Reevaluation of fibronectin-collagen interactions in tissues: an immunocytochemical and immunochemical study

We studied the distribution of fibronectin (FN) in rat uterus, rat tail tendon, and rooster comb, using LM and EM immunocytochemistry. Special attention was paid to the interaction of FN with collagen (COL). Various labeling protocols and dot-blot experiments were performed to confirm the results. Under conditions of labeling specificity, FN distribution over native COL fibrils was usually sparse, especially when these were organized into thick bundles. No labeling was observed over section surfaces of COL fibrils when postembedding methods were used, which indicates that no FN is present within these fibrils. Under conditions in which exogenous FN could react with tissues, e.g., when preincubation with normal serum for background blocking was performed, artifactual staining appeared over COL. Such a reaction also occurred when anti-FN antiserum completely blocked by liquid-phase adsorption was used. Therefore, the FN present in soluble FN-anti-FN immune complexes must have still been able to react with COL. The artifactual labeling was, in all cases, almost exclusively localized on the section surfaces of COL fibrils. These results suggest that FN has a very low affinity for the surface of native COL fibrils.