Immunoelectron microscopic studies on the distributions of fibronectin and actin in a cellular dense connective tissue: the periodontal ligament of the rat

Mouse monoclonal anti‐fibronectin antibodies and rabbit affinity‐purified actin antibodies were used with gold‐labelled secondary antibodies to study the distributions of fibronectin and actin within the periodontal tissues of the rat. Following perfusion fixation, demineralization, and embedding in glycolmethacrylate, sections 70–80 nm thick were obtained from the gingiva, periodontal ligament and the supporting bone of the first and second mandibular molars. In both gingiva and periodontal ligament, fibronectin was located over the collagen fibers and at certain sites at the cell‐collagen interface. In addition, unbanded fibrillar material 10–20 nm thick was decorated by fibronectin antibodies. Actin was localized primarily in the peripheral cytoplasm and at the cell‐extracellular matrix interface. Labelling for actin was detected at a few sites within the extracellular matrix. In double labelling experiments, fibronectin and actin were observed to co‐distribute at cell‐matrix interfaces. These data support the concept that fibronectin within the dense connective tissue of the rat gingiva and periodontal ligament, as in other soft connective tissues, may mediate the interaction between different components of the extracellular matrix and between these components and the cell membrane. In addition, they strengthen the hypothesis that actin filaments may be active in vivo in the formation of cell‐matrix contacts, and that actin and fibronectin may form fibronexus‐like structures similar to those described for fibroblasts in culture.