MMP9 and rheumatoid arthritis: As expected, fibroblasts produced only small amounts of MMP-9 (on a per-cell basis), with skin fibroblasts and fetal foreskin fibroblasts producing significantly more than RA synovial fibroblasts (mean ± SEM 0.15 ± 0.01, 0.15 ± 0.02, and 0.05 ± 0.01 ng/hour/million cells, respectively).