Tendons and Ligaments: Connecting Developmental Biology to Musculoskeletal Disease Pathogenesis

PDGFRα signaling regulates cartilage and fibrous tissue differentiation during synovial joint development

Synovial joints develop from mesenchymal structures called interzones, with progenitor cells differentiating into specialized cartilaginous and fibrous tissues of the joint. Platelet-derived growth factor receptor-α (PDGFRα) is a tyrosine kinase expressed by cells of the limb bud, but its role in limb development is unknown. To investigate PDGFRα function, we generated mice expressing mutant PDGFRα with a point mutation (D842V) that increases receptor signaling. Mutant hindlimbs are immobile with knee joints fused by cartilage and lacking ligaments and menisci. The interzone marker Gdf5 is initially expressed at E12.5 but is downregulated thereafter, suggesting a defect in interzone maintenance. Omics analysis of the joint tissues identifies ectopic cartilage matrix expressing genes for cartilage and fibrotic tissue. Thus, elevated PDGFRα signaling corrupts joint development by downregulating Gdf5 and redirecting interzone progenitors into a fibrocartilage fate. This suggests that tight regulation of tyrosine kinase activity is necessary for the development of the mouse knee joint.

Fluoroquinolone-Mediated Tendinopathy and Tendon Rupture

The fluoroquinolone (FQ) class of antibiotics includes the world's most prescribed antibiotics such as ciprofloxacin, levofloxacin, and ofloxacin that are known for their low bacterial resistance. This is despite their potential to trigger severe side effects, such as myopathy, hearing loss, tendinopathy, and tendon rupture. Thus, healthcare organizations around the world have recommended limiting the prescription of FQs. Tendinopathy is a common name for maladies that cause pain and degeneration in the tendon tissue, which can result in tendon rupture. Whilst there are several identified effects of FQ on tendons, the exact molecular mechanisms behind FQ-mediated tendon rupture are unclear. Previous research studies indicated that FQ-mediated tendinopathy and tendon rupture can be induced by changes in gene expression, metabolism, and function of tendon resident cells, thus leading to alterations in the extracellular matrix. Hence, this review begins with an update on FQs, their mode of action, and their known side effects, as well as summary information on tendon tissue structure and cellular content. Next, how FQs affect the tendon tissue and trigger tendinopathy and tendon rupture is explored in detail. Lastly, possible preventative measures and promising areas for future research are also discussed. Specifically, follow-up studies should focus on understanding the FQ-mediated tendon changes in a more complex manner and integrating in vitro with in vivo models. With respect to in vitro systems, the field should move towards three-dimensional models that reflect the cellular diversity found in the tissue.

Therapeutic potential and mechanisms of umbilical cord mesenchymal stem cells differentiating into tendon cells and promotion of rotator cuff tendon-bone healing

Rotator cuff tendon injuries often lead to shoulder pain and dysfunction. Traditional treatments such as surgery and physical therapy can provide temporary relief, but it is difficult to achieve complete healing of the tendon, mainly because of the limited repair capacity of the tendon cells. Therefore, it is particularly urgent to explore new treatment methods. In vitro experiments were performed to explore the mechanism of differentiation of umbilical cord mesenchymal stem cells (UCMSCs) to tendon cells and to evaluate their potential in promoting rotator cuff injury repair. Growth factors such as CTGF, GDF-6, and GDF-7 were used to induce the differentiation of UCMSCs, and gene expression changes during the differentiation process were analyzed by single-cell sequencing. Hes1 overexpression and animal models were constructed to study its role in UCMSCs differentiation and rotator cuff injury repair. CTGF was the optimal factor for inducing the differentiation of UCMSCs into tendon cells. With increasing induction time, UCMSCs exhibited obvious tendon cell characteristics, such as changes in cell morphology and increased expression of tendon-specific proteins (MKX, SCX, and TNC). Single-cell sequencing analysis revealed key cellular subpopulations and signaling pathways during differentiation. Furthermore, overexpression of the Hes1 gene significantly promoted the differentiation of UCMSCs to tendon cells and showed its therapeutic effect in rotator cuff injury repair in an animal model. This study confirmed the potential of UCMSCs in tendon injury repair, especially the critical role of Hes1 in promoting UCMSCs differentiation and rotator cuff tendon-bone healing, which provides a theoretical basis and experimental rationale for the development of new cellular therapeutic strategies.

Musculoskeletal Organs-on-Chips: An Emerging Platform for Studying the Nanotechnology-Biology Interface

Nanotechnology-based approaches are promising for the treatment of musculoskeletal (MSK) disorders, which present significant clinical burdens and challenges, but their clinical translation requires a deep understanding of the complex interplay between nanotechnology and MSK biology. Organ-on-a-chip (OoC) systems have emerged as an innovative and versatile microphysiological platform to replicate the dynamics of tissue microenvironment for studying nanotechnology-biology interactions. This review first covers recent advances and applications of MSK OoCs and their ability to mimic the biophysical and biochemical stimuli encountered by MSK tissues. Next, by integrating nanotechnology into MSK OoCs, cellular responses and tissue behaviors may be investigated by precisely controlling and manipulating the nanoscale environment. Analysis of MSK disease mechanisms, particularly bone, joint, and muscle tissue degeneration, and drug screening and development of personalized medicine may be greatly facilitated using MSK OoCs. Finally, future challenges and directions are outlined for the field, including advanced sensing technologies, integration of immune-active components, and enhancement of biomimetic functionality. By highlighting the emerging applications of MSK OoCs, this review aims to advance the understanding of the intricate nanotechnology-MSK biology interface and its significance in MSK disease management, and the development of innovative and personalized therapeutic and interventional strategies.

Therapeutic Controlled Release Strategies for Human Osteoarthritis

Osteoarthritis is a progressive, irreversible debilitating whole joint disease that affects millions of people worldwide. Despite the availability of various options (non-pharmacological and pharmacological treatments and therapy, orthobiologics, and surgical interventions), none of them can definitively cure osteoarthritis in patients. Strategies based on the controlled release of therapeutic compounds via biocompatible materials may provide powerful tools to enhance the spatiotemporal delivery, expression, and activities of the candidate agents as a means to durably manage the pathological progression of osteoarthritis in the affected joints upon convenient intra-articular (injectable) delivery while reducing their clearance, dissemination, or side effects. The goal of this review is to describe the current knowledge and advancements of controlled release to treat osteoarthritis, from basic principles to applications in vivo using therapeutic recombinant molecules and drugs and more innovatively gene sequences, providing a degree of confidence to manage the disease in patients in a close future.

ACL injury management: a comprehensive review of novel biotherapeutics

The anterior cruciate ligament (ACL) is integral to the stability of the knee joint, serving to limit anterior tibial translation and regulate rotational movements. ACL injuries are among the most common and debilitating forms of knee trauma, often resulting in joint effusion, muscular atrophy, and diminished athletic capabilities. Despite the established efficacy of ACL reconstruction as the standard treatment, it is not uniformly successful. Consequently, there is a growing interest in novel biotherapeutic interventions as potential alternatives. This comprehensive review examines the latest advancements in ACL biotherapy, encompassing the application of hyaluronic acid, self-assembled short peptides, growth factors, stem cell therapy, gene therapy, platelet-rich plasma therapy, bone marrow aspirate concentrate cells, extracorporeal shock wave, electrical stimulation and cross bracing protocol. The collective aim of these innovative treatments is to facilitate the restoration of the ACL's native biological and biomechanical integrity, with the ultimate goal of enhancing clinical outcomes and the functional recovery of affected individuals.

Single-nucleus transcriptional and chromatin accessibility analyses of maturing mouse Achilles tendon uncover the molecular landscape of tendon stem/progenitor cells

Tendons and ligaments are crucial connective tissues linking bones and muscles, yet achieving full functional recovery after injury remains challenging. We investigated the characteristics of tendon stem/progenitor cells (TSPCs) by focusing on the declining tendon repair capacity with growth. Using single-cell RNA sequencing on Achilles tendon cells from 2- and 6-week-old mice, we identified Cd55 and Cd248 as novel surface antigen markers for TSPCs. Combining single-nucleus ATAC and RNA sequencing analyses revealed that Cd55 and Cd248 positive fractions in tendon tissue are TSPCs, with this population decreasing at 1 weeks. We also identified candidate upstream transcription factors regulating these fractions. Functional analyses of isolated CD55/CD248 positive cells demonstrated high clonogenic potential and tendon differentiation capacity, forming functional tendon-like tissue in vitro . This study establishes CD55 and CD248 as novel TSPC surface antigens, potentially advancing tendon regenerative medicine and contributing to the development of new treatment strategies for tendon and ligament injuries.

In-vitro study on type I collagen synthesis in low-level laser therapy on the early ligament fibroblasts' healing process

BACKGROUND

Low-level Laser Therapy (LLLT) has demonstrated its potential in promoting fiber matrix maturation, collagen synthesis, and fibroblast proliferation, contributing to tissue regeneration. Our study aimed to investigate the impact of LLLT on collagen type I synthesis, cell proliferation, and viability in human ligament fibroblasts derived from the Anterior Cruciate Ligament (ACL).

METHODS

Tissue samples were obtained from individuals undergoing arthroscopic ACL reconstruction surgery. Primary human fibroblasts were isolated, and immunohistochemical assays confirmed their characteristics. LLLT at 850 nm was administered in three groups: Low dose (1.0 J/cm²), High dose (5.0 J/cm²), and Control (0.0 J/cm²). Cell viability was calculated using a membrane integrity assay, proliferation was determined by automated counting, and collagen type I concentration in cell culture was measured using an immunoassay.

RESULTS

Fibroblasts showed decreased viability after low and high doses of LLLT, increased proliferation at the low dose, and increased collagen synthesis at the high dose on day 10 for both sexes after treatment.

CONCLUSION

Our study demonstrated that LLLT may improve the early ligament healing process by increasing cell proliferation at the low dose and enhancing collagen type I synthesis at the high dose in human ligament fibroblasts.

Melatonin supplementation counteracts fiber loss in knee ligaments of diabetes-induced rats

Diabetes mellitus (DM) is a prevalent metabolic disease. The clinical impact of sustained hyperglycemia on ligament healing has not been well characterized. Diabetes is a known cause of macro-, microvascular, and diabetic ulcer healing difficulties among tissues. Therefore, we aimed to investigate the healing potential occurring in injured and healthy ligaments among diabetic and healthy individuals using a rat model. We hypothesize that DM may contribute to altering the knee medial collateral ligament (MCL), thus its morphology, biochemical fitness, and functionality. The study cohort consisted of 40 rats. The animals were randomized into four equal groups. Groups I and II (20 rats) received saline subcutaneously and served as controls. Groups III and IV (20 rats) were injected with a single dose of streptozotocin (STZ). All animals underwent surgery to cut the left tibial collateral ligament in the hind limb and suture it. The access site was sutured to create inflammation and study the regenerative capacities of animals with normal carbohydrate metabolism and pharmacologically induced diabetes. Each animal then underwent sham surgery to access and suture the right tibial collateral ligament in the hind limb without ligament intervention. After the animals had undergone surgeries, groups II and IV were given melatonin supplementation for 4 weeks. Rats with DM presented with more fibrosis and calcification of the MCL and decreased healing potential. Treatment with melatonin in diabetic rats mitigated alterations and improved the antioxidant status of ligaments from the diabetic group.

Biomineral-Based Composite Materials in Regenerative Medicine

Regenerative medicine aims to address substantial defects by amplifying the body's natural regenerative abilities and preserving the health of tissues and organs. To achieve these goals, materials that can provide the spatial and biological support for cell proliferation and differentiation, as well as the micro-environment essential for the intended tissue, are needed. Scaffolds such as polymers and metallic materials provide three-dimensional structures for cells to attach to and grow in defects. These materials have limitations in terms of mechanical properties or biocompatibility. In contrast, biominerals are formed by living organisms through biomineralization, which also includes minerals created by replicating this process. Incorporating biominerals into conventional materials allows for enhanced strength, durability, and biocompatibility. Specifically, biominerals can improve the bond between the implant and tissue by mimicking the micro-environment. This enhances cell differentiation and tissue regeneration. Furthermore, biomineral composites have wound healing and antimicrobial properties, which can aid in wound repair. Additionally, biominerals can be engineered as drug carriers, which can efficiently deliver drugs to their intended targets, minimizing side effects and increasing therapeutic efficacy. This article examines the role of biominerals and their composite materials in regenerative medicine applications and discusses their properties, synthesis methods, and potential uses.

Anatomical study of the sacrotuberous ligament and the hamstring muscles: A histomorphological analysis

The sacrotuberous ligament (STL) and the hamstrings are important structures that are mutually connected and influenced by the pelvis. However, the anatomical connectivity and histological characteristics of these structures remain unclear. The present study aimed to comprehensively investigate the relationship between the STL and the proximal hamstrings through histological analysis. Sixteen specimens were obtained from eight fresh cadavers (mean age at death, 73.4 years). Verhoeff Van Gieson, Masson's trichrome, and immunohistochemical staining were used to analyze the connectivity between the STL and the hamstrings and to verify the ratios of collagen and elastic fibers. Dense connective tissue that overlapped tightly between the STL and hamstrings was observed. The relative ratios of collagen and elastic fibers between the STL and hamstrings characteristically identified regional differences. The ratio of elastic fibers to collagen in the biceps femoris (BF) was ~38.6 ± 4.7%, and the lowest ratio was 5.9 ± 2.6% observed in the semimembranosus (SM). In the case of the BF, contractibility is well-regulated due to a high content of elastic fibers; however, the muscular structure of the BF is relatively fragile due to the low content of collagen. In the SM, collagen content is higher than that in the STL. This ratio of elastic fibers in the collagen analysis could provide crucial information for understanding the differences in hamstring contractility and maintaining the morphology of these structures.

Investigating and Practicing Orthopedics at the Intersection of Sex and Gender: Understanding the Physiological Basis, Pathology, and Treatment Response of Orthopedic Conditions by Adopting a Gender Lens: A Narrative Overview

In the biomedical field, the differentiation between sex and gender is crucial for enhancing the understanding of human health and personalizing medical treatments, particularly within the domain of orthopedics. This distinction, often overlooked or misunderstood, is vital for dissecting and treating musculoskeletal conditions effectively. This review delves into the sex- and gender-specific physiology of bones, cartilage, ligaments, and tendons, highlighting how hormonal differences impact the musculoskeletal system's structure and function, and exploring the physiopathology of orthopedic conditions from an epidemiological, molecular, and clinical perspective, shedding light on the discrepancies in disease manifestation across sexes. Examples such as the higher rates of deformities (adolescent idiopathic and adult degenerative scoliosis and hallux valgus) in females and osteoporosis in postmenopausal women illustrate the critical role of sex and gender in orthopedic health. Additionally, the review addresses the morbidity-mortality paradox, where women, despite appearing less healthy on frailty indexes, show lower mortality rates, highlighting the complex interplay between biological and social determinants of health. Injuries and chronic orthopedic conditions such osteoarthritis exhibit gender- and sex-specific prevalence and progression patterns, necessitating a nuanced approach to treatment that considers these differences to optimize outcomes. Moreover, the review underscores the importance of recognizing the unique needs of sexual minority and gender-diverse individuals in orthopedic care, emphasizing the impact of gender-affirming hormone therapy on aspects like bone health and perioperative risks. To foster advancements in sex- and gender-specific orthopedics, we advocate for the strategic disaggregation of data by sex and gender and the inclusion of "Sexual Orientation and Gender Identity" (SOGI) data in research and clinical practice. Such measures can enrich clinical insights, ensure tailored patient care, and promote inclusivity within orthopedic treatments, ultimately enhancing the precision and effectiveness of care for diverse patient populations. Integrating sex and gender considerations into orthopedic research and practice is paramount for addressing the complex and varied needs of patients. By embracing this comprehensive approach, orthopedic medicine can move towards more personalized, effective, and inclusive treatment strategies, thereby improving patient outcomes and advancing the field.

Heterotopic mineralization (ossification or calcification) in aged musculoskeletal soft tissues: A new candidate marker for aging

Aging can lead to various disorders in organisms and with the escalating impact of population aging, the incidence of age-related diseases is steadily increasing. As a major risk factor for chronic illnesses in humans, the prevention and postponement of aging have become focal points of research among numerous scientists. Aging biomarkers, which mirror molecular alterations at diverse levels in organs, tissues, and cells, can be used to monitor and evaluate biological changes associated with aging. Currently, aging biomarkers are primarily categorized into physiological traits, imaging characteristics, histological features, cellular-level alterations, and molecular-level changes that encompass the secretion of aging-related factors. However, in the context of the musculoskeletal soft tissue system, aging-related biological indicators primarily involve microscopic parameters at the cellular and molecular levels, resulting in inconvenience and uncertainty in the assessment of musculoskeletal soft tissue aging. To identify convenient and effective indicators, we conducted a comprehensive literature review to investigate the correlation between ectopic mineralization and age-related changes in the musculoskeletal soft tissue system. Here, we introduce the concept of ectopic mineralization as a macroscopic, reliable, and convenient biomarker for musculoskeletal soft tissue aging and present novel targets and strategies for the future management of age-related musculoskeletal soft tissue disorders.

Review paper: The importance of consideration of collagen cross-links in computational models of collagen-based tissues

Collagen as the main protein in Extra Cellular Matrix (ECM) is the main load-bearing component of fibrous tissues. Nanostructure and architecture of collagen fibrils play an important role in mechanical behavior of these tissues. Extensive experimental and theoretical studies have so far been performed to capture these properties, but none of the current models realistically represent the complexity of network mechanics because still less is known about the collagen's inner structure and its effect on the mechanical properties of tissues. The goal of this review article is to emphasize the significance of cross-links in computational modeling of different collagen-based tissues, and to reveal the need for continuum models to consider cross-links properties to better reflect the mechanical behavior observed in experiments. In addition, this study outlines the limitations of current investigations and provides potential suggestions for the future work.

A morphological study on the sphenoid bone ligaments' ossification pattern

PURPOSE

The sphenoid bone (SB) extracranial ligaments (ECRLs) are the pterygoalar and pterygospinous ligaments (PTAL and PTSL) that are located at the SB lateral pterygoid plate, and inferior to the foramen ovale (FO). Their ossification may affect the mandibular nerve's distribution. The intracranial ligaments' (ICRLs) ossification (the caroticoclinoid ligament-CCLL, the anterior and posterior interclinoid ligaments-AICLL and PICLL) may impede the approaches to the sella. This study highlights the incidence of the ossified ECRLs and ICRLs location, their type (partial, or complete), considering laterality, gender, age, and ligaments' simultaneous presence.

METHODS

The sample consisted of 156 Greek adult dried skulls of both genders and variable age.

RESULTS

Ossified ligaments were identified in 57.05%, predominantly extracranially (42.31%, P = 0.003). ECRLs were predominantly identified unilaterally (30.13%, P < 0.001). The majority of the ossified ICRLs were predominantly identified in male skulls (31.1%, P = 0.048) and the majority of the ECRLs (52.8%, P = 0.028) were predominantly identified at the age of 60 years and above. The PTAL was the most ossified (32.69%), followed by the CCLL (24.36%), the PTSL (16.03%), the PICLL (6.41%), and the AICLL (4.49%).

CONCLUSIONS

Detailed knowledge of the SB morphology and ligaments' ossification extent is essential to improve the technique of the FO percutaneous approach, and sellar approaches, to minimize complications.

Current progress in growth factors and extracellular vesicles in tendon healing

Tendon injury healing is a complex process that involves the participation of a significant number of molecules and cells, including growth factors molecules in a key role. Numerous studies have demonstrated the function of growth factors in tendon healing, and the recent emergence of EV has also provided a new visual field for promoting tendon healing. This review examines the tendon structure, growth, and development, as well as the physiological process of its healing after injury. The review assesses the role of six substances in tendon healing: insulin-like growth factor-I (IGF-I), transforming growth factor β (TGFβ), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and EV. Different growth factors are active at various stages of healing and exhibit separate physiological activities. IGF-1 is expressed immediately after injury and stimulates the mitosis of various cells while suppressing the response to inflammation. VEGF, which is also active immediately after injury, accelerates local metabolism by promoting vascular network formation and positively impacts the activities of other growth factors. However, VEGF's protracted action could be harmful to tendon healing. PDGF, the earliest discovered cytokine to influence tendon healing, has a powerful cell chemotaxis and promotes cell proliferation, but it can equally accelerate the response to inflammation and relieve local adhesions. Also useful for relieving tendon adhesion is TGF- β, which is active almost during the entire phase of tendon healing. As a powerful active substance, in addition to its participation in the field of cardiovascular and cerebrovascular vessels, tumour and chronic wounds, TGF- β reportedly plays a role in promoting cell proliferation, activating growth factors, and inhibiting inflammatory response during tendon healing.

[Research progress on bioactive strategies for promoting tendon graft healing after anterior cruciate ligament reconstruction]

Objective

To review the bioactive strategies that enhance tendon graft healing after anterior cruciate ligament reconstruction (ACLR), and to provide insights for improving the therapeutic outcomes of ACLR.

Methods

The domestic and foreign literature related to the bioactive strategies for promoting the healing of tendon grafts after ACLR was extensively reviewed and summarized.

Results

At present, there are several kinds of bioactive materials related to tendon graft healing after ACLR: growth factors, cells, biodegradable implants/tissue derivatives. By constructing a complex interface simulating the matrix, environment, and regulatory factors required for the growth of native anterior cruciate ligament (ACL), the growth of transplanted tendons is regulated at different levels, thus promoting the healing of tendon grafts. Although the effectiveness of ACLR has been significantly improved in most studies, most of them are still limited to the early stage of animal experiments, and there is still a long way to go from the real clinical promotion. In addition, limited by the current preparation technology, the bionics of the interface still stays at the micron and millimeter level, and tends to be morphological bionics, and the research on the signal mechanism pathway is still insufficient.

Conclusion

With the further study of ACL anatomy, development, and the improvement of preparation technology, the research of bioactive strategies to promote the healing of tendon grafts after ACLR is expected to be further promoted.

Nanofiber matrix formulations for the delivery of Exendin-4 for tendon regeneration: In vitro and in vivo assessment

Tendon and ligament injuries are the most common musculoskeletal injuries, which not only impact the quality of life but result in a massive economic burden. Surgical interventions for tendon/ligament injuries utilize biological and/or engineered grafts to reconstruct damaged tissue, but these have limitations. Engineered matrices confer superior physicochemical properties over biological grafts but lack desirable bioactivity to promote tissue healing. While incorporating drugs can enhance bioactivity, large matrix surface areas and hydrophobicity can lead to uncontrolled burst release and/or incomplete release due to binding. To overcome these limitations, we evaluated the delivery of a peptide growth factor (exendin-4; Ex-4) using an enhanced nanofiber matrix in a tendon injury model. To overcome drug surface binding due to matrix hydrophobicity of poly(caprolactone) (PCL)-which would be expected to enhance cell-material interactions-we blended PCL and cellulose acetate (CA) and electrospun nanofiber matrices with fiber diameters ranging from 600 to 1000 nm. To avoid burst release and protect the drug, we encapsulated Ex-4 in the open lumen of halloysite nanotubes (HNTs), sealed the HNT tube endings with a polymer blend, and mixed Ex-4-loaded HNTs into the polymer mixture before electrospinning. This reduced burst release from ∼75% to ∼40%, but did not alter matrix morphology, fiber diameter, or tensile properties. We evaluated the bioactivity of the Ex-4 nanofiber formulation by culturing human mesenchymal stem cells (hMSCs) on matrix surfaces for 21 days and measuring tenogenic differentiation, compared with nanofiber matrices in basal media alone. Strikingly, we observed that Ex-4 nanofiber matrices accelerated the hMSC proliferation rate and elevated levels of sulfated glycosaminoglycan, tendon-related genes (Scx, Mkx, and Tnmd), and ECM-related genes (Col-I, Col-III, and Dcn), compared to control. We then assessed the safety and efficacy of Ex-4 nanofiber matrices in a full-thickness rat Achilles tendon defect with histology, marker expression, functional walking track analysis, and mechanical testing. Our analysis confirmed that Ex-4 nanofiber matrices enhanced tendon healing and reduced fibrocartilage formation versus nanofiber matrices alone. These findings implicate Ex-4 as a potentially valuable tool for tendon tissue engineering.

Optimizing the classification of biological tissues using machine learning models based on polarized data

Polarimetric data is nowadays used to build recognition models for the characterization of organic tissues or the early detection of some diseases. Different Mueller matrix-derived polarimetric observables, which allow a physical interpretation of a specific characteristic of samples, are proposed in literature to feed the required recognition algorithms. However, they are obtained through mathematical transformations of the Mueller matrix and this process may loss relevant sample information in search of physical interpretation. In this work, we present a thorough comparative between 12 classification models based on different polarimetric datasets to find the ideal polarimetric framework to construct tissues classification models. The study is conducted on the experimental Mueller matrices images measured on different tissues: muscle, tendon, myotendinous junction and bone; from a collection of 165 ex-vivo chicken thighs. Three polarimetric datasets are analyzed: (A) a selection of most representative metrics presented in literature; (B) Mueller matrix elements; and (C) the combination of (A) and (B) sets. Results highlight the importance of using raw Mueller matrix elements for the design of classification models.

Osseous Bridges of the Sphenoid Bone: Frequency, Bilateral and Sex Distribution

Sellar (caroticoclinoid and interclinoid), pterygospinous and pterygoalar bridges are osseous bars of the sphenoid bone, which enclose additional foramina in the skull base and could cause entrapment of nerves, occlusion of vessels and obstruction of surgical corridors. This study aimed to investigate the frequency of sphenoid bone bridges in Bulgarians and to assess the bilateral and sex differences in their distribution. This study was performed on head CT scans of 315 Bulgarians, 148 males and 167 females. The sellar bridges were the most common type of sphenoid bridging; particularly the caroticoclinoid bridge. The pterygospinous bridge was a relatively common finding and the pterygoalar bridge was the most infrequent type of bridging. The total frequency of sellar bridges did not differ significantly between both sides and sexes. The pterygospinous bridge did not indicate significant bilateral differences but showed considerable sex differences concerning the left-side occurrence, which was significantly higher in the male series. There were no considerable bilateral and sex differences in the distribution of the pterygoalar bridging. There were no significant correlations between the different types of sphenoid bone bridges, but each type of bridging showed significant positive correlations between the right and left side co-occurrence in males and females.

Mechanisms of skeletal muscle-tendon development and regeneration/healing as potential therapeutic targets

Skeletal muscle contraction is essential for the movement of our musculoskeletal system. Tendons and ligaments that connect the skeletal muscles to bones in the correct position at the appropriate time during development are also required for movement to occur. Since the musculoskeletal system is essential for maintaining basic bodily functions as well as enabling interactions with the environment, dysfunctions of these tissues due to disease can significantly reduce quality of life. Unfortunately, as people live longer, skeletal muscle and tendon/ligament diseases are becoming more common. Sarcopenia, a disease in which skeletal muscle function declines, and tendinopathy, which involves chronic tendon dysfunction, are particularly troublesome because there have been no significant advances in their treatment. In this review, we will summarize previous reports on the development and regeneration/healing of skeletal muscle and tendon tissues, including a discussion of the molecular and cellular mechanisms involved that may be used as potential therapeutic targets.

MR Imaging–Ultrasonography Correlation of Acute and Chronic Foot and Ankle Conditions

Foot and ankle injuries are common musculoskeletal disorders. In the acute setting, ligamentous injuries are most common, whereas fractures, osseous avulsion injuries, tendon and retinaculum tears, and osteochondral injuries are less common. The most common chronic and overuse injuries include osteochondral and articular cartilage defects, tendinopathies, stress fractures, impingement syndromes, and neuropathies. Common forefoot conditions include traumatic and stress fractures, metatarsophalangeal and plantar plate injuries and degenerations, intermittent bursitis, and perineural fibrosis. Ultrasonography is well-suited for evaluating superficial tendons, ligaments, and muscles. MR imaging is best for deeper-located soft tissue structures, articular cartilage, and cancellous bone.

Biological and Mechanical Factors and Epigenetic Regulation Involved in Tendon Healing

Tendons are an important part of the musculoskeletal system. Connecting muscles to bones, tendons convert force into movement. Tendon injury can be acute or chronic. Noticeably, tendon healing requires a long time span and includes inflammation, proliferation, and remodeling processes. The mismatch between endogenous and exogenous healing may lead to adhesion causing further negative effects. Management of tendon injuries and complications such as subsequent adhesion formation are still challenges for clinicians. Due to numerous factors, tendon healing is a complex process. This review introduces the role of various biological and mechanical factors and epigenetic regulation processes involved in tendon healing.

Tendon tissue engineering: An overview of biologics to promote tendon healing and repair

Tendons are dense connective tissues with a hierarchical polarized structure that respond to and adapt to the transmission of muscle contraction forces to the skeleton, enabling motion and maintaining posture. Tendon injuries, also known as tendinopathies, are becoming more common as populations age and participation in sports/leisure activities increases. The tendon has a poor ability to self-heal and regenerate given its intrinsic, constrained vascular supply and exposure to frequent, severe loading. There is a lack of understanding of the underlying pathophysiology, and it is not surprising that disorder-targeted medicines have only been partially effective at best. Recent tissue engineering approaches have emerged as a potential tool to drive tendon regeneration and healing. In this review, we investigated the physiochemical factors involved in tendon ontogeny and discussed their potential application in vitro to reproduce functional and self-renewing tendon tissue. We sought to understand whether stem cells are capable of forming tendons, how they can be directed towards the tenogenic lineage, and how their growth is regulated and monitored during the entire differentiation path. Finally, we showed recent developments in tendon tissue engineering, specifically the use of mesenchymal stem cells (MSCs), which can differentiate into tendon cells, as well as the potential role of extracellular vesicles (EVs) in tendon regeneration and their potential for use in accelerating the healing response after injury.

Are COL22A1 Gene Polymorphisms rs11784270 and rs6577958 Associated with Susceptibility to a Non-Contact Anterior Cruciate Ligament Injury in Polish Athletes?

Understanding the risk factors and etiology of ACL ruptures (anterior cruciate ligament) is crucial due to the injury’s high occurrence, significant financial cost to the healthcare sector, and clinical consequences. In this study, we investigated the hypothesis that rs11784270 A/C and rs6577958 C/T SNPs (single gene polymorphism) within COL22A1 are associated with ACL ruptures (ACLR) in Polish soccer players. Methods: 228 athletes with ACLR (157 male, age 26 ± 4, 71 female, age 26 ± 6) and 202 control athletes (117 male, age 26 ± 6, 85 female, age 29 ± 2) engaged in the study. The buccal cell swabs were genotyped using TaqMan® pre-designed SNP genotyping assays, following the manufacturer’s recommendations. The R program and SNPassoc package were used to determine the genotype and allele frequency distributions under the various inheritance models (co-dominant, dominant, recessive, and over-dominant). Further, p-values of <0.05 were considered statistically significant. We found no association between the analyzed polymorphisms and the risk of non-contact ACL ruptures in any of the studied models. Although the genetic variants investigated in this study were not associated with the risk of non-contact ACL ruptures, we assumed that the COL22A1 gene remains a candidate for further investigations in musculoskeletal injuries.

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.

Wetspun Polymeric Fibrous Systems as Potential Scaffolds for Tendon and Ligament Repair, Healing and Regeneration

Tendon and ligament traumatic injuries are among the most common diagnosed musculoskeletal problems. Such injuries limit joint mobility, reduce musculoskeletal performance, and most importantly, lower people's comfort. Currently, there are various treatments that are used to treat this type of injury, from surgical to conservative treatments. However, they're not entirely effective, as reinjures are frequent and, in some cases, fail to re-establish the lost functionality. Tissue engineering (TE) approaches aim to overcome these disadvantages by stimulating the regeneration and formation of artificial structures that resemble the original tissue. Fabrication and design of artificial fibrous scaffolds with tailored mechanical properties are crucial for restoring the mechanical function of the tissues. Recently, polymeric nanofibers produced by wetspinning have been largely investigated to mimic, repair, and replace the damaged tissue. Wetspun fibrous structures are extensively used due to their exceptional properties, such as the ability to mimic the native tissue, their biodegradability and biocompatibility, and good mechanical properties. In this review, the tendon and ligament structure and biomechanics are presented. Then, promising wetspun multifunctional fibrous structures based on biopolymers, more specifically polyhydroxyalkanoates (PHA), polycaprolactone (PCL), and polyethylenes, will be discussed, as well as reinforcing agents such as cellulose nanocrystals (CNC), nanoparticles, and growth factors.

Effects of local anesthetics and contrast agents on musculoskeletal regenerative medicine procedures

In regenerative medicine, cells, tissues and organs are often replaced, engineered or regrown in order to restore their function after they have been damaged or lost. Local anesthetics, corticosteroids and contrast agents are commonly employed for both diagnostic and therapeutic objectives in interventional pain and musculoskeletal treatments for regenerative medicine. There is growing evidence that routine injectables promote catabolism and disease processes. Thus, understanding the effects of these compounds on regenerative medicine injectates and target tissues such as tenocytes, chondrocytes, nucleus pulposus and ligamentous tissue is critical. This review includes the current research on the effects of local anesthetics and contrast agents, as well as their use and recommendations in regenerative medicine operations.

Intra-flexor retinaculum steroid injection in elderly patients with carpal tunnel syndrome: A randomized clinical trial

Introduction

The efficacy of ultrasonography-guided intra-flexor retinaculum corticosteroid injection is compared to within-carpal tunnel steroid injection, for the treatment of elderly patients with carpal tunnel syndrome (CTS).

Material & methods

In this prospective, double-blind, randomized trial, the elderly patients with CTS are allocated 1:1 into the two treatment groups. Subjects and assessors remained blinded to group allocation throughout the trial. All patients received 40 ​mg triamcinolone (1 ​mL) plus 1 ​mL of 2% lidocaine, either fenestrated in the flexor retinaculum (group 1) or injected within the carpal tunnel between the flexor retinaculum and median nerve (group 2). Patients were instructed to use a wrist splint for two weeks post-treatment. Symptom severity, grip, electrodiagnostic indices, and ultrasonographic features were measured at baseline and 6-weeks thereafter. The primary outcomes were median nerve distal motor and sensory latencies, and those secondary outcomes were Boston Carpal Tunnel Questionnaire (BCTQ) scores, visual analog scale (VAS) pain scores, and the median nerve inlet cross-sectional area (CSA).

Results

Of 92 individuals screened, 50 eligible participants were randomized, all of whom completed the study and were included in the analysis. Patients receiving the intra-flexor retinaculum injection demonstrated significantly greater improvements in their total BCTQ score (p ​= ​0.023), VAS score (p ​= ​0.026), and inlet CSA (p ​= ​0.004), while the electrodiagnostic indices and the grip scale did not differ between groups.

Conclusion

The intra-flexor retinaculum corticosteroid injection can provide better functional recovery and symptom reduction for elderly patients with CTS, compared to the within-carpal tunnel corticosteroid injection.

Tideglusib promotes wound healing in aged skin by activating PI3K/Akt pathway

Background

Aging disturbs the skin morphology and function, manifested as thinned epithelium and impaired wound healing. As a major type of skin cells, epidermal stem cells (EpiSCs) are inevitably affected by aging. The effect of age on EpiSCs and wound healing needs to be further explored.

Methods

Skin RNA-seq data of young (5 months) and old (30 months) CB6F1 mice were obtained from GEO Series GSE35322 with 10 in each age group. Differentially expressed genes were analyzed, and EpiSCs-related pathways were enriched by KEGG. The age-related changes of the screened PI3K/Akt pathway were validated by Western Blot and immunofluorescence of epidermis of SD rats (2, 17, and 23 months, n = 6). The expression of upstream protein EGFR was assessed by immunofluorescence in skin of mice (4, 13, and 23 months, n = 6) and human (respectively, 23, 28, 30 years old in the young group and 69, 73, 78 years old in the old group) skin. Inhibitors of EGFR were used to verify its effects on EpiSCs and wound healing. The small molecule drug Tideglusib was tested for its effects on signaling pathways of EpiSCs and wound healing of aged rats. Western Blot was used for the detection of signaling pathways in in vitro experiments. Cell migration assays were used to assess cell migration ability. Flow cytometry was used to detect changes in cell cycle and apoptosis levels. Sulforhodamine B assay and CCK-8 assay were used to evaluate cell proliferation and viability, respectively. Student’s t test and one-way analysis of variance (ANOVA) followed by the multiple comparisons Bonferroni test were used for statistical analysis. The 0.05 level of confidence was accepted as a significant difference.

Results

EpiSCs-related PI3K/Akt pathway was enriched by KEGG and verified by decreased phosphorylation of Akt (32.1 ± 13.8%, P < 0.01) and mTOR (38.9 ± 11.8%, P < 0.01) in aged epidermis of rats. Furthermore, the expression of PI3K/Akt-upstream EGFR decreased with age in the epidermis of mouse (27.6 ± 5.5%, P < 0.01) and human (25.8 ± 9.3%, P < 0.01). With EGFR blocked by Erlotinib, EpiSCs showed reduced phosphorylation of Akt (30.4 ± 10.6%, P < 0.01) and mTOR (39.8 ± 12.8%, P < 0.01), impaired proliferation and migration after incubated for 24 h and 36 h (P < 0.05), and higher levels of apoptosis (11.9 ± 1.7%, P < 0.05), and rats showed slower wound healing from d7 to d14 after wounding (P < 0.01). In addition to slower wound healing rates, aged rats also showed a decrease in the efficacy of EGF, partly due to the downregulated EGFR expression. By activating PI3K/Akt pathway, Tideglusib promoted the proliferation and migration of EpiSCs with apoptosis inhibited (P < 0.01) and accelerated wound healing in aged rats from d7 to d14 after wounding (P < 0.05). Notably, the combined use of Tideglusib and EGF could further enhance wound healing in aged rats.

Conclusions

The decreased expression of EGFR in epidermis with age resulted in decreased activity of the PI3K/Akt pathway and limited EGF efficacy. Tideglusib could assist wound healing in aged rats via activating PI3K/Akt pathway, which may be considered as an ingredient for medical and cosmetics use.

A Review of Recent Advances in Natural Polymer-Based Scaffolds for Musculoskeletal Tissue Engineering

The musculoskeletal (MS) system consists of bone, cartilage, tendon, ligament, and skeletal muscle, which forms the basic framework of the human body. This system plays a vital role in appropriate body functions, including movement, the protection of internal organs, support, hematopoiesis, and postural stability. Therefore, it is understandable that the damage or loss of MS tissues significantly reduces the quality of life and limits mobility. Tissue engineering and its applications in the healthcare industry have been rapidly growing over the past few decades. Tissue engineering has made significant contributions toward developing new therapeutic strategies for the treatment of MS defects and relevant disease. Among various biomaterials used for tissue engineering, natural polymers offer superior properties that promote optimal cell interaction and desired biological function. Natural polymers have similarity with the native ECM, including enzymatic degradation, bio-resorb and non-toxic degradation products, ability to conjugate with various agents, and high chemical versatility, biocompatibility, and bioactivity that promote optimal cell interaction and desired biological functions. This review summarizes recent advances in applying natural-based scaffolds for musculoskeletal tissue engineering.

Defining the Components of the Deltoid Ligament (DL): A Cadaveric Study

Background: The deltoid ligament (DL) is a strong triangle-shaped ligament with a complex fascicular arrangement. Understanding the morphological and/or functional typing of the DL structure is hindered by a paucity of clear, quantitative, and reproducible data and is further complicated by inconsistent terminology use. The aim of this work was to describe different components of the DL using strict identification criteria.

Methods: Thirty embalmed cadaveric ankles of both sides were dissected on all sides and studied by using gross examination, micro-dissection, and light microscopy by tracing the fascicular pattern of each under 6X magnification.

Results: Six ligamentous bands were identified. The tibiotalocalcaneal ligament (TTC) and the superficial posterior tibiotalar ligament (sPTT) were two superficial variants and the anterior tibiotalar ligament (ATT), the anterior tibiotalonavicular ligament (ATTN), the intermediate tibiotalar ligament (ITT), and the deep posterior tibiotalar ligament (dPTT) were four deep variants. The TTC was identified in all 30 embalmed cadaveric specimens. Five additional ligamentous bands (ITT, sPTT, dPTT, ATT, and ATTN) were variable findings in the current cohort.

Conclusion: This study presents six ligamentous bands as a regular finding and five additional ligamentous bands as variable findings in the dissected specimen. This data could assist in the radiological diagnosis of DL injuries and advanced procedures related to its surgical repair and reconstruction.

Mechanically and biologically promoted cell-laden constructs generated using tissue-specific bioinks for tendon/ligament tissue engineering applications

Tendon and ligament tissues provide stability and mobility crucial for musculoskeletal function, but are particularly prone to injury. Owing to poor innate healing capacity, the regeneration of mature and functional tendon/ligament (T/L) poses a formidable clinical challenge. Advanced bioengineering strategies to develop biomimetic tissue implants are highly desired for the treatment of T/L injuries. Here, we presented a cell-based tissue engineering strategy to generate cell-laden tissue constructs comprising stem cells and tissue-specific bioinks using 3D cell-printing technology. We implemented an in vitro preconditioning approach to guide semi-organized T/L-like formation before the in vivo application of cell-printed implants. During in vitro maturation, tissue-specific decellularized extracellular matrix-based cellular constructs facilitated long-term in vitro culture with high cell viability and promoted tenogenesis with enhanced cellular/structural anisotropy. Moreover, we demonstrated improved cell survival/retention upon in vivo implantation of pre-matured constructs in nude mice with de novo tendon formation and improved mechanical strength. Although in vivo mechanical properties of the cell-printed implants were lower than those of human T/L tissues, the results of this study may have significant implications for future cell-based therapies in tendon and ligament regeneration and translational medicine.

Comparison of Tendon Development Versus Tendon Healing and Regeneration

Tendon is a vital connective tissue in human skeletal muscle system, and tendon injury is very common and intractable in clinic. Tendon development and repair are two closely related but still not fully understood processes. Tendon development involves multiple germ layer, as well as the regulation of diversity transcription factors (Scx et al.), proteins (Tnmd et al.) and signaling pathways (TGFβ et al.). The nature process of tendon repair is roughly divided in three stages, which are dominated by various cells and cell factors. This review will describe the whole process of tendon development and compare it with the process of tendon repair, focusing on the understanding and recent advances in the regulation of tendon development and repair. The study and comparison of tendon development and repair process can thus provide references and guidelines for treatment of tendon injuries.

Analysis of the Stability of the Body in a Standing Position When Shooting at a Stationary Target-A Randomized Controlled Trial

Postural stability of the body depends on many factors. One of them is physical activity. It is especially important in the case of sports or professional work, which combine mobility with the accuracy of a shot in a standing position. The smaller the body fatigue, the more accurate the shot. The aim of the study was the assessment of the impact of physical effort on the center of gravity deflection and length of the COP (center of pressure) path, as well as the reaction of ground forces in people who do not engage in systematic physical activity. The study group included 139 people (23.1 ± 5.2 yr; M: 46.8%; F: 53.2%). The test consisted of performing a static test twice, shooting at the target in a multimedia shooting range. Group X performed the Harvard test between the static tests. Group Y made no effort. The reaction parameters of the ground forces were assessed using the Zebris PDM-L Platform. In Group X performing the Harvard test, an increase in the average COP, VCOP, and 95% confidence ellipse area was noted. The path length and the average velocity of COP speed increased. There were no differences in Group Y (p > 0.05). Physical effort significantly affected the postural stability of the studied people, increasing the average parameters assessing balance when adopting static firing position.

Single-Cell Integration Analysis of Heterotopic Ossification and Fibrocartilage Developmental Lineage: Endoplasmic Reticulum Stress Effector Xbp1 Transcriptionally Regulates the Notch Signaling Pathway to Mediate Fibrocartilage Differentiation

INTRODUCTION

Regeneration of fibrochondrocytes is essential for the healing of the tendon-bone interface (TBI), which is similar to the formation of neurogenic heterotopic ossification (HO). Through single-cell integrative analysis, this study explored the homogeneity of HO cells and fibrochondrocytes.

METHODS

This study integrated six datasets, namely, GSE94683, GSE144306, GSE168153, GSE138515, GSE102929, and GSE110993. The differentiation trajectory and key transcription factors (TFs) for HO occurrence were systematically analyzed by integrating single-cell RNA (scRNA) sequencing, bulk RNA sequencing, and assay of transposase accessible chromatin seq. The differential expression and enrichment pathways of TFs in heterotopically ossified tissues were identified.

RESULTS

HO that mimicked pathological cells was classified into HO1 and HO2 cell subsets. Results of the pseudo-temporal sequence analysis suggested that HO2 is a differentiated precursor cell of HO1. The analysis of integrated scRNA data revealed that ectopically ossified cells have similar transcriptional characteristics to cells in the fibrocartilaginous zone of tendons. The modified SCENIC method was used to identify specific transcriptional regulators associated with ectopic ossification. Xbp1 was defined as a common key transcriptional regulator of ectopically ossified tissues and the fibrocartilaginous zone of tendons. Subsequently, the CellPhoneDB database was completed for the cellular ligand-receptor analysis. With further pathway screening, this study is the first to propose that Xbp1 may upregulate the Notch signaling pathway through Jag1 transcription. Twenty-four microRNAs were screened and were found to be potentially associated with upregulation of XBP1 expression after acute ischemic stroke.

CONCLUSION

A systematic analysis of the differentiation landscape and cellular homogeneity facilitated a molecular understanding of the phenotypic similarities between cells in the fibrocartilaginous region of tendon and HO cells. Furthermore, by identifying Xbp1 as a hub regulator and by conducting a ligand-receptor analysis, we propose a potential Xbp1/Jag1/Notch signaling pathway.

Tendinopathy and tendon material response to load: What we can learn from small animal studies

Tendinopathy is a debilitating disease that causes as much as 30% of all musculoskeletal consultations. Existing treatments for tendinopathy have variable efficacy, possibly due to incomplete characterization of the underlying pathophysiology. Mechanical load can have both beneficial and detrimental effects on tendon, as the overall tendon response depends on the degree, frequency, timing, and magnitude of the load. The clinical continuum model of tendinopathy offers insight into the late stages of tendinopathy, but it does not capture the subclinical tendinopathic changes that begin before pain or loss of function. Small animal models that use high tendon loading to mimic human tendinopathy may be able to fill this knowledge gap. The goal of this review is to summarize the insights from in-vivo animal studies of mechanically-induced tendinopathy and higher loading regimens into the mechanical, microstructural, and biological features that help characterize the continuum between normal tendon and tendinopathy. STATEMENT OF SIGNIFICANCE: This review summarizes the insights gained from in-vivo animal studies of mechanically-induced tendinopathy by evaluating the effect high loading regimens have on the mechanical, structural, and biological features of tendinopathy. A better understanding of the interplay between these realms could lead to improved patient management, especially in the presence of painful tendon.

Boost Tendon/Ligament Repair With Biomimetic and Smart Cellular Constructs

Tendon and ligament are soft connective tissues that play essential roles in transmitting forces from muscle to bone or bone to bone. Despite significant progress made in the field of ligament and tendon regeneration over the past decades, many strategies struggle to recapitulate basic structure-function criteria of native ligament/tendon. The goal here is to provide a fundamental understanding of the structure and composition of ligament/tendon and highlight few key challenges in functional regeneration of these connective tissues. The remainder of the review will examine several biomaterials strategies including biomimetic scaffold with non-linear mechanical behavior, hydrogel patch with anisotropic adhesion and gene-activated scaffold for interactive healing of tendon/ligament. Finally, emerging technologies and research avenues are suggested that have the potential to enhance treatment outcomes of tendon/ligament injuries.

Tissue-specific parameters for the design of ECM-mimetic biomaterials

The extracellular matrix (ECM) is a complex network of biomolecules that mechanically and biochemically directs cell behavior and is crucial for maintaining tissue function and health. The heterogeneous organization and composition of the ECM varies within and between tissue types, directing mechanics, aiding in cell-cell communication, and facilitating tissue assembly and reassembly during development, injury and disease. As technologies like 3D printing rapidly advance, researchers are better able to recapitulate in vivo tissue properties in vitro; however, tissue-specific variations in ECM composition and organization are not given enough consideration. This is in part due to a lack of information regarding how the ECM of many tissues varies in both homeostatic and diseased states. To address this gap, we describe the components and organization of the ECM, and provide examples for different tissues at various states of disease. While many aspects of ECM biology remain unknown, our goal is to highlight the complexity of various tissues and inspire engineers to incorporate unique components of the native ECM into in vitro platform design and fabrication. Ultimately, we anticipate that the use of biomaterials that incorporate key tissue-specific ECM will lead to in vitro models that better emulate human pathologies. STATEMENT OF SIGNIFICANCE: Biomaterial development primarily emphasizes the engineering of new materials and therapies at the expense of identifying key parameters of the tissue that is being emulated. This can be partially attributed to the difficulty in defining the 3D composition, organization, and mechanics of the ECM within different tissues and how these material properties vary as a function of homeostasis and disease. In this review, we highlight a range of tissues throughout the body and describe how ECM content, cell diversity, and mechanical properties change in diseased tissues and influence cellular behavior. Accurately mimicking the tissue of interest in vitro by using ECM specific to the appropriate state of homeostasis or pathology in vivo will yield results more translatable to humans.

Polarimetric data-based model for tissue recognition

We highlight the potential of a predictive optical model method for tissue recognition, based on the statistical analysis of different polarimetric indicators that retrieve complete polarimetric information (selective absorption, retardance and depolarization) of samples. The study is conducted on the experimental Mueller matrices of four biological tissues (bone, tendon, muscle and myotendinous junction) measured from a collection of 157 ex-vivo chicken samples. Moreover, we perform several non-parametric data distribution analyses to build a logistic regression-based algorithm capable to recognize, in a single and dynamic measurement, whether a sample corresponds (or not) to one of the four different tissue categories.

Localized chondro-ossification underlies joint dysfunction and motor deficits in the Fkbp10 mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a genetic disorder that features wide-ranging defects in both skeletal and nonskeletal tissues. Previously, we and others reported that loss-of-function mutations in FK506 Binding Protein 10 (FKBP10) lead to skeletal deformities in conjunction with joint contractures. However, the pathogenic mechanisms underlying joint dysfunction in OI are poorly understood. In this study, we have generated a mouse model in which Fkbp10 is conditionally deleted in tendons and ligaments. Fkbp10 removal substantially reduced telopeptide lysyl hydroxylation of type I procollagen and collagen cross-linking in tendons. These biochemical alterations resulting from Fkbp10 ablation were associated with a site-specific induction of fibrosis, inflammation, and ectopic chondrogenesis followed by joint deformities in postnatal mice. We found that the ectopic chondrogenesis coincided with enhanced Gli1 expression, indicating dysregulated Hedgehog (Hh) signaling. Importantly, genetic inhibition of the Hh pathway attenuated ectopic chondrogenesis and joint deformities in Fkbp10 mutants. Furthermore, Hh inhibition restored alterations in gait parameters caused by Fkbp10 loss. Taken together, we identified a previously unappreciated role of Fkbp10 in tendons and ligaments and pathogenic mechanisms driving OI joint dysfunction.

The glucagon-like peptide 1 receptor agonist Exendin-4 induces tenogenesis in human mesenchymal stem cells

Tendon injuries are common and account for up to 50% of musculoskeletal injuries in the United States. The poor healing nature of the tendon is attributed to poor vascularization and cellular composition. In the absence of FDA-approved growth factors for tendon repair, engineering strategies using bioactive factors, donor cells, and delivery matrices to promote tendon repair and regeneration are being explored. Growth factor alternatives in the form of small molecules, donor cells, and progenitors offer several advantages and enhance the tendon healing response. Small drug molecules and peptides offer stability over growth factors that are known to suffer from relatively short biological half-lives. The primary focus of this study was to assess the ability of the exendin-4 (Ex-4) peptide, a glucagon-like peptide 1 (GLP-1) receptor agonist, to induce tenocyte differentiation in bone marrow-derived human mesenchymal stem cells (hMSCs). We treated hMSCs with varied doses of Ex-4 in culture media to evaluate proliferation and tendonogenic differentiation. A 20 nM Ex-4 concentration was optimal for promoting cell proliferation and tendonogenic differentiation. Tendonogenic differentiation of hMSCs was evaluated via gene expression profile, immunofluorescence, and biochemical analyses. Collectively, the levels of tendon-related transcription factors (Mkx and Scx) and extracellular matrix (Col-I, Dcn, Bgn, and Tnc) genes and proteins were elevated compared to media without Ex-4 and other controls including insulin and IGF-1 treatments. The tendonogenic factor Ex-4 in conjunction with hMSCs appear to enhance tendon regeneration.

Overview of Skeletal Development

Development of cartilage and bone, the core components of the mouse skeletal system, depends on coordinated proliferation and differentiation of skeletogenic cells, including chondrocytes and osteoblasts. These cells differentiate from common progenitor cells originating in the mesoderm and neural crest. Multiple signaling pathways and transcription factors tightly regulate differentiation and proliferation of skeletal cells. In this chapter, we overview the process of mouse skeletal development and discuss major regulators of skeletal cells at each developmental stage.

Potential cross-talk between muscle and tendon in Duchenne muscular dystrophy

PURPOSE

To describe potential signaling (cross-talk) between dystrophic skeletal muscle and tendon in Duchenne muscular dystrophy.

MATERIALS AND METHODS

Review of Duchenne muscular dystrophy and associated literature relevant to muscle-tendon cross-talk.

RESULTS AND CONCLUSIONS

Duchenne muscular dystrophy results from the absence of the protein dystrophin and the associated dystrophin - glycoprotein complex, which are thought to provide both structural support and signaling functions for the muscle fiber. In addition, there are other potential signal pathways that could represent cross-talk between muscle and tendon, particularly at the myotendinous junction. Duchenne muscular dystrophy is characterized by multiple pathophysiologic mechanisms. Herein, we explore three of these: (1) the extracellular matrix, fibrosis, and fat deposition; (2) satellite cells; and (3) tensegrity. A key signaling protein that emerged in each was transforming growth factor - beta one (TGF-β1).].

Stem Cells for the Regeneration of Tendon and Ligament: A Perspective

Tendons are structures that connect muscles to the bones in our body and transmit the force generated by contraction of the muscles to the bones. Ligaments are structures that connect bones to bones, with histological properties similar to tendons. In tendon and ligament tissue, there are very small amounts of cells similar to mesenchymal stem cells (MSCs) called tendon stem/progenitor cells (TSPCs), or tenogenic stem cells. While the role of specific growth factors and transcription factors is well established in the osteogenic and chondrogenic differentiation of stem cells, a consensus has not been established for tenogenic differentiation. Injuries to tendons and ligaments are very common, but natural healing is very slow and inefficient due to limited vascularization. Currently, there is no adequate method for restoring extensive tendon or ligament defects. Procedures addressing the unmet need for regeneration of these tissues are needed. In this review, the current knowledge, as well as the authors’ ideas and perspective on stem cell and regenerative medicine for tendon and ligament defects are presented.

FGF2: a key regulator augmenting tendon-to-bone healing and cartilage repair

Ligament/tendon and cartilage injuries are clinically common diseases that perplex most clinicians. Because of the lack of blood vessels and nerves, their self-repairing abilities are rather poor. Therefore, surgeries are necessary and also widely used to treat ligament/tendon or cartilage injuries. However, after surgery, there are still many problems that affect healing. In recent years, it has been found that exogenous FGF2 plays an important role in the repair of ligament/tendon and cartilage injuries and exerts a synergistic effect with endogenous FGF2. Therefore, FGF2 can be used as a new type of biomolecule to accelerate tendon-to-bone healing and cartilage repair after injury.

Depolarization metric spaces for biological tissues classification

Classification of tissues is an important problem in biomedicine. An efficient tissue classification protocol allows, for instance, the guided-recognition of structures through treated images or discriminating between healthy and unhealthy regions (e.g., early detection of cancer). In this framework, we study the potential of some polarimetric metrics, the so-called depolarization spaces, for the classification of biological tissues. The analysis is performed using 120 biological ex vivo samples of three different tissues types. Based on these data collection, we provide for the first time a comparison between these depolarization spaces, as well as with most commonly used depolarization metrics, in terms of biological samples discrimination. The results illustrate the way to determine the set of depolarization metrics which optimizes tissue classification efficiencies. In that sense, the results show the interest of the method which is general, and which can be applied to study multiple types of biological samples, including of course human tissues. The latter can be useful for instance, to improve and to boost applications related to optical biopsy.

A single cell transcriptional atlas of early synovial joint development

Synovial joint development begins with the formation of the interzone, a region of condensed mesenchymal cells at the site of the prospective joint. Recently, lineage-tracing strategies have revealed that Gdf5-lineage cells native to and from outside the interzone contribute to most, if not all, of the major joint components. However, there is limited knowledge of the specific transcriptional and signaling programs that regulate interzone formation and fate diversification of synovial joint constituents. To address this, we have performed single cell RNA-Seq analysis of 7329 synovial joint progenitor cells from the developing murine knee joint from E12.5 to E15.5. By using a combination of computational analytics, in situ hybridization and in vitro characterization of prospectively isolated populations, we have identified the transcriptional profiles of the major developmental paths for joint progenitors. Our freely available single cell transcriptional atlas will serve as a resource for the community to uncover transcriptional programs and cell interactions that regulate synovial joint development.

Matrix Metalloproteinase Genes (MMP1, MMP10, MMP12) on Chromosome 11q22 and the Risk of Non-Contact Anterior Cruciate Ligament Ruptures

Background: Sequence variants within the matrix metalloproteinases genes remain plausible biological candidates for further investigation of anterior cruciate ligament (ACL) rupture risk. The aim of the present study was to establish whether variants within the MMP1 (rs1799750, ->G), MMP10 (rs486055, C > T) and MMP12 (rs2276109, T > C) genes were associated with non-contact ACL rupture in a Polish cohort. Methods: The unrelated, self-reported Polish Caucasian participants consisted of 228 (157 male) individuals with primary non-contact ACL rupture and 202 (117 male) participants without any history of ACL rupture. All samples were genotyped in duplicate using the Applied Biosystems TaqMan^® methodology. The statistical analyses were involved in determining the distribution of genotype and allele frequencies for the investigated polymorphisms between the diagnostic groups. Furthermore, pseudo-haplotypes were constructed to assess possible gene–gene interactions. Results: All genotype frequencies in the ACL rupture and control groups conformed to Hardy Weinberg Equilibrium expectations. None of the polymorphisms were associated with risk of non-contact ACL rupture under the codominant, dominant, recessive and over-dominant genetic models. Likewise, no genotype–genotype combinations inferred as “haplotypes” as a proxy of gene–gene interactions were associated with the risk of non-contact ACL ruptures. Conclusions: Despite the fact that the current study did not support existing evidence suggesting that variants within the MMP1, MMP10, and MMP12 genes influence non-contact ACL rupture risk, future work should include high-throughput sequencing technologies to identify potential targeted polymorphisms to fully characterize the 11q22 region with susceptibility to non-contact ACL rupture susceptibility in a Polish cohort.