Analysis of initial pre-clinical changes in PIA. A female 10-week-old rat was injected with pristane and monitored by CatWalk. When gait changes suggestive of a beginning onset of PIA were detected at Day 11 (A), the rat was sacrificed. At this time no clinically visible signs of arthritis such as erythema or swelling were noticed. (B) Hematoxylin and eosin-stained paw sections of the pristane-injected rat's hind paws and of a PBS-injected control rat were assessed for signs of inflammation. The very early pristane-induced arthritis is characterized by synovitis and inflammation of the tendon sheaths. Boxes in (B) are shown at 10 × magnification in (C).