Biology of Fibrocartilage Cells

Combined single-cell RNA sequencing and mendelian randomization to identify biomarkers associated with necrotic apoptosis in intervertebral disc degeneration

BACKGROUND

Intervertebral disc degeneration (IDD) is associated with back pain; back pain is a world-wide contributor to poor quality of life, while necroptosis has the characteristics of necroptosis and apoptosis, however, its role in IDD is still unclear. Therefore, the aim of this study was to identify biomarkers associated with necroptosis in IDD.

PURPOSE

To explore biomarkers associated with necroptosis in IDD, reveal the pathogenesis of IDD, as well as provide new directions for the diagnosis and treatment of this disease.

STUDY DESIGN/SETTINGS

Retrospective cohort study. Our study employs scRNA-seq coupled with MR analysis to investigate the causal relationship between necroptosis and IDD, laying a foundational groundwork for unveiling the intricate pathogenic mechanisms of this condition.

METHODS

Data quality control and normalisation was executed in single-cell dataset, GSE205535. Then, different cell types were obtained by cell annotation through marker genes. Subsequently, chi-square test was employed to assess the distribution difference of different cell types between IDD and control to screen key cells. AUCell was applied to calculate necroptosis-related genes (NRGs) scores of all cell types, further key cells were divided into high and low NRGs groups according to the median AUC scores of different cell types. Afterwards, the differentially expressed genes (DEGs) within the two score groups were screened. Then, the genes that had causal relationship with IDD were selected as biomarkers by univariate and multivariate Mendelian randomization (MR) analysis. Finally, the expression of biomarkers in different cell types and pseudo-time analysis was analyzed separately.

RESULTS

In GSE205535, 16 different cell populations identified by UMAP cluster analysis were further annotated to 8 cell types using maker genes. Afterwards, 53 DEGs were screened between the high and low NRGs groups. In addition, 9 genes with causal relationship with IDD were obtained by univariate MR analysis, further multivariate MR analysis proved that NT5E and TMEM158 had a direct causal relationship with IDD, which were used as biomarkers in this study. This study not only found that the expression levels of NT5E and TMEM158 were higher in IDD group, but also found that fibrochondrocytes and inflammatory chondrocytes were the key cells of NT5E and TMEM158, respectively. In the end, the biomarkers had the same expression trend in the quasi-time series, and both of them from high to low and then increased.

CONCLUSION

NT5E and TMEM158, as biomarkers of necroptotic apoptotic IDD, were causally associated with IDD.

CLINICAL SIGNIFICANCE

The understanding of chondrocytes as key cells provides new perspectives for deeper elucidation of the pathogenesis of IDD, improved diagnostic methods, and the development of more effective treatments. These findings are expected to provide a more accurate and personalised approach to clinical diagnosis and treatment, thereby improving the prognosis and quality of life of patients with IDD.

Enhancing Cartilage Repair: Surgical Approaches, Orthobiologics, and the Promise of Exosomes

Treating cartilage damage is challenging as its ability for self-regeneration is limited. Left untreated, it can progress to osteoarthritis (OA), a joint disorder characterized by the deterioration of articular cartilage and other joint tissues. Surgical options, such as microfracture and cell/tissue transplantation, have shown promise as techniques to harness the body's endogenous regenerative capabilities to promote cartilage repair. Nonetheless, these techniques have been scrutinized due to reported inconsistencies in long-term outcomes and the tendency for the defects to regenerate as fibrocartilage instead of the smooth hyaline cartilage native to joint surfaces. Orthobiologics are medical therapies that utilize biologically derived substances to augment musculoskeletal healing. These treatments are rising in popularity because of their potential to enhance surgical standards of care. More recent developments in orthobiologics have focused on the role of exosomes in articular cartilage repair. Exosomes are nano-sized extracellular vesicles containing cargo such as proteins, lipids, and nucleic acids, and are known to facilitate intercellular communication, though their regenerative potential still needs to be fully understood. This review aims to demonstrate the advancements in cartilage regeneration, highlight surgical and biological treatment options, and discuss the recent strides in understanding the precise mechanisms of action involved.

Use of Acellular Matrices as Scaffolds in Cartilage Regeneration: A Systematic Review

Significance: Cartilage regeneration remains a significant challenge in the field of regenerative medicine. Acellular matrix (AM)-based cartilage tissue regeneration offers an innovative approach to repairing cartilage defects by providing a scaffold for new tissue growth. Its significance lies in its potential to restore joint function, mitigate pain, and improve the quality of life for patients suffering from cartilage-related injuries and conditions. Recent Advances: Recent advances in AM-based cartilage regeneration have focused on enhancing scaffold properties for improved cell adhesion, proliferation, and differentiation. Moreover, several scaffold techniques such as combining acellular dermal matrix (ADM) and acellular cartilage matrix (ACM) with cartilage tissue, as well as biphasic scaffolding, enjoy rising research activity. Incorporating bioactive factors and advanced manufacturing techniques holds promise for producing more biomimetic scaffolds, advancing efficient cartilage repair and regeneration. Critical Issues: Obstacles in AM-based cartilage regeneration include achieving proper integration with the surrounding tissue and ensuring long-term durability of the regenerated cartilage. Furthermore, issues such as high costs and limited availability of suitable cells for scaffold seeding must be considered. The heterogeneity and limited regenerative capabilities of cartilage need to be addressed for successful clinical translation. Future Directions: Research should focus on exploring advanced biomaterials and developing new techniques, regarding easily reproducible scaffolds, ideally constructed from clinically validated and readily available commercial products. Findings underline the potential of AM-based approaches, especially the rising exploration of tissue-derived ADM and ACM. In future, the primary objective should not only be the regeneration of small cartilage defects but rather focus on fully regenerating a joint or larger cartilage defect.

Bulk RNA-seq analyses of mandibular condylar cartilage in a post-traumatic TMJ osteoarthritis rabbit model

OBJECTIVE

The temporomandibular joint (TMJ) is anatomically comprised of the mandibular condylar cartilage (CC) lined with fibrocartilaginous superficial zone and is crucial for eating and dental occlusion. TMJ osteoarthritis (OA) leads to pain, joint dysfunction and permanent loss of cartilage tissue. However, there are no drugs clinically available that ameliorate OA and little is known about global profiles of genes that contribute to TMJ OA. Furthermore, animal models that recapitulate the complexity of signalling pathways contributing to OA pathogenesis are crucial for designing novel biologics that thwart OA progression. We have previously developed a New Zealand white rabbit TMJ injury model that demonstrates CC degeneration. Here, we performed genome-wide profiling to identify new signalling pathways critical for cellular functions during OA pathology.

MATERIALS AND METHODS

Temporomandibular joint OA was surgically induced in New Zealand white rabbits. Three months following injury, we performed global gene expression profiling of the TMJ condyle. RNA samples from TMJ condyles were subjected to sequencing. After raw RNA-seq data were mapped to relevant genomes, differential expression was analysed with DESeq2. Gene ontology enrichment analysis and Kyoto Encyclopedia of Genes and Genomes pathway analysis were conducted.

RESULTS/CONCLUSIONS

Our study revealed multiple pathways altered during TMJ OA induction including the Wnt, Notch and PI3K-Akt signalling pathways. We demonstrate an animal model that recapitulates the complexity of the cues and signals underlying TMJ OA pathogenesis, which is essential for developing and testing novel pharmacologic agents to treat OA.

Heterogeneous Characteristics of the CD90+ Progenitors in the Fibrocartilage of Different Joints

OBJECTIVE

This study aimed to isolate and compare the mesenchymal stem cell characteristics of CD90+ cells from different fibrocartilage tissues in the temporomandibular joint (TMJ), the knee joint, and the intervertebral joint to further understand the similarities and differences of these 4 fibrocartilage tissues.

METHODS

CD90+ cells were isolated from TMJ disc, condylar cartilage, meniscus, and intervertebral disc by using magnetic-activated cell sorting. Cellular assays including 4.5-ethynyl-2'-deoxyuridine labeling, multilineage differentiation, colony formation, and cell migration were conducted to compare their mesenchymal stem cell characteristics. Immunofluorescent staining was performed for observing the expression of actively proliferating CD90+ cells within the tissues. H&E staining and Safranine O staining were used to compare the histological features.

RESULTS

The CD90+ cells derived from these 4 fibrocartilage tissues exhibited comparable cell proliferation abilities. However, the cells from the TMJ disc displayed limited multilineage differentiation potential, colony formation, and cell migration abilities in comparison with the cells from the other fibrocartilage tissues. In vivo, there was relatively more abundant expression of CD90+ cells in the TMJ disc during the early postnatal stage. The limited EDU+ cell numbers signified a low proliferation capacity of CD90+ cells in the TMJ disc. In addition, we observed a significant decrease in cell density and a restriction in the synthesis of extracellular proteoglycans in the TMJ disc.

CONCLUSION

Our study highlights the spatial heterogeneity of CD90+ cells in the fibrocartilages of different joint tissues, which may contribute to the limited cartilage repair capacity in the TMJ disc.

Discoidin domain receptors; an ancient family of collagen receptors has major roles in bone development, regeneration and metabolism

The extracellular matrix (ECM) niche plays a critical role in determining cellular behavior during bone development including the differentiation and lineage allocation of skeletal progenitor cells to chondrocytes, osteoblasts, or marrow adipocytes. As the major ECM component in mineralized tissues, collagen has instructive as well as structural roles during bone development and is required for bone cell differentiation. Cells sense their extracellular environment using specific cell surface receptors. For many years, specific β1 integrins were considered the main collagen receptors in bone, but, more recently, the important role of a second, more primordial collagen receptor family, the discoidin domain receptors, has become apparent. This review will specifically focus on the roles of discoidin domain receptors in mineralized tissue development as well as related functions in abnormal bone formation, regeneration and metabolism.

The Contribution of the Nrf2/ARE System to Mechanotransduction in Musculoskeletal and Periodontal Tissues

Mechanosensing plays an essential role in maintaining tissue functions. Across the human body, several tissues (i.e., striated muscles, bones, tendons, ligaments, as well as cartilage) require mechanical loading to exert their physiological functions. Contrary, mechanical unloading triggers pathological remodeling of these tissues and, consequently, human body dysfunctions. At the cellular level, both mechanical loading and unloading regulate a wide spectrum of cellular pathways. Among those, pathways regulated by oxidants such as reactive oxygen species (ROS) represent an essential node critically controlling tissue organization and function. Hence, a sensitive balance between the generation and elimination of oxidants keeps them within a physiological range. Here, the Nuclear Factor-E2-related factor 2/Antioxidant response element (Nrf2/ARE) system plays an essential role as it constitutes the major cellular regulation against exogenous and endogenous oxidative stresses. Dysregulations of this system advance, i.a., liver, neurodegenerative, and cancer diseases. Herein, we extend our comprehension of the Nrf2 system to the aforementioned mechanically sensitive tissues to explore its role in their physiology and pathology. We demonstrate the relevance of it for the tissues' functionality and highlight the imperative to further explore the Nrf2 system to understand the physiology and pathology of mechanically sensitive tissues in the context of redox biology.

Interleukin-1β and methylprednisolone acetate demonstrate differential effects on equine deep digital flexor tendon and navicular bone fibrocartilage cells in vitro

OBJECTIVE

To investigate the effects of interleukin-1β (IL-1β) and methylprednisolone acetate (MPA) on equine intrabursal deep digital flexor tendon (DDFT) and navicular bone fibrocartilage (NBF) cells in vitro.

SAMPLE

Third passage DDFT and NBF cells from 5 healthy donor horses ages 11-17 years euthanized for reasons unrelated to musculoskeletal conditions.

PROCEDURES

Aggregate cultures were incubated with culture medium alone (control), 10 ng/mL IL-1β, 10 ng/mL IL-1β + 0.05 mg/mL MPA, or 10 ng/mL IL-1β + 0.5 mg/mL MPA for 24 hours. Extracellular matrix (ECM) gene expressions were assessed via real-time polymerase chain reaction (rtPCR). Culture media matrix metalloproteinase (MMP) -3 and -13 concentrations were quantified via ELISA. Total glycosaminoglycan (GAG) content in the cell pellets and culture media was also assessed.

RESULTS

IL-1β and IL-1β combined with MPA significantly downregulated ECM gene expression to a greater extent in NBF cells compared with DDFT cells. IL-1β and IL-1β combined with MPA significantly upregulated MMP-3 culture media concentrations in DDFT cells only, and MMP-13 culture media concentrations to a greater extent in NBF cells compared with DDFT cells.

CLINICAL RELEVANCE

NBF cells were more susceptible to IL-1β and MPA-mediated ECM gene expression downregulation in vitro. These results serve as a first step for future work to determine intrabursal corticosteroid regimens that limits or resolve the inflammation as well as take into consideration NBF cell biosynthesis in horses with navicular disease, for which currently no information exists.

Anabolic Response of Intermittent Parathyroid Hormone and Alendronate on the Osteochondral Tissue of TMJ

OBJECTIVE

To characterize the effects of parathyroid hormone (PTH) and alendronate (Alend) on the osteochondral tissue of temporomandibular joint (TMJ).

MATERIALS AND METHODS

Ninety-six male and female transgenic reporter mice, 4 to 5 weeks old were divided into 6 groups: (1) Control group: Saline was injected daily for 14 days; (2) PTH: PTH was injected daily for 14 days; (3) Alend: Alend was injected every alternate days for 14 days; (4) Combined PTH and Alend: PTH was injected daily and Alend injected every alternate days for 14 days; (5) PTH then Alend: PTH was injected daily for 14 days followed by Alend injections in alternate days for 14 days; and (6) PTH wait Alend: PTH was injected daily for 14 days. There was a waiting period of 1 week before administration of Alend in alternate days for 14 days. Mice were injected with 5-ethnyl-2'-deoxyuridine (EdU), 48 and 24 hours prior to euthanization.

RESULTS

There was significant increase in bone volume and decrease in osteoclastic activity in groups in which Alend was administered after PTH in both gender. There was significant increase in cartilage thickness with PTH or Alend alone in females, whereas in males, PTH alone led to increase in cartilage thickness. Chondrocyte apoptosis was significantly decreased with PTH or Alend alone in both male and female. Matrix metallopeptidase 13, and aggreganase-2 (ADAMTS5) expression were significantly decreased with PTH and Alend alone in both gender.

CONCLUSION

PTH and Alend administration causes anabolic effects in the osteochondral tissue of TMJ.

Articular fibrocartilage-targeted therapy by microtubule stabilization

The fibrocartilage presented on the joint surface was caused by cartilage injury or degeneration. There is still a lack of effective strategies for fibrocartilage. Here, we hypothesized that the fibrocartilage could be viewed as a raw material for the renewal of hyaline cartilage and proposed a previously unidentified strategy of cartilage regeneration, namely, "fibrocartilage hyalinization." Cytoskeleton remodeling plays a vital role in modifying the cellular phenotype. We identified that microtubule stabilization by docetaxel repressed cartilage fibrosis and increased the hyaline cartilage extracellular matrix. We further designed a fibrocartilage-targeted negatively charged thermosensitive hydrogel for the sustained delivery of docetaxel, which promoted fibrocartilage hyalinization in the cartilage defect model. Moreover, the mechanism of fibrocartilage hyalinization by microtubule stabilization was verified as the inhibition of Sparc (secreted protein acidic and rich in cysteine). Together, our study suggested that articular fibrocartilage-targeted therapy in situ was a promising strategy for hyaline cartilage repair.

The articular cartilage surface is impaired by a loss of thick collagen fibers and formation of type I collagen in early osteoarthritis

Osteoarthritis (OA) is a joint disease affecting millions of patients worldwide. During OA onset and progression, the articular cartilage is destroyed, but the underlying complex mechanisms remain unclear. Here, we uncover changes in the thickness of collagen fibers and their composition at the onset of OA. For articular cartilage explants from knee joints of OA patients, we find that type I collagen-rich fibrocartilage-like tissue was formed in macroscopically intact cartilage, distant from OA lesions. Importantly, the number of thick fibers (>100 nm) has decreased early in the disease, followed by complete absence of thick fibers in advanced OA. We have obtained these results by a combination of high-resolution atomic force microscopy imaging under near-native conditions, immunofluorescence, scanning electron microscopy and a fluorescence-based classification of the superficial chondrocyte spatial organization. Taken together, our data suggests that the loss of tissue functionality in early OA cartilage is caused by a reduction of thick type II collagen fibers, likely due to the formation of type I collagen-rich fibrocartilage, followed by the development of focal defects in later OA stages. We anticipate that such an integrative characterization will be very beneficial for an in-depth understanding of other native biological tissues and the development of sustainable biomaterials. STATEMENT OF SIGNIFICANCE: In early osteoarthritis (OA) the cartilage appears macroscopically intact. However, this study demonstrates that the collagen network already changes in early OA by collagen fiber thinning and the formation of fibrocartilage-like tissue. Both nanoscopic deficiencies already occur in macroscopically intact regions of the human knee joint and are likely connected to processes that result in a weakened extracellular matrix. This study enhances the understanding of earliest progressive cartilage degeneration in the absence of external damage. The results suggest a determination of the mean collagen fiber thickness as a new target for the detection of early OA and a regulation of type I collagen synthesis as a new path for OA treatment.

Histophysiology of Fibrocartilage

There are 3 types of cartilage found in the human body: hyaline cartilage, elastic cartilage, and fibrocartilage. Fibrocartilage may be found in intervertebral discs, symphysis pubis, tendinous insertions, acetabular labrums, and the temporomandibular joint. Specifically, in the foot and ankle we mainly see fibrocartilage in tendinous insertions and in areas where tendons wrap around boney prominence. Histologically, fibrocartilage is comprised of an extracellular matrix that contains glycosaminoglycans, proteoglycans, and collagens. This composition allows for a hydrophilic environment, which allows tissue to withstand high compressive forces seen in weight bearing.

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.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Time course of 3D fibrocartilage formation by expanded human meniscus fibrochondrocytes in hypoxia

Adult human meniscus fibrocartilage is avascular and nonhealing after injury. Meniscus tissue engineering aims to replace injured meniscus with lab-grown fibrocartilage. Dynamic culture systems may be necessary to generate fibrocartilage of sufficient mechanical properties for implantation; however, the optimal static preculture conditions before initiation of dynamic culture are unknown. This study thus investigated the time course of fibrocartilage formation by human meniscus fibrochondrocytes on a three-dimensional biomaterial scaffold under various static conditions. Human meniscus fibrochondrocytes from partial meniscectomy were expanded to passage 1 (P1) or P2 (3.0 ± 0.4 and 6.5 ± 0.6 population doublings), seeded onto type I collagen scaffolds, and grown in hypoxia (HYP, 3% O₂ ) or normoxia (NRX, 20% O₂ ) for 3, 6, and 9 weeks. Mechanical properties were not different between P1 and P2 cell-based constructs. Mechanical properties were lower in HYP, increased continually in NRX only, and were positively correlated with glycosaminoglycan content and accumulation of hyaline cartilage-like matrix components. The most mechanically competent tissues (NRX/9 weeks) reached 1/5 of the native meniscus instantaneous compression modulus but had an increasingly hypertrophic matrix-forming phenotype. HYP consistently suppressed the hypertrophic phenotype. The results provide baselines of engineered meniscus fibrocartilage properties under static conditions, which can be used to select a preculture strategy for dynamic culture depending on the desired combination of mechanical properties, hyaline cartilage-like matrix abundance, and hypertrophic phenotype.

An updated view on Temporomandibular Joint degeneration: insights from the cell subsets of mandibular condylar cartilage

The high prevalence of temporomandibular joint osteoarthritis (TMJOA), which causes joint dysfunction, indicates the need for more effective methods for treatment and repair. Mandibular condylar cartilage (MCC), a typical fibrocartilage that experiences degenerative changes during the development of TMJOA, has become a research focus and therapeutic target in recent years. MCC is composed of four zones of cells at various stages of differentiation. The cell subsets in MCC exhibit different physiological and pathological characteristics during development and in TMJOA. Most studies of TMJOA are mainly concerned with gene regulation of pathological changes. The corresponding treatment targets with specific cell subsets in MCC may provide more accurate and reliable results for cartilage repair and TMJOA treatment. In this review, we summarized the current research progress on the cell subsets of MCC from the perspective of MCC development and degeneration. We hope to provide a reference for further exploration of the pathological process of TMJOA and improvement of TMJOA treatment.

Cartilage Tissue Engineering Approaches Need to Assess Fibrocartilage When Hydrogel Constructs Are Mechanically Loaded

Chondrocytes that are impregnated within hydrogel constructs sense applied mechanical force and can respond by expressing collagens, which are deposited into the extracellular matrix (ECM). The intention of most cartilage tissue engineering is to form hyaline cartilage, but if mechanical stimulation pushes the ratio of collagen type I (Col1) to collagen type II (Col2) in the ECM too high, then fibrocartilage can form instead. With a focus on Col1 and Col2 expression, the first part of this article reviews the latest studies on hyaline cartilage regeneration within hydrogel constructs that are subjected to compression forces (one of the major types of the forces within joints) in vitro. Since the mechanical loading conditions involving compression and other forces in joints are difficult to reproduce in vitro, implantation of hydrogel constructs in vivo is also reviewed, again with a focus on Col1 and Col2 production within the newly formed cartilage. Furthermore, mechanotransduction pathways that may be related to the expression of Col1 and Col2 within chondrocytes are reviewed and examined. Also, two recently-emerged, novel approaches of load-shielding and synchrotron radiation (SR)–based imaging techniques are discussed and highlighted for future applications to the regeneration of hyaline cartilage. Going forward, all cartilage tissue engineering experiments should assess thoroughly whether fibrocartilage or hyaline cartilage is formed.

Imaging features of cartilaginous tumors of the head and neck

There is a wide spectrum of head and neck cartilaginous lesions which include both neoplastic and nonneoplastic processes. Cartilaginous tumors of the head and neck are uncommon, posing a diagnostic challenge. Benign cartilaginous tumors that may occur in the head and neck include chondroma, chondroblastoma, chondromyxoid fibroma, osteochondroma, and synovial chondromatosis. Chondromesenchymal hamartoma is a rare non-neoplastic cartilaginous lesion that is included for the 1first time in the new WHO classification and radiologically can mimic a tumor. Malignant cartilaginous tumors include chondrosarcoma and chondroid variant of chordoma. Characteristic tumor locations, internal chondroid matrix calcification, and typical T2 hyperintense signal secondary to high-water content within the extracellular matrix of the hyaline cartilage are useful imaging features that narrow the differential diagnosis and help in diagnosing these diseases. This article presents a narrative review of the anatomy of the head and neck cartilaginous structures, discusses the current knowledge and imaging spectrum of benign and malignant cartilaginous tumors and tumor-like lesions of the head and neck.

Anatomy and histomorphology of the flexor digitorum profundus enthesis: functional implications for tissue engineering and surgery

Background

The enthesis possesses morphological adaptations across the soft-hard tissue junction which are not fully restored during surgical avulsion repairs. This loss of anatomical structure, highly related to function, contributes to poor clinical outcomes. Investigating the native macro- and micro-structure of a specific enthesis can provide functional and biomechanical insights to develop specialised, novel tissue-engineered therapeutic options and potentially improve current surgical treatments for avulsion injuries.

Methods

This study examines the anatomy and histomorphology of the flexor digitorum profundus (FDP) enthesis in 96 fresh-frozen human cadaveric fingers, quantitatively and qualitatively analyzing the shape, size, angle of tendon fibres and histological architecture, and explores differences in sex, finger and distance along the enthesis using linear mixed effects models.

Results

Macroscopically, results showed a consistent trapezoidal insertion shape of 29.29 ± 2.35 mm² mean surface area, but with significant morphometric size differences influenced primarily by the smaller dimensions of the little finger. Microscopically, a fibrocartilaginous enthesis was apparent with a 30.05 ± 0.72^o mean angle of inserting tendon fibres, although regional variation in fibrocartilage and the angle change of tendon fibres before insertion existed.

Conclusions

The implication of these findings on native and specific FDP enthesis function is discussed whilst providing recommendations for optimal FDP enthesis recreation for interfacial tissue engineers and hand surgeons. The study emphasizes the importance of region-specific knowledge whilst also describing methods applicable to assessing any soft tissue insertion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04922-1.

BMP2 Is Required for Postnatal Maintenance of Osteochondral Tissues of the Temporomandibular Joint

OBJECTIVE

Bone morphogenetic protein 2 (BMP2) plays important roles in cartilage growth and development. Paradoxically, elevated levels of BMP2 leads to hypertrophic differentiation and osteoarthritis of cartilage. We examined the in vivo loss of BMP2 in cells expressing aggrecan of the mandibular condyle and knee.

DESIGN

Three-week-old BMP2 flox/flox-CreER-positive mice and their Cre-negative littermates were treated with tamoxifen and raised until 3 or 6 months. We also investigated the direct effects of BMP2 on chondrocytes in vitro. Cells from the mandibular condyle of mice were treated with recombinant human BMP2 (rhBMP2) or rhNoggin (inhibitor of BMP2 signaling).

RESULTS

Conditional deletion of BMP2 caused breakage of the cartilage integrity in the mandibular condyle of mice from both age groups, accompanied by a decrease in cartilage thickness, matrix synthesis, mineralization, chondrocyte proliferation, and increased expression of degeneration markers, while the effects at articular cartilage were not significant. In vitro results revealed that rhBMP2 increased chondrocyte proliferation, mineralization, and differentiation, while noggin induced opposite effects.

CONCLUSIONS

In conclusion, BMP2 is essential for postnatal maintenance of the osteochondral tissues of the mandibular condyle.

The role of tendon derived stem/progenitor cells and extracellular matrix components in the bone tendon junction repair

Fibrocartilage enthesis is the junction between bone and tendon with a typical characteristics of fibrocartilage transition zones. The regeneration of this transition zone is the bottleneck for functional restoration of bone tendon junction (BTJ). Biomimetic approaches, especially decellularized extracellular matrix (ECM) materials, are strategies which aim to mimic the components of tissues to the utmost extent, and are becoming popular in BTJ healing because of their ability not only to provide scaffolds to allow cells to attach and migrate, but also to provide a microenvironment to guide stem/progenitor cells lineage-specific differentiation. However, the cellular and molecular mechanisms of those approaches, especially the ECM proteins, remain unclear. For BTJ reconstruction, fibrocartilage regeneration is the key for good integrity of bone and tendon as well as its mechanical recovery, so the components which can guide stem cells to a chondrogenic commitment in biomimetic approaches might well be the key for fibrocartilage regeneration and eventually for the better BTJ healing. In this review, we firstly discuss the importance of cartilage-like formation in the healing process of BTJ. Next, we explore the possibility of tendon-derived stem/progenitor cells as cell sources for BTJ regeneration due to their multi-differentiation potential. Finally, we summarize the role of extracellular matrix components of BTJ in guiding stem cell fate to a chondrogenic commitment, so as to provide cues for understanding the mechanisms of lineage-specific potential of biomimetic approaches as well as to inspire researchers to incorporate unique ECM components that facilitate BTJ repair into design.

Testing Hypoxia in Pig Meniscal Culture: Biological Role of the Vascular-Related Factors in the Differentiation and Viability of Neonatal Meniscus

Menisci play an essential role in shock absorption, joint stability, load resistance and its transmission thanks to their conformation. Adult menisci can be divided in three zones based on the vascularization: an avascular inner zone with no blood supply, a fully vascularized outer zone, and an intermediate zone. This organization, in addition to the incomplete knowledge about meniscal biology, composition, and gene expression, makes meniscal regeneration still one of the major challenges both in orthopedics and in tissue engineering. To overcome this issue, we aimed to investigate the role of hypoxia in the differentiation of the three anatomical areas of newborn piglet menisci (anterior horn (A), central body (C), and posterior horn (P)) and its effects on vascular factors. After sample collection, menisci were divided in A, C, P, and they were cultured in vitro under hypoxic (1% O₂) and normoxic (21% O₂) conditions at four different experimental time points (T0 = day of explant; T7 = day 7; T10 = day 10; T14 = day 14); samples were then evaluated through immune, histological, and molecular analyses, cell morpho-functional characteristics; with particular focus on matrix composition and expression of vascular factors. It was observed that hypoxia retained the initial phenotype of cells and induced extracellular matrix production resembling a mature tissue. Hypoxia also modulated the expression of angiogenic factors, especially in the early phase of the study. Thus, we observed that hypoxia contributes to the fibro-chondrogenic differentiation with the involvement of angiogenic factors, especially in the posterior horn, which corresponds to the predominant weight-bearing portion.

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.

Senolytics alleviate the degenerative disorders of temporomandibular joint in old age

Aging is one of the major risk factors for degenerative joint disorders, including those involving the temporomandibular joint (TMJ). TMJ degeneration occurs primarily in the population over 65, significantly increasing the risk of joint discomfort, restricted joint mobility, and reduced quality of life. Unfortunately, there is currently no effective mechanism‐based treatment available in the clinic to alleviate TMJ degeneration with aging. We now demonstrate that intermittent administration of senolytics, drugs which can selectively clear senescent cells, preserved mandibular condylar cartilage thickness, improved subchondral bone volume and turnover, and reduced Osteoarthritis Research Society International (OARSI) histopathological score in both 23‐ to 24‐month‐old male and female mice. Senolytics had little effect on 4 months old young mice, indicating age‐specific benefits. Our study provides proof‐of‐concept evidence that age‐related TMJ degeneration can be alleviated by pharmaceutical intervention targeting cellular senescence. Since the senolytics used in this study have been proven relatively safe in recent human studies, our findings may help justify future clinical trials addressing TMJ degeneration in old age.

Negative impact of disuse and unloading on tendon enthesis structure and function

Exposure to chronic skeletal muscle disuse and unloading that astronauts experience results in muscle deconditioning and bone remodeling. Tendons involved in the transmission of force from muscles to skeleton are also affected. Understanding the changes that occur in muscle, tendon, and bone is an essential step toward limiting or preventing the deleterious effects of chronic reduction in mechanical load. Numerous reviews have reported the effects of this reduction on both muscle and bone, and to a lesser extent on the tendon. However, none focused on the tendon enthesis, the tendon-to-bone attachment site. While the enthesis structure appears to be determined by mechanical stress, little is known about enthesis plasticity. Our review first looks at the relationship between entheses and mechanical stress, exploring how tensile and compressive loads determine and influence enthesis structure and composition. The second part of this review addresses the deleterious effects of skeletal muscle disuse and unloading on enthesis structure, composition, and function. We discuss the possibility that spaceflight-induced enthesis remodeling could impact both the capacity of the enthesis to withstand compressive stress and its potential weakness. Finally, we point out how altered compressive strength at entheses could expose astronauts to the risk of developing enthesopathies.

Fibrocartilage Stem Cells in the Temporomandibular Joint: Insights From Animal and Human Studies

Temporomandibular disorders (TMD) are diseases involving the temporomandibular joint (TMJ), masticatory muscles, and osseous components. TMD has a high prevalence, with an estimated 4.8% of the U.S. population experiencing signs and symptoms, and represents a financial burden to both individuals and society. During TMD progression, the most frequently affected site is the condylar cartilage. Comprising both fibrous and cartilaginous tissues, condylar cartilage has restricted cell numbers but lacks a vascular supply and has limited regenerative properties. In 2016, a novel stem cell niche containing a reservoir of fibrocartilage stem cells (FCSCs) was discovered in the condylar cartilage of rats. Subsequently, FCSCs were identified in mouse, rabbit, and human condylar cartilage. Unlike mesenchymal stem cells or other tissue-specific stem/progenitor cells, FCSCs play a unique role in the development and regeneration of fibrocartilage. More importantly, engraftment treatment of FCSCs has been successfully applied in animal models of TMD. In this context, FCSCs play a major role in the regeneration of newly formed cartilage. Furthermore, FCSCs participate in the regeneration of intramembranous bone by interacting with endothelial cells in bone defects. This evidence highlights the potential of FCSCs as an ideal stem cell source for the regeneration of oral maxillofacial tissue. This review is intended to detail the current knowledge of the characteristics and function of FCSCs in the TMJ, as well as the potential therapeutic applications of FCSCs. A deep understanding of the properties of FCSCs can thus inform the development of promising, biologically based strategies for TMD in the future.

Understanding Early-Stage Posttraumatic Osteoarthritis for Future Prospects of Diagnosis: from Knee to Temporomandibular Joint

PURPOSE OF REVIEW

Many mechanical load-bearing joints of the body are prone to posttraumatic osteoarthritis (PTOA), including the knee joint and temporomandibular joint (TMJ). Early detection of PTOA can be beneficial in prevention or alleviating further progression of the disease.

RECENT FINDINGS

Various mouse models, similar to those used in development of novel diagnosis strategies for early stages of OA, have been proposed to study early PTOA. While many studies have focused on OA and PTOA in the knee joint, early diagnostic methods for OA and PTOA of the TMJ are still not well established. Previously, we showed that fluorescent near-infrared imaging can diagnose inflammation and cartilage damage in mouse models of knee PTOA. Here we propose that the same approach can be used for early diagnosis of TMJ-PTOA. In this review, we present a brief overview of PTOA, application of relevant mouse models, current imaging methods available to examine TMJ-PTOA, and the prospects of near-infrared optical imaging to diagnose early-stage TMJ-OA.

Trends in Articular Cartilage Tissue Engineering: 3D Mesenchymal Stem Cell Sheets as Candidates for Engineered Hyaline-Like Cartilage

Articular cartilage defects represent an inciting factor for future osteoarthritis (OA) and degenerative joint disease progression. Despite multiple clinically available therapies that succeed in providing short term pain reduction and restoration of limited mobility, current treatments do not reliably regenerate native hyaline cartilage or halt cartilage degeneration at these defect sites. Novel therapeutics aimed at addressing limitations of current clinical cartilage regeneration therapies increasingly focus on allogeneic cells, specifically mesenchymal stem cells (MSCs), as potent, banked, and available cell sources that express chondrogenic lineage commitment capabilities. Innovative tissue engineering approaches employing allogeneic MSCs aim to develop three-dimensional (3D), chondrogenically differentiated constructs for direct and immediate replacement of hyaline cartilage, improve local site tissue integration, and optimize treatment outcomes. Among emerging tissue engineering technologies, advancements in cell sheet tissue engineering offer promising capabilities for achieving both in vitro hyaline-like differentiation and effective transplantation, based on controlled 3D cellular interactions and retained cellular adhesion molecules. This review focuses on 3D MSC-based tissue engineering approaches for fabricating “ready-to-use” hyaline-like cartilage constructs for future rapid in vivo regenerative cartilage therapies. We highlight current approaches and future directions regarding development of MSC-derived cartilage therapies, emphasizing cell sheet tissue engineering, with specific focus on regulating 3D cellular interactions for controlled chondrogenic differentiation and post-differentiation transplantation capabilities.

3D Bioprinted Highly Elastic Hybrid Constructs for Advanced Fibrocartilaginous Tissue Regeneration

Advanced strategies to bioengineer a fibrocartilaginous tissue to restore the function of the meniscus are necessary. Currently, 3D bioprinting technologies have been employed to fabricate clinically relevant patient-specific complex constructs to address unmet clinical needs. In this study, a highly elastic hybrid construct for fibrocartilaginous regeneration is produced by co-printing a cell-laden gellan gum/fibrinogen (GG/FB) composite bioink together with a silk fibroin methacrylate (Sil-MA) bioink in an interleaved crosshatch pattern. We characterize each bioink formulation by measuring the rheological properties, swelling ratio, and compressive mechanical behavior. For in vitro biological evaluations, porcine primary meniscus cells (pMCs) are isolated and suspended in the GG/FB bioink for the printing process. The results show that the GG/FB bioink provides a proper cellular microenvironment for maintaining the cell viability and proliferation capacity, as well as the maturation of the pMCs in the bioprinted constructs, while the Sil-MA bioink offers excellent biomechanical behavior and structural integrity. More importantly, this bioprinted hybrid system shows the fibrocartilaginous tissue formation without a dimensional change in a mouse subcutaneous implantation model during the 10-week postimplantation. Especially, the alignment of collagen fibers is achieved in the bioprinted hybrid constructs. The results demonstrate this bioprinted mechanically reinforced hybrid construct offers a versatile and promising alternative for the production of advanced fibrocartilaginous tissue.

Viscoelasticity of hydrazone crosslinked poly(ethylene glycol) hydrogels directs chondrocyte morphology during mechanical deformation

Chondrocyte deformation influences disease progression and tissue regeneration in load-bearing joints. In this work, we found that viscoelasticity of dynamic covalent crosslinks temporally modulates the biophysical transmission of physiologically relevant compressive strains to encapsulated chondrocytes. Chondrocytes in viscoelastic alky-hydrazone hydrogels demonstrated (91.4 ± 4.5%) recovery of native rounded morphologies during mechanical deformation, whereas primarily elastic benzyl-hydrazone hydrogels significantly limited morphological recovery (21.2 ± 1.4%).

Atelocollagen promotes chondrogenic differentiation of human adipose-derived mesenchymal stem cells

Effective engineering approaches for cartilage regeneration involve a combination of cells and biomaterial scaffolds. Multipotent mesenchymal stem cells (MSCs) are important sources for cartilage regeneration. Atelocollagen provides a suitable substrate for MSC attachment and enhancing chondrogenic differentiation. Here, we assessed the chondrogenic potential of adipose tissue derived human MSCs (hMSCs) mixed with atelocollagen gel. We observed cell attachment, viability, and microstructures by electron microscopy over 21 days. The levels of Sox9, type II collagen, aggrecan, type I collagen, Runx2, type X collagen, ALP, Osterix, and MMP13 were measured by RT-qPCR. Cartilage matrix-related proteins were assessed by enzyme-linked immunosorbent assay (ELISA), histology, and immunohistochemistry. hMSCs of all groups exhibited well-maintained cell survival, distribution and morphology. Abundant type II collagen fibers developed on day 21; while Sox9, type II collagen, and aggrecan expression increased over time in the atelocollagen group. However, type I collagen, RUNX2, type X collagen (CoL10A1), Osterix, and ALP were not expressed. These results corroborated the protein expression detected by ELISA. Further, histological analysis revealed lacunae-like structures, while staining demonstrated glycosaminoglycan accumulation. Cumulatively, these results indicate that atelocollagen scaffolds improve hMSC chondrogenic differentiation and are a potential approach for cartilage regeneration.

Adult chondrogenesis and spontaneous cartilage repair in the skate, Leucoraja erinacea

Mammalian articular cartilage is an avascular tissue with poor capacity for spontaneous repair. Here, we show that embryonic development of cartilage in the skate (Leucoraja erinacea) mirrors that of mammals, with developing chondrocytes co-expressing genes encoding the transcription factors Sox5, Sox6 and Sox9. However, in skate, transcriptional features of developing cartilage persist into adulthood, both in peripheral chondrocytes and in cells of the fibrous perichondrium that ensheaths the skeleton. Using pulse-chase label retention experiments and multiplexed in situ hybridization, we identify a population of cycling Sox5/6/9+ perichondral progenitor cells that generate new cartilage during adult growth, and we show that persistence of chondrogenesis in adult skates correlates with ability to spontaneously repair cartilage injuries. Skates therefore offer a unique model for adult chondrogenesis and cartilage repair and may serve as inspiration for novel cell-based therapies for skeletal pathologies, such as osteoarthritis.

Aortic heterogeneity across segments and under high fat/salt/glucose conditions at the single-cell level

The aorta, with ascending, arch, thoracic and abdominal segments, responds to the heartbeat, senses metabolites and distributes blood to all parts of the body. However, the heterogeneity across aortic segments and how metabolic pathologies change it are not known. Here, a total of 216 612 individual cells from the ascending aorta, aortic arch, and thoracic and abdominal segments of mouse aortas under normal conditions or with high blood glucose levels, high dietary salt, or high fat intake were profiled using single-cell RNA sequencing. We generated a compendium of 10 distinct cell types, mainly endothelial (EC), smooth muscle (SMC), stromal and immune cells. The distributions of the different cells and their intercommunication were influenced by the hemodynamic microenvironment across anatomical segments, and the spatial heterogeneity of ECs and SMCs may contribute to differential vascular dilation and constriction that were measured by wire myography. Importantly, the composition of aortic cells, their gene expression profiles and their regulatory intercellular networks broadly changed in response to high fat/salt/glucose conditions. Notably, the abdominal aorta showed the most dramatic changes in cellular composition, particularly involving ECs, fibroblasts and myeloid cells with cardiovascular risk factor-related regulons and gene expression networks. Our study elucidates the nature and range of aortic cell diversity, with implications for the treatment of metabolic pathologies.

Emerging Potential of Exosomes in Regenerative Medicine for Temporomandibular Joint Osteoarthritis

Exosomes are nanosized vesicles (30–140 nm) of endocytic origin that play important roles in regenerative medicine. They are derived from cell membranes during endocytic internalization and stabilize in biological fluids such as blood and synovia. Temporomandibular joint osteoarthritis (TMJ OA) is a degenerative disease, which, in addition to chronic pain, is characterized by progressive cartilage breakdown, condylar bone remodeling, and synovitis. However, traditional clinical treatments have limited symptom- and structure-modifying effects to restore damaged cartilage and other TMJ tissues. This is due to the limited self-healing capacity of condylar cartilage. Recently, stem-cell-derived exosomes have been studied as an alternative therapeutic approach to tissue repair and regeneration. It is known that trophic regulation of mesenchymal stem cells (MSCs) has anti-inflammatory and immunomodulatory effects under pathological conditions, and research on MSC-derived exosomes is rapidly accumulating. MSC-derived exosomes mimic the major therapeutic effects of MSCs. They affect the activity of immune effector cells and possess multilineage differentiation potential, including chondrogenic and osteogenic differentiation. Furthermore, exosomes are capable of regenerating cartilage or osseous compartments and restoring injured tissues and can treat dysfunction and pain caused by TMJ OA. In this review, we looked at the uniqueness of TMJ, the pathogenesis of TMJ OA, and the potential role of MSC-derived exosomes for TMJ cartilage and bone regeneration.

Temporomandibular Joint Bioengineering Conference: Working Together Toward Improving Clinical Outcomes

The sixth temporomandibular joint (TMJ) Bioengineering Conference (TMJBC) was held on June 14-15 2018, in Redondo Beach, California, 12 years after the first TMJBC. Speakers gave 30 presentations and came from the United States, Europe, Asia, and Australia. The goal of the conference has remained to foster a continuing forum for bioengineers, scientists, and surgeons and veterinarians to advance technology related to TMJ disorders. These collective multidisciplinary interactions over the past decade have made large strides in moving the field of TMJ research forward. Over the past 12 years, in vivo approaches for tissue engineering have emerged, along with a wide variety of degeneration models, as well as with models occurring in nature. Furthermore, biomechanical tools have become more sensitive and new biologic interventions for disease are being developed. Clinical directives have evolved for specific diagnoses, along with patient-specific biological and immunological responses to TMJ replacement devices alloplastic and/or bioengineered devices. The sixth TMJBC heralded many opportunities for funding agencies to advance the field: (1) initiatives on TMJ that go beyond pain research, (2) more training grants focused on graduate students and fellows, (3) partnership funding with government agencies to translate TMJ solutions, and (4) the recruitment of a critical mass of TMJ experts to participate on grant review panels. The TMJ research community continues to grow and has become a pillar of dental and craniofacial research, and together we share the unified vision to ultimately improve diagnoses and treatment outcomes in patients affected by TMJ disorders.

Crosstalk between Fas and S1P1 signaling via NF-kB in osteoclasts controls bone destruction in the TMJ due to rheumatoid arthritis

Rheumatoid arthritis (RA) mainly affects various joints of the body, including the temporomandibular joint (TMJ), and it involves an infiltration of autoantibodies and inflammatory leukocytes into articular tissues and the synovium. Initially, the synovial lining tissue becomes engaged with several kinds of infiltrating cells, including osteoclasts, macrophages, lymphocytes, and plasma cells. Eventually, bone degradation occurs. In order to elucidate the best therapy for RA, a comprehensive study of RA pathogenesis needs to be completed. In this article, we discuss a Fas-deficient condition which develops into RA, with an emphasis on the role of sphingosine 1-phosphate (S1P)/S1P receptor 1 signaling which induces the migration of osteoclast precursor cells. We describe that Fas/S1P1 signaling via NF-κB activation in osteoclasts is a key factor in TMJ-RA severity and we discuss a strategy for blocking nuclear translocation of the p50 NF-κB subunit as a potential therapy for attenuating osteoclastogenesis.

Fibrochondrogenic differentiation potential of tendon-derived stem/progenitor cells from human patellar tendon

Background

Bone–tendon junction (BTJ) is a unique structure connecting tendon and bone through a fibrocartilage zone. Owing to its unique structure, the regeneration of BTJ remains a challenge. Here, we study the fibrochondrogenic differentiation of human tendon-derived stem/progenitor cells (TSPCs) both in vitro and in vivo.

Methods

TSPCs were isolated from human patellar tendon tissues and investigated for their multidifferentiation potential. TSPCs were cultured in chondrogenic medium with transforming growth factor beta 3 (TGF-β3) and BMP-2 in vitro ​and examined for the expression of fibrochondrogenic marker genes by quantitative real-time reverse transcription polymerase chain reaction, enzyme-linked immunosorbent assay, and immunofluorescence. TSPCs pretreated were also seeded in collage II sponge and then transplanted in immunocompromised nude mice to examine if the fibrochondrogenic characteristics were conserved in vivo.

Results

We found that TSPCs were differentiated towards fibrochondrogenic lineage, accompanied by the expression of collagen I, collagen II, SRY-box transcription factor 9 (Sox 9), and tenascin C. Furthermore, after TSPCs were seeded in collagen II sponge and transplanted in immunocompromised nude mice, they expressed fibrochondrogenic genes, including proteoglycan, collagen I, and collagen II.

Conclusion

Taken together, this study showed that TSPCs are capable of differentiating towards fibrocartilage-like cells, and the fibrochondrogenic characteristics were conserved even in vivo, and thus might have the potential application for fibrocartilage regeneration in BTJ repair.

The translational potential of this article

TSPCs are able to differentiate into fibrocartilage-like cells and thus might well be one potential cell source for fibrocartilage regeneration in a damaged BTJ repair.

Rotator Cuff Tenocytes Differentiate into Hypertrophic Chondrocyte-Like Cells to Produce Calcium Deposits in an Alkaline Phosphatase-Dependent Manner

Calcific tendonitis is a frequent cause of chronic shoulder pain. Its cause is currently poorly known. The objectives of this study were to better characterize the cells and mechanisms involved in depositing apatite crystals in human tendons. Histologic sections of cadaveric calcified tendons were analyzed, and human calcific deposits from patients undergoing lavage of their calcification were obtained to perform infrared spectroscopy and mass spectrometry-based proteomic characterizations. In vitro, the mineralization ability of human rotator cuff cells from osteoarthritis donors was assessed by alizarin red or Von Kossa staining. Calcifications were amorphous areas surrounded by a fibrocartilaginous metaplasia containing hypertrophic chondrocyte-like cells that expressed tissue non-specific alkaline phosphatase (TNAP) and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), which are two key enzymes of the mineralization process. Calcific deposits were composed of apatite crystals associated with proteins involved in bone and cartilage development and endochondral bone growth. In vitro, tenocyte-like cells extracted from the rotator cuff were able to mineralize in osteogenic cultures, and expressed TNAP, type X COLLAGEN, and MMP13, which are hypertrophic chondrocytes markers. The use of a TNAP inhibitor significantly prevented mineral deposits. We provide evidence that tenocytes have a propensity to differentiate into hypertrophic chondrocyte-like cells to produce TNAP-dependent calcium deposits. We believe that these results may pave the way to identifying regulating factors that might represent valuable targets in calcific tendonitis.

An Anatomical Study of the Anterosuperior Capsular Attachment Site on the Acetabulum

BACKGROUND

Despite the fact that many surgeons perform partial capsular detachment from the anterosuperior aspect of the acetabulum to correct acetabular deformities during hip arthroscopy, few studies have focused on whether these detachments influence hip joint stability. The aim of this study was to investigate the capsular attachment on the anterosuperior aspect of the acetabulum. We hypothesized that the attachment on the inferior aspect of the anterior inferior iliac spine (AIIS) is wide and fibrocartilaginous and might have a substantial role in hip joint stability.

METHODS

Fifteen hips from 9 cadavers of Japanese donors were analyzed. Eleven hips were analyzed macroscopically, and the other 4 were analyzed histologically. In all specimens, the 3-dimensional morphology of the acetabulum and AIIS was examined using micro-computed tomography (micro-CT).

RESULTS

Macroscopic analysis showed that the widths of the capsular attachments varied according to the location, and the attachment width on the inferior edge of the AIIS was significantly larger than that on the anterosuperior aspect of the acetabulum. Moreover, the capsular attachment on the inferior edge of the AIIS corresponded with the impression, which was identified by micro-CT. Histological analysis revealed that the hip joint capsule on the inferior edge of the AIIS attached to the acetabulum adjacent to the proximal margin of the labrum. In addition, the hip joint capsule attached to the inferior edge of the AIIS via the fibrocartilage.

CONCLUSIONS

The capsular attachment on the inferior edge of the AIIS was characterized by an osseous impression, large attachment width, and distributed fibrocartilage.

CLINICAL RELEVANCE

It appeared that the capsular attachment on the inferior edge of the AIIS was highly adaptive to mechanical stress, on the basis of its osseous impression, attachment width, and histological features. Anatomical knowledge of the capsular attachment on the inferior edge of the AIIS provides a better understanding of the pathological condition of hip joint instability.

The effects on the mandibular condyle of Botox injection into the masseter are not transient

OBJECTIVES

To evaluate whether the effects on the mandibular condylar cartilage (MCC) and subchondral bone are transient of botulinum neurotoxin (Botox) injection into the masseter muscle.

METHODS

Botox (0.3 U) was injected into the right masseter of 6-week-old female mice (C57BL/6; n = 16). In addition, 16 mice were used as control and received no injections. Experimental and matching control mice were killed 4 or 8 weeks after the single Botox injection. Mandibles and mandibular condyles were analyzed by means of microscopic computed tomography (microCT) and histology. Sagittal sections of condyles were stained for tartrate-resistant acid phosphatase (TRAP), toluidine blue, 5-ethynyl-2'-deoxyuridine (EdU), and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling.

RESULTS

Bone volume fraction was significantly decreased on the subchondral bone of the Botox-injected side, compared with the control side and control mice, 4 and 8 weeks after injection. Furthermore, histologic analysis revealed decrease in mineralization, cartilage thickness, TRAP activity, and EdU-positive cells in the MCC of the Botox-injected side 4 and 8 weeks after injection.

CONCLUSIONS

The effects on the MCC and subchondral bone of Botox injection into the masseter muscle persisted for 8 weeks after injection and were not considered to be transient.

Articular fibrocartilage - Why does hyaline cartilage fail to repair?

Once damaged, articular cartilage has a limited potential to repair. Clinically, a repair tissue is formed, yet, it is often mechanically inferior fibrocartilage. The use of monolayer expanded versus naïve cells may explain one of the biggest discrepancies in mesenchymal stromal/stem cell (MSC) based cartilage regeneration. Namely, studies utilizing monolayer expanded MSCs, as indicated by numerous in vitro studies, report as a main limitation the induction of type X collagen and hypertrophy, a phenotype associated with endochondral bone formation. However, marrow stimulation and transfer studies report a mechanically inferior collagen I/II fibrocartilage as the main outcome. Therefore, this review will highlight the collagen species produced during the different therapeutic approaches. New developments in scaffold design and delivery of therapeutic molecules will be described. Potential future directions towards clinical translation will be discussed. New delivery mechanisms are being developed and they offer new hope in targeted therapeutic delivery.

Sox9+ messenger cells orchestrate large-scale skeletal regeneration in the mammalian rib

Most bones in mammals display a limited capacity for natural large-scale repair. The ribs are a notable exception, yet the source of their remarkable regenerative ability remains unknown. Here, we identify a Sox9-expressing periosteal subpopulation that orchestrates large-scale regeneration of murine rib bones. Deletion of the obligate Hedgehog co-receptor, Smoothened, in Sox9-expressing cells prior to injury results in a near-complete loss of callus formation and rib bone regeneration. In contrast to its role in development, Hedgehog signaling is dispensable for the proliferative expansion of callus cells in response to injury. Instead, Sox9-positive lineage cells require Hh signaling to stimulate neighboring cells to differentiate via an unknown signal into a skeletal cell type with dual chondrocyte/osteoblast properties. This type of callus cell may be critical for bridging large bone injuries. Thus despite contributing to only a subset of callus cells, Sox9-positive progenitors play a major role in orchestrating large-scale bone regeneration.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

The Zonal Architecture of the Mandibular Condyle Requires ADAMTS5

Temporomandibular joint (TMJ) osteoarthritis (TMJOA) disrupts extracellular matrix (ECM) homeostasis, leading to cartilage degradation. Upregulated a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-5 leads to cleavage of its substrate aggrecan (Acan) and is considered a hallmark of TMJOA. However, most research on ADAMTS5-Acan turnover has focused on hyaline cartilage, not fibrocartilage, which comprises the TMJ. The mandibular condylar cartilage (MCC) of the TMJ is organized in zones, and chondrocytes are arranged in axial rows, yet the molecular mechanisms required to generate the MCC zonal architecture have not been elucidated. Here, we test the hypothesis that ADAMTS5 is required for development of the TMJ MCC. Adamts5+/+ and Adamts5-/- murine TMJs were harvested at postnatal day 7 (P7), P21, 2 mo, and 6 mo of age; histomorphometrics indicated increased ECM. Immunohistochemistry and Western blots demonstrated the expanded ECM correlated with increased Acan localization in Adamts5-/- compared to Adamts5+/+. Cell volume was also decreased in the MCC of Adamts5-/- due to both a reduction in cell size and less mature hypertrophic chondrocytes. Analysis of chondrogenic maturation markers by quantitative real-time polymerase chain reaction indicated Col2a1, Col10a1, and Sox9 were significantly reduced in Adamts5-/- MCC compared to that of Adamts5+/+. The older (6 mo) Adamts5-/- MCC exhibited changes in chondrogenic cell arrangements, including clustering and chondrogenic atrophy, that correlated with early stages of TMJOA using modified Mankin scoring. These data indicate a potentially novel and critical role of ADAMTS5 for maturation of hypertrophic chondrocytes and establishment of the zonal architecture that, when disrupted, may lead to early onset of TMJOA.

Recent advances on 3D printing of patient-specific implants for fibrocartilage tissue regeneration

Fibrocartilage is a very peculiar type of tissue found in intervertebral disc and meniscus. It is characterized by its avascular nature and for the shear and compressive forces that it can be subjected to. The number of individuals affected by the degeneration of fibrocartilaginous tissues has been growing and the poor outcomes of current treatments have led to an increased interest in new alternative approaches. Therefore, the combination of reverse engineering with 3D printing has been extensively explored in order to produce patient-specific implants capable of improving the current clinical outcomes. This review outlines the recent advances achieved in the tissue engineering field, especially focusing on fibrocartilaginous tissue.

PTH [1-34]-induced alterations predispose the mandibular condylar cartilage to mineralization

OBJECTIVE

To study the effects of intermittent parathyroid hormone (PTH [1-34]) on the mandibular condylar cartilage (MCC) and subchondral bone in adult female mice.

MATERIALS AND METHODS

Twenty-two, 20-week-old female mice were used for in vivo experiments. The experimental mice (n=11) received daily intraperitoneal injections of PTH [1-34] for 3 weeks, while control mice (n=11) received intraperitoneal injections of 0.9% saline solution. Mice were euthanized and then micro-computed tomography (micro-CT); histology and immunostaining were carried out to assess the response.

RESULTS

Intermittent PTH [1-34] led to early MCC breakdown and surface irregularities. Micro-CT analyses indicated that PTH [1-34] treatment led to increased bone volume fraction, tissue density and trabecular thickness, while decreasing the trabecular spacing. Histological analyses showed decreased proteoglycan secretion, increased bone turnover (TRAP staining) and increased mineralization. Furthermore, PTH [1-34] treatment showed increased apoptosis of the cells. Our immunohistochemistry showed increased expression of pSMAD158 in the MCC and subchondral bone with PTH administration, whereas sclerostin (SOST) expression was decreased.

CONCLUSIONS

Intermittent PTH [1-34] results in early mineralization of the MCC, which may result in cartilage degeneration. Our results identified a novel mechanism by which PTH [1-34] induces alteration in the microarchitecture of the MCC and the subchondral bone.

Pericellular collagen I coating for enhanced homing and chondrogenic differentiation of mesenchymal stem cells in direct intra-articular injection

Background

Direct intra-articular injection (DIAI) of mesenchymal stem cells (MSCs) is a promising technique for cartilage repair. However, the repair process was hindered by the absence of scaffold and poor cell–matrix interactions.

Methods

In this study, we developed a pericellular collagen I coating (PCC) on MSCs. The overall performances of MSC-PCC homing, chondrogenic differentiation, and cartilage regeneration have been comprehensively evaluated in a New Zealand rabbit model. Firstly, we examined the morphology and physical characteristics of PCC. Secondly, MSC ex-vivo cartilage slice adhesion and in-vivo cartilage defect homing were observed using multiscale methods. Thirdly, the precartilage condensation of cell pellets formed by aggregation of MSCs was examined to evaluate the cartilage-inducing potential of PCC. Finally, the cartilage regeneration by DIAI of PCC-coated MSCs was observed and scored macroscopically and histologically.

Results

In general, the cell adhesion and homing assay revealed that PCC facilitated MSC adhesion on cartilage slices, enhancing MSC homing and retention to cartilage defect. This increased homing ratio was accompanied by an increasing cell–cell contact. Compared with naked MSCs, the cell pellets formed by PCC-coated MSCs exhibited more evident appearance of condensation. In pellets, cell–cell interaction has been significantly stimulated, inducing the expression of condensation marker N-cadherin, and subsequent chondrogenic marker collagen II and aggrecan. By 12 weeks after DIAI, cartilage defects have been repaired by MSCs to varying degrees. Overall, PCC significantly enhances the quality of cartilage regeneration judging from macroscopic observation, ICRS score, histological examination, and collagen type I, II, and X immunohistochemical staining.

Conclusions

The capacity and viability of MSCs can be enhanced by collagen I coating, which provides cues for enhancing cell homing and differentiation. Our method provides a novel strategy for stem cell therapy.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0916-z) contains supplementary material, which is available to authorized users.

Quality Assessment of Surgical Disc Samples Discriminates Human Annulus Fibrosus and Nucleus Pulposus on Tissue and Molecular Level

A discrimination of the highly specialised annulus fibrosus (AF) and nucleus pulposus (NP) cells in the mature human intervertebral disc (IVD) is thus far still not possible in a reliable way. The aim of this study was to identify molecular markers that distinguish AF and NP cells in human disc tissue using microarray analysis as a screening tool. AF and NP samples were obtained from 28 cervical discs. First, all samples underwent quality sorting using two novel scoring systems for small-sized disc tissue samples including macroscopic, haptic and histological evaluation. Subsequently, samples with clear disc characteristics of either AF or NP that were free from impurities of foreign tissue (IVD score) and with low signs of disc degeneration on cellular level (DD score) were selected for GeneChip analysis (HGU1332P). The 11 AF and 9 NP samples showed distinctly different genome-wide transcriptomes. The majority of differentially expressed genes (DEGs) could be specifically assigned to the AF, whereas no DEG was exclusively expressed in the NP. Nevertheless, we identified 11 novel marker genes that clearly distinguished AF and NP, as confirmed by quantitative gene expression analysis. The novel established scoring systems and molecular markers showed the identity of AF and NP in disc starting material and are thus of great importance in the quality assurance of cell-based therapeutics in regenerative treatment of disc degeneration.