Rheumatoid nodules. A lightmicroscopical study with special reference to fibrin and fibronectin.

The presence and localization of fibrin and fibronectin in rheumatoid nodules were studied using an indirect immunoperoxidase technique on tissue specimens fixed in formaldehyde, embedded in paraffin and pretreated with pepsin and testicular hyaluronidase. Three zones characteristic for rheumatoid nodules was recognized. Central area with necrosis, containing at least in part fibrinogen-antigenic material and fibronectin especially in the peripheral part of the necrotic area. Around the necrosis a layer of mesenchymal cells in a palisade arrangement was found. Especially in the external part of this layer fibronectin was demonstrated around and between the cells, where fibrin was absent. Peripherally, a zone of non-specific granulation tissue containing moderate amount of fibronectin decreasing towards the surround mature connective tissue, was seen. In the border of the cellular layer vessels were found in variable amount. In some of the vessels vasculitis was demonstrated with the presence of inflammatory cell infiltration, fibrin deposition and occasionally thrombosis. The pathogenesis of the inflammatory reaction in rheumatoid nodules is discussed.