The signs and symptoms of arthritis are caused by distinct processes in the joint. Synovitis with extensive inflammation is characteristic. Formation of pannus tissue and activation of osteoclasts contributes to joint destruction. Tissue remodeling is characterized by new cartilage and bone formation eventually leading to ankylosis. The images presented were obtained from mice with methylated bovine serum albumin-induced arthritis (inflammation and destruction) and from mice with spontaneous ankylosing enthesitis (remodeling).