This study investigated which of the reciprocal stimuli between articular chondrocytes (ACs) and mesenchymal stem cells (MSCs) played the more important role in enhancing cartilage matrix formation, and examined the relative importance of physical contact and soluble factors in the co-culture system.Rat ACs and bone marrow MSCs with green fluorescent protein (GFP-BMSCs) were co-cultured in vitro with or without direct cell-cell contact at the ratio of 2:1. After co-culturing in direct cell-cell contact, ACs and GFP-BMSCs were separated by flow cytometry. The effects of different co-culture methods were analysed by quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) and western blotting.SOX-9, COL2 and aggrecan mRNA levels and protein expression in ACs co-cultured with direct cell-cell contact were significantly higher than in ACs co-cultured without direct cell-cell contact; and similar results were found in GFP-BMSCs. After co-culture either with or without direct cell-cell contact, mRNA levels and protein expression of SOX-9, COL2 and aggrecan in GFP-BMSCs were significantly lower than in ACs in the equivalent co-culture systems. Though the expression of chondrocyte-specific proteins in GFP-BMSCs was enhanced, the protein expression was still much lower than in ACs cultured alone.Reciprocal interactions exist between ACs and BMSCs in co-culture. The stimulating and supporting effects of BMSCs on ACs were more important in enhancing cartilage-matrix formation than the reciprocal effect of ACs on BMSCs. Both soluble factors and direct physical contact occur in AC/BMSC co-cultures, with physical contact playing a predominant, or at least very important role.
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