The identification and partial characterisation of an articular cartilage progenitor/stem cell

We have previously shown that articular cartilage grows by apposition from cells occupying the upper zones of the tissue (surface and transitional). Using the South American opposum as a model system, we showed that cells associated with the articular surface were immunopositive for PCNA, IGF I and II together with 3 isoforms of TGFβ. Further, by repeated intra-articular injection of the thymidine analogue bromo-deoxyuridine and subsequent immunodetection, we showed initial incorporation into the transitional zone cells followed by incorporation into the flattened cells of the articular surface indicating that these cells were in extended cycle compared with transitional zone cells. Using 7-day bovine articular cartilage, we have isolated articular surface chondrocytes by scalpel dissection followed by sequential enzyme digestion. Isolated chondrocytes (4,000/ml) were then subjected to a differential adhesion assay on fibronectin coated dishes that is preferentially expressed in the articular surface. We found that cells that had high affinity for the ligand (20 min attachment time) represented 10% of the initial population compared with 4 % that adhered in the following 20 min and the remaining 75+% that adhered after 40 min. FACS analysis showed that the initial 10% cohort with high affinity for fibronectin had significantly elevated levels of α5β1 integrins (fibronectin receptor) than the remaining two cohorts. In addition, it was found that the high affinity cohort had a 400% higher colony forming efficiency than the lower affinity cohorts. Taken together, these data suggest that there is a sub-population of cells that reside in the articular surface that show classical features of a progenitor/stem cell that has been described in other tissues.