Rebecca Williams

Identification and Clonal Characterisation of a Progenitor Cell Sub-Population in Normal Human Articular Cartilage

Background Articular 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 Findings Human 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. Conclusions In 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.

The Development of Synovial Joints

Synovial joints arise through two main processes. In long bone elements, cartilaginous differentiation occurs across the locations of the prospective joints that then segment secondarily. This process occurs through the development of a noncartilaginous region known as the interzone. The interzone becomes an important signaling center to the opposing elements, which can regulate growth through such factors as GDF-5. The interzone also expresses bone morphogenetic proteins (BMPs) and their antagonists, such as noggin. Overexpression of BMPs, or the loss of noggin leads to joint fusions. The interzone also expresses Wnt-14, which appears to be specific for this region in the developing anlagen, and regulates its nonchondrogenic nature. Cavitation of the joint follows, driven by selective high-level synthesis of hyaluronan by interzone cells and presumptive synovial cells. In addition, as the interzone disperses during cavity enlargement, data are now accruing that suggest that both the synovium and articular cartilage develop from this population. Finally, the development of articular cartilage progresses through appositional growth driven by a progenitor/stem cell subpopulation that resides in the articular surface. The individual elements of the skeleton are connected together at regions termed joints or articulations. Classically, there are three broad categories of joints: immovable joints (syntharthroses); mixed articulations, in which the range of movement is limited (amphiarthroses); and the movable, or synovial, joints (diarthroses). This review concentrates on the development of the synovial joints.

One Flew over the Progenitor's Nest: Migratory Cells Find a Home in Osteoarthritic Cartilage

Articular cartilage is the target tissue of osteoarthritis (OA), a degenerative disease with no cure. In this issue of Cell Stem Cell, Miosge and colleagues (Koelling et al., 2009) report that migratory progenitor cells occupy degenerating OA tissue but that this population is not present in healthy cartilage.

‘It’s all my interpretation’: Reading Spike through the subcultural celebrity of James Marsters

This article considers how fans of Buffy the Vampire Slayer and Angel interpret the character of Spike through meanings attached to actor James Marsters as a ‘subcultural celebrity’. Work on TV’s celebrity actors has stressed how character and actor can become semiotically blurred. Rather than approaching this blurring of textual and extra-textual connotations as an essential property of television celebrity, we analyse how Marsters displays situated agency by discursively constructing ‘himself’ in publicity materials as ‘like Spike’. We then consider Marsters as a reader of Buffy. As a subcultural celebrity, we argue that Marsters is positioned between media producers and media fans, and therefore is able to offer up privileged interpretations of ‘his’ character, Spike, while simultaneously observing the symbolic power of producers’ preferred readings. Marsters supports certain fan readings of Spike, acting as a textual poacher who nevertheless is ‘inside’ the texts of Buffy the Vampire Slayer and Angel.

Notch receptor and Notch ligand expression in developing avian cartilage

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.

Evidence of a Viable Pool of Stem Cells within Human Osteoarthritic Cartilage

Objectives Osteoarthritis (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. Design Clonally 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. Results Colony 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. Conclusions A 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.

Characterisation of a divergent progenitor cell sub-populations in human osteoarthritic cartilage: the role of telomere erosion and replicative senescence

In recent years it has become increasingly clear that articular cartilage harbours a viable pool of progenitor cells and interest has focussed on their role during development and disease. Analysis of progenitor numbers using fluorescence-activated sorting techniques has resulted in wide-ranging estimates, which may be the result of context-dependent expression of cell surface markers. We have used a colony-forming assay to reliably determine chondroprogenitor numbers in normal and osteoarthritic cartilage where we observed a 2-fold increase in diseased tissue (P  < 0.0001). Intriguingly, cell kinetic analysis of clonal isolates derived from single and multiple donors of osteoarthritic cartilage revealed the presence of a divergent progenitor subpopulation characterised by an early senescent phenotype. Divergent sub-populations displayed increased senescence-associated β–galactosidase activity, lower average telomere lengths but retained the capacity to undergo multi-lineage differentiation. Osteoarthritis is an age-related disease and cellular senescence is predicted to be a significant component of the pathological process. This study shows that although early senescence is an inherent property of a subset of activated progenitors, there is also a pool of progenitors with extended viability and regenerative potential residing within osteoarthritic cartilage.

Human Articular Cartilage Progenitor Cells Are Responsive to Mechanical Stimulation and Adenoviral-Mediated Overexpression of Bone-Morphogenetic Protein 2

Articular cartilage progenitor cells (ACPCs) represent a new and potentially powerful alternative cell source to commonly used cell sources for cartilage repair, such as chondrocytes and bone-marrow derived mesenchymal stem cells (MSCs). This is particularly due to the apparent resistance of ACPCs to hypertrophy. The current study opted to investigate whether human ACPCs (hACPCs) are responsive towards mechanical stimulation and/or adenoviral-mediated overexpression of bone morphogenetic protein 2 (BMP-2). hACPCs were cultured in fibrin-polyurethane composite scaffolds. Cells were cultured in a defined chondro-permissive medium, lacking exogenous growth factors. Constructs were cultured, for 7 or 28 days, under free-swelling conditions or with the application of complex mechanical stimulation, using a custom built bioreactor that is able to generate joint-like movements. Outcome parameters were quantification of BMP-2 and transforming growth factor beta 1 (TGF-β1) concentration within the cell culture medium, biochemical and gene expression analyses, histology and immunohistochemistry. The application of mechanical stimulation alone resulted in the initiation of chondrogenesis, demonstrating the cells are mechanoresponsive. This was evidenced by increased GAG production, lack of expression of hypertrophic markers and a promising gene expression profile (significant up-regulation of cartilaginous marker genes, specifically collagen type II, accompanied by no increase in the hypertrophic marker collagen type X or the osteogenic marker alkaline phosphatase). To further investigate the resistance of ACPCs to hypertrophy, overexpression of a factor associated with hypertrophic differentiation, BMP-2, was investigated. A novel, three-dimensional, transduction protocol was used to transduce cells with an adenovirus coding for BMP-2. Over-expression of BMP-2, independent of load, led to an increase in markers associated with hypertropy. Taken together ACPCs represent a potential alterative cell source for cartilage tissue engineering applications.

“This Is the Night TV Died”: Television Post-Object Fandom and the Demise of The West Wing

This article examines fan identity and ontological security through the work of sociologist Anthony Giddens (1991, 1992) to explore how fans articulated their responses to the ending of the American political drama series The West Wing. The article discusses how fans negotiated their reactions to this cessation through a “reiteration discourse” in which they linked the show to their self-narratives and often interpreted and understood the show and its characters through their own real-life experiences. The article also examines how fans responded in the years after the shows cancellation, considering how they responded at the moment that their favored object ceased to be ongoing, and when their fandom moved into an era of what can be termed “post-object fandom.”

Remembering Ourselves, Viewing the Others: Historical Reality Television and Celebrity in the Small Nation

This article explores the specificity of media participation in a small nation, Wales, through empirical research on participants in historical reality television. It takes as its focus the case study of BBC Wales’s multiplatform project, Coal House (Indus, 2007) and Coal House at War (Indus, 2008), which exemplifies how public service broadcasters in the digital era seek to cultivate diverse forms of participation from national and regional audiences. Drawing on interviews, text-in-action participant observation, and online postings, the authors examine how participants and their families negotiate issues of experience and embodiment, engaging in unpaid media labor to protect and promote their own experiences and interpretations of the show. In contrast to theories of celebrity emerging from analyses of globalized formats such as Big Brother, the authors propose the concept of the “localebrity” to explain how celebrity functions in the local and regional context of the small nation.