FN1 and rheumatoid arthritis: However, the 1 to 2 pg/ml amount of IL-7 we detected in chondrocytes stimulated with fibronectin fragments or IL-1 plus IL-6 is higher than the 0.33 pg/ml IL-7 reported to be produced by cultured RA synovial fibroblasts and is the same as the amounts made by these cells after stimulation with IL-1β or TNF-α [11].