Chondrocytes kept in monolayer culture at low density seem to lose their phenotype within four passages or approximately one month in culture. They change from a polygonal or round to a more flattened ameboid shape and synthesize type I collagen instead of cartilage collagen II.51 The relationship between the shape of the cells and the type of collagen produced is not clear. In ACI, cartilage is harvested from the non-weightbearing part of the patient's own knee and chondrocytes are cultured and expanded and returned to the same patient for repair of chondral defects. Martin and Buckwalter52 have shown that the telomeres shorten with age in chon-drocytes as they do in other cells. This implies that in an older patient the autologous chondrocytes that are re-implanted may suffer the fate of earlier degeneration. Animal experiments in our laboratory comparing chondro-cyte to mesenchymal stem cell transfer for repair of cartilage defects have clearly demonstrated that the joints repaired with chondrocytes showed degenerative changes in the repaired cartilage within 36 weeks, whereas the joints repaired by stem cells were still intact at that time.90 As the longevity of the repair of cartilage defects is under question, it may be worthwhile examining the role of stem cells in cartilage repair, as stem cells or progenitor cells may in fact differentiate into chondrocytes which may produce a long lasting repair.
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