Identification of superficial zone articular chondrocyte stem/progenitor cells

The Actin Cytoskeleton as a Regulator of Proteoglycan 4

OBJECTIVE

The superficial zone (SZ) of articular cartilage is responsible for distributing shear forces for optimal cartilage loading and contributes to joint lubrication through the production of proteoglycan 4 (PRG4). PRG4 plays a critical role in joint homeostasis and is chondroprotective. Normal PRG4 production is affected by inflammation and irregular mechanical loading in post-traumatic osteoarthritis (PTOA). THe SZ chondrocyte (SZC) phenotype, including PRG4 expression, is regulated by the actin cytoskeleton in vitro. There remains a limited understanding of the regulation of PRG4 by the actin cytoskeleton in native articular chondrocytes. The filamentous (F)-actin cytoskeleton is a potential node in crosstalk between mechanical stimulation and cytokine activation and the regulation of PRG4 in SZCs, therefore developing insights in the regulation of PRG4 by actin may identify molecular targets for novel PTOA therapies.

MATERIALS AND METHODS

A comprehensive literature search on PRG4 and the regulation of the SZC phenotype by actin organization was performed.

RESULTS

PRG4 is strongly regulated by the actin cytoskeleton in isolated SZCs in vitro. Biochemical and mechanical stimuli have been characterized to regulate PRG4 and may converge upon actin cytoskeleton signaling.

CONCLUSION

Actin-based regulation of PRG4 in native SZCs is not fully understood and requires further elucidation. Understanding the regulation of PRG4 by actin in SZCs requires an in vivo context to further potential of leveraging actin arrangement to arthritic therapeutics.

Harnessing knee joint resident mesenchymal stem cells in cartilage tissue engineering

Osteoarthritis (OA) is a widespread clinical disease characterized by cartilage degeneration in middle-aged and elderly people. Currently, there is no effective treatment for OA apart from total joint replacement in advanced stages. Mesenchymal stem cells (MSCs) are a type of adult stem cell with diverse differentiation capabilities and immunomodulatory potentials. MSCs are known to effectively regulate the cartilage microenvironment, promote cartilage regeneration, and alleviate OA symptoms. As a result, they are promising sources of cells for OA therapy. Recent studies have revealed the presence of resident MSCs in synovial fluid, synovial membrane, and articular cartilage, which can be collected as knee joint-derived MSCs (KJD-MSC). Several preclinical and clinical studies have demonstrated that KJD-MSCs have great potential for OA treatment, whether applied alone, in combination with biomaterials, or as exocrine MSCs. In this article, we will review the characteristics of MSCs in the joints, including their cytological characteristics, such as proliferation, cartilage differentiation, and immunomodulatory abilities, as well as the biological function of MSC exosomes. We will also discuss the use of tissue engineering in OA treatment and introduce the concept of a new generation of stem cell-based tissue engineering therapy, including the use of engineering, gene therapy, and gene editing techniques to create KJD-MSCs or KJD-MSC derivative exosomes with improved functionality and targeted delivery. These advances aim to maximize the efficiency of cartilage tissue engineering and provide new strategies to overcome the bottleneck of OA therapy. STATEMENT OF SIGNIFICANCE: This research will provide new insights into the medicinal benefit of Joint resident Mesenchymal Stem Cells (MSCs), specifically on its cartilage tissue engineering ability. Through this review, the community will further realize promoting joint resident mesenchymal stem cells, especially cartilage progenitor/MSC-like progenitor cells (CPSC), as a preventive measure against osteoarthritis and cartilage injury. People and medical institutions may also consider cartilage derived MSC as an alternative approach against cartilage degeneration. Moreover, the discussion presented in this study will convey valuable information for future research that will explore the medicinal benefits of cartilage derived MSC.

The emerging studies on mesenchymal progenitors in the long bone

Mesenchymal progenitors (MPs) are considered to play vital roles in bone development, growth, bone turnover, and repair. In recent years, benefiting from advanced approaches such as single-cell sequence, lineage tracing, flow cytometry, and transplantation, multiple MPs are identified and characterized in several locations of bone, including perichondrium, growth plate, periosteum, endosteum, trabecular bone, and stromal compartment. However, although great discoveries about skeletal stem cells (SSCs) and progenitors are present, it is still largely obscure how the varied landscape of MPs from different residing sites diversely contribute to the further differentiation of osteoblasts, osteocytes, chondrocytes, and other stromal cells in their respective destiny sites during development and regeneration. Here we discuss recent findings on MPs' origin, differentiation, and maintenance during long bone development and homeostasis, providing clues and models of how the MPs contribute to bone development and repair.

Nanoengineered hydrogels as 3D biomimetic extracellular matrix with injectable and sustained delivery capability for cartilage regeneration

The regeneration of articular cartilage remains a great challenge due to the difficulty in effectively enhancing spontaneous healing. Recently, the combination of implanted stem cells, suitable biomaterials and bioactive molecules has attracted attention for tissue regeneration. In this study, a novel injectable nanocomposite was rationally designed as a sustained release platform for enhanced cartilage regeneration through integration of a chitosan-based hydrogel, articular cartilage stem cells (ACSCs) and mesoporous SiO₂ nanoparticles loaded with anhydroicaritin (AHI). The biocompatible engineered nanocomposite acting as a novel 3D biomimetic extracellular matrix exhibited a remarkable sustained release effect due to the synergistic regulation of the organic hydrogel framework and mesopore channels of inorganic mSiO₂ nanoparticles (mSiO₂ NPs). Histological assessment and biomechanical tests showed that the nanocomposites exhibited superior performance in inducing ACSCs proliferation and differentiation in vitro and promoting extracellular matrix (ECM) production and cartilage regeneration in vivo. Such a novel multifunctional biocompatible platform was demonstrated to significantly enhance cartilage regeneration based on the sustained release of AHI, an efficient bioactive natural small molecule for ACSCs chondrogenesis, within the hybrid matrix of hydrogel and mSiO₂ NPs. Hence, the injectable nanocomposite holds great promise for use as a 3D biomimetic extracellular matrix for tissue regeneration in clinical diagnostics.

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The anhydroicaritin (AHI) was identified as a bioactive factor for promoting cartilage repair.

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The hydrogel was designed to achieve sustained AHI release and optimize the microenvironment of cartilage defect sites.

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The hydrogel exhibited superior advantages for chondrogenic differentiation and cartilage regeneration.

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The hydrogel holds a great promise for use as functional scaffold for tissue and organ regeneration in the future.

Lin28a induces SOX9 and chondrocyte reprogramming via HMGA2 and blunts cartilage loss in mice

Articular cartilage has low regenerative capacity despite permanent stress. Irreversible cartilage lesions characterize osteoarthritis (OA); this is not followed by tissue repair. Lin28a, an RNA binding protein, is detected in damaged cartilage in humans and mice. We investigated the role of LIN28a in cartilage physiology and in osteoarthritis. Lin28a-inducible conditional cartilage deletion up-regulated Mmp13 in intact mice and exacerbated the cartilage destruction in OA mice. Lin28a-specific cartilage overexpression protected mice against cartilage breakdown, stimulated chondrocyte proliferation and the expression of Prg4 and Sox9, and down-regulated Mmp13. Lin28a overexpression inhibited Let-7b and Let-7c miRNA levels while RNA-sequencing analysis revealed five genes of transcriptional factors regulated by Let-7. Moreover, Lin28a overexpression up-regulated HMGA2 and activated SOX9 transcription, a factor required for chondrocyte reprogramming. HMGA2 siRNA knockdown inhibited the cartilage protective effect of Lin28a overexpression. This study provides insights into a new pathway including the Lin28a-Let7 axis, thus promoting chondrocyte anabolism in injured cartilage in mice.

Senescence-Associated Cell Transition and Interaction (SACTAI): A Proposed Mechanism for Tissue Aging, Repair, and Degeneration

Aging is a broad process that occurs as a time-dependent functional decline and tissue degeneration in living organisms. On a smaller scale, aging also exists within organs, tissues, and cells. As the smallest functional unit in living organisms, cells “age” by reaching senescence where proliferation stops. Such cellular senescence is achieved through replicative stress, telomere erosion and stem cell exhaustion. It has been shown that cellular senescence is key to tissue degradation and cell death in aging-related diseases (ARD). However, senescent cells constitute only a small percentage of total cells in the body, and they are resistant to death during aging. This suggests that ARD may involve interaction of senescent cells with non-senescent cells, resulting in senescence-triggered death of non-senescent somatic cells and tissue degeneration in aging organs. Here, based on recent research evidence from our laboratory and others, we propose a mechanism—Senescence-Associated Cell Transition and Interaction (SACTAI)—to explain how cell heterogeneity arises during aging and how the interaction between somatic cells and senescent cells, some of which are derived from aging somatic cells, results in cell death and tissue degeneration.

The clinical potential of articular cartilage-derived progenitor cells: a systematic review

Over the past two decades, evidence has emerged for the existence of a distinct population of endogenous progenitor cells in adult articular cartilage, predominantly referred to as articular cartilage-derived progenitor cells (ACPCs). This progenitor population can be isolated from articular cartilage of a broad range of species, including human, equine, and bovine cartilage. In vitro, ACPCs possess mesenchymal stromal cell (MSC)-like characteristics, such as colony forming potential, extensive proliferation, and multilineage potential. Contrary to bone marrow-derived MSCs, ACPCs exhibit no signs of hypertrophic differentiation and therefore hold potential for cartilage repair. As no unique cell marker or marker set has been established to specifically identify ACPCs, isolation and characterization protocols vary greatly. This systematic review summarizes the state-of-the-art research on this promising cell type for use in cartilage repair therapies. It provides an overview of the available literature on endogenous progenitor cells in adult articular cartilage and specifically compares identification of these cell populations in healthy and osteoarthritic (OA) cartilage, isolation procedures, in vitro characterization, and advantages over other cell types used for cartilage repair. The methods for the systematic review were prospectively registered in PROSPERO (CRD42020184775).

Collagen Growth Pattern in Human Articular Cartilage of the Knee

OBJECTIVE

During skeletal growth, the articular cartilage expands to maintain its cover of bones in joints, however, it is unclear when and how cartilage grows. We aim to determine the expanding growth pattern and timing across the tibia plateau in human knees.

DESIGN

Six human tibia plateaus (2 healthy, 2 with osteoarthritis, and 2 with posttraumatic osteoarthritis) were used for full-depth cartilage sampling systematically across the joint surface at 12 medial and 4 lateral sites. Methodologically, we took advantage of the performed nuclear bomb tests in the years 1955 to 1963, which increased the atmospheric ¹⁴C that was incorporated into human tissues. Cartilage was treated enzymatically to extract collagen, analyzed for ¹⁴C content, and year at formation was determined from historical atmospheric ¹⁴C concentrations.

RESULTS

By age-determination, each tibia condyle had central points of formation surrounded by later-formed cartilage toward the periphery. Furthermore, the tibia plateaus contained collagen with ¹⁴C levels corresponding to mean donor age of 11.7 years (±3.8 SD). Finally, the medial condyle had lower ¹⁴C levels corresponding to formation 1 year later than the lateral condyle (P = 0.009).

CONCLUSIONS

Human cartilage on the tibia plateau contains collagen that has experienced little if any turnover since school-age. The cartilage formation develops from 2 condyle centers and radially outward with the medial condyle finishing slightly later than the lateral condyle. This suggests a childhood programmed cartilage formation with a very limited adulthood collagen turnover.

Human adipose-derived stromal/stem cells expressing doublecortin improve cartilage repair in rabbits and monkeys

Localized cartilage lesions in early osteoarthritis and acute joint injuries are usually treated surgically to restore function and relieve pain. However, a persistent clinical challenge remains in how to repair the cartilage lesions. We expressed doublecortin (DCX) in human adipose-derived stromal/stem cells (hASCs) and engineered hASCs into cartilage tissues using an in vitro 96-well pellet culture system. The cartilage tissue constructs with and without DCX expression were implanted in the knee cartilage defects of rabbits (n = 42) and monkeys (n = 12). Cohorts of animals were euthanized at 6, 12, and 24 months after surgery to evaluate the cartilage repair outcomes. We found that DCX expression in hASCs increased expression of growth differentiation factor 5 (GDF5) and matrilin 2 in the engineered cartilage tissues. The cartilage tissues with DCX expression significantly enhanced cartilage repair as assessed macroscopically and histologically at 6, 12, and 24 months after implantation in the rabbits and 24 months after implantation in the monkeys, compared to the cartilage tissues without DCX expression. These findings suggest that hASCs expressing DCX may be engineered into cartilage tissues that can be used to treat localized cartilage lesions.

Type V Collagen Regulates the Structure and Biomechanics of TMJ Condylar Cartilage: A Fibrous-Hyaline Hybrid

This study queried the role of type V collagen in the post-natal growth of temporomandibular joint (TMJ) condylar cartilage, a hybrid tissue with a fibrocartilage layer covering a secondary hyaline cartilage layer. Integrating outcomes from histology, immunofluorescence imaging, electron microscopy and atomic force microscopy-based nanomechanical tests, we elucidated the impact of type V collagen reduction on TMJ condylar cartilage growth in the type V collagen haploinsufficiency and inducible knockout mice. Reduction of type V collagen led to significantly thickened collagen fibrils, decreased tissue modulus, reduced cell density and aberrant cell clustering in both the fibrous and hyaline layers. Post-natal growth of condylar cartilage involves the chondrogenesis of progenitor cells residing in the fibrous layer, which gives rise to the secondary hyaline layer. Loss of type V collagen resulted in reduced proliferation of these cells, suggesting a possible role of type V collagen in mediating the progenitor cell niche. When the knockout of type V collagen was induced in post-weaning mice after the start of physiologic TMJ loading, the hyaline layer exhibited pronounced thinning, supporting an interplay between type V collagen and occlusal loading in condylar cartilage growth. The phenotype in hyaline layer can thus be attributed to the impact of type V collagen on the mechanically regulated progenitor cell activities. In contrast, knee cartilage does not contain the progenitor cell population at post-natal stages, and develops normal structure and biomechanical properties with the loss of type V collagen. Therefore, in the TMJ, in addition to its established role in regulating the assembly of collagen I fibrils, type V collagen also impacts the mechanoregulation of progenitor cell activities in the fibrous layer. We expect such knowledge to establish a foundation for understanding condylar cartilage matrix development and regeneration, and to yield new insights into the TMJ symptoms in patients with classic Ehlers-Danlos syndrome, a genetic disease due to autosomal mutation of type V collagen.

Comparison of the major cell populations among osteoarthritis, Kashin-Beck disease and healthy chondrocytes by single-cell RNA-seq analysis

Chondrocytes are the key target cells of the cartilage degeneration that occurs in Kashin–Beck disease (KBD) and osteoarthritis (OA). However, the heterogeneity of articular cartilage cell types present in KBD and OA patients and healthy controls is still unknown, which has prevented the study of the pathophysiology of the mechanisms underlying the roles of different populations of chondrocytes in the processes leading to KBD and OA. Here, we aimed to identify the transcriptional programmes and all major cell populations in patients with KBD, patients with OA and healthy controls to identify the markers that discriminate among chondrocytes in these three groups. Single-cell RNA sequencing was performed to identify chondrocyte populations and their gene signatures in KBD, OA and healthy cells to investigate their differences as related to the pathogenetic mechanisms of these two osteochondral diseases. We performed immunohistochemistry and quantitative reverse-transcription PCR (qRT-PCR) assays to validate the markers for chondrocyte population. Ten clusters were labelled by cell type according to the expression of previously described markers, and one novel population was identified according to the expression of a new set of markers. The homeostatic and mitochondrial chondrocyte populations, which were identified by the expression of the unknown markers MT1X and MT2A and MT-ND1 and MT-ATP6, were markedly expanded in KBD. The regulatory chondrocyte population, identified by the expression of CHI3L1, was markedly expanded in OA. Our study allows us to better understand the heterogeneity of chondrocytes in KBD and OA and provides new evidence of differences in the pathogenetic mechanisms between these two diseases.

Coaction of TGF-β1 and CDMP1 in BMSCs-induced laryngeal cartilage repair in rabbits

Bone marrow mesenchymal stem cells (BMSCs) are well-known for tissue regeneration and bone repair. This study intended to evaluate the potential efficiency BMSCs in poly(lactide-co-glycolide) (PLGA) scaffolds for the treatment of laryngeal cartilage defects. BMSCs were isolated and identified, and added with 10 ng/mL transforming growth factor-beta1 (TGF-β1) or/and 300 ng/mL CDMP1 to coculture with PLGA scaffolds. The chondrogenic differentiation, migration, and apoptosis of BMSCs were detected under the action of TGF-β1 or/and CDMP1. After successful modeling of laryngeal cartilage defects, PLGA scaffolds were transplanted into the rabbits correspondingly. After 8 weeks, laryngeal cartilage defects were assessed. Levels of collagen II, aggrecan, Sox9, Smad2, Smad3, ERK, and JNK were detected. The TGF-β1 or/and CDMP1-induced BMSCs expressed collagen II, aggrecan, and Sox9, with enhanced cell migration and inhibited apoptosis. In addition, laryngeal cartilage defect in rabbits with TGF-β1 or/and CDMP1 was alleviated, and levels of specific cartilage matrix markers were decreased. The combined effects of TGF-β1 and CDMP1 were more significant. The TGF-β1/Smad and ERK/JNK pathways were activated after TGF-β1 or/and CDMP1 were added to BMSCs or rabbits. In summary, BMSCs and PLGA scaffolds repair laryngeal cartilage defects in rabbits by activating the TGF-β1/Smad and ERK/JNK pathways under the coaction of TGF-β1 and CDMP1.

Senescent Mesenchymal Stem Cells: Disease Mechanism and Treatment Strategy

Purpose of review

Mesenchymal stem cells (MSCs) have been extensively studied for therapeutic application in tissue engineering and regenerative medicine. Despite their promise, recent findings suggest that MSC replication during repair process may lead to replicative senescence and stem cell exhaustion. Here, we review the basic mechanisms of MSC senescence, how it leads to degenerative diseases, and potential treatments for such diseases.

Recent findings

Emerging evidence has shown a link between senescent MSCs and degenerative diseases, especially age-related diseases such as osteoarthritis and idiopathic pulmonary fibrosis. During these disease processes, MSCs undergo cell senescence and mediate Senescence Associated Secretory Phenotypes (SASP) to affect the surrounding microenvironment. Thus, senescent MSCs can accelerate tissue aging by increasing the number of senescent cells and spreading inflammation to neighboring cells.

Summary

Senescent MSCs not only hamper tissue repair through cell senescence associated stem cell exhaustion, but also mediate tissue degeneration by initiating and spreading senescence-associated inflammation. It suggests new strategies of MSC-based cell therapy to remove, rejuvenate, or replace (3Rs) the senescent MSCs.

Characterization of heterogeneous primary human cartilage-derived cell population using non-invasive live-cell phase-contrast time-lapse imaging

Reliable and reproducible cell therapy strategies to treat osteoarthritis demand an improved characterization of the cell and heterogeneous cell population resident in native cartilage tissue. Using live-cell phase-contrast time-lapse imaging (PC-TLI), this study investigates the morphological attributes and biological performance of the three primary biological objects enzymatically isolated from primary human cartilage: connective tissue progenitors (CTPs), non-progenitors (NPs) and multi-cellular structures (MCSs). The authors' results demonstrated that CTPs were smaller in size in comparison to NPs (P < 0.001). NPs remained part of the adhered cell population throughout the cell culture period. Both NPs and CTP progeny on day 8 increased in size and decreased in circularity in comparison to their counterparts on day 1, although the percent change was considerably less in CTP progeny (P < 0.001). PC-TLI analyses indicated three colony types: single-CTP-derived (29%), multiple-CTP-derived (26%) and MCS-derived (45%), with large heterogeneity with respect to cell morphology, proliferation rate and cell density. On average, clonal (CL) (P = 0.009) and MCS (P = 0.001) colonies exhibited higher cell density (cells per colony area) than multi-clonal (MC) colonies; however, it is interesting to note that the behavior of CL (less cells per colony and less colony area) and MCS (high cells per colony and high colony area) colonies was quite different. Overall effective proliferation rate (EPR) of the CTPs that formed CL colonies was higher than the EPR of CTPs that formed MC colonies (P = 0.02), most likely due to CTPs with varying EPR that formed the MC colonies. Finally, the authors demonstrated that lag time before first cell division of a CTP (early attribute) could potentially help predict its proliferation rate long-term. Quantitative morphological characterization using non-invasive PC-TLI serves as a reliable and reproducible technique to understand cell heterogeneity. Size and circularity parameters can be used to distinguish CTP from NP populations. Morphological cell and colony features can also be used to reliably and reproducibly identify CTP subpopulations with preferred proliferation and differentiation potentials in an effort to improve cell manufacturing and therapeutic outcomes.

Identification of human temporomandibular joint fibrocartilage stem cells with distinct chondrogenic capacity

OBJECTIVE

This study was aimed to identify the residence of human fibrocartilage stem cells (hFCSCs), characterize their stem cell properties and investigate the functional mechanisms which regulate fibrocartilage stem cells (FCSCs) toward chondrogenic differentiation during cartilage homeostasis and repairing.

METHODS

Cytological characteristics of hFCSCs and human orofacial mesenchymal stem cells (hOFMSCs) were analyzed. Chondrogenic potential of hFCSCs was compared with hOFMSCs both in vitro and in vivo. Regulatory role of SOX9 during FCSCs chondrogenesis was studied by shRNA interference in vitro, and by GFP⁺ FCSCs treatment in rat condylar cartilage defect model. SOX9 expression was also examined in temporomandibular joint osteoarthritis (TMJOA) patients' cartilage surface.

RESULTS

hFCSCs exhibited typical mesenchymal stem cell characteristics, with significantly stronger chondrogenic capability compared to hOFMSCs. Moreover, hFCSCs showed remarkably increased expression of SOX9. During cartilage pellet culture, there was stronger SOX9 expression in hFCSCs than hOFMSCs. SOX9 shRNA interference downregulated chondrogenic capability of hFCSCs in vitro, as well as disrupting migration and chondrogenic differentiation of GFP⁺ FCSCs toward mature chondrocytes in rat condylar cartilage defect. Of note, SOX9 expression was also found suppressed in the condylar superficial zone of TMJOA patients.

CONCLUSION

We found the existence of FCSCs in human TMJ cartilage, and characterized their distinct stem cell features. SOX9 is essential for hFCSCs chondrogenic differentiation, and a comprehensive understanding of the regulatory role of SOX9 in hFCSCs would be important for exploring potential intervention strategy of condylar cartilage degradation during TMJ disorders.

Expression and function of cartilage-derived pluripotent cells in joint development and repair

Cartilage-derived pluripotent cells reside in hyaline cartilage and fibrocartilage. These cells have the potential for multidirectional differentiation; can undergo adipogenesis, osteogenesis, and chondrogenesis; and have been classified as mesenchymal stem cells (MSCs) conforming to the minimal criteria of the International Society for Cellular Therapy. Cartilage tissue is prone to injury and is difficult to repair. As cartilage-derived pluripotent cells are the closest cell source to cartilage tissue, they are expected to have the strongest ability to differentiate into cartilage compared to other MSCs. This review focuses on the organizational distribution, expression, and function of cartilage-derived pluripotent cells in joint development and repair to help explore the therapeutic potential of in situ cartilage-derived pluripotent cells for joint cartilage repair.

Single-cell mass cytometry reveals cross-talk between inflammation-dampening and inflammation-amplifying cells in osteoarthritic cartilage

Aging or injury leads to degradation of the cartilage matrix and the development of osteoarthritis (OA). Because of a paucity of single-cell studies of OA cartilage, little is known about the interpatient variability in its cellular composition and, more importantly, about the cell subpopulations that drive the disease. Here, we profiled healthy and OA cartilage samples using mass cytometry to establish a single-cell atlas, revealing distinct chondrocyte progenitor and inflammation-modulating subpopulations. These rare populations include an inflammation-amplifying (Inf-A) population, marked by interleukin-1 receptor 1 and tumor necrosis factor receptor II, whose inhibition decreased inflammation, and an inflammation-dampening (Inf-D) population, marked by CD24, which is resistant to inflammation. We devised a pharmacological strategy targeting Inf-A and Inf-D cells that significantly decreased inflammation in OA chondrocytes. Using our atlas, we stratified patients with OA in three groups that are distinguished by the relative proportions of inflammatory to regenerative cells, making it possible to devise precision therapeutic approaches.

Articular cartilage regeneration: The role of endogenous mesenchymal stem/progenitor cell recruitment and migration

BACKGROUND

Trauma- or osteoarthritis-related cartilage damage resulted in functional decline of joints and heavy burden of public health. Recently, the reparative role of mesenchymal stem/progenitor cells (MSCs) in articular cartilage (AC) reconstruction is drawing more and more attention.

OBJECTIVE

To provide a review on (1) the locations and categories of joint-resident MSCs, (2) the regulation of chondrogenic capacities of MSCs, (3) the migratory approaches of MSCs to diseased AC and regulatory mechanisms.

METHODS

PubMed and Web of Science were searched for English-language articles related to MSC recruitment and migration for AC repair until June 2019. The presence of various MSCs in or around joints, the potential approaches to diseased AC` and the regenerative capacities of MSCs were reviewed.

RESULTS

Various intra- and peri-articular MSCs, with inherent migratory potentials, are present in multiple stem cell niches in or around joints. The recruitment and migration of joint-resident MSCs play crucial roles in endogenous AC repair. Multiple recruiting signals, such as chemokines, growth factors, etc., emerge during the development of AC diseases and participate in the regulation of MSC mobilization. Motivated MSCs could migrate into cartilage lesions and then exert multiple reparative potentials, including extracellular matrix (ECM) reconstruction and microenvironment modulation.

CONCLUSION

In general, AC repair based on endogenous MSC recruitment and migration is a feasible strategy, and a promising research field. Furthermore, endogenous AC repair mediated by native MSCs would provide new opportunities to efficient preventative or therapeutic options for AC diseases.

Chondrogenic induction of human osteoarthritic cartilage-derived mesenchymal stem cells activates mineralization and hypertrophic and osteogenic gene expression through a mechanomiR

BACKGROUND

While bone marrow-derived mesenchymal stem cells (BMSC) are established sources for stem cell-based cartilage repair therapy, articular cartilage-derived mesenchymal stem cells from osteoarthritis patients (OA-MSC) are new and potentially attractive candidates. We compared OA-MSC and BMSC in chondrogenic potentials, gene expression, and regulation by miR-365, a mechanical-responsive microRNA in cartilage (Guan et al., FASEB J 25: 4457-4466, 2011).

METHODS

To overcome the limited number of OA-MSC, a newly established human OA-MSC cell line (Jayasuriya et al., Sci Rep 8: 7044, 2018) was utilized for analysis and comparison to BMSC. Chondrogenesis was induced by the chondrogenic medium in monolayer cell culture. After chondrogenic induction, chondrogenesis and mineralization were assessed by Alcian blue and Alizarin red staining respectively. MiRNA and mRNA levels were quantified by real-time PCR while protein levels were determined by western blot analysis at different time points. Immunohistochemistry was performed with cartilage-specific miR-365 over-expression transgenic mice.

RESULTS

Upon chondrogenic induction, OA-MSC underwent rapid chondrogenesis in comparison to BMSC as shown by Alcian blue staining and activation of ACAN and COL2A1 gene expression. Chondrogenic induction also activated mineralization and the expression of hypertrophic and osteogenic genes in OA-MSC while only hypertrophic genes were activated in BMSC. MiR-365 expression was activated by chondrogenic induction in both OA-MSC and BMSC. Transfection of miR-365 in OA-MSC induced chondrogenic, hypertrophic, and osteogenic genes expression while miR-365 inhibition suppressed the expression of these genes. Over-expression of miR-365 upregulated markers of OA-MSC and hypertrophy and increased OA scores in adult mouse articular cartilage.

CONCLUSIONS

Induction of chondrogenesis can activate mineralization, hypertrophic, and osteogenic genes in OA-MSC. MiR-365 appears to be a master regulator of these differentiation processes in OA-MSC during OA pathogenesis. These findings have important implications for cartilage repair therapy using cartilage derived stem cells from OA patients.

Chondrogenic Progenitor Cells Exhibit Superiority Over Mesenchymal Stem Cells and Chondrocytes in Platelet-Rich Plasma Scaffold-Based Cartilage Regeneration

BACKGROUND

Platelet-rich plasma (PRP) has been considered a promising tool for cartilage regeneration. However, increasing evidence has demonstrated the controversial effects of PRP on tissue regeneration, partially due to the unsatisfactory cell source. Chondrogenic progenitor cells (CPCs) have gained increasing attention as a potential cell source due to their self-renewal and multipotency, especially toward the chondrogenic lineage, and, thus, may be an appropriate alternative for cartilage engineering.

PURPOSE

To compare the effects of PRP on CPC, mesenchymal stem cell (MSC), and chondrocyte proliferation, chondrogenesis, and cartilage regeneration.

STUDY DESIGN

Controlled laboratory study.

METHODS

Whole blood samples were obtained from 5 human donors to create PRPs (0, 1000 × 10⁹, and 2000 × 10⁹ platelets per liter). The proliferation and chondrogenesis of CPCs, bone marrow-derived MSCs (BMSCs), and chondrocytes were evaluated via growth kinetic and CCK-8 assays. Immunofluorescence, cytochemical staining, and gene expression analyses were performed to assess chondrogenic differentiation and cartilaginous matrix formation. The in vivo effects of CPCs, BMSCs, and chondrocytes on cartilage regeneration after PRP treatment were measured by use of histopathological, biochemical, and biomechanical techniques in a cartilage defect model involving mature male New Zealand White rabbits (critical size, 5 mm).

RESULTS

The CPCs possessed migration abilities and proliferative capacities superior to those of the chondrocytes, while exhibiting a chondrogenic predisposition stronger than that of the BMSCs. The growth kinetic, CCK-8, cytochemical staining, and biochemical analyses revealed that the CPCs simultaneously displayed a higher cell density than the chondrocytes and stronger chondrogenesis than the BMSCs after PRP stimulation. In addition, the in vivo study demonstrated that the PRP+CPC construct yielded better histological (International Cartilage Repair Society [ICRS] score, mean ± SEM, 1197.2 ± 163.2) and biomechanical (tensile modulus, 1.523 ± 0.194) results than the PRP+BMSC (701.1 ± 104.9, P < .05; 0.791 ± 0.151, P < .05) and PRP+chondrocyte (541.6 ± 98.3, P < .01; 0.587 ± 0.142, P < .01) constructs at 12 weeks after implantation.

CONCLUSION

CPCs exhibit superiority over MSCs and chondrocytes in PRP scaffold-based cartilage regeneration, and PRP+CPC treatment may be a favorable strategy for cartilage repair.

CLINICAL RELEVANCE

These findings provide evidence highlighting the preferable role of CPCs as a cell source in PRP-mediated cartilage regeneration and may help researchers address the problem of unsatisfactory cell sources in cartilage engineering.

Agili-C implant promotes the regenerative capacity of articular cartilage defects in an ex vivo model

PURPOSE

Osteochondral implants are currently adopted for the treatment of symptomatic full-thickness chondral and osteochondral defects. Agili-C™ is a cell-free aragonite-based scaffold which aims to reproduce the original structure and function of the articular joint while directing the growth and regeneration of both cartilage and its underlying subchondral bone. The goal of the present study was to investigate the ex vivo mechanisms of action (MOA) of the Agili-C™ implant in the repair of full-thickness cartilage defects. In particular, we tested whether Agili-C™ implant has the potential to stimulate cartilage ingrowth through chondrocytes migration into the 3D interconnected porous structure of the scaffold, along with maintaining their viability and phenotype and the deposition of hyaline cartilage matrix.

METHODS

Articular cartilage samples were collected through the Gift of Hope Organ and Tissue Donor Network (Itasca, IL) within 24 h from death. For this study, cartilage from a total of 14 donors was used. To model a chondral defect, donut-shaped cartilage explants were prepared from each tissue specimen. The chondral phase of the Agili-C™ implant was placed inside the tissue in full contact and press fit manner. Cartilage explants with the Agili-C™ implant inside were cultured for 60 days. As a control, the same donut-shaped cartilage explants were cultured without Agili-C™, under the same culture conditions.

RESULTS

Using fresh human cadaveric articular cartilage tissue in a 60-day culture, it was demonstrated that chondrocytes were able to migrate into the Agili-C™ scaffold and contribute to the deposition of the extracellular matrix (ECM) rich in collagen type II and aggrecan, and lacking collagen type I. Additionally, we were able to show the formation of a layer populated by progenitor-like cells on the articular surface of the implant.

CONCLUSIONS

The analysis of samples taken from knee and ankle joints of human donors with a wide age range and both genders supports the potential of Agili-C™ scaffold to stimulate cartilage regeneration and repair. Based on these results, the present scaffold can be used in the clinical practice as a one-step procedure to treat full-thickness chondral defects.

Up-regulation of FOXD1 by YAP alleviates senescence and osteoarthritis

Cellular senescence is a driver of various aging-associated disorders, including osteoarthritis. Here, we identified a critical role for Yes-associated protein (YAP), a major effector of Hippo signaling, in maintaining a younger state of human mesenchymal stem cells (hMSCs) and ameliorating osteoarthritis in mice. Clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR associated protein 9 nuclease (Cas9)-mediated knockout (KO) of YAP in hMSCs resulted in premature cellular senescence. Mechanistically, YAP cooperated with TEA domain transcriptional factor (TEAD) to activate the expression of forkhead box D1 (FOXD1), a geroprotective protein. YAP deficiency led to the down-regulation of FOXD1. In turn, overexpression of YAP or FOXD1 rejuvenated aged hMSCs. Moreover, intra-articular administration of lentiviral vector encoding YAP or FOXD1 attenuated the development of osteoarthritis in mice. Collectively, our findings reveal YAP–FOXD1, a novel aging-associated regulatory axis, as a potential target for gene therapy to alleviate osteoarthritis.

The Hippo signalling effector YAP and the transcription factor FOXD1 play a role in alleviating cellular senescence and osteoarthritis, identifying the YAP-FOXD1 axis as a potential therapeutic target for aging-associated disorders.

Stem cell aging contributes to aging-associated degenerative diseases. Studies aiming to characterize the mechanisms of stem cell aging are critical for obtaining a comprehensive understanding of the aging process and developing novel strategies to treat aging-related diseases. As a prevalent aging-associated chronic joint disorder, osteoarthritis is a leading cause of disability. Senescent mesenchymal stem cells (MSCs) residing in the joint may be a critical target for the prevention of osteoarthritis; however, the key regulators of MSC senescence are little known, and targeting aging regulatory genes for the treatment of osteoarthritis has not yet been reported. Here, we show that Yes-associated protein (YAP), a major effector of Hippo signaling, represses human mesenchymal stem cell (hMSC) senescence through transcriptional up-regulation of forkhead box D1 (FOXD1). Lentiviral gene transfer of YAP or FOXD1 can rejuvenate aged hMSCs and ameliorate osteoarthritis symptoms in mouse models. We propose that the YAP–FOXD1 axis is a novel target for combating aging-associated diseases.

Spontaneous calcium signaling of cartilage cells: from spatiotemporal features to biophysical modeling

Intracellular calcium ([Ca²⁺]_i) oscillation is a fundamental signaling response of cartilage cells under mechanical loading or osmotic stress. Chondrocytes are usually considered as nonexcitable cells with no spontaneous [Ca²⁺]_i signaling. This study proved that chondrocytes can exhibit robust spontaneous [Ca²⁺]_i signaling without explicit external stimuli. The intensity of [Ca²⁺]_i peaks from individual chondrocytes maintain a consistent spatiotemporal pattern, acting as a unique "fingerprint" for each cell. Statistical analysis revealed lognormal distributions of the temporal parameters of [Ca²⁺]_i peaks, as well as strong linear correlations between their means and sds. Based on these statistical findings, we hypothesized that the spontaneous [Ca²⁺]_i peaks may result from an autocatalytic process and that [Ca²⁺]_i oscillation is controlled by a threshold-regulating mechanism. To test these 2 mechanisms, we established a multistage biophysical model by assuming the spontaneous [Ca²⁺]_i signaling of chondrocytes as a combination of deterministic and stochastic processes. The theoretical model successfully explained the lognormal distribution of the temporal parameters and the fingerprint feature of [Ca²⁺]_i peaks. In addition, by using antagonists for 10 pathways, we revealed that the initiation of spontaneous [Ca²⁺]_i peaks in chondrocytes requires the presence of extracellular Ca²⁺, and that the PLC-inositol 1,4,5-trisphosphate pathway, which controls the release of calcium from the endoplasmic reticulum, can affect the initiation of spontaneous [Ca²⁺]_i peaks in chondrocytes. The purinoceptors and transient receptor potential vanilloid 4 channels on the plasma membrane also play key roles in the spontaneous [Ca²⁺]_i signaling of chondrocytes. In contrast, blocking the T-type or L-type voltage-gated calcium channel promoted the spontaneous calcium signaling. This study represents a systematic effort to understand the features and initiation mechanisms of spontaneous [Ca²⁺]_i signaling in chondrocytes, which are critical for chondrocyte mechanobiology.-Zhou, Y., Lv, M., Li, T., Zhang, T., Duncan, R., Wang, L., Lu, X. L. Spontaneous calcium signaling of cartilage cells: from spatiotemporal features to biophysical modeling.

Targeting of chondrocyte plasticity via connexin43 modulation attenuates cellular senescence and fosters a pro-regenerative environment in osteoarthritis

Osteoarthritis (OA), a chronic disease characterized by articular cartilage degeneration, is a leading cause of disability and pain worldwide. In OA, chondrocytes in cartilage undergo phenotypic changes and senescence, restricting cartilage regeneration and favouring disease progression. Similar to other wound-healing disorders, chondrocytes from OA patients show a chronic increase in the gap junction channel protein connexin43 (Cx43), which regulates signal transduction through the exchange of elements or recruitment/release of signalling factors. Although immature or stem-like cells are present in cartilage from OA patients, their origin and role in disease progression are unknown. In this study, we found that Cx43 acts as a positive regulator of chondrocyte-mesenchymal transition. Overactive Cx43 largely maintains the immature phenotype by increasing nuclear translocation of Twist-1 and tissue remodelling and proinflammatory agents, such as MMPs and IL-1β, which in turn cause cellular senescence through upregulation of p53, p16^INK4a and NF-κB, contributing to the senescence-associated secretory phenotype (SASP). Downregulation of either Cx43 by CRISPR/Cas9 or Cx43-mediated gap junctional intercellular communication (GJIC) by carbenoxolone treatment triggered rediferentiation of osteoarthritic chondrocytes into a more differentiated state, associated with decreased synthesis of MMPs and proinflammatory factors, and reduced senescence. We have identified causal Cx43-sensitive circuit in chondrocytes that regulates dedifferentiation, redifferentiation and senescence. We propose that chondrocytes undergo chondrocyte-mesenchymal transition where increased Cx43-mediated GJIC during OA facilitates Twist-1 nuclear translocation as a novel mechanism involved in OA progression. These findings support the use of Cx43 as an appropriate therapeutic target to halt OA progression and to promote cartilage regeneration.

Human Cartilage-Derived Progenitors Resist Terminal Differentiation and Require CXCR4 Activation to Successfully Bridge Meniscus Tissue Tears

Meniscus injuries are among the most common orthopedic injuries. Tears in the inner one‐third of the meniscus heal poorly and present a significant clinical challenge. In this study, we hypothesized that progenitor cells from healthy human articular cartilage (chondroprogenitor cells [C‐PCs]) may be more suitable than bone‐marrow mesenchymal stem cells (BM‐MSCs) to mediate bridging and reintegration of fibrocartilage tissue tears in meniscus. C‐PCs were isolated from healthy human articular cartilage based on their expression of mesenchymal stem/progenitor marker activated leukocyte cell adhesion molecule (ALCAM) (CD166). Our findings revealed that healthy human C‐PCs are CD166+, CD90+, CD54+, CD106‐ cells with multilineage differentiation potential, and elevated basal expression of chondrogenesis marker SOX‐9. We show that, similar to BM‐MSCs, C‐PCs are responsive to the chemokine stromal cell‐derived factor‐1 (SDF‐1) and they can successfully migrate to the area of meniscal tissue damage promoting collagen bridging across inner meniscal tears. In contrast to BM‐MSCs, C‐PCs maintained reduced expression of cellular hypertrophy marker collagen X in monolayer culture and in an explant organ culture model of meniscus repair. Treatment of C‐PCs with SDF‐1/CXCR4 pathway inhibitor AMD3100 disrupted cell localization to area of injury and prevented meniscus tissue bridging thereby indicating that the SDF‐1/CXCR4 axis is an important mediator of this repair process. This study suggests that C‐PCs from healthy human cartilage may potentially be a useful tool for fibrocartilage tissue repair/regeneration because they resist cellular hypertrophy and mobilize in response to chemokine signaling. stem cells2019;37:102–114

Isolation and characterization of stem cells from differentially degenerated human lumbar zygapophyseal articular cartilage

The present study aimed to verify the presence of stem cells with multilineage differentiation potential in human lumbar zygapophyseal articular cartilage (LZAC) and to compare the chondrogenic potential of cells obtained from differentially degenerated articular cartilage samples. Surgically obtained human lumbar zygapophyseal joint tissues were classified into the normal, mildly degenerated and severely degenerated groups, according to their pathological characteristics. Primary chondrocytes from these groups were cultured, and stem cells were selected using a monoclonal cell culture method. Differences in stem cell morphology between the three groups were observed using inverted microscopy and phalloidin staining. In addition, stem cell chondrogenic potential was determined through induced differentiation and cellular staining. Gene and protein expression levels of the chondrogenic‑specific markers aggrecan, collagen type‑II and SRY‑related high‑mobility‑group box 9 were determined using reverse transcription‑quantitative polymerase chain reaction and western blotting. The clonogenic ability of stem cells in the three groups was determined using a clonogenic assay. It was revealed that stem cells with multilineage differentiation potential were isolated from all three cartilage groups; however, the cells obtained from severely degenerated articular cartilage resulted in severe fibrosis, whilst those obtained from mildly degenerated articular cartilage possessed stronger chondrogenic and clonogenic abilities. Taken together, stem cells with multilineage differentiation potential and clonal properties were identified in human LZAC, and these characteristics were more prominent in mildly degenerated as compared with severely degenerated articular cartilage.

Mesenchymal Stem/Progenitor Cells Derived from Articular Cartilage, Synovial Membrane and Synovial Fluid for Cartilage Regeneration: Current Status and Future Perspectives

Large articular cartilage defects remain an immense challenge in the field of regenerative medicine because of their poor intrinsic repair capacity. Currently, the available medical interventions can relieve clinical symptoms to some extent, but fail to repair the cartilaginous injuries with authentic hyaline cartilage. There has been a surge of interest in developing cell-based therapies, focused particularly on the use of mesenchymal stem/progenitor cells with or without scaffolds. Mesenchymal stem/progenitor cells are promising graft cells for tissue regeneration, but the most suitable source of cells for cartilage repair remains controversial. The tissue origin of mesenchymal stem/progenitor cells notably influences the biological properties and therapeutic potential. It is well known that mesenchymal stem/progenitor cells derived from synovial joint tissues exhibit superior chondrogenic ability compared with those derived from non-joint tissues; thus, these cell populations are considered ideal sources for cartilage regeneration. In addition to the progress in research and promising preclinical results, many important research questions must be answered before widespread success in cartilage regeneration is achieved. This review outlines the biology of stem/progenitor cells derived from the articular cartilage, the synovial membrane, and the synovial fluid, including their tissue distribution, function and biological characteristics. Furthermore, preclinical and clinical trials focusing on their applications for cartilage regeneration are summarized, and future research perspectives are discussed.

Current Concepts in Meniscus Tissue Engineering and Repair

The meniscus is the most commonly injured structure in the human knee. Meniscus deficiency has been shown to lead to advanced osteoarthritis (OA) due to abnormal mechanical forces, and replacement strategies for this structure have lagged behind other tissue engineering endeavors. The challenges include the complex 3D structure with individualized size parameters, the significant compressive, tensile and shear loads encountered, and the poor blood supply. In this progress report, a review of the current clinical treatments for different types of meniscal injury is provided. The state-of-the-art research in cellular therapies and novel cell sources for these therapies is discussed. The clinically available cell-free biomaterial implants and the current progress on cell-free biomaterial implants are reviewed. Cell-based tissue engineering strategies for the repair and replacement of meniscus are presented, and the current challenges are identified. Tissue-engineered meniscal biocomposite implants may provide an alternative solution for the treatment of meniscal injury to prevent OA in the long run, because of the limitations of the existing therapies.

Molecular characterization of mesenchymal stem cells in human osteoarthritis cartilage reveals contribution to the OA phenotype

Adult human articular cartilage harbors a population of CD166+ mesenchymal stem cell-like progenitors that become more numerous during osteoarthritis (OA). While their role is not well understood, here we report that they are indeed part of cellular clusters formed in OA cartilage, which is a pathological hallmark of this disease. We hypothesize that these cells, termed OA mesenchymal stem cells (OA-MSCs), contribute to OA pathogenesis. To test this hypothesis, we generated and characterized multiple clonally derived stable/immortalized human OA-MSC cell lines, which exhibited the following properties. Firstly, two mesenchymal stem cell populations exist in human OA cartilage. While both populations are multi-potent, one preferentially undergoes chondrogenesis while the other exhibits higher osteogenesis potential. Secondly, both OA-MSCs exhibit significantly higher expression of hypertrophic OA cartilage markers COL10A1 and RUNX2, compared to OA chondrocytes. Induction of chondrogenesis in OA-MSCs further stimulated COL10A1 expression and MMP-13 release, suggesting that they contribute to OA phenotypes. Finally, knocking down RUNX2 is insufficient to inhibit COL10A1 in OA-MSCs and also requires simultaneous knockdown of NOTCH1 thereby suggesting altered gene regulation in OA stem cells in comparison to chondrocytes. Overall, our findings suggest that OA-MSCs may drive pathogenesis of cartilage degeneration and should therefore be a novel cell target for OA therapy.

Activation of β-catenin signaling in aggrecan-expressing cells in temporomandibular joint causes osteoarthritis-like defects

β-Catenin plays a critical role in cartilage formation and development. To further understand the role of β-catenin in osteoarthritis (OA) development in temporomandibular joint (TMJ), we have generated β-catenin conditional activation mice (β-cat(ex3)^Agc1CreER) by breeding Agc1-CreER mice with β-catenin^flox(ex3)/+ mice. Results of histologic analysis showed the progressive TMJ defects in 3- and 6-month-old β-cat(ex3)^Agc1CreER mice (tamoxifen induction was performed at 2 weeks of age), including decreased chondrocyte numbers in the superficial layer associated with less Alcian blue staining, increased numbers of hypertrophic chondrocytes in deep layers, and rough articular surface. Compared to the TMJ phenotype of β-cat(ex3)^Col2CreER mice, β-cat(ex3)^Agc1CreER mice showed much severe morphological defects in the superficial layer of TMJ. This may reflect that Agc1-CreER mice could efficiently target cells in the superficial layer of TMJ. Results of immunostaining showed significantly increased expression of MMP13, Col-X, Adamts4, and Adamts5 in TMJ of β-cat(ex3)^Agc1CreER mice. Results of proliferating cell nuclear antigen (PCNA), Ki67, and terminal deoxinucleotidyl transferase-mediated dUTP-fluorescein nick end labeling (TUNEL) staining further demonstrated that cell proliferation was decreased and cell apoptosis was increased in condylar cartilage of β-cat(ex3)^Agc1CreER mice. Our findings indicate that abnormal upregulation of β-catenin in TMJ leads to defects assembling to OA-like phenotype, further demonstrating that β-catenin plays a critical role in TMJ pathogenesis.

Therapies targeting a cartilage-regulating signaling protein could mitigate painful damage inflicted on the jaw by age-related osteoarthritis. Many older individuals experience degeneration of the temporomandibular joint (TMJ), where the upper and lower jaw connect. Researchers led by Di Chen of Rush University Medical School in Chicago, USA, have developed a genetically-modified mouse model that reveals a likely molecular driver for TMJ osteoarthritis. Previous studies have implicated a protein called β-catenin in this process, and Chen’s team generated mice in which β-catenin levels can be selectively boosted in cartilage-forming cells at skeletal joints. This increased β-catenin markedly altered the organization of TMJ cartilage, with decreased cell proliferation and increased cell death. The effects were strikingly similar to human osteoarthritis, and the researchers hypothesize that compounds that counter β-catenin could offer useful treatments for this condition.

Isolation and biological characteristic evaluation of a novel type of cartilage stem/progenitor cell derived from Small‑tailed Han sheep embryos

Cartilage stem/progenitor cells (CSPCs) are a novel stem cell population and function as promising therapeutic candidates for cell‑based cartilage repair. Until now, numerous existing research materials have been obtained from humans, horses, cows and other mammals, but rarely from sheep. In the present study, CSPCs with potential applications in repairing tissue damage and cell‑based therapy were isolated from 45‑day‑old Small‑tailed Han Sheep embryos, and examined at the cellular and molecular level. The expression level of characteristic surface markers of the fetal sheep CSPCs were also evaluated by immunofluorescence, reverse transcription‑polymerase chain reaction analysis and flow cytometric assays. Biological growth curves were drawn in accordance with cell numbers. Additionally, karyotype analysis showed no marked differences in the in vitro cultured CSPCs and they were genetically stable among different passages. The CSPCs were also capable of adipogenic, osteogenic and chondrogenic lineage progression under the appropriate induction medium in vitro. Together, these findings provide a theoretical basis and experimental evidence for cellular transplant therapy in tissue engineering.

Reserve or Resident Progenitors in Cartilage? Comparative Analysis of Chondrocytes versus Chondroprogenitors and Their Role in Cartilage Repair

Introduction Articular cartilage is made up of hyaline tissue embodying chondrocytes, which arise from mesenchymal stromal cells (MSCs) and specialized extracellular matrix. Despite possessing resident progenitors in and around the joint primed for chondrogenesis, cartilage has limited intrinsic capacity of repair and cell turnover. Advances in isolation, culture, and characterization of these progenitors have raised the possibility for their use in cell-based cartilage repair. Chondroprogenitors (CPCs) have been classified as MSCs and have been postulated to play a vital role in injury response and are identified by their colony forming ability, proliferative potential, telomere dynamics, multipotency, and expression of stem cell markers. The combined presence of CPCs and chondrocytes within the same tissue compartments and the ability of chondrocytes to dedifferentiate and acquire stemness during culture expansion has obscured our ability to define and provide clear-cut differences between these 2 cell populations. Objective This review aims to evaluate and summarize the available literature on CPCs in terms of their origin, growth kinetics, molecular characteristics, and differential and therapeutic potential with emphasis on their difference from daughter chondrocytes. Design For this systematic review, a comprehensive electronic search was performed on PubMed and Google Scholar using relevant terms such as chondrocytes, chondroprogenitors, and surface marker expression. Results and Conclusion Our comparative analysis shows that there is an ill-defined distinction between CPCs and chondrocytes with respect to their cell surface expression (MSC markers and CPC-specific markers) and differentiation potential. Accumulating evidence indicates that the 2 subpopulations may be distinguished based on their growth kinetics and chondrogenic marker.

Cartilage regeneration and ageing: Targeting cellular plasticity in osteoarthritis

Ageing processes play a major contributing role for the development of Osteoarthritis (OA). This prototypic degenerative condition of ageing is the most common form of arthritis and is accompanied by a general decline, chronic pain and mobility deficits. The disease is primarily characterized by articular cartilage degradation, followed by subchondral bone thickening, osteophyte formation, synovial inflammation and joint degeneration. In the early stages, osteoarthritic chondrocytes undergo phenotypic changes that increase cell proliferation and cluster formation and enhance the production of matrix-remodelling enzymes. In fact, chondrocytes exhibit differentiation plasticity and undergo phenotypic changes during the healing process. Current studies are focusing on unravelling whether OA is a consequence of an abnormal wound healing response. Recent investigations suggest that alterations in different proteins, such as TGF-ß/BMPs, NF-Kß, Wnt, and Cx43, or SASP factors involved in signalling pathways in wound healing response, could be directly implicated in the initiation of OA. Several findings suggest that osteoarthritic chondrocytes remain in an immature state expressing stemness-associated cell surface markers. In fact, the efficacy of new disease-modifying OA drugs that promote chondrogenic differentiation in animal models indicates that this may be a drug-sensible state. In this review, we highlight the current knowledge regarding cellular plasticity in chondrocytes and OA. A better comprehension of the mechanisms involved in these processes may enable us to understand the molecular pathways that promote abnormal repair and cartilage degradation in OA. This understanding would be advantageous in identifying novel targets and designing therapies to promote effective cartilage repair and successful joint ageing by preventing functional limitations and disability.

Biomaterials for articular cartilage tissue engineering: Learning from biology

Articular cartilage is commonly described as a tissue that is made of up to 80% water, is devoid of blood vessels, nerves, and lymphatics, and is populated by only one cell type, the chondrocyte. At first glance, an easy tissue for clinicians to repair and for scientists to reproduce in a laboratory. Yet, chondral and osteochondral defects currently remain an open challenge in orthopedics and tissue engineering of the musculoskeletal system, without considering osteoarthritis. Why do we fail in repairing and regenerating articular cartilage? Behind its simple and homogenous appearance, articular cartilage hides a heterogeneous composition, a high level of organisation and specific biomechanical properties that, taken together, make articular cartilage a unique material that we are not yet able to repair or reproduce with high fidelity. This review highlights the available therapies for cartilage repair and retraces the research on different biomaterials developed for tissue engineering strategies. Their potential to recreate the structure, including composition and organisation, as well as the function of articular cartilage, intended as cell microenvironment and mechanically competent replacement, is described. A perspective of the limitations of the current research is given in the light of the emerging technologies supporting tissue engineering of articular cartilage.

STATEMENT OF SIGNIFICANCE

The mechanical properties of articular tissue reflect its functionally organised composition and the recreation of its structure challenges the success of in vitro and in vivo reproduction of the native cartilage. Tissue engineering and biomaterials science have revolutionised the way scientists approach the challenge of articular cartilage repair and regeneration by introducing the concept of the interdisciplinary approach. The clinical translation of the current approaches are not yet fully successful, but promising results are expected from the emerging and developing new generation technologies.

The Potential of microRNAs for Stem Cell-based Therapy for Degenerative Skeletal Diseases

Purpose of Review

Degenerative skeletal disorders including osteoarthritis (OA) and osteoporosis (OP) are the result of attenuation of tissue regeneration and lead to painful conditions with limited treatment options. Preventative measures to limit the onset of OA and OP remain a significant unmet clinical need. MicroRNAs (miRNAs) are known to be involved in the differentiation of stem cells, and in combination with stem cell therapy could induce skeletal regeneration and potentially prevent OA and OP onset.

Recent Findings

The combination of stem cells and miRNA has been successful at regenerating the bone and cartilage in vivo. MiRNAs, including miR-146b known to be involved in chondrogenic differentiation, could provide innovative targets for stem cell-based therapy, for the repair of articular cartilage defects forestalling the onset of OA or in the generation of a stem cell-based therapy for OP.

Summary

This review discusses the combination of skeletal stem cells (SSCs) and candidate miRNAs for application in a cell-based therapy approach for skeletal regenerative medicine.

Infrapatellar Fat Pad Stem Cells: From Developmental Biology to Cell Therapy

The ideal cell type to be used for cartilage therapy should possess a proven chondrogenic capacity, not cause donor-site morbidity, and should be readily expandable in culture without losing their phenotype. There are several cell sources being investigated to promote cartilage regeneration: mature articular chondrocytes, chondrocyte progenitors, and various stem cells. Most recently, stem cells isolated from joint tissue, such as chondrogenic stem/progenitors from cartilage itself, synovial fluid, synovial membrane, and infrapatellar fat pad (IFP) have gained great attention due to their increased chondrogenic capacity over the bone marrow and subcutaneous adipose-derived stem cells. In this review, we first describe the IFP anatomy and compare and contrast it with other adipose tissues, with a particular focus on the embryological and developmental aspects of the tissue. We then discuss the recent advances in IFP stem cells for regenerative medicine. We compare their properties with other stem cell types and discuss an ontogeny relationship with other joint cells and their role on in vivo cartilage repair. We conclude with a perspective for future clinical trials using IFP stem cells.

Characteristics of meniscus progenitor cells migrated from injured meniscus

Serious meniscus injuries seldom heal and increase the risk for knee osteoarthritis; thus, there is a need to develop new reparative therapies. In that regard, stimulating tissue regeneration by autologous stem/progenitor cells has emerged as a promising new strategy. We showed previously that migratory chondrogenic progenitor cells (CPCs) were recruited to injured cartilage, where they showed a capability in situ tissue repair. Here, we tested the hypothesis that the meniscus contains a similar population of regenerative cells. Explant studies revealed that migrating cells were mainly confined to the red zone in normal menisci: However, these cells were capable of repopulating defects made in the white zone. In vivo, migrating cell numbers increased dramatically in damaged meniscus. Relative to non-migrating meniscus cells, migrating cells were more clonogenic, overexpressed progenitor cell markers, and included a larger side population. Gene expression profiling showed that the migrating population was more similar to CPCs than other meniscus cells. Finally, migrating cells equaled CPCs in chondrogenic potential, indicating a capacity for repair of the cartilaginous white zone of the meniscus. These findings demonstrate that, much as in articular cartilage, injuries to the meniscus mobilize an intrinsic progenitor cell population with strong reparative potential. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1966-1972, 2017.

Mesenchymal Stem Cell-Based Cartilage Regeneration Approach and Cell Senescence: Can We Manipulate Cell Aging and Function?

Aging is the most prominent risk factor triggering several degenerative diseases, such as osteoarthritis (OA). Due to its poor self-healing capacity, once injured cartilage needs to be reestablished. This process might be approached through resorting to cell-based therapies and/or tissue engineering. Human mesenchymal stem cells (MSCs) represent a promising approach due to their chondrogenic differentiation potential. Presently, in vitro chondrogenic differentiation of MSCs is limited by two main reasons as follows: aging of MSCs, which determines the loss of cell proliferative and differentiation capacity and MSC-derived chondrocyte hypertrophic differentiation, which limits the use of these cells in cartilage tissue regeneration approach. The effect of aging on MSCs is fundamental for stem cell-based therapy development, especially in older subjects. In the present review we focus on homeostasis alterations occurring in MSC-derived chondrocytes during in vitro aging. Moreover, we deal with potential cell aging regulation approaches, such as cell stimulation through telomerase activators, mechanical strain, and epigenetic regulation. Future investigations in this field might provide new insights into innovative strategies for cartilage regeneration and potentially inspire novel therapeutic approaches for OA treatment.

Expression of Notch signaling pathway during osteoarthritis in the temporomandibular joint

PURPOSE

Our study aim was to characterize the expression of Notch molecules during temporomandibular joint osteoarthritis (TMJOA), thus exploring the mechanism and roles that Notch signaling possibly plays in the initiation and progression of TMJOA.

MATERIALS AND METHODS

A total of 126 mice were divided randomly into experimental groups, a sham-surgery group and a normal control group. In the experimental group, total discectomy was performed in the right temporomandibular joint (TMJ) to induce TMJOA; the sham-operation group underwent the same procedure without disc removal, and the normal control group was left undisturbed. Fourteen mice in each group were sacrificed in batches respectively at 1, 2, and 4 weeks postoperatively. Histology was performed to examine TMJOA in eight condyles each group, and a modified Mankin scoring system was used for evaluation. Immunohistochemistry was carried out to characterize the expression of the Notch markers Notch1, Jagged1, Hes1, and Hes5. Real-time polymerase chain reaction (RT-PCR) was performed for further detection and analysis of Notch markers in six condyles in each group.

RESULTS

Notch1, Jagged1, and Hes5 were activated in the experimental group, with expression levels that increased dramatically over time, whereas the control group showed no fluctuation. Hes1 expression was suppressed at the beginning but was up-regulated afterward.

CONCLUSIONS

Our data suggest that Notch signaling is activated in TMJOA with a much more abundant expression in osteoarthritis cartilage.

Chondrogenic progenitor cells promote vascular endothelial growth factor expression through stromal-derived factor-1

OBJECTIVE

Vascular endothelial growth factor (VEGF) is elevated in joint fluids from patients diagnosed with osteoarthritis (OA). VEGF is known to contribute to vascular tidemark invasion and osteophyte formation, which are classic features of advanced OA. Among the factors that may drive VEGF accumulation in diseased joints, stromal cell-derived factor-1α (SDF-1α) is a likely culprit, as it is enriched in synovial fluids from osteoarthritic joints and is a potent inducer of VEGF expression. Chondrogenic progenitor cells (CPCs) that overexpress SDF-1α are abundant in osteoarthritic cartilage, implicating them in elevating synovial SDF-1α levels. Here we conducted a series of experiments to determine the potential for CPCs to stimulate VEGF expression via autocrine and paracrine mechanisms.

DESIGN

Immunohistochemistry, immunoblotting, and PCR were used to evaluate the effects of SDF-1α on VEGF expression in CPCs and chondrocytes, and the effects of CPC-conditioned medium on chondrocytes. An SDF-1α receptor antagonist and inhibitors of mitogen-activated protein kinases (MAPKs) were used to probe the pathway linking SDF-1 with VEGF expression in CPCs.

RESULTS

SDF-1α and CPC-conditioned medium stimulated VEGF expression in chondrocytes. In both chondrocytes and CPCs, SDF-1α stimulated increased VEGF expression via C-X-C chemokine receptor type 4 (CXCR4), a cell-surface SDF-1α receptor. This response in CPCs is dependent on p38 MAPK activation, but not on ERK or c-Jun N-terminal kinase (JNK) activation.

CONCLUSIONS

By secreting SDF-1α, CPCs stimulate VEGF expression in nearby cells. The co-expression of SDF-1 and its receptor by CPCs indicates they are capable of self-sustained VEGF expression via an autocrine mechanism.

Meniscus, articular cartilage and nucleus pulposus: a comparative review of cartilage-like tissues in anatomy, development and function

The degradation of cartilage in the human body is impacted by aging, disease, genetic predisposition and continued insults resulting from daily activity. The burden of cartilage defects (osteoarthritis, rheumatoid arthritis, intervertebral disc damage, knee replacement surgeries, etc.) is daunting in light of substantial economic and social stresses. This review strives to broaden the scope of regenerative medicine and tissue engineering approaches used for cartilage repair by comparing and contrasting the anatomical and functional nature of the meniscus, articular cartilage (AC) and nucleus pulposus (NP). Many review papers have provided detailed evaluations of these cartilages and cartilage-like tissues individually but none have comprehensively examined the parallels and inconsistencies in signaling, genetic expression and extracellular matrix composition between tissues. For the first time, this review outlines the importance of understanding these three tissues as unique entities, providing a comparative analysis of anatomy, ultrastructure, biochemistry and function for each tissue. This novel approach highlights the similarities and differences between tissues, progressing research toward an understanding of what defines each tissue as distinctive. The goal of this paper is to provide researchers with the fundamental knowledge to correctly engineer the meniscus, AC and NP without inadvertently developing the wrong tissue function or biochemistry.

Modulation of Superficial Zone Protein/Lubricin/PRG4 by Kartogenin and Transforming Growth Factor-β1 in Surface Zone Chondrocytes in Bovine Articular Cartilage

OBJECTIVE

Superficial zone protein (SZP)/lubricin/PRG4 functions as a boundary lubricant in articular cartilage to decrease friction and wear. As articular cartilage lubrication is critical for normal joint function, the accumulation of SZP at the surface of cartilage is important for joint homeostasis. Recently, a heterocyclic compound called kartogenin (KGN) was found to induce chondrogenic differentiation and enhance mRNA expression of lubricin. The objective of this study was to determine whether KGN can stimulate synthesis of SZP in superficial zone, articular chondrocytes.

DESIGN

We investigated the effects of KGN and transforming growth factor-β1 (TGF-β1) on articular cartilage and synovium of the bovine knee joint by evaluating SZP secretion by enzyme-linked immunosorbent assay analysis. Monolayer, micromass, and explant cultures of articular cartilage, and monolayer culture of synoviocytes, were treated with KGN. SZP accumulation in the medium was evaluated and mRNA expression was measured through quantitative polymerase chain reaction.

RESULTS

TGF-β1 stimulated SZP secretion by superficial zone chondrocytes in monolayer, explant, and micromass cultures as expected. In addition, SZP secretion was inhibited by IL-1β in explant cultures, and enhanced by TGF-β1 in synoviocyte monolayer cultures. Although KGN elicited a 1.2-fold increase in SZP mRNA expression in combination with TGF-β1, KGN neither stimulated any significant increases in SZP synthesis nor prevented catabolic decreases in SZP production from IL-1β.

CONCLUSIONS

These data suggest that the chondrogenic effects of KGN depend on cellular phenotype and differentiation status, as KGN did not alter SZP synthesis in differentiated, superficial zone articular chondrocytes.

Enhanced phagocytic capacity endows chondrogenic progenitor cells with a novel scavenger function within injured cartilage

OBJECTIVE

Articular cartilage harbors chondrogenic progenitor cells (CPCs), a population that responds chemotactically to cell death. Because this behavior is reminiscent of macrophages, we hypothesized that CPCs have macrophage-like capabilities for scavenging cell and tissue debris through phagocytosis.

DESIGN

CPCs, chondrocytes, synoviocytes, and macrophages were cultured with fluorophore-labeled chondrocyte debris for 3, 6, 12, or 24 h. Debris internalization was quantified by confocal microscopy and flow cytometry. Confocal microscopy was also used to test CPCs and chondrocytes for uptake of fluorophore-labeled fibronectin fragments (Fn-fs), a form of extracellular matrix debris. Lysosome activity and mass in CPCs and chondrocytes were measured using fluorescent probes. The relative expression of phagocytosis-related genes and proteins was evaluated by polymerase chain reaction (PCR) and immunoblotting, respectively. Pulse-chase experiments were performed to determine if the debris internalized by CPCs and chondrocytes was cleared, and if clearance was affected by a cathepsin B inhibitor.

RESULTS

More macrophages, synoviocytes, and CPCs internalized cell debris than chondrocytes at all time points. While uptake remained flat in chondrocytes at ∼10%, in the other cell types it peaked at more than 60% after 12-24 h. Relative to chondrocytes, CPCs showed significantly higher rates of Fn-fs engulfment, greater lysosome activity and mass, and over-expressed phagocytosis-related genes and proteins. Pulse-chase experiments revealed time- and cathepsin B-dependent clearance of cell debris in CPCs, but not in chondrocytes.

CONCLUSIONS

CPCs phagocytized cell and matrix debris much more efficiently than chondrocytes, supporting the hypothesis that they play a macrophage-like role in injured cartilage.

Articular cartilage repair: Current needs, methods and research directions

Articular cartilage is a highly specialized tissue whose remarkable properties of deformability, resistance to mechanical loading, and low-friction gliding are essential to joint function. Due to its role as a cushion in bone articulation, articular cartilage is subject to many types of damaging insults, including decades of wear and tear, and acute joint injuries. However, this built-for-life tissue has a very poor intrinsic ability in adulthood to durably heal defects created by damaging insults. Consequently, articular cartilage progressively deteriorates and is eventually eroded, exposing the subchondral bone to the joint space, triggering inflammation and osteophyte development, and generating severe pain and joint incapacitation. The disease is called osteoarthritis (OA) and is today the leading cause of pain and disability in the human population. Researchers and clinicians have worked for decades to develop strategies to treat OA and restore joint function, but they are still far from being able to offer patients effective preventive or restorative treatments. Novel ideas, knowledge and technologies that nurture hope for major new breakthroughs are therefore sought. In this review, we first outline the composition, structure, and functional properties of normal human adult articular cartilage, as a reference for tissue conservation and regenerative strategies. We then describe current options that have been used clinically and in pre-clinical trials to treat osteoarthritic patients, and we discuss the benefits and inadequacies of these treatment options. Next, we review research efforts that are currently ongoing to try and achieve durable repair of functional cartilage tissue. Methods include engineering of tissue implants and we discuss the needs and options for tissue scaffolds, cell sources, and growth and differentiation factors to generate de novo or repair bona fide articular cartilage.

Distinguishing characteristics of stem cells derived from different anatomical regions of human degenerated intervertebral discs

INTRODUCTION

Several types of stem cells have been successfully demonstrated to exist in the human degenerated intervertebral disc (IVD), which is composed of annulus fibrosus (AF), nucleus pulposus (NP) and cartilage endplate (CEP). However, the differences in the biological characteristics among these and bone marrow derived mesenchymal stem cells (BM-MSCs) remain unclear.

MATERIALS AND METHODS

To investigate this issue, cells were harvested from human AF, NP, CEP, and bone marrow, respectively; passage 2 cells were selected using the agarose suspension culture system to obtain stem cell clones. Following expansion in vitro, stem cells from different anatomical regions were compared regarding the morphology, proliferation ability, immunophenotypic expression, and multi-lineage differentiation capacity. In addition, stem cell-alginate bead compositions were constructed for the comparison of DNA and sGAG content.

RESULTS

There were subtle differences regarding cell morphology, but no significant differences in proliferation ability among the four types of stem cells. For the immunophenotypic analysis, all stem cells basically fulfilled the criteria for mesenchymal stem cells (MSCs), which have been published by the International Society for Cellular Therapy (ISCT), with a significant difference in CD105 expression. A comparison of the osteogenic capacities indicated: cartilage endplate-derived stem cells (CESCs) > annulus fibrosus-derived stem cells (AFSCs) > BM-MSCs > nucleus pulposus-derived stem cells (NPSCs). The chondrogenesis difference was similar to osteogenesis. For adipogenesis: BM-MSCs >NPSCs >CESCs >AFSCs. In the stem cell/alginate composition, the CESCs consistently showed the superior chondrogenic potential among all those cell types.

CONCLUSIONS

Our data indicated that all the four types of stem cells shared some similar biological properties (regarding shape, proliferation ability and immunophenotypic expression). CESCs, which had the strongest osteogenic and chondrogenic potentials, may serve as excellent seed cells for NP/cartilage or bone tissue engineering.

Aberrant expression of Twist1 in diseased articular cartilage and a potential role in the modulation of osteoarthritis severity

The bHLH transcription factor Twist1 has emerged as a negative regulator of chondrogenesis in skeletal progenitor cells and as an inhibitor of maturation in growth plate chondrocytes. However, its role in articular cartilage remains obscure. Here we examine Twist1 expression during re-differentiation of expanded human articular chondrocytes, the distribution of Twist1 proteins in normal versus OA human articular cartilage, and its role in modulating OA development in mice. High levels of Twist1 transcripts were detected by qPCR analyses of expanded de-differentiated human articular chondrocytes that had acquired mesenchymal-like features. The induction of hallmark cartilage genes by Bmp-2 mediated chondrogenic differentiation was paralleled by the dramatic suppression of Twist1 in vitro. In normal human articular cartilage, Twist1-expressing chondrocytes were most abundant in the superficial zone with little to no expression in the middle and deep zones. However, our analyses revealed a higher proportion of deep zone articular chondrocytes expressing Twist1 in human OA cartilage as compared to normal articular cartilage. Moreover, Twist1 expression was prominent within proliferative cell clusters near fissure sites in more severely affected OA samples. To assess the role of Twist1 in OA pathophysiology, we subjected wild type mice and transgenic mice with gain of Twist1 function in cartilage to surgical destabilization of the medial meniscus. At 12 weeks post-surgery, micro-CT and histological analyses revealed attenuation of the OA phenotype in Twist1 transgenic mice compared to wild type mice. Collectively, the data reveal a role for Twist in articular cartilage maintenance and the attenuation of cartilage degeneration.

Intermittent hydrostatic pressure maintains and enhances the chondrogenic differentiation of cartilage progenitor cells cultivated in alginate beads

The objective of this study was to explore the effects of intermittent hydrostatic pressure (IHP) on the chondrogenic differentiation of cartilage progenitor cells (CPCs) cultivated in alginate beads. CPCs were isolated from the knee joint cartilage of rabbits, and infrapatellar fat pad-derived stem cells (FPSCs) and chondrocytes (CCs) were included as the control cell types. Cells embedded in alginate beads were treated with IHP at 5 Mpa and 0.5 Hz for 4 h/day for 1, 2, or 4 weeks. The cells' migratory and proliferative capacities were evaluated using the scratch and Live/Dead assays, respectively. Hematoxylin and eosin staining, safranin O staining, and immunohistochemical staining were performed to determine the effects of IHP on the synthesis of extracellular matrix (ECM) proteins. Real-time polymerase chain reaction analysis was performed to measure the expression of genes related to chondrogenesis. The scratch and Live/Dead assays revealed that IHP significantly promoted the migration and proliferation of FPSCs and CPCs to different extents. The staining experiments showed greater production of cartilage ECM components (glycosaminoglycans and collagen II) by cells exposed to IHP, and the gene expression analysis demonstrated that IHP stimulated the expression of chondrocyte-related genes. Importantly, these effects of IHP were more prominent in CPCs than in FPSCs and CCs. Considering all of our experimental results combined, we conclude that CPCs demonstrated a stronger chondrogenic differentiation capacity than the FPSCs and CCs under stimulation with IHP. Thus, the use of CPCs, combined with mechanical stimulation, may represent a valuable strategy for cartilage tissue engineering.

Isolation, Culture, and Characterization of Chicken Cartilage Stem/Progenitor Cells

A chondrocyte progenitor population isolated from the surface zone of articular cartilage has become a promising cell source for cell-based cartilage repair. The cartilage-derived stem/progenitor cells are multipotent stem cells, which can differentiate into three cell types in vitro including adipocytes, osteoblasts, and chondrocytes. Much work has been done on cartilage stem/progenitor cells (CSPCs) from people, horses, and cattle, but the relatively little literature has been published about these cells in chickens. In our work, CSPCs were isolated from chicken embryos in incubated eggs for 20 days. In order to inquire into the biological characteristics of chicken CSPCs, immunofluorescence, reverse transcription-polymerase chain reaction (RT-PCR), and flow cytometry were adopted to detect the characteristic surface markers of CSPCs. Primary CSPCs were subcultured to passage 22 and, for purpose of knowing the change of cell numbers, we drew the growth curves. Isolated CSPCs were induced to adipocytes, osteoblasts, and chondrocytes. Our results suggest that we have identified and characterised a novel cartilage progenitor population resident in chicken articular cartilage and CSPCs isolated from chickens possess similar biological characteristics to those from other species, which will greatly benefit future cell-based cartilage repair therapies.

Cartilage regeneration for treatment of osteoarthritis: a paradigm for nonsurgical intervention

Osteoarthritis (OA) is associated with articular cartilage abnormalities and affects people of older age: preventative or therapeutic treatment measures for OA and related articular cartilage disorders remain challenging. In this perspective review, we have integrated multiple biological, morphological, developmental, stem cell and homeostasis concepts of articular cartilage to develop a paradigm for cartilage regeneration. OA is conceptually defined as an injury of cartilage that initiates chondrocyte activation, expression of proteases and growth factor release from the matrix. This regenerative process results in the local activation of inflammatory response genes in cartilage without migration of inflammatory cells or angiogenesis. The end results are catabolic and anabolic responses, and it is the balance between these two outcomes that controls remodelling of the matrix and regeneration. A tantalizing clinical clue for cartilage regrowth in OA joints has been observed in surgically created joint distraction. We hypothesize that cartilage growth in these distracted joints may have a biological connection with the size of organs and regeneration. Therefore we propose a novel, practical and nonsurgical intervention to validate the role of distraction in cartilage regeneration in OA. The approach permits normal wake-up activity while during sleep; the index knee is subjected to distraction with a pull traction device. Comparison of follow-up magnetic resonance imaging (MRI) at 3 and 6 months of therapy to those taken before therapy will provide much-needed objective evidence for the use of this mode of therapy for OA. We suggest that the paradigm presented here merits investigation for treatment of OA in knee joints.

Cartilage issues in football-today's problems and tomorrow's solutions

Articular cartilage injury is prevalent in football players and results from chronic joint stress or acute traumatic injuries. Articular cartilage injury can often result in progressive painful impairment of joint function and limit sports participation. Management of articular cartilage injury in athletes aims to return the player to competition, and requires effective and durable joint surface restoration that resembles normal hyaline articular cartilage that can withstand the high joint stresses of football. Existing articular cartilage repair techniques can return the athlete with articular cartilage injury to high-impact sports, but treatment does not produce normal articular cartilage, and this limits the success rate and durability of current cartilage repair in athletes. Novel scientific concepts and treatment techniques that apply modern tissue engineering technologies promise further advancement in the treatment of these challenging injuries in the high demand athletic population. We review the current knowledge of cartilage injury pathophysiology, epidemiology and aetiology, and outline existing management algorithms, developing treatment options and future strategies to manage articular cartilage injuries in football players.