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The effects of synovial conditioned medium (SCM) on DNA, proteoglycan (PG), and protein-collagen synthesis and respective gene expressions, in human articular chondrocytes (AC) and DNA synthesis in synovial fibroblasts (SFb), were studied in monolayer culture. All SCM exhibited concentration-dependent inhibition of [3H]thymidine incorporation in both AC and SFb. In contrast, SCM from three OA patients stimulated [35S]SO4 and [3H]glycine incorporations and the expression (RT-PCR) of aggrecan- and type II collagen-specific mRNAs in AC. The production of agents that inhibit DNA synthesis was blocked by indomethacin and dexamethasone and stimulated by IL-1 beta and TNF-alpha. The inhibitory substances were not produced by heat-inactivated tissue nor cultured SFb or AC and were completely solubles in methanol. It is postulated that synovial tissue secretes lipids, most probably arachidonic acid metabolites. These may counteract growth of an inflammatory synovial pannus by inhibiting SFb proliferation and enhance repair of damaged tissues by stimulating the matrix synthesis.

Chondrocytes were released from articular cartilage fragments of 6-week-old Wistar rats by a 2-hour treatment with bacterial collagenase. The cells from one animal were seeded in a 25-cm² culture flask at a density of 10⁵ cells/cm². After 1 h, the flask was gently shaken and the medium, containing nonadherent cells, was transferred to a new flask. The attached cells were incubated with 5 ml of fresh medium. This procedure was repeated after 3, 24, 48 and 96 h. Resulting cell populations were then analyzed. The earlier cells attached, the more rapidly they proliferated, and the less collagen and proteoglycan (PG) they produced. The cells that attached after 24 h grew much more slowly, piled up in many areas, exhibited strong alkaline phosphatase activity and calcified extracellular matrix (ECM). Differences in deoxyribonucleic acid (DNA) and protein/PG synthesis were also observed when these cell populations were challenged with growth factors and 12-myristate 13-acetate (PMA). Pretreatment of cells for 2 h with PMA strongly enhanced DNA and PG synthesis only in cultures containing insulin-like growth factor-1. Nonselected rat articular chondrocytes (AC) subcultured at least four times as monolayers still expressed mRNA specific for aggrecan and type II collagen, suggesting conservation of the chondrogenic phenotype. In conclusion, AC of young individuals seem to be heterogeneous with respect to their capacity to proliferate and synthesize ECM. By selecting and expanding in vitro the appropriate cell population, this method could be potentially useful for studies aimed at repairing damaged cartilage.