Plasticity and interactions of articular cartilage progenitor cells
Marcus, Paula Louise
Articular cartilage is an avascular and aneural tissue and this is, in part, attributable to its low intrinsic capacity for repair after injury. Research is now focusing on alternate cell sources for tissue engineering of damaged cartilage, and recently a population of progenitor cells has been identified within the surface zone of bovine articular cartilage. These cells are capable of differentiating along a variety of mesenchymal lineages and are thought to be required for the appositional growth of the cartilage. The aims of this thesis were to further characterise these cells and determine factors affecting their differentiation. Prolonged growth of the clonal cells in culture was found to alter the ability of the cells to differentiate into a hyaline-like tissue, although these changes didn't always result in a decrease in the chondrogenic capacity. The rate of cell growth was also found to slightly affect the ability of the cells to differentiate, with more rapidly growing cells producing a matrix high in glycosaminoglycans. After short term culture, the cells also altered their expression of three different glycosaminoglycans sulphate epitopes 3B3(-), 4C3 and 7D4. When injected intramuscularly, the chondroprogenitor cells failed to form cartilage pellets despite expressing cartilage related genes. The progenitor cells also appeared unable to functionally engraft into the surrounding tissue, although one clonal cell line expressed the endothelial marker PECAM-1. Within this study we also assessed the ability of the chondroprogenitor cells to express connexins, and form functional gap junctions. The cells were found to fluctuate their connexin expression, although they maintained Cx43 expression throughout culture. Using a novel ultrasound standing wave trap, it was found that the cells failed to upregulate connexin after cell contact resulting in non-functional junctions, whilst the cells were able to form functional gap junctions with terminally differentiated chondrocytes. Treating the clonal cells with growth factors to enhance chondrogenesis also failed to cause the cells to functionally communicate. Finally we looked at the cellular organisation of the tissue to determine if paired cells within the surface zone of the cartilage may contain a progenitor population. These paired cells labelled positively for Notch-1, which is known to affect the clonality of the progenitor cells and could possibly signify the presence of the progenitor cell population. Cellular interactions are vital for controlling and coordinating cell differentiation, and manipulating cellular interactions could be an excellent way to enhance the chondrogenic differentiation of the cells and possibly improve tissue integration.