Effect of prostaglandin E2 (PGE2) on chondrocyte proliferation. (a) Proliferation of cultured chondrocytes was determined by [3H]-thymidine incorporation. Subconfluent chondrocytes were synchronized in serum-free medium for 24 hours. Medium was renewed and PGE2 or solvent was added in the indicated concentrations for 24 hours. Data are presented as mean ± standard error of the mean, n = 5. (b) Relative quantification of DNA in cultured chondrocytes was used as a measure for proliferation. Chondrocytes were grown in 96-well-plates until subconfluency. After synchronization, PGE2 or solvent was added for 24 hours. Thereafter, medium was aspirated, DNA was extracted by freeze-thawing and 200 μl of the staining solution (containing a fluorescent nucleic acid stain) were added and DNA-bound fluorophore was determined by fluorescence spectroscopy, expressed as OD at 530 nm. Data are presented as mean ± standard error of the mean of four parallel experiments, given as percent of the control. Excitation of the control was 14,705 ± 2,675 after 24 hours. *p value < 0.05.