Dense Connective Tissues Research Articles

Chitosan/Fibroin Biopolymer-Based Hydrogels for Potential Angiogenesis in Developing Chicks and Accelerated Wound Healing in Mice.

Potential therapies for wound management remain one of the most challenging affairs to date. Biopolymer hydrogels possess inherent properties that facilitate the healing of damaged tissue by creating a supportive and hydrated environment. Chitosan/fibroin hydrogels were formulated with poly (vinyl pyrrolidone) and cross-linked using 3-aminopropyl (diethoxy) methylsilane (APDEMS) for the aforementioned function. The hydrogels were characterized through Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy, and their swelling response was observed using a variety of solvents. Additionally, hydrogels were investigated for biomedical applications. As the amount of fibroin added to the hydrogels increased, the swelling ratio decreased. The analysis of chorioallantoic membrane (CAM) assay revealed that higher concentrations of fibroin in the hydrogel were directly correlated with increased angiogenesis. The intragroup comparison showed that the vascular number in the CPF5 group was significantly increased (p ≤ 0.05) compared to other hydrogel groups. The wound healing efficiency of the prepared hydrogels showed that the rate of wound reduction (99.06%) was remarkably (p ≤ 0.05) high in the hydrogel group with a greater fibroin content against control (67.03%). Histological findings of wounded tissues corroborate the abovementioned results, showing dense fibrous connective tissues in the fibroin group compared to the control. The results of this work provide thorough preclinical evidence that chitosan-fibroin biopolymers are involved in enhanced angiogenesis in growing chicks and speed up wound healing in mice without any obvious toxicity.

Target NF-κB p65 for preventing posttraumatic joint contracture in rats.

RelA/p65 is as a crucial component of the nuclear factor κB (NF-κB) signaling pathway that has a significant impact on various fibrotic diseases. However, its role in the fibrosis of tissues surrounding the joint after traumatic injury remains unclear. In this study, rats were divided into three groups: non-operated control (NC) group, p65-siRNA treated (siRNA-p65) group, and negative siRNA treated (siRNA-neg) group. Then, 10 μL (10 nmol) of p65-siRNAwas injected into the joint of the siRNA-p65 group. Meanwhile, 10 μL of negative siRNA was administered to the knee joint of the operated siRNA-neg group for comparison. The rats in the NC group did not receive surgery or drug intervention. After 4 weeks of right knee fixation in each group, X-ray measurements revealed significantly reduced degree of knee flexion contracture following p65-siRNA treatment (siRNA-neg: 77.73° ± 2.799°; siRNA-p65: 105.7° ± 2.629°, p < 0.0001). Histopathological examination revealed that the number of dense fibrous connective tissues decreased following p65-siRNA inhibition. Western blot analysis revealed significantly different expression levels of fibrosis-related proteins between the siRNA-p65 and siRNA-neg groups. Immunohistochemical analysis revealed a reduction in the average number of myofibroblasts in the siRNA-p65 group compared with that in the siRNA-neg group. Thus, intra-articular p65-siRNA injection could attenuate fibroblast activation and fibrosis-related protein production, suppress periarticular tissue fibrosis, and prevent joint contracture by downregulating the NF-κB p65 pathway. Statement of clinical significance: Intra-articular injection of p65-siRNA could reduce myofibroblast proliferation and fibrosis-related protein expressionby downregulating the NF-κB p65 pathway, inhibit periarticular tissue fibrosis, and prevent joint adhesion, which representsa potential therapy in the prevention of joint fibrosis following traumatic injury.

Accurate Spatio-Temporal Delivery of Nitric Oxide Facilitates the Programmable Repair of Avascular Dense Connective Tissues Injury.

Avascular dense connective tissues (e.g., the annulus fibrosus (AF) rupture, the meniscus tear, and tendons and ligaments injury) repair remains a challenge due to the "biological barrier" that hinders traditional drug permeation and limits self-healing of the injured tissue. Here, accurate delivery of nitric oxide (NO) to penetrate the "AF biological barrier" is achieved thereby enabling programmable AF repair. NO-loaded BioMOFs are synthesized and mixed in a modified polyvinyl alcohol and PCL-composited electrospun fiber membrane with excellent reactive oxygen species-responsive capability (LN@PM). The results show that LN@PM could respond to the high oxidative stress environment at the injured tissue and realize continuous and substantial NO release. Based on low molecular weight and lipophilicity, NO could penetrate through the "biological barrier" for accurate AF drug delivery. Moreover, the dynamic characteristics of the LN@PM reaction can be matched with the pathological microenvironment to initiate programmable tissue repair including sequential remodeling microenvironment, reprogramming the immune environment, and finally promoting tissue regeneration. This tailored programmable treatment strategy that matches the pathological repair process significantly repairs AF, ultimately alleviating intervertebral disc degeneration. This study highlights a promising approach for avascular dense connective tissue treatment through intelligent NO release, effectively overcoming "AF biological barriers" and programmable treatment.

Functional anatomy of the orbit in strabismus surgery: Connective tissues, pulleys, and the modern surgical implications of the "arc of contact" paradigm.

Oculomotricity is a multidimensional domain characterised by a delicate interplay of anatomical structures and physiological processes. This manuscript meticulously dissects the nuances of this interplay, bringing to the fore the integral role of the extraocular muscles (EOMs) and their intricate relationship with the myriad orbital connective tissues as it harmoniously orchestrates binocular movements, ensuring synchronised and fluid visual tracking. Historically, the peripheral oculomotor apparatus was conceptualised as a rudimentary system predominantly driven by neural directives. While widely accepted, this perspective offered a limited view of the complexities inherent in ocular movement mechanics. The twentieth century heralded a paradigm shift in this understanding. With advances in anatomical research and imaging techniques, a much clearer picture of the gross anatomy of the EOMs emerged. This clarity challenged traditional viewpoints, suggesting that the inherent biomechanical properties of the EOMs, coupled with their associated tissue pulleys, play a pivotal role in dictating eye movement dynamics. Central to this revised understanding is the "arc of contact" paradigm. This concept delves deep into the mechanics of eye rotation, elucidating the significance of the point of contact between the EOMs and the eyeball. The arc of contact is not just a static anatomical feature; its length and orientation play a crucial role in determining the effective torque generated by a muscle, thereby influencing the amplitude and direction of eye rotation. The dynamic nature of this arc, influenced by the position and tension of the muscle pulleys, offers a more comprehensive model for understanding ocular kinematics. Previously overlooked in traditional models, muscle pulleys have now emerged as central players in the biomechanics of eye movement. These anatomical structures, formed by dense connective tissues, guide the paths of the EOMs, ensuring that their pulling angles remain optimal across a range of gaze directions. The non-linear paths resulting from these pulleys provide a more dynamic and intricate understanding of eye movement, challenging two-dimensional, linear models of orbital anatomy. The implications of these revelations extend beyond mere theoretical knowledge. The insights garnered from this research promise transformative potential in the realm of strabismus surgery. Recognising the pivotal role of muscle pulleys and the "arc of contact" paradigm allows for more precise surgical interventions, ensuring better post-operative outcomes and minimising the risk of complications. Surgical procedures that previously relied on basic mechanical principles now stand to benefit from a more nuanced understanding of the underlying anatomical and physiological dynamics. In conclusion, this manuscript serves as a testament to the ever-evolving nature of scientific knowledge. Challenging established norms and introducing fresh perspectives pave the way for more effective and informed clinical interventions in strabismus surgery.

Spatial gene expression in the adult rat patellar tendon

Tendons are dense connective tissues with relatively few cells which makes studying the molecular profile of the tissue challenging. There is not a consensus on the spatial location of various cell types within a tendon, nor the accompanying transcriptional profile. In the present study, we used two male rat patellar tendon samples for sequencing-based spatial transcriptomics to determine the gene expression profile. We integrated our data with a mouse Achilles single cell dataset to predict the cell type composition of the patellar tendon as a function of location within the tissue. The spatial location of the predicated cell types suggested that there were two populations of tendon fibroblasts, one located in the tendon midsubstance, while the other localized with red blood cells, pericytes, and immune cells to the tendon peripheral connective tissue. Of the highest expressed spatially variable genes, there were multiple genes with known function in tendon: Col1a1, Col1a2, Dcn, Fmod, Sparc, and Comp. Further, a novel spatially regulated gene (AABR07000398.1) with no known function was identified. The spatial gene expression of tendon associated genes (Scx, Thbs4, Tnmd, Can, Bgn, Lum, Adamts2, Lox, Ppib, Col2a1, Col3a1, Col6a2) was also visualized. Both patellar tendon samples had similar expression patterns for all these genes. This dataset provides new spatial insights into gene expression in a healthy tendon.

Complement factor D regulates collagen type I expression and fibroblast migration to enhance human tendon repair and healing outcomes.

Dense connective tissues (DCTs) such as tendon, ligament, and cartilage are important stabilizers and force transmitters in the musculoskeletal system. The healing processes after DCT injuries are highly variable, often leading to degenerative changes and poor clinical outcome. Biomarkers in relation to repair quality for human DCTs, especially tendon are lacking. This study expands our previous findings and aimed to characterize the mechanisms by which a potential biomarker of good outcomes, complement factor D (CFD), regulates tendon healing. Quantitative mass spectrometry (QMS) profiling of tissue biopsies from the inflammatory phase of healing (n = 40 patients) and microdialysates from the proliferative phase of healing (n = 28 patients) were used to identify specific biomarkers for tendon healing. Further bioinformatic and experimental investigations based on primary fibroblasts and fibroblast cell line were used to confirm the identified biomarkers. The QMS profiling of tissue biopsies from the inflammatory phase of healing identified 769 unique proteins, and microdialysates from the proliferative phase of healing identified 1423 unique proteins in Achilles tendon rupture patients. QMS-profiling showed that CFD expression was higher during the inflammatory- and lower during the proliferative healing phase in the good outcome patients. Further bioinformatic and experimental explorations based on both inflammatory and proliferative fibroblast models demonstrated that CFD potentially improved repair by regulating cell migration and modulating collagen type I (Col1a1) expression. Moreover, it was shown that the enhanced Col1a1 expression, through increased fibroblast migration, was correlated with the validated clinical outcome. The results of the current studies characterized underlying inflammatory- and proliferative healing mechanisms by which CFD potentially improved tendon repair. These findings may lead to improved individualized treatment options, as well the development of effective therapies to promote good long-term clinical outcomes after tendon and other DCT injuries. http://clinicaltrials.gov, identifiers NCT02318472, NCT01317160.

Lack of skeletal muscle contraction disrupts fibrous tissue morphogenesis in the developing murine knee.

Externally applied forces, such as those generated through skeletal muscle contraction, are important to embryonic joint formation, and their loss can result in gross morphologic defects including joint fusion. While the absence of muscle contraction in the developing chick embryo leads to dissociation of dense connective tissue structures of the knee and ultimately joint fusion, the central knee joint cavitates whereas the patellofemoral joint does not in murine models lacking skeletal muscle contraction, suggesting a milder phenotype. These differential results suggest that muscle contraction may not have as prominent of a role in the growth and development of dense connective tissues of the knee. To explore this question, we investigated the formation of the menisci, tendon, and ligaments of the developing knee in two murine models that lack muscle contraction. We found that while the knee joint does cavitate, there were multiple abnormalities in the menisci, patellar tendon, and cruciate ligaments. The initial cellular condensation of the menisci was disrupted and dissociation was observed at later embryonic stages. The initial cell condensation of the tendon and ligaments were less affected than the meniscus, but these tissues contained cells with hyper-elongated nuclei and displayed diminished growth. Interestingly, lack of muscle contraction led to the formation of an ectopic ligamentous structure in the anterior region of the joint as well. These results indicate that muscle forces are essential for the continued growth and maturation of these structures during this embryonic period.

Equine tendon mechanical behaviour: Prospects for repair and regeneration applications.

Tendons are dense connective tissues that play an important role in the biomechanical function of the musculoskeletal system. The mechanical forces have been implicated in every aspect of tendon biology. Tendon injuries are frequently occurring and their response to treatments is often unsatisfactory. A better understanding of tendon biomechanics and mechanobiology can help develop treatment options to improve clinical outcomes. Recently, tendon tissue engineering has gained more attention as an alternative treatment due to its potential to overcome the limitations of current treatments. This review first provides a summary of tendon mechanical properties, focusing on recent findings of tendon mechanobiological responses. In the next step, we highlight the biomechanical parameters of equine energy-storing and positional tendons. The final section is devoted to how mechanical loading contributes to tenogenic differentiation using bioreactor systems. This study may help develop novel strategies for tendon injury prevention or accelerate and improve tendon healing.

Histological and immunohistochemical guide to tendon tissue.

Tendons are unique dense connective tissues with discrete zones having specific structure and function. They are juxtaposed with other tissues (e.g., bone, muscle, and fat) with different compositional, structural, and mechanical properties. Additionally, tendon properties change drastically with growth and development, disease, aging, and injury. Consequently, there are unique challenges to performing high quality histological assessment of this tissue. To address this need, histological assessment was one of the breakout session topics at the 2022 Orthopaedic Research Society (ORS) Tendon Conference hosted at the University of Pennsylvania. The purpose of the breakout session was to discuss needs from members of the ORS Tendon Section related to histological procedures, data presentation, knowledge dissemination, and guidelines for future work. Therefore, this review provides a brief overview of the outcomes of this discussion and provides a set of guidelines, based on the perspectives from our laboratories, for histological assessment to assist researchers in their quest to utilize these techniques to enhance the outcomes and interpretations of their studies.

Histological study of the thin membranous dense connective tissues around the curving portion of the bilateral recurrent laryngeal nerves.

We aimed to demonstrate the anatomical relationship between the recurrent laryngeal nerves (RLNs), thin membranous dense connective tissue (TMDCT [e.g., the visceral or vascular sheaths around the esophagus]), and the lymph nodes around the esophagus at the curving portion of the RLNs for rational and efficient lymph node dissection. Transverse sections of the mediastinum at 5mm or 1mm intervals were obtained from four cadavers. Hematoxylin and eosin staining and Elastica van Gieson staining were performed. The visceral sheaths could not be clearly observed the curving portions of the bilateral RLNs, which were observed on the cranial and medial side of the great vessels (aortic arch and right subclavian artery [SCA]). The vascular sheaths could be clearly observed. The bilateral RLNs diverged from the bilateral vagus nerves, which ran along with the vascular sheaths, went up around the caudal side of the great vessels and the vascular sheath, and ran cranially on the medial side of the visceral sheath. Visceral sheaths were not observed around the region containing the left tracheobronchial lymph nodes (No. 106tbL) or the right recurrent nerve lymph nodes (No. 106recR). The regions containing the left recurrent nerve lymph nodes (No. 106recL) and the right cervical paraesophageal lymph nodes (No. 101R) were observed on the medial side of the visceral sheath, with the RLN. The recurrent nerve, which branched off from the vagus nerve descending along the vascular sheath, ascended the medial side of the visceral sheath after inversion. However, no clear visceral sheath could be identified in the inverted area. Therefore, during radical esophagectomy, the visceral sheath along No. 101R or 106recL may be recognized and available.

Collagen/κ-Carrageenan-Based Scaffolds as Biomimetic Constructs for In Vitro Bone Mineralization Studies.

Tissue engineering offers attractive strategies to develop three-dimensional scaffolds mimicking the complex hierarchical structure of the native bone. The bone is formed by cells incorporated in a molecularly organized extracellular matrix made of an inorganic phase, called biological apatite, and an organic phase mainly made of collagen and noncollagenous macromolecules. Although many strategies have been developed to replicate the complexity of bone at the nanoscale in vitro, a critical challenge has been to control the orchestrated process of mineralization promoted by bone cells in vivo and replicate the anatomical and biological properties of native bone. In this study, we used type I collagen to fabricate mineralized scaffolds mimicking the microenvironment of the native bone. The sulfated polysaccharide κ-carrageenan was added to the scaffolds to fulfill the role of noncollagenous macromolecules in the organization and mineralization of the bone matrix and cell adhesion. Scanning electron microscopy images of the surface of the collagen/κ-carrageenan scaffolds showed the presence of a dense and uniform network of intertwined fibrils, while images of the scaffolds' lateral sides showed the presence of collagen fibrils with a parallel alignment, which is characteristic of dense connective tissues. MC3T3-E1 osteoblasts were cultured in the collagen scaffolds and were viable after up to 7 days of culture, both in the absence and in the presence of κ-carrageenan. The presence of κ-carrageenan in the collagen scaffolds stimulated the maturation of the cells to a mineralizing phenotype, as suggested by the increased expression of key genes related to bone mineralization, including alkaline phosphatase (Alp), bone sialoprotein (Bsp), osteocalcin (Oc), and osteopontin (Opn), as well as the ability to mineralize the extracellular matrix after 14 and 21 days of culture. Taken together, the results described in this study shed light on the potential use of collagen/κ-carrageenan scaffolds to study the role of the structural organization of bone-mimetic synthetic matrices in cell function.

Optimizing repair of tendon ruptures and chronic tendinopathies: Integrating the use of biomarkers with biological interventions to improve patient outcomes and clinical trial design.

Tendons are dense connective tissues of the musculoskeletal system that link bones with muscles to foster mobility. They have complex structures and exist in varying biomechanical, metabolic and biological environments. In addition, tendon composition and mechanical properties can change over the lifespan as an individual ages. Many tendons function in high stress conditions with a low vascular and neuronal supply, conditions often leading to development of chronic tendinopathies, and in some cases, overt rupture of the tissues. Given their essential nature for human mobility and navigation through the environment, the effective repair and regeneration of different tendons after injury or damage is critical for quality of life, and for elite athletes, the return to sport participation at a high level. However, for mainly unknown reasons, the outcomes following injury are not always successful and lead to functional compromise and risk for re-injury. Thus, there is a need to identify those patients who are at risk for developing tendon problems, as well those at risk for poor outcomes after injury and to design interventions to improve outcomes after injury or rupture to specific tendons. This review will discuss recent advances in the identification of biomarkers prognostic for successful and less successful outcomes after tendon injury, and the mechanistic implications of such biomarkers, as well as the potential for specific biologic interventions to enhance outcomes to improve both quality of life and a return to participation in sports. In addition, the implication of these biomarkers for clinical trial design is discussed, as is the issue of whether such biomarkers for successful healing of one tendon can be extended to all tendons or are valid only for tendons in specific biomechanical and biological environments. As maintaining an active lifestyle is critical for health, the successful implementation of these advances will benefit the large number of individuals at risk.

An open-source OpenSim® ankle-foot musculoskeletal model for assessment of strains and forces in dense connective tissues

BackgroundThe ankle and foot are among the most critical load-bearing joints in the human anatomy. Anatomically accurate human body models are imperative to understanding the mechanics of injury and musculoskeletal disorders. A typical human ankle-foot anatomy consists of 25 DOFs, 112 dense connective tissues (DCTs) (92 ligaments, one capsule and 19 fasciae), 30 tendons, and 65 muscles. Existing models possess less than half of the DOFs and physiological elements. In this work, we have developed an ankle-foot joint complex musculoskeletal model for the OpenSim® platform by incorporating 24 degrees of freedom (DOF) comprising of 66 DCTs (46 ligaments, one 1 capsule and 19 fasciae), 30 tendons, and 65 muscles. MethodsComputed tomography (CT) data of human ankle joint-foot complex was segmented using Mimics ® (Version 17.0, Materialise, Belgium) to obtain models of the cartilages and bones of the ankle joint-foot complex. The position and resting lengths of the DCTs were attained from the MRI data and literature. Five joints, namely, tibiotalar, subtalar, chopart, tarsometatarsal (TMT), and metatarsophalangeal (MTP) joints and their joint axes were formulated to yield 24 DOFs. A forward simulation was carried out at each joint of the ankle-foot complex within their respective range of motions. The strains, instantaneous strain rates, and forces developed in the ligaments during the simulation were studied. ResultsDuring plantar-dorsiflexion of the tibiotalar joint, the anterior tibio-talar ligament (aTTL) yielded the maximum strain compared to all other ligaments. Anterior tibio-fibular ligament (aTFL) experienced extreme strain during subtalar inversion. Hence, the coupled kinematics of subtalar inversion and plantar flexion are failure-prone activities for aTFL. The chopart, TMT, and MTP joints yielded maximum strains or forces for several bundles at the extremes of the range of motion. This signifies that rotations of these joints to their extreme range of motion are prone to failure for the bundles attached to the joint complex. ConclusionThe results illustrate the potential application of the proposed OpenSim® ankle-foot model in understanding the ligament injury mechanism during sports activity and its prevention. Researchers can use the proposed model or customise it to study complex kinematics, understanding injury mechanisms, testing fixtures, orthosis or prosthesis, and many more in the domain of musculoskeletal research.

Rugae Patterns in Dentulous and Edentulous Models - A Comparative Study

INTRODUCTION: Palatal rugae are asymmetrical irregular elevations of dense connective tissues located on the anterior third of the palate extending on either side of the mid palatine raphae, behind the incisive papilla. Palatal rugoscopy is defined as the study of palatal rugae patterns. The uniqueness of the palatal rugae suggests their important role in personal identification and they are well protected by the lips, buccal pad of fat and teeth. They are considered to be stable throughout life even after the completion of growth and hence are suggested for use in forensics.AIM AND OBJECTIVE: This study aims to analyse rugae patterns in dentulous and edentulous models of both genders.METHODOLOGY: The study group consisted of 100 study models of which 50 were dentulous and 50 were edentulous. Thomas and Kotze classification was followed based on form and position to assess the edentulous and dentulous rugae pattern.RESULTS: The present study concludes that there are variations in rugae pattern between the dentulous and edentulous models. However, the most predominant patterns were straight, wavy, and circular.

The regenerative capacity of neonatal tissues.

It is well established that humans and other mammals are minimally regenerative compared with organisms such as zebrafish, salamander or amphibians. In recent years, however, the identification of regenerative potential in neonatal mouse tissues that normally heal poorly in adults has transformed our understanding of regenerative capacity in mammals. In this Review, we survey the mammalian tissues for which regenerative or improved neonatal healing has been established, including the heart, cochlear hair cells, the brain and spinal cord, and dense connective tissues. We also highlight common and/or tissue-specific mechanisms of neonatal regeneration, which involve cells, signaling pathways, extracellular matrix, immune cells and other factors. The identification of such common features across neonatal tissues may direct therapeutic strategies that will be broadly applicable to multiple adult tissues.

Pathomorphological characteristics of the wound bed prior to skin autografting

Objective: to conduct a comparative pathomorphological analysis of wounds of various origins requiring full-thickness skin autografting. Materials and Methods. Histomorphological comparison of the wound bed prior to plastic surgery with full-thickness skin autografts was performed in three groups of patients: (1) during excision of scar tissue in elective surgery; (2) in case of traumatic skin detachments with autografting sensu Krasovitov; (3) when excising the granulation tissue to the fibrous layer. The object of the study included biopsy specimens from patients of three study groups. Results. The histological picture of wounds after removal of scars was characterized by well-developed dense fibrocellular connective tissue and had signs of chronic inflammation. In contrast to the cicatricial wound, acute lesions were characterized by granulation and mature dense fibrous connective tissues with pronounced inflammatory changes, each of which had its own characteristics. Conclusion. The results of the comparative analysis revealed the features of the morphological picture of wounds depending on the type of damage. In the group of acute injuries, traumatic and burn wounds, the most pronounced tissue damage was revealed. Given the obtained data, it should be assumed that full-thickness skin autografting will yield the best result in the group of patients after the planned excision of scar tissue.

Bioprinting of Collagen Type I and II via Aerosol Jet Printing for the Replication of Dense Collagenous Tissues.

Collagen has grown increasingly present in bioprinting, however collagen bioprinting has mostly been limited to the extrusion printing of collagen type I to form weak collagen hydrogels. While these weak collagen hydrogels have their applications, synthetic polymers are often required to reinforce gel-laden constructs that aim to replicate dense collagenous tissues found in vivo. In this study, aerosol jet printing (AJP) was used to print and process collagen type I and II into dense constructs with a greater capacity to replicate the dense collagenous ECM found in connective tissues. Collagen type I and II was isolated from animal tissues to form solutions for printing. Collagen type I and II constructs were printed with 576 layers and measured to have average effective elastic moduli of 241.3 ± 94.3 and 196.6 ± 86.0 kPa (±SD), respectively, without any chemical modification. Collagen type II solutions were measured to be less viscous than type I and both collagen type I and II exhibited a drop in viscosity due to AJP. Circular dichroism and SDS-PAGE showed collagen type I to be more vulnerable to structural changes due to the stresses of the aerosol formation step of aerosol jet printing while the collagen type II triple helix was largely unaffected. SEM illustrated that distinct layers remained in the aerosol jet print constructs. The results show that aerosol jet printing should be considered an effective way to process collagen type I and II into stiff dense constructs with suitable mechanical properties for the replication of dense collagenous connective tissues.

An adhesion G protein-coupled receptor is required in cartilaginous and dense connective tissues to maintain spine alignment.

Adolescent idiopathic scoliosis (AIS) is the most common spine disorder affecting children worldwide, yet little is known about the pathogenesis of this disorder. Here, we demonstrate that genetic regulation of structural components of the axial skeleton, the intervertebral discs, and dense connective tissues (i.e., ligaments and tendons) is essential for the maintenance of spinal alignment. We show that the adhesion G protein-coupled receptor ADGRG6, previously implicated in human AIS association studies, is required in these tissues to maintain typical spine alignment in mice. Furthermore, we show that ADGRG6 regulates biomechanical properties of tendon and stimulates CREB signaling governing gene expression in cartilaginous tissues of the spine. Treatment with a cAMP agonist could mirror aspects of receptor function in culture, thus defining core pathways for regulating these axial cartilaginous and connective tissues. As ADGRG6 is a key gene involved in human AIS, these findings open up novel therapeutic opportunities for human scoliosis.

Transcriptional profiling of mESC-derived tendon and fibrocartilage cell fate switch

The transcriptional regulators underlying induction and differentiation of dense connective tissues such as tendon and related fibrocartilaginous tissues (meniscus and annulus fibrosus) remain largely unknown. Using an iterative approach informed by developmental cues and single cell RNA sequencing (scRNA-seq), we establish directed differentiation models to generate tendon and fibrocartilage cells from mouse embryonic stem cells (mESCs) by activation of TGFβ and hedgehog pathways, achieving 90% induction efficiency. Transcriptional signatures of the mESC-derived cells recapitulate embryonic tendon and fibrocartilage signatures from the mouse tail. scRNA-seq further identify retinoic acid signaling as a critical regulator of cell fate switch between TGFβ-induced tendon and fibrocartilage lineages. Trajectory analysis by RNA sequencing define transcriptional modules underlying tendon and fibrocartilage fate induction and identify molecules associated with lineage-specific differentiation. Finally, we successfully generate 3-dimensional engineered tissues using these differentiation protocols and show activation of mechanotransduction markers with dynamic tensile loading. These findings provide a serum-free approach to generate tendon and fibrocartilage cells and tissues at high efficiency for modeling development and disease.

The Potential Use of Mesenchymal Stem Cells and Their Derived Exosomes for Orthopedic Diseases Treatment.

Musculoskeletal disorders (MSDs) are conditions that can affect muscles, bones, and joints. These disorders are very painful and severely limit patients' mobility and are more common in the elderly. MSCs are multipotent stem cells isolated from embryonic (such as the umbilical cord) and mature sources (such as adipose tissue and bone marrow). These cells can differentiate into various cells such as osteoblasts, adipocytes, chondrocytes, NP-like cells, Etc. Due to MSC characteristics such as immunomodulatory properties, ability to migrate to the site of injury, recruitment of cells involved in repair, production of growth factors, and large amount production of extracellular vesicles, these cells have been used in many regenerative-related medicine studies. Also, MSCs produce different types of EVs, such as exosomes, to the extracellular environment. Exosomes reflect MSCs' characteristics and do not have cell therapy-associated problems because they are cell-free. These vesicles carry proteins, nucleic acids, and lipids to the host cell and change their function. This review focuses on MSCs and MSCs exosomes' role in repairing dense connective tissues such as tendons, cartilage, invertebrate disc, bone fracture, and osteoporosis treatment.Graphical abstract