The localization of fibronectin in rheumatoid arthritis synovium by light and electron microscopic...

The distribution of fibronectin in rheumatoid arthritis (RA) synovium has been investigated by light and electron microscopic immunohistochemistry using an antihuman fibronectin antibody. Heavy accumulation of fibronectin was observed in the lining layer and the areas of proliferation of fibroblasts. Rough endoplasmic reticulum (RER) and peripheral vesicles of proliferated type B lining cells and fibroblasts contained large amounts of fibronectin. Thus these cells seem to participate actively in the local synthesis and secretion of this glycoprotein. Type A lining cells and migrated mononuclear phagocytes contained many phagolysosomes in some of which dense accumulation of fibronectin was observed. Some of the materials in the phagolysosomes, with dense accumulation of fibronectin, resembled the fibrinous material-fibronectin complexes frequently seen in the pericellular spaces. Accordingly fibronectin seems to play a role in the clearance of fibrinous materials by these phagocytes. The proliferated capillaries and small vessels possessed multilamellated basement membranes with heavy accumulation of fibronectin. However, RER or Golgi apparatus of the endothelial cells contained no detectable amounts of fibronectin. This indicates that these cells do not actively participate in the synthesis of fibronectin and that the majority of this glycoprotein in the basement membranes originates in fibronectin from blood vessel exudate. Fibrinous material-fibronectin complexes were frequently seen on the endothelial cell surfaces. Circulating platelets and mononuclear cells occasionally came in contact with these complexes, suggesting an association of fibronectin with the formation and clearance of thrombi in the vascular lumina at the inflammatory sites of RA synovium.