DEXAMETHASONE PROMOTES VON KOSSA-POSITIVE NODULE FORMATION AND INCREASED ALKALINE-PHOSPHATASE ACTIVITY IN COSTOCHONDRAL CHONDROCYTE CULTURES

This study examined the effect of dexamethasone on von Kossa-positive nodule formation and alkaline phosphate specific activity of costochon dral chondrocytes at two distinct stages of maturation. The nodules fo rmed by the more mature growth zone chondrocyte cultures contained von Kossa-positive deposits in the extracellular matrix that had a puncta te morphology. The nodules formed by the less mature resting zone cell s also contained von Kossa-positive deposits, but differentiation was delayed by three-to-five days compared to the growth zone cell culture s. Dexamethasone stimulated the number of nodules formed and shortened the length of time required for von Kossa-positive nodule formation i n both types of cultures. During the first 48 h of exposure to dexamet hasone, alkaline phosphatase specific activity in the Cell layer of bo th resting zone and growth zone cultures was increased in a dose-depen dent manner. At 12 days post-confluence and thereafter, enzyme activit y was inhibited in the dexamethasone-treated cultures. Changes in matr ix vesicle alkaline phosphatase specific activity reflected those chan ges seen in the cell layer after dexamethasone treatment, but with hig her magnitude, suggesting that one effect of dexamethasone might be to regulate matrix vesicle function. With the exception of one culture, the chondrocytes did not synthesize type X collagen under any of the e xperimental conditions used. Fourier transform infrared spectroscopy ( FT-IR) failed to detect the presence of calcium phosphates in any of t he cultures exposed to dexamethasone except one. These results demonst rate that dexamethasone promotes early differentiation events, includi ng nodule formation and increased alkaline phosphatase activity, in co stochondral chondrocyte cultures. The failure to detect type X collage n synthesis and mineralization in both dexamamethasone-treated and con trol cultures suggests that these cultures lack the factors necessary for terminal differentiation and mineralization.