Immunocytochemical demonstration of the synovial membrane in experimentally induced arthritis of the rat temporomandibular joint

The present study is first to report an experimental model of adjuvant-indu ced arthritis in the rat temporomandibular joint (TMJ). Arthritis was induc ed by simultaneous intradermal administrations of Freund's complete adjuvan t, one at the parietal scalp and the other at the base of the tail. In this model, we demonstrated responses of the synovial membrane by immunocytoche mistry using antibodies to OX6 and ED1 which recognize Ia antigen in MHC cl ass II antigen-expressing cells and the macrophage/monocyte lineage, respec tively. Three weeks after administration, no remarkable signs of inflammati on were macroscopically recognizable in the TMJ, but microscopically the sy novial membrane in the TMJ revealed marked changes such as enhanced vascula rization and hemostasis in the sublining layer and a thickening in the syno vial lining cell layer. Intense OX6-immunoreactivity was found in the synov ial lining cells at lesions in the experimental group but not in the contro l group. Immunoelectron microscopy revealed that these OX6-immunopositive s ynovial lining cells developed dense cytoplasmic processes and numerous vac uoles and vesicles, resembling type A cells. Part of the type A cells also showed ED1-immunoreactivity. The expression of OX6 or ED1 immunoreactivity in the synovial lining cells might be involved in the initial immune respon ses in this arthritis model because the synovial membranes are exposed to t he synovial fluids which have been believed to contain antigenic substances .