Tendon and ligament: development, repair and disease

An engineered in vitro model of the human myotendinous junction

The myotendinous junction (MTJ) is a vulnerable region at the interface of skeletal muscle and tendon that forms an integrated mechanical unit. This study presents a technique for the spatially restrictive co-culture of human embryonic stem cell (hESC)-derived skeletal myocytes and primary tenocytes for two-dimensional modeling of the MTJ. Micropatterned lanes of extracellular matrix and a 2-well culture chamber define the initial regions of occupation. On day 1, both lines occupy less than 20 % of the initially vacant interstitial zone, referred to henceforth as the junction. Myocyte-tenocyte interdigitations are observed by day 7. Immunocytochemistry reveals enhanced organization and alignment of patterned myocyte and tenocyte features, as well as differential expression of multiple MTJ markers. On day 24, electrically stimulated junction myocytes demonstrate negative contractile strains, while positive tensile strains are exhibited by mechanically passive tenocytes at the junction. Unpatterned tenocytes distal to the junction experience significantly decreased strains in comparison to cells at the interface. Unpatterned myocytes have impaired organization and uncoordinated contractile behavior. These findings suggest that this platform is capable of inducing myocyte-tenocyte junction formation and mechanical coupling similar to the native MTJ, showing transduction of force across the cell-cell interface. STATEMENT OF SIGNIFICANCE: The myotendinous junction (MTJ) is an integrated structure that transduces force across the muscle-tendon boundary, making the region vulnerable to strain injury. Despite the clinical relevance, previous in vitro models of the MTJ lack the structure and mechanical accuracy of the native tissue and have difficulty transmitting force across the cell-cell interface. This study demonstrates an in vitro model of the MTJ, using spatially restrictive cues to inform human myocyte-tenocyte interactions and architecture. The model expressed MTJ markers and developed anisotropic myocyte-tenocyte integrations that resemble the native tissue and allow for force transduction from contracting myocytes to passive tenocyte regions. As such, this study presents a system capable of investigating development, injury, and pathology in the human MTJ.

Fluoroquinolones: Neurological Complications and Side Effects in Clinical Practice

Fluoroquinolones, a popular antibiotic class that inhibits nucleic acid synthesis of bacteria by disrupting the activity of the enzyme's topoisomerase IV and DNA gyrase, are used to treat bacterial infections. However, the widespread use of these drugs has allowed for the development of microbial resistance in recent years. Quinolones also have many clinically relevant side effects, including psychosis, confusion, seizures, headaches, dizziness, and nausea. Common side effects include tendinitis, myopathy, depression, and fatigue. Cardiovascular side effects include increased risk of aortic aneurysm, aortic dissection, and QT interval prolongation. Overall, quinolones can be an effective choice for treating bacterial infections. Still, the side effect profile and decreased efficacy secondary to microbial resistance no longer make the quinolone class an ideal choice for many types of infection. A better understanding of the role of quinolone-mediated or neurological damage, cardiovascular impairment, and musculoskeletal involvement is imperative to determine the risks/benefits for the clinician.

Skeletal Muscle Satellite Cells Co-Opt the Tenogenic Gene Scleraxis to Instruct Regeneration

Skeletal muscles connect bones and tendons for locomotion and posture. Understanding the regenerative processes of muscle, bone and tendon is of importance to basic research and clinical applications. Despite their interconnections, distinct transcription factors have been reported to orchestrate each tissue's developmental and regenerative processes. Here we show that Scx expression is not detectable in adult muscle stem cells (also known as satellite cells, SCs) during quiescence. Scx expression begins in activated SCs and continues throughout regenerative myogenesis after injury. By SC-specific Scx gene inactivation (ScxcKO), we show that Scx function is required for SC expansion/renewal and robust new myofiber formation after injury. We combined single-cell RNA-sequencing and CUT&RUN to identify direct Scx target genes during muscle regeneration. These target genes help explain the muscle regeneration defects of ScxcKO, and are not overlapping with Scx -target genes identified in tendon development. Together with a recent finding of a subpopulation of Scx -expressing connective tissue fibroblasts with myogenic potential during early embryogenesis, we propose that regenerative and developmental myogenesis co-opt the Scx gene via different mechanisms.

A single-cell atlas depicting the cellular and molecular features in human anterior cruciate ligamental degeneration: A single cell combined spatial transcriptomics study

Background

To systematically identify cell types in the human ligament, investigate how ligamental cell identities, functions, and interactions participated in the process of ligamental degeneration, and explore the changes of ligamental microenvironment homeostasis in the disease progression.

Methods

Using single-cell RNA sequencing and spatial RNA sequencing of approximately 49,356 cells, we created a comprehensive cell atlas of healthy and degenerated human anterior cruciate ligaments. We explored the variations of the cell subtypes' spatial distributions and the different processes involved in the disease progression, linked them with the ligamental degeneration process using computational analysis, and verified findings with immunohistochemical and immunofluorescent staining.

Results

We identified new fibroblast subgroups that contributed to the disease, mapped out their spatial distribution in the tissue and revealed two dynamic trajectories in the process of the degenerative process. We compared the cellular interactions between different tissue states and identified important signaling pathways that may contribute to the disease.

Conclusions

This cell atlas provides the molecular foundation for investigating how ligamental cell identities, biochemical functions, and interactions contributed to the ligamental degeneration process. The discoveries revealed the pathogenesis of ligamental degeneration at the single-cell and spatial level, which is characterized by extracellular matrix remodeling. Our results provide new insights into the control of ligamental degeneration and potential clues to developing novel diagnostic and therapeutic strategies.

Funding

This study was funded by the National Natural Science Foundation of China (81972123, 82172508, 82372490) and 1.3.5 Project for Disciplines of Excellence of West China Hospital Sichuan University (ZYJC21030, ZY2017301).

Electroconductivity, a regenerative engineering approach to reverse rotator cuff muscle degeneration

Muscle degeneration is one the main factors that lead to the high rate of retear after a successful repair of rotator cuff (RC) tears. The current surgical practices have failed to treat patients with chronic massive rotator cuff tears (RCTs). Therefore, regenerative engineering approaches are being studied to address the challenges. Recent studies showed the promising outcomes of electroactive materials (EAMs) on the regeneration of electrically excitable tissues such as skeletal muscle. Here, we review the most important biological mechanism of RC muscle degeneration. Further, the review covers the recent studies on EAMs for muscle regeneration including RC muscle. Finally, we will discuss the future direction toward the application of EAMs for the augmentation of RCTs.

Intraspecific anatomical variations of the extensor tendons of the carpus and digits with a reexamination of their insertion sites in the domestic dog (Canis lupus familiaris): a cadaveric study

BACKGROUND

The aim of the current study was to investigate the frequency of variations of the extensor tendons of the carpus and digits in the domestic dog (Canis lupus familiaris) with a reexamination of their insertions as well as the morphometric measurements of the tendons and the brachioradialis muscle. In total, we investigated 68 paired thoracic limbs of the domestic dog (16 females and 18 males) which were fixed in a 10% formalin solution.

RESULTS

The extensor carpi radialis (ECR) tendons showed striking variations in both splitting and insertion sites. In 4.4% of dissections, ECR had three tendons. Of these tendons, the extra tendon either attached independently on the fourth metacarpal bone (one right) or joined its counterpart tendon at the distal end (cross-connections) (one bilateral). It is worth mentioning that one of the ECR tendons split into two or three slips which inserted on the first, second, third, or fourth metacarpal bone in 11 (16.2%) of the specimens. In addition, we found a long tendinous slip originating from the ECR tendons to digit II or III in 7.4% of the distal limbs. The most common type of contribution to digit III was a third tendon of the extensor digiti I et II (ED III) joining the extensor digitorum lateralis (EDL III) with a frequency of 17.6%. In other types of variations, the contribution to digit III was incomplete. A part of the abductor pollicis longus (APL) deep to the superficial part of the flexor retinaculum seemed to continue up to the flexor digitorum superficialis (FDS) tendon.

CONCLUSIONS

The rare intraspecific variations of the extensor tendons of the manus described in the current research are valuable from both clinical and phylogenetic perspectives. Nonetheless, their functional importance needs more studies.

Influence of Staphylococcus epidermidis on Collagen Crimp Patterns of Soft Tissue Allograft

BACKGROUND

Postoperative infections, commonly from Staphylococcus epidermidis, may result in anterior cruciate ligament graft failure and necessitate revision surgery. In biomechanical studies, S. epidermidis has been shown to establish biofilms on tendons and reduce graft strength.

PURPOSE/HYPOTHESIS

The goal of this study was to determine the effect of bacterial bioburden on the collagen structure of tendon. It was hypothesized that an increase in S. epidermidis biofilm would compromise tendon crimp, a pattern necessary for mechanical integrity, of soft tissue allografts.

STUDY DESIGN

Controlled laboratory study.

METHODS

Cultures of S. epidermidis were used to inoculate tibialis anterior cadaveric tendons. Conditions assessed included 5 × 10⁵ colony-forming units or concentrated spent media from culture (no living bacteria). Incubation times of 30 minutes, 3 hours, 6 hours, and 24 hours were utilized. Second-harmonic generation imaging allowed for visualization of collagen autofluorescence. Crimp lengths were determined using ImageJ and compared based on incubation time.

RESULTS

Incubation time positively correlated with increasing S. epidermidis bioburden. Both fine and coarse crimp patterns lengthened with increasing incubation time. Significant coarse crimp changes were observed after only 30-minute incubations (P < .029), whereas significant fine crimp lengthening occurred after 6 hours (P < .0001). No changes in crimp length were identified after incubation in media lacking living bacteria.

CONCLUSION

The results of this study demonstrate that exposure to S. epidermidis negatively affects collagen crimp structure. Structural alterations at the collagen fiber level occur within 30 minutes of exposure to media containing S. epidermidis.

CLINICAL RELEVANCE

Our study highlights the need for antimicrobial precautions to prevent graft colonization and maximize graft mechanical strength.

Preparation of mineralized pericardium by alternative soaking for soft-hard interregional tissue application

Recent applications of decellularized tissues include the ectopic use of sheets and powders for three-dimensional (3D) tissue reconstruction. Decellularized tissues are modified (or fabricated) with the desired functions for application to the target (transplanted or used) tissue, including soft-hard interregional tissues, such as ligaments, tendons, and periodontal ligaments. This study aimed to prepare a mineralized decellularized pericardium to construct a soft-hard interregional tissue by 3D fabrication of decellularized pericardium, for example, rolling up to a cylindrical form. The decellularized pericardial tissue was prepared using the high hydrostatic pressurization (HHP) and surfactants method. The pericardium consisted of bundles of aligned fibers, and the bundles were slightly disordered when prepared with the surfactant decellularization method compared with that prepared using the HHP decellularization method. Mineralization of the decellularized pericardium was performed using an alternate soaking process with various cycles. The surface of the decellularized pericardium was covered with calcium phosphate precipitates, which accumulated on the surface with an increasing number of soaking cycles. The inside of the HHP decellularized pericardium was mineralized uniformly, whereas the mineralization of the decellularized pericardium decreased toward the interior. These findings suggest that the decellularization method strongly affects the structure and mineralized parts of the decellularized pericardium. The mineralized decellularized pericardium could be a candidate material for reconstructing alternative interregional tissues, such as ligaments and tendons.

Advanced Gene Therapy Strategies for the Repair of ACL Injuries

The anterior cruciate ligament (ACL), the principal ligament for stabilization of the knee, is highly predisposed to injury in the human population. As a result of its poor intrinsic healing capacities, surgical intervention is generally necessary to repair ACL lesions, yet the outcomes are never fully satisfactory in terms of long-lasting, complete, and safe repair. Gene therapy, based on the transfer of therapeutic genetic sequences via a gene vector, is a potent tool to durably and adeptly enhance the processes of ACL repair and has been reported for its workability in various experimental models relevant to ACL injuries in vitro, in situ, and in vivo. As critical hurdles to the effective and safe translation of gene therapy for clinical applications still remain, including physiological barriers and host immune responses, biomaterial-guided gene therapy inspired by drug delivery systems has been further developed to protect and improve the classical procedures of gene transfer in the future treatment of ACL injuries in patients, as critically presented here.

Histoarchitecture of the fibrillary matrix of human fetal posterior tibial tendons

Adult tendons are highly differentiated. In mature individuals, tendon healing after an injury occurs through fibrotic tissue formation. Understanding the intrinsic reparative properties of fetal tendons would help to understand the maturation tissue process and tendon tissue repair. The present study evaluated the evolution of histoarchitecture, cellularity and the distribution of collagens I, III and V in the posterior tibial tendon in human fetuses at different gestational ages. Morphological profiles were assessed in nine fresh spontaneously aborted fetuses (Group I: five fetuses aged between 22 and 28 weeks of gestation; Group II: four fetuses aged between 32 and 38 weeks of gestation), characterized by a combination of histology, fluorescence and immunohistochemistry. In Group I, the posterior tibial tendon showed statistically significant greater cellularity and presence of collagen III and V than in Group II tendon, which showed a predominance of collagenous I and a better organization of the extracellular matrix compared with Group I tendons. In addition, a statistically significant higher rate of CD90, a marker of mesenchymal cells, was found in Group I tendons. In fetuses with gestational age between 22 and 28 weeks, the posterior tibialis tendons showed a thin and disorganized fibrillar structure, with an increase in collagen III and V fibers and mesenchymal cells. In the posterior tibialis tendons of fetuses with gestational age between 32 and 38 weeks, the fibrillar structure was thicker with a statistically significant increase in type I collagen and decreased cellularity.

In Vitro Tissue Reconstruction Using Decellularized Pericardium Cultured with Cells for Ligament Regeneration

Recent applications of decellularized tissues have included the ectopic use of their sheets and powders for three-dimensional (3D) tissue reconstruction. Decellularized tissues are fabricated with the desired functions to employ them to a target tissue. The aim of this study was to develop a 3D reconstruction method using a recellularized pericardium to overcome the difficulties in cell infiltration into tight and dense tissues, such as ligament and tendon tissues. Decellularized pericardial tissues were prepared using the high hydrostatic pressurization (HHP) and surfactant methods. The pericardium consisted of bundles of aligned fibers. The bundles were slightly disordered in the surfactant decellularization method compared to the HHP decellularization method. The mechanical properties of the pericardium were maintained after the HHP and surfactant decellularizations. The HHP-decellularized pericardium was rolled up into a cylindrical formation. Its mechanical behavior was similar to that of a porcine anterior cruciate ligament in tensile testing. NIH3T3, C2C12, and mesenchymal stem cells were adhered with elongation and alignment on the HHP- and surfactant-decellularized pericardia, with dependences on the cell type and decellularization method. When the recellularized pericardium was rolled up into a cylinder formation and cultured by hanging circulation for 2 days, the cylinder formation and cellular elongation and alignment were maintained on the decellularized pericardium, resulting in a layer structure of cells in a cross-section. According to these results, the 3D-reconstructed decellularized pericardium with cells has the potential to be an attractive alternative to living tissues, such as ligament and tendon tissues.

Playing Position is Associated with Injury Incidence Rate in Male Academy Soccer Players

CONTEXT

It is unclear whether playing position influences injury in male academy soccer players (ASP).

OBJECTIVE

To determine if playing position is associated with injury in ASP.

DESIGN

Descriptive Epidemiology Study.

SETTING

English, Spanish, Uruguayan and Brazilian soccer academies.

PARTICIPANTS

369 ASP from Under 14 (U14) to U23 age groups, classified as 'post-peak height velocity' using maturity offset, and grouped as goalkeepers (GK), lateral defenders (LD), central defenders (CD), lateral midfielders (LM), central midfielders (CM) and forwards (FWD). Additional analysis compared central (CENT) with lateral/forward (LAT/FWD) positions.

MAIN OUTCOME MEASURES

Injuries were recorded prospectively over one season. Injury prevalence proportion (IPP), days missed and injury incidence rate (IIR, injuries per 1000 training/match hours, n=116) were analysed according to playing position.

RESULTS

No association with playing position was observed for any injury type/location regarding IPP (P≥0.089) or days missed (P≥0.235). The IIR was higher in CD than LD for general (9.30 vs. 4.18 injuries/1000h, P=0.009), soft-tissue (5.14 vs. 1.95 injuries/1000h, P=0.026) and ligament/tendon injuries (2.69 vs. 0.56 injuries/1000h, P=0.040). Regarding CENT vs. LAT/FWD, there were no associations with IPP (P≥0.051) or days missed (P≥0.083), but general IIR was greater in CENT than LAT/FWD (8.67 vs. 6.12 injuries/1000h, P=0.047).

CONCLUSIONS

ASP playing position was not associated with IPP or days missed but the higher general, soft-tissue and ligament/tendon IIR in CD suggests this position warrants specific attention regarding injury prevention strategies. These novel findings highlight the importance of including training/match exposure when investigating the influence of playing position on injury in ASP.

Changes in the Synovial Fluid Cytokine Profile of the Knee Between an Acute Anterior Cruciate Ligament Injury and Surgical Reconstruction

BACKGROUND

Changes in the intra-articular inflammatory state during the immediate period after an acute anterior cruciate ligament (ACL) rupture are not well defined.

PURPOSE

To evaluate changes in the concentration of select proinflammatory and anti-inflammatory synovial fluid cytokines during the interval between an ACL injury and surgical reconstruction.

STUDY DESIGN

Descriptive laboratory study.

METHODS

In patients with an acute ACL injury, a synovial fluid sample was obtained from the injured knee during the initial office visit within 2 weeks of the inciting traumatic event. An additional synovial fluid sample was collected at the time of ACL reconstruction just before the surgical incision. Synovial fluid samples from both the acute injury and the surgery time points were processed with a protease inhibitor, and the concentrations of 10 cytokines of interest were measured using a multiplex magnetic bead immunoassay. The primary outcome was the change in cytokine concentrations between time points.

RESULTS

A total of 20 patients with a mean age of 30.2 ± 8.3 years were included. The acute injury synovial fluid samples were collected at 6.6 ± 3.8 days after the injury. The surgical synovial fluid samples were collected at 31.6 ± 15.6 days after the acute injury samples. Based on a series of linear mixed-effects models to control for the effect of concomitant meniscal injuries and by-patient variability, there was a statistically significant increase in the concentrations of RANTES and bFGF and a statistically significant decrease in the concentrations of IL-6, MCP-1, MIP-1β, TIMP-1, IL-1Ra, and VEGF between time points.

CONCLUSION

This study demonstrates the ongoing alterations in the intra-articular microenvironment during the initial inflammatory response in the acute postinjury period. We identified 6 synovial fluid cytokines that significantly decreased and 2 that significantly increased between the first clinical presentation shortly after the injury and the time of surgery 1 month later.

CLINICAL RELEVANCE

This study describes the molecular profile of the inflammatory changes between the time of an acute ACL injury and the time of surgical reconstruction 1 month later. A greater understanding of the acute inflammatory response within the knee may be helpful in identifying the optimal timing for a surgical intervention that balances the risk of chondral damage with the likelihood of successful, well-healed reconstruction.

Oxo-M and 4-PPBP Delivery via Multi-Domain Peptide Hydrogel Toward Tendon Regeneration

We have recently identified novel small molecules, Oxo-M and 4-PPBP, which specifically stimulate endogenous tendon stem/progenitor cells (TSCs), leading to potential regenerative healing of fully transected tendons. Here, we investigated an injectable, multidomain peptide (MDP) hydrogel providing controlled delivery of the small molecules for regenerative tendon healing. We investigated the release kinetics of Oxo-M and 4-PPBP from MDP hydrogels and the effect of MDP-released small molecules on tenogenic differentiation of TSCs and in vivo tendon healing. In vitro, MDP showed a sustained release of Oxo-M and 4-PPBP and a slower degradation than fibrin. In addition, tenogenic gene expression was significantly increased in TSC with MDP-released Oxo-M and 4-PPBP as compared to the fibrin-released. Invivo, MDP releasing Oxo-M and 4-PPBP significantly improved tendon healing, likely associated with prolonged effects of Oxo-M and 4-PPBP on suppression of M1 macrophages and promotion of M2 macrophages. Comprehensive analyses including histomorphology, digital image processing, and modulus mapping with nanoindentation consistently suggested that Oxo-M and 4-PPBP delivered via MDP further improved tendon healing as compared to fibrin-based delivery. In conclusion, MDP delivered with Oxo-M and 4-PPBP may serve as an efficient regenerative therapeutic for in situ tendon regeneration and healing.

Finite element analysis of the performance of additively manufactured scaffolds for scapholunate ligament reconstruction

Rupture of the scapholunate interosseous ligament can cause the dissociation of scaphoid and lunate bones, resulting in impaired wrist function. Current treatments (e.g., tendon-based surgical reconstruction, screw-based fixation, fusion, or carpectomy) may restore wrist stability, but do not address regeneration of the ruptured ligament, and may result in wrist functional limitations and osteoarthritis. Recently a novel multiphasic bone-ligament-bone scaffold was proposed, which aims to reconstruct the ruptured ligament, and which can be 3D-printed using medical-grade polycaprolactone. This scaffold is composed of a central ligament-scaffold section and features a bone attachment terminal at either end. Since the ligament-scaffold is the primary load bearing structure during physiological wrist motion, its geometry, mechanical properties, and the surgical placement of the scaffold are critical for performance optimisation. This study presents a patient-specific computational biomechanical evaluation of the effect of scaffold length, and positioning of the bone attachment sites. Through segmentation and image processing of medical image data for natural wrist motion, detailed 3D geometries as well as patient-specific physiological wrist motion could be derived. This data formed the input for detailed finite element analysis, enabling computational of scaffold stress and strain distributions, which are key predictors of scaffold structural integrity. The computational analysis demonstrated that longer scaffolds present reduced peak scaffold stresses and a more homogeneous stress state compared to shorter scaffolds. Furthermore, it was found that scaffolds attached at proximal sites experience lower stresses than those attached at distal sites. However, scaffold length, rather than bone terminal location, most strongly influences peak stress. For each scaffold terminal placement configuration, a basic metric was computed indicative of bone fracture risk. This metric was the minimum distance from the bone surface to the internal scaffold bone terminal. Analysis of this minimum bone thickness data confirmed further optimisation of terminal locations is warranted.

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

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

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

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Comments on current research methods concerning the structure of ECMs.

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

The development of the tensor vastus intermedius during the human embryonic period and its clinical implications

The tensor vastus intermedius (TVI) is a newly discovered muscle located in the anterolateral thigh area and is considered the fifth component of the quadriceps femoris muscle. There have been several papers describing its anatomical and morphological features in detail; however, many features of this muscle, such as its ontology or kinetic functions, remain unknown. The purpose of this study was to determine the initial appearance of the TVI muscle in human embryonic development and to investigate its growth and development. Histological observations were performed on 30 lower limbs of 15 human embryos from Carnegie stage (CS) 21, 22, and 23 (with crown-rump length ranging from 18.7 to 28.7 mm). Myocyte clusters of the TVI were observed between the vastus lateralis and intermedius muscles in 7 out of 10 limbs in CS 22, indicating that the TVI arises during this stage. In CS 23, the TVI was clearly present in all specimens except one. However, neither the aponeurosis nor the tendonous structure of the TVI were observed in these embryonic stages. Formation of the conventional four components of the quadriceps muscle is completed within CS 21; therefore, our results suggest that the TVI is the last element to develop in the quadriceps femoris complex. It is posited that after the embryonic period, the TVI continues to grow, while forming the tendinous structure toward the patella and receiving vascular supply from certain vascular branches. The clinical significance of these findings is that orthopedists and plastic surgeons who perform surgical procedures within the anterolateral thigh (ALT) area should be aware of the anatomy and development of the TVI in order to reduce surgical complications. Our present research aims to contribute to a deeper understanding of the morphogenesis of the TVI and the other femoral extensor muscles.

The dipeptide prolyl-hydroxyproline promotes cellular homeostasis and lamellipodia-driven motility via active β1-integrin in adult tendon cells

Collagen-derived hydroxyproline (Hyp)-containing peptides have a variety of biological effects on cells. These bioactive collagen peptides are locally generated by the degradation of endogenous collagen in response to injury. However, no comprehensive study has yet explored the functional links between Hyp-containing peptides and cellular behavior. Here, we show that the dipeptide prolyl-4-hydroxyproline (Pro-Hyp) exhibits pronounced effects on mouse tendon cells. Pro-Hyp promotes differentiation/maturation of tendon cells with modulation of lineage-specific factors and induces significant chemotactic activity in vitro. In addition, Pro-Hyp has profound effects on cell proliferation, with significantly upregulated extracellular signal-regulated kinase phosphorylation and extracellular matrix production and increased type I collagen network organization. Using proteomics, we have predicted molecular transport, cellular assembly and organization, and cellular movement as potential linked-network pathways that could be altered in response to Pro-Hyp. Mechanistically, cells treated with Pro-Hyp demonstrate increased directional persistence and significantly increased directed motility and migration velocity. They are accompanied by elongated lamellipodial protrusions with increased levels of active β1-integrin-containing focal contacts, as well as reorganization of thicker peripheral F-actin fibrils. Pro-Hyp-mediated chemotactic activity is significantly reduced (p < 0.001) in cells treated with the mitogen-activated protein kinase kinase 1/2 inhibitor PD98059 or the α5β1-integrin antagonist ATN-161. Furthermore, ATN-161 significantly inhibits uptake of Pro-Hyp into adult tenocytes. Thus, our findings document the molecular basis of the functional benefits of the Pro-Hyp dipeptide in cellular behavior. These dynamic properties of collagen-derived Pro-Hyp dipeptide could lead the way to its application in translational medicine.

Anatomical Considerations of Embryology and Development of the Musculoskeletal System: Basic Notions for Musculoskeletal Radiologists

The musculoskeletal (MSK) system begins to form in the third week of intrauterine development. Multiple genes are involved in the complex different processes to form the skeleton, muscles and joints. The embryonic period, from the third to the eighth week of development, is critical for normal development and therefore the time when most structural defects are induced. Many of these defects have a genetic origin, but environmental factors may also play a very important role. This review summarizes the embryology of the different components of the MSK system and their configuration as an organ-system, analyzes the clinical implications resulting from failures in the process of organogenesis, and describes the first approach to diagnosis of skeletal abnormalities using prenatal ultrasound.

Development and maintenance of tendons and ligaments

Tendons and ligaments are fibrous connective tissues vital to the transmission of force and stabilization of the musculoskeletal system. Arising in precise regions of the embryo, tendons and ligaments share many properties and little is known about the molecular differences that differentiate them. Recent studies have revealed heterogeneity and plasticity within tendon and ligament cells, raising questions regarding the developmental mechanisms regulating tendon and ligament identity. Here, we discuss recent findings that contribute to our understanding of the mechanisms that establish and maintain tendon progenitors and their differentiated progeny in the head, trunk and limb. We also review the extent to which these findings are specific to certain anatomical regions and model organisms, and indicate which findings similarly apply to ligaments. Finally, we address current research regarding the cellular lineages that contribute to tendon and ligament repair, and to what extent their regulation is conserved within tendon and ligament development.

Melt Electrospinning of Polymers: Blends, Nanocomposites, Additives and Applications

Melt electrospinning has been developed in the last decade as an eco-friendly and solvent-free process to fill the gap between the advantages of solution electrospinning and the need of a cost-effective technique for industrial applications. Although the benefits of using melt electrospinning compared to solution electrospinning are impressive, there are still challenges that should be solved. These mainly concern to the improvement of polymer melt processability with reduction of polymer degradation and enhancement of fiber stability; and the achievement of a good control over the fiber size and especially for the production of large scale ultrafine fibers. This review is focused in the last research works discussing the different melt processing techniques, the most significant melt processing parameters, the incorporation of different additives (e.g., viscosity and conductivity modifiers), the development of polymer blends and nanocomposites, the new potential applications and the use of drug-loaded melt electrospun scaffolds for biomedical applications.

Murine Limb Bud Organ Cultures for Studying Musculoskeletal Development

The biological signals that coordinate the three-dimensional outgrowth and patterning of the vertebrate limb bud have been well delineated. These include a number of vital embryonic signaling pathways, including the fibroblast growth factor, WNT, transforming growth factor, and hedgehog. Collectively these signals converge on multiple progenitor populations to drive the formation of a variety of tissues that make up the limb musculoskeletal system, such as muscle, tendon, cartilage, stroma, and bone. The basic mechanisms regulating the commitment and differentiation of diverse limb progenitor populations has been successfully modeled in vitro using high density primary limb mesenchymal or micromass cultures. However, this approach is limited in its ability to more faithfully recapitulate the assembly of progenitors into organized tissues that span the entire musculoskeletal system. Other biological systems have benefitted from the development and availability of three-dimensional organoid cultures which have transformed our understanding of tissue development, homeostasis and regeneration. Such a system does not exist that effectively models the complexity of limb development. However, limb bud organ cultures while still necessitating the use of collected embryonic tissue have proved to be a powerful model system to elucidate the molecular underpinning of musculoskeletal development. In this methods article, the derivation and use of limb bud organ cultures from murine limb buds will be described, along with strategies to manipulate signaling pathways, examine gene expression and for longitudinal lineage tracking.

The twisted structure of the fetal calcaneal tendon is already visible in the second trimester

INTRODUCTION

The progress in morphological science results from the greater possibilities of intra-pubic diagnosis and treatment of congenital disabilities, including the motor system. However, the structure and macroscopic development of the calcaneal tendon have not been investigated in detail. Studies on the adult calcaneal tendon showed that the calcaneal tendon is composed of twisted subtendons. This study aimed to investigate the internal structure of the fetal calcaneal tendon in the second trimester.

MATERIALS AND METHODS

Thirty-six fetuses fixed in 10% formaldehyde were dissected using the layer-by-layer method and a surgical microscope.

RESULTS

The twisted structure of the calcaneal tendon was revealed in all specimens. The posterior layer of the calcaneal tendon is formed by the subtendon from the medial head of the gastrocnemius muscle. In contrast, the anterior layer is formed by the subtendon from the lateral head of the gastrocnemius muscle. The subtendon from the soleus muscle constitutes the anteromedial outline of the calcaneal tendon. The lateral outline of the calcaneal tendon is formed by the subtendon originating from the medial head of the gastrocnemius muscle. In contrast, the medial outline is formed by the subtendon from the soleus muscle. In most of the examined limbs, the plantaris tendon attached to the tuber calcanei was not directly connected to the calcaneal tendon.

CONCLUSIONS

The twisted structure of the subtendons of the fetal calcaneal tendon is already visible in the second trimester and is similar to that seen in adults.

Effects of dextrose prolotherapy on tendinopathy, fasciopathy, and ligament injuries, fact or myth?: A systematic review and meta-analysis

OBJECTIVES

Prolotherapy or proliferative therapy is a treatment option for damaged connective tissues involving the injection of a solution (proliferant) which theoretically causes an initial cell injury and a subsequent "proliferant" process of wound healing via modulation of the inflammatory process. Nonetheless, the benefits of dextrose prolotherapy have not been adequately evaluated. Therefore, the present study assesses the effectiveness and superiority of prolotherapy separately in treating dense fibrous connective tissue injuries.

METHODS

PubMed, Scopus, and Embase were searched from the earliest record to February 18, 2019. This study included randomized controlled trials whichBoth analysis at individual studies level and pooled meta-analysis were performed.

RESULTS

Ten trials involving 358 participants were included for review. At study level, the majority of comparisons did not reveal significant differences between dextrose prolotherapy and no treatment (or placebo) regarding pain control. The meta-analysis showed dextrose prolotherapy was effective in improving activity only at immediate follow-up (i.e., 0-1 month) (standardized mean difference [SMD]: 0.98; 95% confidence interval [CI]: 0.40-1.50; I = 0%); and superior to corticosteroid injections only in pain reduction at short-term follow-up (i.e., 1-3 month) (SMD: 0.70; 95% CI: 0.14-1.27; I = 51%). No other significant SMDs were found in this analysis.

CONCLUSIONS

There is insufficient evidence to support the clinical benefits of dextrose prolotherapy in managing dense fibrous tissue injuries. More high-quality randomized controlled trials are warranted to establish the benefits of dextrose prolotherapy.

REVIEW REGISTRATION

PROSPERO (CRD42019129044).

In Vitro Innovation of Tendon Tissue Engineering Strategies

Tendinopathy is the term used to refer to tendon disorders. Spontaneous adult tendon healing results in scar tissue formation and fibrosis with suboptimal biomechanical properties, often resulting in poor and painful mobility. The biomechanical properties of the tissue are negatively affected. Adult tendons have a limited natural healing capacity, and often respond poorly to current treatments that frequently are focused on exercise, drug delivery, and surgical procedures. Therefore, it is of great importance to identify key molecular and cellular processes involved in the progression of tendinopathies to develop effective therapeutic strategies and drive the tissue toward regeneration. To treat tendon diseases and support tendon regeneration, cell-based therapy as well as tissue engineering approaches are considered options, though none can yet be considered conclusive in their reproduction of a safe and successful long-term solution for full microarchitecture and biomechanical tissue recovery. In vitro differentiation techniques are not yet fully validated. This review aims to compare different available tendon in vitro differentiation strategies to clarify the state of art regarding the differentiation process.

Comparative Analysis of the Extracellular Matrix Proteome across the Myotendinous Junction

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

Injectable hydrogels for tendon and ligament tissue engineering

The problem of tendon and ligament (T/L) regeneration in musculoskeletal diseases has long constituted a major challenge. In situ injection of formable biodegradable hydrogels, however, has been demonstrated to treat T/L injury and reduce patient suffering in a minimally invasive manner. An injectable hydrogel is more suitable than other biological materials due to the special physiological structure of T/L. Most other materials utilized to repair T/L are cell-based, growth factor-based materials, with few material properties. In addition, the mechanical property of the gel cannot reach the normal T/L level. This review summarizes advances in natural and synthetic polymeric injectable hydrogels for tissue engineering in T/L and presents prospects for injectable and biodegradable hydrogels for its treatment. In future T/L applications, it is necessary develop an injectable hydrogel with mechanics, tissue damage-specific binding, and disease response. Simultaneously, the advantages of various biological materials must be combined in order to achieve personalized precision therapy.

Implementation of Endogenous and Exogenous Mesenchymal Progenitor Cells for Skeletal Tissue Regeneration and Repair

Harnessing adult mesenchymal stem/progenitor cells to stimulate skeletal tissue repair is a strategy that is being actively investigated. While scientists continue to develop creative and thoughtful ways to utilize these cells for tissue repair, the vast majority of these methodologies can ultimately be categorized into two main approaches: (1) Facilitating the recruitment of endogenous host cells to the injury site; and (2) physically administering into the injury site cells themselves, exogenously, either by autologous or allogeneic implantation. The aim of this paper is to comprehensively review recent key literature on the use of these two approaches in stimulating healing and repair of different skeletal tissues. As expected, each of the two strategies have their own advantages and limitations (which we describe), especially when considering the diverse microenvironments of different skeletal tissues like bone, tendon/ligament, and cartilage/fibrocartilage. This paper also discusses stem/progenitor cells commonly used for repairing different skeletal tissues, and it lists ongoing clinical trials that have risen from the implementation of these cells and strategies. Lastly, we discuss our own thoughts on where the field is headed in the near future.

An injury audit in high-level male youth soccer players from English, Spanish, Uruguayan and Brazilian academies

OBJECTIVES

To identify the most common injury types/locations in high-level male youth soccer players (YSP).

DESIGN

Prospective cohort surveillance study.

SETTING

Professional soccer club academies.

PARTICIPANTS

Six hundred and twenty-four high-level male YSP [Under 9 (U9) to U23 year-old age groups] from academies in England, Spain, Uruguay and Brazil.

MAIN OUTCOME MEASURES

Injury type, location and severity were recorded during one season. Injury severity was compared between age groups, while injury type and location were compared between nations.

RESULTS

Four hundred and forty-three training or match injuries were recorded, giving an injury rate of 0.71 per player. Non-contact injuries were most common (58.5%), with most (44.2%) resolved between 8 and 28 days. Most injuries (75.4%) occurred in the lower limbs, with muscle (29.6%) the most commonly injured tissue. U14 and U16 suffered a greater number of severe injuries relative to U12 and U19/U20/U23/Reserves. Tendon injury rate was higher in Brazil vs. Spain (p < 0.05), with low back/sacrum/pelvis injury rate highest in Spain (p < 0.05).

CONCLUSIONS

The proportion of severe injuries in U14 and U16 suggests YSP injury risk is maturation-dependent. Minimal differences in type and location between high-level YSP from four different countries suggest injury rates in this population are geographically similar.

Dexamethasone causes calcium deposition and degeneration in human anterior cruciate ligament cells through endoplasmic reticulum stress

BACKGROUND

Dexamethasone is widely used in the treatment of joint diseases due to its anti-inflammatory properties. However, it can cause serious adverse effects. The anterior cruciate ligament (ACL) is an important stabilizer of the knee joint. However, the effect of dexamethasone treatment on the ACL is unclear.

OBJECTIVE

This study aims to explore the effects of dexamethasone on ACL tissues and cells through in vitro and in vivo experiments.

RESULTS

In vitro, we found that after treatment with dexamethasone, human ACL cell apoptosis was increased, type I collagen (COL1A1) content was decreased, mineralization related genes (ENPP1 and ANKH) and calcified nodules were increased, and endoplasmic reticulum stress (ERS) was enhanced. However, ERS inhibitors could significantly inhibit the increase in calcification and the decrease in COL1A1 induced by dexamethasone. In vivo, Wistar rats received the infra-articular injection with dexamethasone (0.5 mg/kg) for 8 weeks. We found that dexamethasone treatment decreased the COL1A1 content and increased the COL2A1 content in the ACL tissues of rats and that chondroid differentiation and mineralization occurred. Meanwhile, the expression of ERS-related proteins was increased.

CONCLUSION

Dexamethasone increased the calcification of ACL cells and caused ACL degeneration through ERS, suggesting that long-term treatment with dexamethasone may cause adverse effects on ACL tissue and increase the risk of long-term rupture.

The Effect of Growth Differentiation Factor 8 (Myostatin) on Bone Marrow-Derived Stem Cell-Coated Bioactive Sutures in a Rabbit Tendon Repair Model

Background: We have reported that bioactive sutures coated with bone marrow-derived mesenchymal stem cells (BMSCs) enhance tendon repair strength in an in vivo rat model. We have additionally shown that growth differentiation factor 8 (GDF-8, also known as myostatin) simulates tenogenesis in BMSCs in vitro. The purpose of this study was to determine the possibility of BMSC-coated bioactive sutures treated with GDF-8 to increase tendon repair strength in an in vivo rabbit tendon repair model. Methods: Rabbit BMSCs were grown and seeded on to 4-0 Ethibond sutures and treated with GDF-8. New Zealand white rabbits' bilateral Achilles tendons were transected and randomized to experimental (BMSC-coated bioactive sutures treated with GDF-8) or plain suture repaired control groups. Tendons were harvested at 4 and 7 days after the surgery and subjected to tensile mechanical testing and quantitative polymerase chain reaction. Results: There were distinguishing differences of collagen and matrix metalloproteinase RNA level between the control and experimental groups in the early repair periods (day 4 and day 7). However, there were no significant differences between the experimental and control groups in force to 1-mm or 2-mm gap formation or stiffness at 4 or 7 days following surgery. Conclusions: BMSC-coated bioactive sutures with GDF-8 do not appear to affect in vivo rabbit tendon healing within the first week following repair despite an increased presence of quantifiable RNA level of collagen. GDF-8's treatment efficacy of the early tendon repair remains to be defined.

Scleraxis genes are required for normal musculoskeletal development and for rib growth and mineralization in zebrafish

Tendons are an essential part of the musculoskeletal system, connecting muscle and skeletal elements to enable force generation. The transcription factor scleraxis marks vertebrate tendons from early specification. Scleraxis-null mice are viable and have a range of tendon and bone defects in the trunk and limbs but no described cranial phenotype. We report the expression of zebrafish scleraxis orthologs: scleraxis homolog (scx)-a and scxb in cranial and intramuscular tendons and in other skeletal elements. Single mutants for either scxa or scxb, generated by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), are viable and fertile as adult fish. Although scxb mutants show no obvious phenotype, scxa mutant embryos have defects in cranial tendon maturation and muscle misalignment. Mutation of both scleraxis genes results in more severe defects in cranial tendon differentiation, muscle and cartilage dysmorphogenesis and paralysis, and lethality by 2-5 wk, which indicates an essential function of scleraxis for craniofacial development. At juvenile and adult stages, ribs in scxa mutants fail to mineralize and/or are small and heavily fractured. Scxa mutants also have smaller muscle volume, abnormal swim movement, and defects in bone growth and composition. Scleraxis function is therefore essential for normal craniofacial form and function and vital for fish development.-Kague, E., Hughes, S. M., Lawrence, E. A., Cross, S., Martin-Silverstone, E., Hammond, C. L., Hinits, Y. Scleraxis genes are required for normal musculoskeletal development and for rib growth and mineralization in zebrafish.

Isolation and characterization of turkey bone marrow-derived mesenchymal stem cells

Flexor tendon injury is often associated with suboptimal outcomes and results in substantial digit dysfunction. Stem cells have been isolated from several experimental animals for the growing interest and needs of utilizing cell-based therapies. Recently, turkey has been developed as a new large animal model for flexor tendon research. In the present study, we reported the isolation and characterization of bone marrow-derived mesenchymal stem cells (BMSCs) from 8- to 12-month-old heritage-breed turkeys. The isolated cells demonstrated fibroblast-like morphology, clonogenic capacity, and high proliferation rate. These cells were positive for surface antigens CD90, CD105, and CD44, but were negative for CD45. The multipotency of turkey BMSCs was determined by differentiating cells into osteogenic, adipogenic, chondrogenic, and tenogenic lineages. There was upregulated gene expression of tenogenic markers, including mohawk, tenomodulin, and EGR1 as well as increased collagen synthesis in BMP12 induced cells. The successful isolation and verification of bone marrow-derived MSCs from turkey would provide opportunities of studying cell-based therapies and developing new treatments for tendon injuries using this novel preclinical large animal model. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1419-1428, 2019.

A Combination of Oxo-M and 4-PPBP as a potential regenerative therapeutics for tendon injury

Tendons injuries frequently result in scar-like tissue with poor biochemical structure and mechanical properties. We have recently reported that CD146⁺ perivascular originated tendon stem/progenitor cells (TSCs), playing critical roles in tendon healing. Here, we identified highly efficient small molecules that selectively activate endogenous TSCs for tendon regeneration.

Methods: From a pool of ERK1/2 and FAK agonists, Oxo-M and 4-PPBP were identified, and their roles in tenogenic differentiation of TSCs and in vivo tendon healing were investigated. Controlled delivery of Oxo-M and 4-PPBP was applied via PLGA µS. Signaling studies were conducted to determine the mechanism for specificity of Oxo-M and 4-PPBP to CD146⁺ TSCs.

Results: A combination of Oxo-M and 4-PPBP synergistically increased the expressions of tendon-related gene markers in TSCs. In vivo, delivery of Oxo-M and 4-PPBP significantly enhanced healing of fully transected rat patellar tendons (PT), with functional restoration and reorganization of collagen fibrous structure. Our signaling study suggested that Oxo-M and 4-PPBP specifically targets CD146⁺ TSCs via non-neuronal muscarinic acetylcholine receptors (AChR) and σ1 receptor (σ1) signaling.

Principal conclusions: Our findings demonstrate a significant potential of Oxo-M and 4-PPBP as a regenerative therapeutics for tendon injuries.

Platelet-Rich Fibrin Scaffolds for Cartilage and Tendon Regenerative Medicine: From Bench to Bedside

Nowadays, research in Tissue Engineering and Regenerative Medicine is focusing on the identification of instructive scaffolds to address the requirements of both clinicians and patients to achieve prompt and adequate healing in case of injury. Among biomaterials, hemocomponents, and in particular Platelet-rich Fibrin matrices, have aroused widespread interest, acting as delivery platforms for growth factors, cytokines and immune/stem-like cells for immunomodulation; their autologous origin and ready availability are also noteworthy aspects, as safety- and cost-related factors and practical aspects make it possible to shorten surgical interventions. In fact, several authors have focused on the use of Platelet-rich Fibrin in cartilage and tendon tissue engineering, reporting an increasing number of in vitro, pre-clinical and clinical studies. This narrative review attempts to compare the relevant advances in the field, with particular reference being made to the regenerative role of platelet-derived growth factors, as well as the main pre-clinical and clinical research on Platelet-rich Fibrin in chondrogenesis and tenogenesis, thereby providing a basis for critical revision of the topic.

Understanding Tendons: Lessons from Transgenic Mouse Models

Tendons and ligaments are connective tissues that have been comparatively less studied than muscle and cartilage/bone, even though they are crucial for proper function of the musculoskeletal system. In tendon biology, considerable progress has been made in identifying tendon-specific genes (Scleraxis, Mohawk, and Tenomodulin) in the past decade. However, besides tendon function and the knowledge of a small number of important players in tendon biology, neither the ontogeny of the tenogenic lineage nor signaling cascades have been fully understood. This results in major drawbacks in treatment and repair options following tendon degeneration. In this review, we have systematically evaluated publications describing tendon-related genes, which were studied in depth and characterized by using knockout technologies and the subsequently generated transgenic mouse models (Tg) (knockout mice, KO). We report in a tabular manner, that from a total of 24 tendon-related genes, in 22 of the respective knockout mouse models, phenotypic changes were detected. Additionally, in some of the models it was described at which developmental stages these changes appeared and progressed. To summarize, only loss of Scleraxis and TGFβ signaling led to severe tendon developmental phenotypes, while mice deficient for various proteoglycans, Mohawk, EGR1 and 2, and Tenomodulin presented mild phenotypes. These data suggest that the tendon developmental system is well organized, orchestrated, and backed up; this is even more evident among the members of the proteoglycan family, where the compensatory effects are much clearer. In future, it will be of great importance to discover additional master tendon transcription factors and the genes that play crucial roles in tendon development. This would improve our understanding of the genetic makeup of tendons, and will increase the chances of generating tendon-specific drugs to advance overall treatment strategies.

Obesity/Type II Diabetes Promotes Function-limiting Changes in Murine Tendons that are not reversed by Restoring Normal Metabolic Function

Type II Diabetes (T2DM) negatively alters baseline tendon function, including decreased range of motion and mechanical properties; however, the biological mechanisms that promote diabetic tendinopathy are unknown. To facilitate identification of therapeutic targets we developed a novel murine model of diabetic tendinopathy. Mice fed a High Fat Diet (HFD) developed diet induced obesity and T2DM and demonstrated progressive impairments in tendon gliding function and mechanical properties, relative to mice fed a Low Fat Diet (LFD). We then determined if restoration of normal metabolic function, by switching mice from HFD to LFD, was sufficient to halt the pathological changes in tendon due to obesity/T2DM. However, switching from a HFD to LFD resulted in greater impairments in tendon gliding function than mice maintained on a HFD. Mechanistically, IRβ signaling is decreased in obese/T2DM murine tendons, suggesting altered IRβ signaling as a driver of diabetic tendinopathy. However, knock-down of IRβ expression in S100a4-lineage cells (IRcKO^S100a4) was not sufficient to induce diabetic tendinopathy as no impairments in tendon gliding function or mechanical properties were observed in IRcKO^S100a4, relative to WT. Collectively, these data define a murine model of diabetic tendinopathy, and demonstrate that restoring normal metabolism does not slow the progression of diabetic tendinopathy.

Transcription factor scleraxis vitally contributes to progenitor lineage direction in wound healing of adult tendon in mice

Tendon is a dense connective tissue that transmits high mechanical forces from skeletal muscle to bone. The transcription factor scleraxis (Scx) is a highly specific marker of both precursor and mature tendon cells (tenocytes). Mice lacking scx exhibit a specific and virtually complete loss of tendons during development. However, the functional contribution of Scx to wound healing in adult tendon has not yet been fully characterized. Here, using ScxGFP-tracking and loss-of-function systems, we show in an adult mouse model of Achilles tendon injury that paratenon cells, representing a stem cell antigen-1 (Sca-1)-positive and Scx-negative progenitor subpopulation, display Scx induction, migrate to the wound site, and produce extracellular matrix (ECM) to bridge the defect, whereas resident tenocytes exhibit a delayed response. Scx induction in the progenitors is initiated by transforming growth factor β (TGF-β) signaling. scx-deficient mice had migration of Sca-1-positive progenitor cell to the lesion site but impaired ECM assembly to bridge the defect. Mechanistically, scx-null progenitors displayed higher chondrogenic potential with up-regulation of SRY-box 9 (Sox9) coactivator PPAR-γ coactivator-1α (PGC-1α) in vitro, and knock-in analysis revealed that forced expression of full-length scx significantly inhibited Sox9 expression. Accordingly, scx-null wounds formed cartilage-like tissues that developed ectopic ossification. Our findings indicate a critical role of Scx in a progenitor-cell lineage in wound healing of adult mouse tendon. These progenitor cells could represent targets in strategies to facilitate tendon repair. We propose that this lineage-regulatory mechanism in tissue progenitors could apply to a broader set of tissues or biological systems in the body.

Molecular Characteristics of the Equine Periodontal Ligament

The equine periodontal ligament (PDL) is a fibrous connective tissue that covers the intra-alveolar parts of the tooth and anchors it to the alveolar bone-it, therefore, provides a similar function to a tendinous structure. While several studies have considered the formation and structure of tendons, there is insufficient information particularly on the molecular composition of the PDL. Especially for the equine PDL, there is limited knowledge concerning the expression of genes commonly regarded as typical for tendon tissue. In this study, the gene expression of, e.g., collagen type 1 alpha 1 (COL1), collagen type 3 alpha 1 (COL3), scleraxis (SCX), and fibrocartilage markers was examined in the functional mature equine PDL compared with immature and mature equine tendon tissue. PDL samples were obtained from incisor, premolar, and molar teeth from seven adult horses. Additionally, tendon samples were collected from four adult horses and five foals at different sampling locations. Analyses of gene expression were performed using real-time quantitative polymerase chain reaction (qRT-PCR). Significantly higher expression levels of COL1 and 3 were found in the mature equine PDL in comparison with mature tendon, indicating higher rates of collagen production and turnover in the mature equine PDL. The expression levels of SCX, a specific marker for tenogenic-differentiated cells, were on a similar level in functional mature PDL and in mature tendon tissue. Evidence of chondrogenic metaplasia, often found in tendon entheses or in pressurized regions of tendons, was not found in the mature equine PDL. The obtained results justify further experiments focused on the possible use of equine PDL cells for cell-based regenerative therapies.

Evolution of the muscular system in tetrapod limbs

While skeletal evolution has been extensively studied, the evolution of limb muscles and brachial plexus has received less attention. In this review, we focus on the tempo and mode of evolution of forelimb muscles in the vertebrate history, and on the developmental mechanisms that have affected the evolution of their morphology. Tetrapod limb muscles develop from diffuse migrating cells derived from dermomyotomes, and the limb-innervating nerves lose their segmental patterns to form the brachial plexus distally. Despite such seemingly disorganized developmental processes, limb muscle homology has been highly conserved in tetrapod evolution, with the apparent exception of the mammalian diaphragm. The limb mesenchyme of lateral plate mesoderm likely plays a pivotal role in the subdivision of the myogenic cell population into individual muscles through the formation of interstitial muscle connective tissues. Interactions with tendons and motoneuron axons are involved in the early and late phases of limb muscle morphogenesis, respectively. The mechanism underlying the recurrent generation of limb muscle homology likely resides in these developmental processes, which should be studied from an evolutionary perspective in the future.

Fos Promotes Early Stage Teno-Lineage Differentiation of Tendon Stem/Progenitor Cells in Tendon

Stem cells have been widely used in tendon tissue engineering. The lack of refined and controlled differentiation strategy hampers the tendon repair and regeneration. This study aimed to find new effective differentiation factors for stepwise tenogenic differentiation. By microarray screening, the transcript factor Fos was found to be expressed in significantly higher amounts in postnatal Achilles tendon tissue derived from 1 day as compared with 7‐days‐old rats. It was further confirmed that expression of Fos decreased with time in postnatal rat Achilles tendon, which was accompanied with the decreased expression of multiply tendon markers. The expression of Fos also declined during regular in vitro cell culture, which corresponded to the loss of tendon phenotype. In a cell‐sheet and a three‐dimensional cell culture model, the expression of Fos was upregulated as compared with in regular cell culture, together with the recovery of tendon phenotype. In addition, significant higher expression of tendon markers was found in Fos‐overexpressed tendon stem/progenitor cells (TSPCs), and Fos knock‐down gave opposite results. In situ rat tendon repair experiments found more normal tendon‐like tissue formed and higher tendon markers expression at 4 weeks postimplantation of Fos‐overexpressed TSPCs derived nonscaffold engineering tendon (cell‐sheet), as compared with the control group. This study identifies Fos as a new marker and functional driver in the early stage teno‐lineage differentiation of tendon, which paves the way for effective stepwise tendon differentiation and future tendon regeneration. Stem Cells Translational Medicine2017;6:2009–2019

Development of Multilayered Chlorogenate-Peptide Based Biocomposite Scaffolds for Potential Applications in Ligament Tissue Engineering - An <i>In Vitro</i> Study

In this work, for the first time, chlorogenic acid, a natural phytochemical, was conjugated to a lactoferrin derived antimicrobial peptide sequence RRWQWRMKKLG to develop a self-assembled template. To mimic the components of extracellular matrix, we then incorporated Type I Collagen, followed by a sequence of aggrecan peptide (ATEGQVRVNSIYQDKVSL) onto the self-assembled templates for potential applications in ligament tissue regeneration. Mechanical properties and surface roughness were studied and the scaffolds displayed a Young’s Modulus of 169 MP and an average roughness of 72 nm respectively. Thermal phase changes were studied by DSC analysis. Results showed short endothermic peaks due to water loss and an exothermic peak due to crystallization of the scaffold caused by rearrangement of the components. Biodegradability studies indicated a percent weight loss of 27.5 % over a period of 37 days. Furthermore, the scaffolds were found to adhere to fibroblasts, the main cellular component of ligament tissue. The scaffolds promoted cell proliferation and displayed actin stress fibers indicative of cell motility and attachment. Collagen and proteoglycan synthesis were also promoted, demonstrating increased expression and deposition of collagen and proteoglycans. Additionally, the scaffolds exhibited antimicrobial activity against Staphylococcus epidermis bacteria, which is beneficial for minimizing biofilm formation if potentially used as implants. Thus, we have developed a novel biocomposite that may open new avenues to enhance ligament tissue regeneration.

Tendons and Ligaments: Connecting Developmental Biology to Musculoskeletal Disease Pathogenesis

Tendons and ligaments provide connections between muscle and bone or bone and bone to enable locomotion. Damage to tendons and ligaments caused by acute or chronic injury or associated with aging and arthritis is a prevalent cause of disability. Improvements in approaches for the treatment of these conditions depend on a better understanding of tendon and ligament development, cell biology, and pathophysiology. This review focuses on recent advances in the discovery of transcription factors that control ligament and tendon cell differentiation, how cell and extracellular matrix homeostasis are altered in disease, and how this new insight can lead to novel therapeutic approaches. © 2017 American Society for Bone and Mineral Research.