BackgroundArticular cartilage displays a poor repair capacity. The aim of cell-based therapies for cartilage defects is to repair damaged joint surfaces with a functional replacement tissue. Currently, chondrocytes removed from a healthy region of the cartilage are used but they are unable to retain their phenotype in expanded culture. The resulting repair tissue is fibrocartilaginous rather than hyaline, potentially compromising long-term repair. Mesenchymal stem cells, particularly bone marrow stromal cells (BMSC), are of interest for cartilage repair due to their inherent replicative potential. However, chondrocyte differentiated BMSCs display an endochondral phenotype, that is, can terminally differentiate and form a calcified matrix, leading to failure in long-term defect repair. Here, we investigate the isolation and characterisation of a human cartilage progenitor population that is resident within permanent adult articular cartilage.Methods and FindingsHuman articular cartilage samples were digested and clonal populations isolated using a differential adhesion assay to fibronectin. Clonal cell lines were expanded in growth media to high population doublings and karyotype analysis performed. We present data to show that this cell population demonstrates a restricted differential potential during chondrogenic induction in a 3D pellet culture system. Furthermore, evidence of high telomerase activity and maintenance of telomere length, characteristic of a mesenchymal stem cell population, were observed in this clonal cell population. Lastly, as proof of principle, we carried out a pilot repair study in a goat in vivo model demonstrating the ability of goat cartilage progenitors to form a cartilage-like repair tissue in a chondral defect.ConclusionsIn conclusion, we propose that we have identified and characterised a novel cartilage progenitor population resident in human articular cartilage which will greatly benefit future cell-based cartilage repair therapies due to its ability to maintain chondrogenicity upon extensive expansion unlike full-depth chondrocytes that lose this ability at only seven population doublings.

ObjectivesOsteoarthritis (OA) is a debilitating disease affecting more than 4 million people in the United Kingdom. Despite its prevalence, there is no successful cell-based therapy currently used to treat patients whose cartilage is deemed irrecoverable. The present study aimed to isolate stem cells from tibial plateaux cartilage obtained from patients who underwent total knee replacements for OA and investigate their stem cell characteristics.DesignClonally derived cell lines were selected using a differential adhesion assay to fibronectin and expanded in monolayer culture. Colony forming efficiencies and growth kinetics were investigated. The potential for tri-lineage differentiation into chondrogenic, osteogenic, and adipogenic phenotypes were analyzed using histological stains, immunocytochemistry, and reverse transcriptase polymerase chain reaction.ResultsColony forming cells were successfully isolated from osteoarthritic cartilage and extensively expanded in monolayer culture. Colony forming efficiencies were consistently below 0.1%. Clonal cell lines were expanded beyond 40 population doublings but disparities were observed in the number of population doublings per day. Clonally derived cell lines also demonstrated in vitro multilineage potential via successful differentiation into chondrogenic, osteogenic, and adipogenic lineages. However, variation in the degree of differentiation was observed between these clonal cell lines.ConclusionsA viable pool of cells with stem cell characteristics have been identified within human osteoarthritic cartilage. Variation in the degree of differentiation suggests the possibility of further subpopulations of cells. The identification of this stem cell population highlights the reparative potential of these cells in osteoarthritic cartilage, which could be further exploited to aid the field of regenerative medicine.

The development of limb cartilage involves complex signalling pathways allowing the formation of distinct segments of cartilage that are maintained in the fully developed joint. In this study, we investigated the Notch signalling pathway and its role in cartilage development. The differential distribution of the Notch signalling family of receptors and their corresponding ligands in developing avian ( gallus gallus ) cartilage revealed expression of Notch 1, Delta 1, Jagged 1 and Jagged 2 in all limb mesenchyme cells at the early stages of cartilage anlagen development, which were subsequently restricted to the developing cartilage element. Expression of both Notch 1 and Jagged 1 became increasingly restricted to the surface cartilage once joint cavity formation had occurred. Delta 1 and Jagged 1 were restricted to a layer of cells underneath the surface cartilage and were also observed in the hypertrophic chondrocytes, where Notch 1 expression was evident in stage 40-44 limbs. Notch 2, Notch 3 and Notch 4 were not evident in early stage limbs but were present after cavitation, although expression was lost in late stage limbs (stage 40-44). We also demonstrated that inhibition of the Notch pathway leads to altered Notch receptor expression, disrupting cartilage differentiation. From these data it is clear that Notch signalling is a necessary and critical factor in regulating cell fate decisions allowing controlled chondrogenesis, elongation and subsequent maintenance of limb cartilage.

Although work is still in its infancy, the use of stem cells in the biological repair of articular cartilage provides a promising outlook onto future developments; advancing from strategies and techniques that are already in use.

The poor reparative potential of articular cartilage is largely attributed to its avascular and aneural status combined with low cellularity; chondrocytes only occupy 10% of the tissue volume. Consequently, there have been a number of strategies developed to augment repair most notably through microfracture and Autologous Chondrocyte Implanation (ACI). However, both of these techniques have limitations. In the case of microfracture, the repair tissue is often fibrocartilaginous and in ACI, the number of cells that can be generated that maintain chondrogenicity is limited thus restricting the size of defect that can be treated. Consequently, there has been increasing interest in the use of Mesenchymal Stem Cells (MSCs) for the treatment of cartilage defects. Here, we discuss the isolation and characterization of a tissue-specific stem cell population from articular cartilage and the potential for its use in intrinsic and extrinsic repair of articular cartilage lesions. Significantly, unlike MSCs isolated from bone marrow, upon differentiation into the chondrogenic lineage, these cells fail to terminally differentiate i.e. are not endochondral, failing to synthesize both collagen type X and alkaline phosphatase. Senescence of chondrocytes following injury or as part of aging is hypothesized to be integral to degenerative disease and correlates with significant telomere erosion within chondrocytes. Articular cartilage-derived stem cells exhibit detectable telomerase activity and exhibit reduced erosion of telomeres; therefore, we hypothesize maintenance of this population is critical to tissue homeostasis.