Histopathology (haematoxylin and eosin) in Chlamydia trachomatis-induced arthritis (CtIA) alone and after a prior exposure to mercuric chloride (HgCl2). CtIA in the non-HgCl2-exposed control rats is accompanied by only moderate synovial hypertrophy and hyperplasia and a mononuclear cell infiltration (a-c). Eight animals were studied in each group. The joints of the HgCl2-exposed rats demonstrated more marked synovial hypertrophy and hyperplasia and a marked infiltration of the synovial tissues, primarily by neutrophils. This is accompanied by bone and cartilage destruction and by pannus formation, which invades subchondral bone (d-f). Panel (g) is representative of a rat that received the HgCl2 injections alone and shows no pathological change in the joint. Original magnifications × 200.