Inhibition of chondrocyte terminal differentiation and matrix calcification by soluble factors released by articular chondrocytes

Chondrocytes do not undergo terminal differentiation in normal articular cartilage, whereas growth plate chondrocytes synthesize ALPase and induce matrix calcification terminally. Articular chondrocytes in osteoarthritic joints have been reported to express the terminal differentiation phenotypes, suggesting that terminal differentiation of articular chondrocytes is inhibited in normal joints, In the present study, we investigated the underlying inhibitory mechanism of the terminal differentiation in articular cartilage using a culture on type II collagen-coated dishes or a novel culture model on Millipore filters, ALPase activity increased from day 7 to day 8 in growth plate chondrocyte cultures on the collagen-coated dishes, but not in articular chondrocyte cultures. The ALPase expression of growth plate chondrocytes on the collagen-coated dish was completely inhibited when the same number of articular chondrocytes was mixed in the growth plate chondrocyte cultures. When articular chondrocytes or growth plate chondrocytes were maintained on Millipore Tillers held in 16-mm dishes, they started to synthesize ALPase. The ALPase expression of the chondrocytes on Millipore filters was inhibited by the presence of articular chondrocytes maintained on the bottom collagen-coated substratum in the same dishes. These results indicate that factors that diffused into the medium through the Millipore filters are involved in the inhibition of terminal differentiation. Since the conditioned medium from articular chondrocyte cultures did not affect the ALPase expression, it is considered that the soluble factors, which are continuously released from articular chondrocytes, are responsible for the inhibition of terminal differentiation.