Fibronectin is not present in the focal adhesions formed between normal cultured fibroblasts and their substrata.

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Abstract
Fibronectin is an extracellular matrix protein that has been implicated in the spreading and adhesion of cultured fibroblasts to their substrate. Double immunoelectron microscopic labeling experiments for fibronectin and for concanavalin [Con] A-binding proteins on the cell surface were carried out on ultrathin frozen sections of cultures of embryonic chicken heart fibroblasts. On cross-sections through the focal adhesions of the cell to the substratum there was substantial labeling for Con A-binding proteins but no detectable labeling for fibronectin; the binding proteins and fibronectin were extensively labeled elsewhere on the cell surface and substratum. Apparently, fibronectin is not present within the sites of focal adhesions. Therefore, the functions of fibronectin in cell spreading and adhesion are not directly mediated through its binding at focal adhesion sites. An alternative model is presented which can account for such fibronectin functions.