The pro-inflammatory cytokines, such as TNF-α, IL-1β, and IL-6, as well as the chemokines, such as CXCL12 and CCL2, which are abnormally secreted by RA-FLSs, contribute a complex inflammatory network, can not only attract the migration of OC precursors to migrate toward the site of inflammation and promote their differentiation into mature OC, but also regulate OC migration, to form a positive feedback loop that exacerbates bone destruction through its interaction with inflammatory cells (51). Here, TNF is linked to rheumatoid arthritis.