Effects of thrombin on proliferation and cell cycle in SFs from RA and OA. Cells were cultured for 48 hours in DMEM containing 1% FCS and then stimulated with 10 units/ml thrombin. (a) Time-course of proliferation of synovial fibroblasts (SFs) from patients with rheumatoid arthritis (RA) (dark symbols) and patients with osteoarthritis (OA) (light symbols). Continuous lines, thrombin-stimulated cells; dotted lines, unstimulated cells. The optical density (OD) was measured by ELISA plate reader at 450 nm. (b) Comparison of cell cycle in RA SFs and OA SFs. Numbers represent the percentage of cells exhibiting mean channel fluorescence (FL2-H) in the S/G2/M phase of the cell-division cycle. Data are expressed as mean ± standard deviation of five experiments, using five independent donors. **P < 0.01.