A time course study of the changes that occur to the subcutaneous model of synovium following polyarthritogen.

The synovial-like structure lining the cavity of a subcutaneous air pouch in rats was examined macroscopically and microscopically for changes during the induction and progression of adjuvant polyarthritis. The earliest event observed was the infiltration of the lining by inflammatory cells which occurred about 6 days after inoculation with adjuvant. Deposition of fibrin and proliferation of blood vessels and connective tissue soon followed, associated with an even greater infiltration by inflammatory cells and the formation of oedema in the hyperplastic fibrous tissue. These changes resulted in a substantial thickening of the pouch wall (and consequently in air pouch skin thickness) that was most marked on day 9, when few rats had visible signs of joint swelling. As arthritis developed and became established (day 12-14), pouch wall thickness declined to below pre-adjuvant thickness. Histological changes were still evident but at a much reduced intensity. Challenge of pouches with tuberculin at various times showed reactivity of the delayed type, beginning not earlier than day 5 post-adjuvant inoculation. Pouches however failed to respond to tuberculin or to the non-specific irritant carrageenan as arthritis developed.