TNF and rheumatoid arthritis: Osteoclasts progenitors are recruited under physiological conditions from the haematopoyetic system and can get stimulated by some cytokines and hormones to form precursors that will eventually express cell surface markers of a well-differentiated osteoclast; furthermore, some inflammatory synovial cells, present in RA, particularly mononuclear cells and giant multinucleated cells can be differentiated into osteoclasts [5], under proinflammatory cytokines stimuli, including tumor necrosis factor alpha (TNF-α), interleukin 1 (IL-1), and interleukin 6 (IL-6) [6].