. The designation 'patellar ligament' as opposed to 'patellar tendon' is questioned. It is argued that the term patellar tendon reflects its structure more accurately than patellar ligament.

Basement-membrane stromal relationships: interactions between collagen fibrils and the lamina densa

Collagens, the most abundant molecules in the extracellular space, predominantly form either fibrillar or sheet-like structures-the two major supramolecular conformations that maintain tissue integrity. In connective tissues, other than cartilage, collagen fibrils are mainly composed of collagens I, III, and V at different molecular ratios, exhibiting a D-periodic banding pattern, with diameters ranging from 30 to 150 nm, that can form a coarse network in the extracellular matrix in comparison with a fine meshwork of lamina densa. The lamina densa represents a stable sheet-like meshwork composed of collagen IV, laminin, nidogen, and perlecan compartmentalizing tissue from one another. We hypothesize that the interactions between collagen fibrils and the lamina densa are crucial for maintaining tissue-tissue interactions. A detailed analysis of these interactions forms the basis of this review article. Here, we demonstrate that there is a direct connection between collagen fibrils and the lamina densa and propose that collagen V may play a crucial role in this connection. Collagen V might also be involved in regulation of collagen fibril diameter and anchoring of epithelia to underlying connective tissues.

Tendon degeneration and chronic shoulder pain: changes in the collagen composition of the human rotator cuff tendons in rotator cuff tendinitis

OBJECTIVES

To analyse the collagen composition of normal adult human supraspinatus tendon and to compare with: (1) a flexor tendon (the common biceps tendon) which is rarely involved in any degenerative pathology; (2) degenerate tendons from patients with chronic rotator cuff tendinitis.

METHODS

Total collagen content, collagen solubility and collagen type were investigated by hydroxyproline analysis, acetic acid and pepsin digestion, cyanogen bromide peptide analysis, SDS-PAGE and Western blotting.

RESULTS

The collagen content of the normal cadaver supraspinatus tendons (n = 60) was 96.3 micrograms HYPRO/mg dry weight (range 79.3-113.3) and there was no significant change across the age range 11 to 95 years. There was no significant difference from the common biceps tendon [93.3 (13.5) micrograms HYPRO/mg dry weight, n = 24]. Although extremely insoluble in both acetic acid and pepsin, much of the collagen was soluble after cyanogen bromide digestion [mean 47.9% (29.8)]. Seventeen per cent (10/60) of the 'normal' cadaver supraspinatus tendon sample contained more than 5% type III collagen, although none of the common biceps tendons had significant amounts. Degenerate supraspinatus and subscapularis tendons had a reduced collagen content [83.8 (13.9) micrograms/mg dry weight and 76.9 (16.8) micrograms/mg dry wt respectively) and were more soluble in acetic acid, pepsin and cyanogen bromide (p < 0.001). Eighty two per cent (14/17) of supraspinatus tendons and 100% (8/8) of subscapularis tendons from patients with tendinitis contained more than 5% type III collagen.

CONCLUSIONS

The changes in collagen composition in rotator cuff tendinitis are consistent with new matrix synthesis, tissue remodelling and wound healing, in an attempt to repair the tendon defect, even in old and degenerate tendons. An increase in type III collagen in some 'normal' cadaver supraspinatus tendons is evidence that changes in collagen synthesis and turnover may precede tendon rupture. These changes may be the result of repeated minor injury and microscopic fibre damage or a consequence of local factors such as reduced vascular perfusion, tissue hypoxia, altered mechanical forces and the influence of cytokines. These collagenous changes may accumulate with age and substantially weaken the tendon structure, predisposing the tendon to rotator cuff tendinitis and eventual tendon rupture.

Confocal immunofluorescence localization of collagen types I, III, IV, V and VI and their ultrastructural organization in term human fetal membranes

The distribution of collagen types I, III, IV, V and VI in term human fetal membranes was examined using conventional and confocal indirect immunofluorescence techniques. Collagens I and III were present in most of the layers of fetal membranes except in the trophoblast layer contrary to what has been previously reported. Although collagen IV is considered to be a basement membrane component our study, using monoclonal and polyclonal antibodies, showed its consistent presence in the spongy and reticular layers in high intensity. This was first report on the distribution of type V collagen in the chorion where it was found in the reticular and in the trophoblast layers. Type VI collagen was present mainly in the amnion and the reticular layer. The ultrastructural examination of the extracellular matrix showed that the main fibrous skeleton of the fetal membranes was formed of large banded fibres (Ultrastructurally identical to collagens types I and III) connected together and to the epithelial basement membranes by networks of unbanded filaments (collagen types V, VI and other components). The extensive and continuous networks formed by these collagens may be a major factor responsible for the mechanical integrity of the fetal membranes.

The effects of early mobilisation and immobilisation on the healing process following muscle injuries

The biological processes following muscle injury include 2 competitive events; regeneration of muscle fibres and the simultaneous production of granulation tissue. We have studied the effects of early mobilisation and immobilisation on the healing of rat gastrocnemius muscle following partial rupture by a controlled contusion mechanism. Muscle fibre regeneration is inhibited by the formation of dense connective tissue scar. Immobilisation following injury limits the size of the connective tissue area formed within the site of injury; the penetration of muscle fibres through the connective tissue is prominent but their orientation is complex and not parallel with the uninjured muscle fibres. Immobilisation for longer than 1 week is followed by marked atrophy of the injured gastrocnemius muscle. Mobilisation started immediately after injury is followed by a dense scar formation in the injury area prohibiting muscle regeneration. When mobilisation is started after a short period of immobilisation a better penetration of muscle fibre through the connective tissue is found and the orientation of regenerated muscle fibres is aligned with the uninjured muscle fibres. Although a little delay in healing processes in muscles mobilised after short immobilisation was found morphologically, the gain in strength and energy absorption capacity was quite similar and as good as that of muscles treated by early mobilisation alone.

Tendon collagens: extracellular matrix composition in shear stress and tensile components of flexor tendons

Outer synovial tissues were separated from the remainder of avian flexor tendon and the collagens characterized biochemically and compared with those of the internal portion of tendon and sheath. The collagen content of tendon synovium was 23%, whereas that of tendon and sheath were 78% and 73%, respectively, based on dry weight. Four genetic types of collagen were found in the pepsin solubilized matrices: in the synovium, types I (78%) and III (19%) predominated; types V and possibly VI were present as minor collagens. Purified synovial type V collagen was a heterotrimer, with chain composition [alpha 1(V)]2 alpha 2(V). In contrast, the internal portion of tendon and sheath were comprised of only type I collagen. There was a large amount (41%) of ethanol extractable, noncollagenous material present in synovium, a part of which was proteoglycans. In addition, collagen cross-links of these tissues were quantified: the internal tendon had an abundant concentration of pyridinoline; synovium exhibited high amounts of labile, reducible cross-links, particularly dihydroxylysinonorleucine. In the case of sheath, lysine aldehyde-derived cross-links appeared to be predominant. These results indicate that each tissue has its own collagen type distribution as well as cross-linking pattern reflecting their maturational and functional differences.