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The assembly of fibronectin into fibrils was examined by high-voltage immunoelectron microscopy in subconfluent cultures of ascorbate-treated human skin fibroblasts. Cells grown in the presence of ascorbic acid for 24, 48 or 72 h were labeled with Ist-9, a monoclonal antibody specific for the EIIIA site in fibronectin, and polyclonal antibodies to type I collagen. Cells were then labeled with goat anti-mouse IgG and goat anti-rabbit IgG coupled to 5 or 18 nm colloidal gold beads. Our results show that by 24 h, fibronectin is observed in fibrils in the extracellular matrix. The majority of fibronectin in fibrils does not co-localize with type I collagen. Morphometric analysis of the distance between EIIIA sites in fibronectin fibrils (less than 12 nm in diameter) show that the EIIIA sites appear to be spaced approximately 84 nm apart. The distance of 84 nm suggests that fibronectin is fully extended in fibrils and that the amino termini of adjacent fibronectin dimers overlap by 20 nm. As fibronectin fibrils become thicker, the average distance between EIIIA sites in fibronectin dimers decreases to 42 nm. This decrease in the distance between EIIIA sites may be due to a staggering of fibronectin dimers within the fibril as the fibril matures.
Dzamba et al. (Fri,) studied this question.