Immunocytochemical in vivo localization of fibronectin‐rich contact sites on fibroblasts of normal periodontal ligament and inflamed gingiva

Fibroblast‐to‐matrix attachment sites were studied by routine electron microscopy and immunocytochemistry. Rabbit antibodies to beagle dog plasma fibronectin and sheep antirabbit antibodies conjugated with horseradish peroxidase or ferritin were used to localize fibronectin at fibroblast‐to‐matrix attachment sites. In fibroblasts of healthy periodontal ligament, the attachment sites consisted of rectangular patches of amorphous material juxtaposed to the external surface of the plasma membrane. At these sites, the cell membrane was more densely stained and the adjacent cytoplasm was characterized by increased density and a high concentration of cytoplasmic filaments. The extracellular plaques contained fibronectin. Morphometric analysis indicated that the attachment plaques were approximately 90 nm thick, 250 nm wide, and 550 nm long, and distributed uniformly over both the cell body and peripheral cytoplasmic processes. In inflamed gingiva, the attachment sites were larger, irregular in shape, and with greater amounts of extracellular amorphous material and fibronectin associated to the cell surface. Cytoplasmic filaments were more often bundled as stress fibers which terminated in fibronexus‐type junctions with extracellular fibronectin‐coated filaments.