ANALYSIS OF THE CHONDROCYTE PHENOTYPE IN CULTURE

Cartilage differentiation is a multistep process starting from mesench ymal cells and eventually resulting in mature hypertrophic chondrocyte s, a stage reach only in cartilage undergoing endochondral ossificatio n. This differentiation is characterized by a switch of gene activatio n. Mesenchymal cells produce collagens I and III, whereas chondrocytes synthesize the large proteoglycan agrecan and collagens II, IX, and X I. Hypertrophic chondrocytes produce collagen X, a biochemical marker for this stage of cell maturation. This critical-developmental pathway of the skeleton is under environmental control of growth factors, hor mones, cytokines, extracellular matrix, which been extensively studied these past years. However, the understanding of the molecular mechani sms of chondrocyte gene regulation or of the precise function of carti lage matrix components has been hindered because of the instability of primary chondrocytes In traditional culture. Suspension culture metho ds and immortalization strategies have been developed and have allowed a better insight into the mechanisms of synthesis and assembly of car tilage macromolecules. The use of immortalizing oncogenes have also sh own that growth and maturation are mutually exclusive phenomena for ch ondrocytes. These chondrocyte cell lines could be used to clone genes controling the pathway of cartilage differentiation. Furthermore, immo rtalization of normal human chondrocytes represent a valuable tool to test pharmacological factors, and the potential exists for immortalizi ng chondrocytes from diseased cartilage. This approach should be usefu l to identify genes involved in maintenance or disruption of the carti lage phenotype. Taken together, in vitro chondrocyte studies supported by the recent developments of cell and molecular biology should help to better understand the mechanisms of skeletogenesis.