Chondroprogenitor cells of synovial tissue

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Abstract
Objective To assess the chondrogenic potential of cells within the synovium. Methods Explants of synovium taken from various sites in the joint were embedded in agarose and cultured with transforming growth factor β1 (TGFβ1) to assess their chondrogenic potential. Isolated synovial cells were also tested for their chondrogenic potential by culturing them as aggregates in a chemically defined medium with TGFβ1. Cartilage formation was determined with histologic staining and immunohistochemistry. The osteochondral potential of the isolated cells was also assessed after subcutaneous implantation of the cells, loaded into porous calcium phosphate ceramic cubes, in athymic mice. Results A total of 48 synovial explants were cultured in agarose with TGFβ1. The formation of cartilage was observed in the outer region of 21 explants, and type II collagen was localized in that region by immunohistochemistry. A larger percentage of TGFβ1+ explants from the inner synovium sites formed cartilage compared with those from the outer synovium sites. Chondrogenesis occurred in aggregates incubated with TGFβ1 as early as day 7, and by day 14, all TGFβ1+ aggregates demonstrated chondrogenesis. In contrast with the results of the in vitro aggregate assay for chondrogenesis, no formation of cartilage or bone was evident in any section containing synovial cell–loaded ceramic cubes that were harvested at either 3 or 6 weeks after implantation subcutaneously in athymic mice. Conclusion Synovial explants and isolated synovial cells will undergo chondrogenesis when cultured in the presence of TGFβ1. The data indicate a possible synovial origin for the chondrocytic cells found in rheumatoid pannus. Furthermore, these data are consistent with the clinical findings of synovial chondrogenesis leading to synovial chondromatosis.

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