Potential benefits and limitations of utilizing chondroprogenitors in cell-based cartilage therapy

Transcriptional profiling of human cartilage endplate cells identifies novel genes and cell clusters underlying degenerated and non-degenerated phenotypes

BACKGROUND

Low back pain is a leading cause of disability worldwide and is frequently attributed to intervertebral disc (IVD) degeneration. Though the contributions of the adjacent cartilage endplates (CEP) to IVD degeneration are well documented, the phenotype and functions of the resident CEP cells are critically understudied. To better characterize CEP cell phenotype and possible mechanisms of CEP degeneration, bulk and single-cell RNA sequencing of non-degenerated and degenerated CEP cells were performed.

METHODS

Human lumbar CEP cells from degenerated (Thompson grade ≥ 4) and non-degenerated (Thompson grade ≤ 2) discs were expanded for bulk (N=4 non-degenerated, N=4 degenerated) and single-cell (N=1 non-degenerated, N=1 degenerated) RNA sequencing. Genes identified from bulk RNA sequencing were categorized by function and their expression in non-degenerated and degenerated CEP cells were compared. A PubMed literature review was also performed to determine which genes were previously identified and studied in the CEP, IVD, and other cartilaginous tissues. For single-cell RNA sequencing, different cell clusters were resolved using unsupervised clustering and functional annotation. Differential gene expression analysis and Gene Ontology, respectively, were used to compare gene expression and functional enrichment between cell clusters, as well as between non-degenerated and degenerated CEP samples.

RESULTS

Bulk RNA sequencing revealed 38 genes were significantly upregulated and 15 genes were significantly downregulated in degenerated CEP cells relative to non-degenerated cells (|fold change| ≥ 1.5). Of these, only 2 genes were previously studied in CEP cells, and 31 were previously studied in the IVD and other cartilaginous tissues. Single-cell RNA sequencing revealed 11 unique cell clusters, including multiple chondrocyte and progenitor subpopulations with distinct gene expression and functional profiles. Analysis of genes in the bulk RNA sequencing dataset showed that progenitor cell clusters from both samples were enriched in "non-degenerated" genes but not "degenerated" genes. For both bulk- and single-cell analyses, gene expression and pathway enrichment analyses highlighted several pathways that may regulate CEP degeneration, including transcriptional regulation, translational regulation, intracellular transport, and mitochondrial dysfunction.

CONCLUSIONS

This thorough analysis using RNA sequencing methods highlighted numerous differences between non-degenerated and degenerated CEP cells, the phenotypic heterogeneity of CEP cells, and several pathways of interest that may be relevant in CEP degeneration.

Recent development in multizonal scaffolds for osteochondral regeneration

Osteochondral (OC) repair is an extremely challenging topic due to the complex biphasic structure and poor intrinsic regenerative capability of natural osteochondral tissue. In contrast to the current surgical approaches which yield only short-term relief of symptoms, tissue engineering strategy has been shown more promising outcomes in treating OC defects since its emergence in the 1990s. In particular, the use of multizonal scaffolds (MZSs) that mimic the gradient transitions, from cartilage surface to the subchondral bone with either continuous or discontinuous compositions, structures, and properties of natural OC tissue, has been gaining momentum in recent years. Scrutinizing the latest developments in the field, this review offers a comprehensive summary of recent advances, current hurdles, and future perspectives of OC repair, particularly the use of MZSs including bilayered, trilayered, multilayered, and gradient scaffolds, by bringing together onerous demands of architecture designs, material selections, manufacturing techniques as well as the choices of growth factors and cells, each of which possesses its unique challenges and opportunities.

Generation of human immortalized chondrocytes from osteoarthritic and healthy cartilage : a new tool for cartilage pathophysiology studies

AIMS

After a few passages of in vitro culture, primary human articular chondrocytes undergo senescence and loss of their phenotype. Most of the available chondrocyte cell lines have been obtained from cartilage tissues different from diarthrodial joints, and their utility for osteoarthritis (OA) research is reduced. Thus, the goal of this research was the development of immortalized chondrocyte cell lines proceeded from the articular cartilage of patients with and without OA.

METHODS

Using telomerase reverse transcriptase (hTERT) and SV40 large T antigen (SV40LT), we transduced primary OA articular chondrocytes. Proliferative capacity, degree of senescence, and chondrocyte surface antigen expression in transduced chondrocytes were evaluated. In addition, the capacity of transduced chondrocytes to synthesize a tissue similar to cartilage and to respond to interleukin (IL)-1β was assessed.

RESULTS

Coexpression of both transgenes (SV40 and hTERT) were observed in the nuclei of transduced chondrocytes. Generated chondrocyte cell lines showed a high proliferation capacity and less than 2% of senescent cells. These cell lines were able to form 3D aggregates analogous to those generated by primary articular chondrocytes, but were unsuccessful in synthesizing cartilage-like tissue when seeded on type I collagen sponges. However, generated chondrocyte cell lines maintained the potential to respond to IL-1β stimulation.

CONCLUSION

Through SV40LT and hTERT transduction, we successfully immortalized chondrocytes. These immortalized chondrocytes were able to overcome senescence in vitro, but were incapable of synthesizing cartilage-like tissue under the experimental conditions. Nonetheless, these chondrocyte cell lines could be advantageous for OA investigation since, similarly to primary articular chondrocytes, they showed capacity to upregulate inflammatory mediators in response to the IL-1β cytokine.Cite this article: Bone Joint Res 2023;12(1):46-57.

Close-to-native bone repair via tissue-engineered endochondral ossification approaches

In order to solve the clinical challenges related to bone grafting, several tissue engineering (TE) strategies have been proposed to repair critical-sized defects. Generally, the classical TE approaches are designed to promote bone repair via intramembranous ossification. Although promising, strategies that direct the osteogenic differentiation of mesenchymal stem/stromal cells are usually characterized by a lack of functional vascular supply, often resulting in necrotic cores. A less explored alternative is engineering bone constructs through a cartilage-mediated approach, resembling the embryological process of endochondral ossification. The remodeling of an intermediary hypertrophic cartilaginous template triggers vascular invasion and bone tissue deposition. Thus, employing this knowledge can be a promising direction for the next generation of bone TE constructs. This review highlights the most recent biomimetic strategies for applying endochondral ossification in bone TE while discussing the plethora of cell types, culture conditions, and biomaterials essential to promote a successful bone regeneration process.

From regeneration to osteoarthritis in the knee joint: The role shift of cartilage-derived progenitor cells

A mount of growing evidence has proven that cartilage-derived progenitor cells (CPCs) harbor strong proliferation, migration, andmultiple differentiation potentials over the past 2 decades. CPCs in the stage of immature tissue play an important role in cartilage development process and injured cartilage repair in the young and active people. However, during maturation and aging, cartilage defects cannot be completely repaired by CPCs in vivo. Recently, tissue engineering has revealed that repaired cartilage defects with sufficient stem cell resources under good condition and bioactive scaffolds in vitro and in vivo. Chronic inflammation in the knee joint limit the proliferation and chondrogenesis abilities of CPCs, which further hampered cartilage healing and regeneration. Neocartilage formation was observed in the varus deformity of osteoarthritis (OA) patients treated with offloading technologies, which raises the possibility that organisms could rebuild cartilage structures spontaneously. In addition, nutritionmetabolismdysregulation, including glucose and free fatty acid dysregulation, could influence both chondrogenesis and cartilage formation. There are a few reviews about the advantages of CPCs for cartilage repair, but few focused on the reasons why CPCs could not repair the cartilage as they do in immature status. A wide spectrum of CPCs was generated by different techniques and exhibited substantial differences. We recently reported that CPCs maybe are as internal inflammation sources during cartilage inflammaging. In this review, we further streamlined the changes of CPCs from immature development to maturation and from healthy status to OA advancement. The key words including “cartilage derived stem cells”, “cartilage progenitor cells”, “chondroprogenitor cells”, “chondroprogenitors” were set for latest literature searching in PubMed and Web of Science. The articles were then screened through titles, abstracts, and the full texts in sequence. The internal environment including long-term inflammation, extendedmechanical loading, and nutritional elements intake and external deleterious factors were summarized. Taken together, these results provide a comprehensive understanding of the underlying mechanism of CPC proliferation and differentiation during development, maturation, aging, injury, and cartilage regeneration in vivo.

Phenotypic characterization of regional human meniscus progenitor cells

Stimulating meniscus regeneration using meniscal progenitor cells has been suggested as a promising new strategy. However, there is a lack of studies which decisively identify and characterize progenitor cell populations in human meniscus tissues. In this study, donor-matched progenitor cells were isolated via selective fibronectin adhesion from the avascular and vascular regions of the meniscus and chondroprogenitors from articular cartilage (n = 5). The mixed populations of cells from these regions were obtained by standard isolation techniques for comparison. The colony formation efficacy of avascular progenitors, vascular progenitors and chondroprogenitors was monitored using Cell-IQ® live cell imaging. Proliferation rates of progenitors were compared with their mixed population counterparts. Cell surface markers indicative of mesenchymal stromal cells profile and progenitor markers were characterized by flow cytometry in all populations. The fibrochondrogenic capacity was assessed via fibrochondrogenic differentiation and measuring GAG/DNA content and morphology. All meniscal progenitor and chondroprogenitor populations showed superior colony forming efficacy and faster proliferation rates compare to their mixed populations. Progenitor populations showed significantly higher positivity for CD49b and CD49c compared to their mixed population counterparts and chondroprogenitors had a higher positivity level of CD166 compared to mixed chondrocytes. GAG/DNA analysis demonstrated that progenitor cells generally produced more GAG than mixed populations. Our study demonstrates that the human meniscus contains meniscal progenitor populations in both the avascular and vascular regions. Meniscal progenitors derived from the vascular region exhibit enhanced proliferative and fibrochondrogenic characteristics compared to those from the avascular region; this may associate with the enhanced meniscal healing potential in the vascular region. These findings build on the body of evidence which suggests that meniscal progenitors represent an attractive cell therapy strategy for meniscal regeneration.

Stable human cartilage progenitor cell line stimulates healing of meniscal tears and attenuates post-traumatic osteoarthritis

Meniscal tearing in the knee increases the risk of post-traumatic osteoarthritis (OA) in patients. The therapeutic application of tissue-specific mesenchymal progenitor cells is currently being investigated as an emerging biologic strategy to help improve healing of musculoskeletal tissues like meniscal fibrocartilage and articular hyaline cartilage. However, many of these approaches involve isolating cells from healthy tissues, and the low yield of rare progenitor populations (< 1% of total cells residing in tissues) can make finding a readily available cell source for therapeutic use a significant logistical challenge. In the present study, we investigated the therapeutic efficacy of using expanded cartilage-derived and bone marrow-derived progenitor cell lines, which were stabilized using retroviral SV40, for repair of meniscus injury in a rodent model. Our findings indicate that these cell lines express the same cell surface marker phenotype of primary cells (CD54+, CD90+, CD105+, CD166+), and that they exhibit improved proliferative capacity that is suitable for extensive expansion. Skeletally mature male athymic rats treated with 3.2 million cartilage-derived progenitor cell line exhibited approximately 79% greater meniscal tear reintegration/healing, compared to injured animals that left untreated, and 76% greater compared to animals treated with the same number of marrow-derived stromal cells. Histological analysis of articular surfaces also showed that cartilage-derived progenitor cell line treated animals exhibited reduced post-traumatic OA associated articular cartilage degeneration. Stable cell line treatment did not cause tumor formation or off-target engraftment in animals. Taken together, we present a proof-of-concept study demonstrating, for the first time, that intra-articular injection of a stable human cartilage-derived progenitor cell line stimulates meniscus tear healing and provide chondroprotection in an animal model. These outcomes suggest that the use of stable cell lines may help overcome cell source limitations for cell-based medicine.

Improved biological performance of human cartilage-derived progenitors in platelet lysate xenofree media in comparison to fetal bovine serum media

Primary articular cartilage-derived cells are among the preferred contenders for cell-based therapy approaches for cartilage repair. Limited access to primary human cartilage tissue necessitates the process of in vitro cell expansion to obtain sufficient cells for therapeutic purposes. Therapeutic outcomes of such cell-based approaches become highly dependent on the quality of the in vitro culture-expanded cells. The objective of this study was to determine the differential biological effects of human platelet lysate (hPL) xeno-free defined media vs FBS containing traditional media on primary human cartilage-derived cells. Our goal in pursuing this work was to identify a preferred xenofree media alternative, that can be used as a platform for expansion of cells intended for clinical applications. Primary cartilage-derived cells obtained from five patients were simultaneously cultured in two expansion media's: (1) traditional (DMEM+10%FBS+1%P/S) and (2) defined xenofree (Nutristem® complete media+0.5%hPL). Connective tissue progenitors (CTPs) were assayed by standard colony forming unit assay, morphology, proliferation in early and late passages, expression of MSC associated cell-surface markers (CD73, CD90 and CD105) and trilineage differentiation (adipogenesis, osteogenesis and chondrogenesis) were considered for comparison of biological performance. Early biological performance of primary cartilage-derived cells was significantly improved in Nutristem® expansion media in comparison to traditional expansion media with respect to (1) Colony forming efficiency tended to be higher (p = 0.058) and (2) CTPs formed larger colonies with respect to total cells per colony and colony area (p < 0.01). In the culture expanded cell population, Nutristem® expansion media was superior to traditional expansion media with respect to: (1) overall proliferation rate through passages 1-4 (p = 0.027), (2) total cells harvested at end of passage 4 (p = 0.028) and (3) total positive stain area of CD73 (p = 0.006), CD90 (p = 0.001) and CD105 (p = 0.049). Nutristem®-hPL expanded cells when differentiated in respective xenofree serum-free defined MSCgo™ differentiated media's, also showed significant improvement in adipogenic, osteogenic and chondrogenic marker expression. Overall, we convincingly demonstrated that a low concentration of hPL in combination with defined xenofree media is an effective and economic growth supplement to culture expand primary cartilage-derived cells. It can be manufactured under cGMP conditions to improve clinical-grade cell products' quality for therapeutic applications.

Articulation inspired by nature: a review of biomimetic and biologically active 3D printed scaffolds for cartilage tissue engineering

In the human body, articular cartilage facilitates the frictionless movement of synovial joints. However, due to its avascular and aneural nature, it has a limited ability to self-repair when damaged due to injury or wear and tear over time. Current surgical treatment options for cartilage defects often lead to the formation of fibrous, non-durable tissue and thus a new solution is required. Nature is the best innovator and so recent advances in the field of tissue engineering have aimed to recreate the microenvironment of native articular cartilage using biomaterial scaffolds. However, the inability to mirror the complexity of native tissue has hindered the clinical translation of many products thus far. Fortunately, the advent of 3D printing has provided a potential solution. 3D printed scaffolds, fabricated using biomimetic biomaterials, can be designed to mimic the complex zonal architecture and composition of articular cartilage. The bioinks used to fabricate these scaffolds can also be further functionalised with cells and/or bioactive factors or gene therapeutics to mirror the cellular composition of the native tissue. Thus, this review investigates how the architecture and composition of native articular cartilage is inspiring the design of biomimetic bioinks for 3D printing of scaffolds for cartilage repair. Subsequently, we discuss how these 3D printed scaffolds can be further functionalised with cells and bioactive factors, as well as looking at future prospects in this field.

Decellularised Cartilage ECM Culture Coatings Drive Rapid and Robust Chondrogenic Differentiation of Human Periosteal Cells

The control of cell behaviour in an effort to create highly homogeneous cultures is becoming an area of intense research, both to elucidate fundamental biology and for regenerative applications. The extracellular matrix (ECM) controls many cellular processes in vivo, and as such is a rich source of cues that may be translated in vitro. Herein, we describe the creation of cell culture coatings from porcine decellularised hyaline cartilage through enzymatic digestion. Surprisingly, heat-mediated sterilisation created a coating with the capacity to rapidly and robustly induce chondrogenic differentiation of human periosteal cells. This differentiation was validated through the alteration of cell phenotype from a fibroblastic to a cuboidal/cobblestone chondrocyte-like appearance. Moreover, chondrogenic gene expression further supported this observation, where cells cultured on heat sterilised ECM-coated plastic displayed higher expression of COL2A1, ACAN and PRG4 (p < 0.05) compared to non-coated plastic cultures. Interestingly, COL2A1 and ACAN expression in this context were sensitive to initial cell density; however, SOX9 expression appeared to be mainly driven by the coating independent of seeding density. The creation of a highly chondrogenic coating may provide a cost-effective solution for the differentiation and/or expansion of human chondrocytes aimed towards cartilage repair strategies.

Cell Sources for Cartilage Repair-Biological and Clinical Perspective

Cell-based therapy represents a promising treatment strategy for cartilage defects. Alone or in combination with scaffolds/biological signals, these strategies open many new avenues for cartilage tissue engineering. However, the choice of the optimal cell source is not that straightforward. Currently, various types of differentiated cells (articular and nasal chondrocytes) and stem cells (mesenchymal stem cells, induced pluripotent stem cells) are being researched to objectively assess their merits and disadvantages with respect to the ability to repair damaged articular cartilage. In this paper, we focus on the different cell types used in cartilage treatment, first from a biological scientist’s perspective and then from a clinician’s standpoint. We compare and analyze the advantages and disadvantages of these cell types and offer a potential outlook for future research and clinical application.

Zone-Dependent Architecture and Biochemical Composition of Decellularized Porcine Nasal Cartilage Modulate the Activity of Adipose Tissue-Derived Stem Cells in Cartilage Regeneration

Previous anatomical studies have shown different functional zones in human nasal septal cartilage (NC). These zones differ in respect to histological architecture and biochemical composition. The aim of this study was to investigate the influence of these zones on the fate of stem cells from a regenerative perspective. Therefore, decellularized porcine septal cartilage was prepared and subjected to histological assessment to demonstrate its equivalence to human cartilage. Decellularized porcine NC (DPNC) exposed distinct surfaces depending on two different histological zones: the outer surface (OS), which is equivalent to the superficial zone, and the inner surface (IS), which is equivalent to the central zone. Human adipose tissue-derived stem cells (ASCs) were isolated from the abdominal fat tissue of five female patients and were seeded on the IS and OS of DPNC, respectively. Cell seeding efficiency (CSE), vitality, proliferation, migration, the production of sulfated glycosaminoglycans (sGAG) and chondrogenic differentiation capacity were evaluated by histological staining (DAPI, Phalloidin, Live-Dead), biochemical assays (alamarBlue^®, PicoGreen^®, DMMB) and the quantification of gene expression (qPCR). Results show that cell vitality and CSE were not influenced by DPNC zones. ASCs, however, showed a significantly higher proliferation and elevated expression of early chondrogenic differentiation, as well as fibrocartilage markers, on the OS. On the contrary, there was a significantly higher upregulation of hypertrophy marker MMP13 (p < 0.0001) and GAG production (p = 0.0105) on the IS, whereas cell invasion into the three-dimensional DPNC was higher in comparison to the OS. We conclude that the zonal-dependent distinct architecture and composition of NC modulates activities of ASCs seeded on DPNC. These findings might be used for engineering of cartilage substitutes needed in facial reconstructive surgery that yield an equivalent histological and functional structure, such as native NC.

Comparison of the efficiency of laminin versus fibronectin as a differential adhesion assay for isolation of human articular cartilage derived chondroprogenitors

Purpose: Cartilage repair following trauma or degeneration is poor, making cell-based therapy an important avenue of treatment. Chondrocytes and mesenchymal stem cells have been extensively studied as potential candidates, although tendency toward hypertrophy and formation of mixed hyaline-fibrocartilage necessitates further optimization. Chondroprogenitors, isolated using fibronectin adhesion assay are reported to show reduced hypertrophy and enhanced chondrogenesis. Laminin, an essential component of extracellular matrix, has been shown to positively modulate chondrocyte proliferation, migration, and survival. The aim of our study was to evaluate the effect of laminin as a differential adhesion assay and obtain an enriched population of chondroprogenitors and assess its efficiency when compared to progenitors obtained via fibronectin.Materials and methods: Chondrocytes were isolated from three osteoarthritic knee joints and subjected to fibronectin and laminin adhesion to obtain chondroprogenitors. After expansion in culture, they were assessed for differences in their biological characteristics based on growth kinetics, surface marker expression, gene expression for assessing markers of chondrogenesis and hypertrophy, and potential for tri-lineage differentiation.Results: Our results showed that cells isolated by laminin and fibronectin both displayed comparable characteristics except in terms of proliferative potential (higher in laminin), gene expression of COL2A1 (lower in laminin) and trilineage potential where the laminin group showed higher osteogenic and adipogenic differentiation.Conclusion: This was the first attempt to successfully isolate human articular cartilage derived chondroprogenitor clones using laminin, which retained stem cell like characteristics. Further evaluation to optimize this method will help enhance chondroprogenitor characteristics, for use in cartilage repair.

Human nasal septal chondrocytes (NSCs) preconditioned on NSC-derived matrix improve their chondrogenic potential

Background

Extracellular matrix (ECM) has a profound effect on cell behaviors. In this study, we prepare a decellularized human nasal septal chondrocyte (NSC)-derived ECM (CHDM), as a natural (N-CHDM) or soluble form (S-CHDM), and investigate their impact on NSCs differentiation.

Methods

N-CHDM, S-CHDM were obtained from NSC. To evaluate function of NSC cultured on each substrate, gene expression using chondrogenic marker, and chondrogenic protein expression were tested. Preconditioned NSCs-loaded scaffolds were transplanted in nude mice for 3 weeks and analyzed.

Results

When cultivated on each substrate, NSCs exhibited similar cell spread area but showed distinct morphology on N-CHDM with significantly lower cell circularity. They were highly proliferative on N-CHDM than S-CHDM and tissue culture plastic (TCP), and showed more improved cell differentiation, as assessed via chondrogenic marker (Col2, Sox9, and Aggrecan) expression and immunofluorescence of COL II. We also investigated the effect of NSCs preconditioning on three different 2D substrates while NSCs were isolated from those substrates, subsequently transferred to 3D mesh scaffold, then cultivated them in vitro or transplanted in vivo. The number of cells in the scaffolds was similar to each other at 5 days but cell differentiation was notably better with NSCs preconditioned on N-CHDM, as assessed via real-time q-PCR, Western blot, and immunofluorescence. Moreover, when those NSCs-loaded polymer scaffolds were transplanted subcutaneously in nude mice for 3 weeks and analyzed, the NSCs preconditioned on the N-CHDM showed significantly advanced cell retention in the scaffold, more cells with a chondrocyte lacunae structure, and larger production of cartilage ECM (COL II, glycosaminoglycan).

Conclusions

Taken together, a natural form of decellularized ECM, N-CHDM would present an advanced chondrogenic potential over a reformulated ECM (S-CHDM) or TCP substrate, suggesting that N-CHDM may hold more diverse signaling cues, not just limited to ECM component.

Differential Production of Cartilage ECM in 3D Agarose Constructs by Equine Articular Cartilage Progenitor Cells and Mesenchymal Stromal Cells

Identification of articular cartilage progenitor cells (ACPCs) has opened up new opportunities for cartilage repair. These cells may be used as alternatives for or in combination with mesenchymal stromal cells (MSCs) in cartilage engineering. However, their potential needs to be further investigated, since only a few studies have compared ACPCs and MSCs when cultured in hydrogels. Therefore, in this study, we compared chondrogenic differentiation of equine ACPCs and MSCs in agarose constructs as monocultures and as zonally layered co-cultures under both normoxic and hypoxic conditions. ACPCs and MSCs exhibited distinctly differential production of the cartilaginous extracellular matrix (ECM). For ACPC constructs, markedly higher glycosaminoglycan (GAG) contents were determined by histological and quantitative biochemical evaluation, both in normoxia and hypoxia. Differential GAG production was also reflected in layered co-culture constructs. For both cell types, similar staining for type II collagen was detected. However, distinctly weaker staining for undesired type I collagen was observed in the ACPC constructs. For ACPCs, only very low alkaline phosphatase (ALP) activity, a marker of terminal differentiation, was determined, in stark contrast to what was found for MSCs. This study underscores the potential of ACPCs as a promising cell source for cartilage engineering.

Characterization of human articular chondrocytes and chondroprogenitors derived from non-diseased and osteoarthritic knee joints to assess superiority for cell-based therapy

PURPOSE

Cell based therapy is constantly underway since regeneration of genuine hyaline cartilage is under par. Much attention has been afforded to chondroprogenitors recently, as an alternative cell substitute for cartilage repair. Although single source derivation of chondrocytes and chondroprogenitors is advantageous, lack of a characteristic differentiating marker obscures clear identification, which is essential to create a biological profile and is also required to assess cell type superiority for cartilage repair.

METHODS

Cells obtained from three non-diseased/osteoarthritic human knee joints each, were expanded in culture up to passage 10. Characterization studies were performed using flow cytometry; gene expression was studied using RT-PCR; growth kinetics and tri-lineage differentiation was also studied to construct a better profile of chondroprogenitors as well as chondrocytes.

RESULTS AND CONCLUSION

Our results showed that both cell populations exhibited similar cell surface characteristics except for non-diseased chondroprogenitors, which showed markedly low expression of CD34 and high expression of CD166. Trilineage data was suggestive of multilineage potential for both cell types with chondroprogenitors showing notably higher glycosaminoglycan and lower calcified matrix deposition. Data acquired from this study aided in describing cellular behavior of human articular cartilage derived chondroprogenitors in conditions not reported earlier. Our comparative analysis suggests that sorting based on a combination of markers (CD34- and CD166+) would yield a population of cells with minimal contamination by chondrocytes, which may provide translatable results in terms of enhanced chondrogenesis and reduced hypertrophy; both indispensable for the field of cartilage regeneration.

Comparison of Electrophysiological Properties and Gene Expression between Human Chondrocytes and Chondroprogenitors Derived from Normal and Osteoarthritic Cartilage

OBJECTIVES

Bone-marrow mesenchymal stem cells (MSCs) and chondrocytes are currently used for cell-based therapy in cartilage repair. Chondroprogenitors (CPs), resident cells of articular cartilage, demonstrate likeness to stem cells. Reports suggest that chondrocytes phenotype is altered in culture, thus making differentiation between the two cell populations difficult. Our objectives were to electrophysiologically assess chondrocytes and CPs, compare their mRNA expression with that of ionic channels already reported in MSCs, and to observe the effect of time in culture and osteoarthritic damage on cells.

DESIGN AND RESULTS

Chondrocytes and CPs at passages 0 (p0) and 5 (p5) derived from normal and osteoarthritic (OA) knee joints were used. Ionic currents were recorded by subjecting cells to depolarizing voltage pulses, and reverse transcriptase-polymerase chain reaction (RT-PCR) was used for studying ion channel expression. Our results demonstrated that both chondrocytes and CPs showed the presence of similar currents belonging to voltage-gated potassium channel subfamily, with RT-PCR confirming high mRNA expression of Maxi K, HKv1.1, HKv1.4, HKv4.2, and hEAG1 channels. Our finding also suggested that CPs were comparatively more sensitive to increased time in culture and inflammatory processes as observed in OA, as was evidenced by the significant decrease in mean current density (p0 normal CP: 183.171 ± 50.80 pA/pF; p5 normal CP: 50.225 ± 17.63 pA/pF; P = 0.0280) and significant increase in cellular size (p0 normal CP: 21.564 ± 2.98 pF; p0 OA CP: 37.939 ± 3.55 pF; P = 0.0057).

CONCLUSION

Both cell types appear to be optimal candidates for cell-based therapy although initial seeding density, cell source (normal vs. OA), and time in culture are matters of concern, prior to cell-type selection.

Evaluation of CD49e as a distinguishing marker for human articular cartilage derived chondroprogenitors

BACKGROUND

Cell-based therapy in cartilage repair can benefit from the use of chondroprogenitors; a cell type classified as mesenchymal stem cells, demonstrating reduced hypertrophy. Fibronectin, routinely used to isolate chondroprogenitors, classically binds to α5β1 integrins (CD49e + CD29), of which CD49e is said to be highly expressed in progenitors. The aim of our study was to assess the specificity of CD49e as a distinguishing marker for chondroprogenitors; because studies report low expression in fresh chondrocytes (FCs), but recent conflicting data has exhibited incremental expression of CD49e in cultured chondrocytes.

METHODS

FCs were isolated from three human osteoarthritic knee joints and CD49e- cells (sorted by flow cytometry) were cultured in adherent and non-adherent conditions and reassessed for CD49e and CD29 at multiple time points. Colony-forming efficiency (CFE) following fibronectin adhesion assay was calculated for FC, CD49e+ and CD49e- cells.

RESULTS

A statistically significant increase in CD49e and CD29 expression was seen in both adherent and non-adherent cultures of CD49e- cells (P < 0.01), as early as 24 h. All groups grew clonally and CFE was similar without any significant difference. CD49e- chondrocytes turned positive when cultured, possibly due to an inherent phenotypic drift, seen after release from cartilage and not because of plastic adherence or chondroprogenitor overgrowth, as non-adherent cultures also showed high expression.

CONCLUSIONS

As the specificity of CD49e is questionable, there is a pressing need for a specific differentiating marker, to isolate a pure population of chondroprogenitors, as this cell type shows inherent chondrogenesis and reduced hypertrophy, both requisites for cartilage repair.

Meniscus Repair and Regeneration: A Systematic Review from a Basic and Translational Science Perspective

Meniscus injuries are among the most common athletic injuries and result in functional impairment in the knee. Repair is crucial for pain relief and prevention of degenerative joint diseases like osteoarthritis. Current treatments, however, do not produce long-term improvements. Thus, recent research has been investigating new therapeutic options for regenerating injured meniscal tissue. This review comprehensively details the current methodologies being explored in the basic sciences to stimulate better meniscus injury repair. Furthermore, it describes how these preclinical strategies may improve current paradigms of how meniscal injuries are clinically treated through a unique and alternative perspective to traditional clinical methodology.

Three dimensional printed degradable and conductive polymer scaffolds promote chondrogenic differentiation of chondroprogenitor cells

We report a conductive and biodegradable 3D printed polymer scaffold that promotes chondrogenic differentiation of chondroprogenitor cells. The conductive material consists of tetraniline-b-polycaprolactone-b-tetraaniline and polycaprolactone.

Comparative analysis of fresh chondrocytes, cultured chondrocytes and chondroprogenitors derived from human articular cartilage

INTRODUCTION

Interest in chondroprogenitors arose due to their inherent stem cell like properties, and their initial characterization was based on identification of a small percentage of CD49e positive cells in cultured chondrocytes (CC). It was further noted that when fresh chondrocytes (FC; reported to express low CD49e) were subjected to fibronectin adhesion assay, an isolate of chondroprogenitors was obtained, which was highly positive for CD49e, thus making it a distinguishing marker for this cell population. However, this notion was challenged when reports demonstrated high CD49e expression in CC as well. Therefore, our aim was to compare CD49e expression in FC, CC and chondroprogenitors.

METHODS

Chondrocytes and chondroprogenitors were isolated from articular cartilage of osteoarthritic joints from three patients. Assessment of classic fibronectin receptor (CD49e, CD29), positive (CD105, CD73, CD90) and negative (CD45, CD34) mesenchymal stem cell marker expression in all groups was performed, as chondroprogenitors fulfill the minimal criteria laid down by International Society for Cellular Therapy. Following this, adipogenic, osteogenic and chondrogenic differentiation was assessed by Oil red O, Alizarin Red and Alcian Blue staining respectively.

RESULTS AND CONCLUSION

Our observations indicate that FC show significantly low surface marker expression as compared to CC and chondroprogenitors, whereas no significant difference was seen in values when CC and chondroprogenitors were compared. Moreover, comparable results were exhibited when trilineage differentiation potential was compared across groups. Since CC and chondroprogenitors show similar characteristics, there is a pressing need for a specific differentiating marker to isolate a pure population of chondroprogenitors.

Donor-matched comparison of chondrogenic progenitors resident in human infrapatellar fat pad, synovium, and periosteum - implications for cartilage repair

Purpose: There is a clinical need to better characterize tissue sources being used for stem cell therapies. This study focuses on comparison of cells and connective tissue progenitors (CTPs) derived from native human infrapatellar fatpad (IPFP), synovium (SYN), and periosteum (PERI). Materials and Methods: IPFP, SYN, PERI were harvested from twenty-eight patients undergoing arthroplasty. CTPs were quantitatively characterized using automated colony-forming-unit assay to compare total nucleated cell concentration-[Cell], cells/mg; prevalence-(P_CTP), CTPs/million nucleated cells; CTP concentration-[CTP], CTPs/mg; proliferation and differentiation potential; and correlate outcomes with patient's age and gender. Results: [Cell] did not differ between IPFP, SYN, and PERI. P_CTP was influenced by age and gender: patients >60 years, IPFP and SYN had higher P_CTP than PERI (p < 0.001) and females had higher P_CTP in IPFP (p < 0.001) and SYN (p = 0.001) than PERI. [CTP] was influenced by age: patients <50 years, SYN (p = 0.0165) and PERI (p < 0.001) had higher [CTP] than IPFP; patients between 60 and 69 years, SYN (p < 0.001) had higher [CTP] than PERI; patients >70 years, IPFP (p = 0.006) had higher [CTP] than PERI. In patients >60 years, proliferation potential of CTPs differed significantly (SYN>IPFP>PERI); however, differentiation potentials were comparable between all three tissue sources. Conclusion: SYN and IPFP may serve as a preferred tissue source for patients >60 years, and PERI along with SYN and IPFP may serve as a preferred tissue source for patients <60 years for cartilage repair. However, the heterogeneity among the CTPs in any given tissue source suggests performance-based selection might be useful to optimize cell-sourcing strategies to improve efficacy of cellular therapies for cartilage repair.

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.

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

Lentiviral-based reporter constructs for profiling chondrogenic activity in primary equine cell populations

Successful clinical translation of mesenchymal stem cell (MSC)-based therapies for cartilage repair will likely require the implementation of standardised protocols and broadly applicable tools to facilitate the comparisons among cell types and chondroinduction methods. The present study investigated the utility of recombinant lentiviral reporter vectors as reliable tools for comparing chondrogenic potential among primary cell populations and distinguishing cellular-level variations of chondrogenic activity in widely used three-dimensional (3D) culture systems. Primary equine MSCs and chondrocytes were transduced with vectors containing combinations of fluorescent and luciferase reporter genes under constitutive cytomeglavirus (CMV) or chondrocyte-lineage (Col2) promoters. Reporter activity was measured by fluorescence imaging and luciferase assay. In 3D cultures of MSC aggregates and polyethylene glycol-hyaluronic acid (PEG-HA) hydrogels, transforming growth factor beta 3 (TGF-β3)-mediated chondroinduction increased Col2 reporter activity, demonstrating close correlation with histology and mRNA expression levels of COL2A1 and SOX9. Comparison of chondrogenic activities among MSC populations using a secretable luciferase reporter revealed enhanced chondrogenesis in bone-marrow-derived MSCs relative to MSC populations from synovium and adipose tissues. A dual fluorescence reporter - enabling discrimination of highly chondrogenic (Col2-GFP) cells within an MSC population (CMV-tdTomato) - revealed marked heterogeneity in differentiating aggregate cultures and identified chondrogenic cells in chondrocyte-seeded PEG-HA hydrogels after 6 weeks in a subcutaneous implant model - indicating stable, long-term reporter expression in vivo. These results suggested that lentiviral reporter vectors may be used to address fundamental questions regarding chondrogenic activity in chondroprogenitor cell populations and accelerate clinical translation of cell-based cartilage repair strategies.

Sheep as a model for evaluating mesenchymal stem/stromal cell (MSC)-based chondral defect repair

Osteoarthritis results from the degradation of articular cartilage and is one of the leading global causes of pain and immobility. Cartilage has a limited capacity for self-repair. While repair can be enhanced through surgical intervention, current methods often generate inferior fibrocartilage and repair is transient. The development of tissue engineering strategies to improve repair outcomes is an active area of research. While small animal models such as rodents and rabbits are often used in early pre-clinical work, larger animals that better recapitulate the anatomy and loading of the human joint are required for late-stage preclinical evaluation. Because of their physiological similarities to humans, and low cost relative to other large animals, sheep are routinely used in orthopedic research, including cartilage repair studies. In recent years, there has been considerable research investment into the development of cartilage repair strategies that utilize mesenchymal stem/stromal cells (MSC). In contrast to autologous chondrocytes derived from biopsies of articular cartilage, MSC offer some benefits including greater expansion capacity and elimination of the risk of morbidity at the cartilage biopsy site. The disadvantages of MSC are related to the challenges of inducing and maintaining a stable chondrocyte-like cell population capable of generating hyaline cartilage. Ovine MSC (oMSC) biology and their utility in sheep cartilage repair models have not been reviewed. Herein, we review the biological properties of MSC derived from sheep tissues, and the use of these cells to study articular cartilage repair in this large animal model.

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.

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.

The bio in the ink: cartilage regeneration with bioprintable hydrogels and articular cartilage-derived progenitor cells

Cell-laden hydrogels are the primary building blocks for bioprinting, and, also termed bioinks, are the foundations for creating structures that can potentially recapitulate the architecture of articular cartilage. To be functional, hydrogel constructs need to unlock the regenerative capacity of encapsulated cells. The recent identification of multipotent articular cartilage-resident chondroprogenitor cells (ACPCs), which share important traits with adult stem cells, represents a new opportunity for cartilage regeneration. However, little is known about the suitability of ACPCs for tissue engineering, especially in combination with biomaterials. This study aimed to investigate the potential of ACPCs in hydrogels for cartilage regeneration and biofabrication, and to evaluate their ability for zone-specific matrix production. Gelatin methacryloyl (gelMA)-based hydrogels were used to culture ACPCs, bone marrow mesenchymal stromal cells (MSCs) and chondrocytes, and as bioinks for printing. Our data shows ACPCs outperformed chondrocytes in terms of neo-cartilage production and unlike MSCs, ACPCs had the lowest gene expression levels of hypertrophy marker collagen type X, and the highest expression of PRG4, a key factor in joint lubrication. Co-cultures of the cell types in multi-compartment hydrogels allowed generating constructs with a layered distribution of collagens and glycosaminoglycans. By combining ACPC- and MSC-laden bioinks, a bioprinted model of articular cartilage was generated, consisting of defined superficial and deep regions, each with distinct cellular and extracellular matrix composition. Taken together, these results provide important information for the use of ACPC-laden hydrogels in regenerative medicine, and pave the way to the biofabrication of 3D constructs with multiple cell types for cartilage regeneration or in vitro tissue models.

STATEMENT OF SIGNIFICANCE

Despite its limited ability to repair, articular cartilage harbors an endogenous population of progenitor cells (ACPCs), that to date, received limited attention in biomaterials and tissue engineering applications. Harnessing the potential of these cells in 3D hydrogels can open new avenues for biomaterial-based regenerative therapies, especially with advanced biofabrication technologies (e.g. bioprinting). This study highlights the potential of ACPCs to generate neo-cartilage in a gelatin-based hydrogel and bioink. The ACPC-laden hydrogel is a suitable substrate for chondrogenesis and data shows it has a bias in directing cells towards a superficial zone phenotype. For the first time, ACPC-hydrogels are evaluated both as alternative for and in combination with chondrocytes and MSCs, using co-cultures and bioprinting for cartilage regeneration in vitro. This study provides important cues on ACPCs, indicating they represent a promising cell source for the next generation of cartilage constructs with increased biomimicry.

Adipose, Bone Marrow and Synovial Joint-Derived Mesenchymal Stem Cells for Cartilage Repair

Current cell-based repair strategies have proven unsuccessful for treating cartilage defects and osteoarthritic lesions, consequently advances in innovative therapeutics are required and mesenchymal stem cell-based (MSC) therapies are an expanding area of investigation. MSCs are capable of differentiating into multiple cell lineages and exerting paracrine effects. Due to their easy isolation, expansion, and low immunogenicity, MSCs are an attractive option for regenerative medicine for joint repair. Recent studies have identified several MSC tissue reservoirs including in adipose tissue, bone marrow, cartilage, periosteum, and muscle. MSCs isolated from these discrete tissue niches exhibit distinct biological activities, and have enhanced regenerative potentials for different tissue types. Each MSC type has advantages and disadvantages for cartilage repair and their use in a clinical setting is a balance between expediency and effectiveness. In this review we explore the challenges associated with cartilage repair and regeneration using MSC-based cell therapies and provide an overview of phenotype, biological activities, and functional properties for each MSC population. This paper also specifically explores the therapeutic potential of each type of MSC, particularly focusing on which cells are capable of producing stratified hyaline-like articular cartilage regeneration. Finally we highlight areas for future investigation. Given that patients present with a variety of problems it is unlikely that cartilage regeneration will be a simple "one size fits all," but more likely an array of solutions that need to be applied systematically to achieve regeneration of a biomechanically competent repair tissue.

Responses to altered oxygen tension are distinct between human stem cells of high and low chondrogenic capacity

BACKGROUND

Lowering oxygen from atmospheric level (hyperoxia) to the physiological level (physioxia) of articular cartilage promotes mesenchymal stem cell (MSC) chondrogenesis. However, the literature is equivocal regarding the benefits of physioxic culture on preventing hypertrophy of MSC-derived chondrocytes. Articular cartilage progenitors (ACPs) undergo chondrogenic differentiation with reduced hypertrophy marker expression in hyperoxia but have not been studied in physioxia. This study sought to delineate the effects of physioxic culture on both cell types undergoing chondrogenesis.

METHODS

MSCs were isolated from human bone marrow aspirates and ACP clones were isolated from healthy human cartilage. Cells were differentiated in pellet culture in physioxia (2 % oxygen) or hyperoxia (20 % oxygen) over 14 days. Chondrogenesis was characterized by biochemical assays and gene and protein expression analysis.

RESULTS

MSC preparations and ACP clones of high intrinsic chondrogenicity (termed high-GAG) produced abundant matrix in hyperoxia and physioxia. Poorly chondrogenic cells (low-GAG) demonstrated a significant fold-change matrix increase in physioxia. Both high-GAG and low-GAG groups of MSCs and ACPs significantly upregulated chondrogenic genes; however, only high-GAG groups had a concomitant decrease in hypertrophy-related genes. High-GAG MSCs upregulated many common hypoxia-responsive genes in physioxia while low-GAG cells downregulated most of these genes. In physioxia, high-GAG MSCs and ACPs produced comparable type II collagen but less type I collagen than those in hyperoxia. Type X collagen was detectable in some ACP pellets in hyperoxia but reduced or absent in physioxia. In contrast, type X collagen was detectable in all MSC preparations in hyperoxia and physioxia.

CONCLUSIONS

MSC preparations and ACP clones had a wide range of chondrogenicity between donors. Physioxia significantly enhanced the chondrogenic potential of both ACPs and MSCs compared with hyperoxia, but the magnitude of response was inversely related to intrinsic chondrogenic potential. Discrepancies in the literature regarding MSC hypertrophy in physioxia can be explained by the use of low numbers of preparations of variable chondrogenicity. Physioxic differentiation of MSC preparations of high chondrogenicity significantly decreased hypertrophy-related genes but still produced type X collagen protein. Highly chondrogenic ACP clones had significantly lower hypertrophic gene levels, and there was little to no type X collagen protein in physioxia, emphasizing the potential advantage of these cells.

Cartilage defect repair in horses: Current strategies and recent developments in regenerative medicine of the equine joint with emphasis on the surgical approach

Chondral and osteochondral lesions due to injury or other pathology are highly prevalent conditions in horses (and humans) and commonly result in the development of osteoarthritis and progression of joint deterioration. Regenerative medicine of articular cartilage is an emerging clinical treatment option for patients with articular cartilage injury or disease. Functional articular cartilage restoration, however, remains a major challenge, but the field is progressing rapidly and there is an increasing body of supportive clinical and scientific evidence. This review gives an overview of the established and emerging surgical techniques employed for cartilage repair in horses. Through a growing insight in surgical cartilage repair possibilities, surgeons might be more stimulated to explore novel techniques in a clinical setting.