Fibronectin (LETS protein) is a major cell surface glycoprotein component of a variety of nontransformed, substrate-attached cells in culture. Its presence has been related to increased adhesive properties. Using the peroxidase-antiperoxidase method to localize antibodies to fibronectin, we have observed that the distribution of fibronectin on L6 myoblasts varies with the density of the culture and the differentiative state of the cells. Low density, undifferentiated cultures of L6 myoblasts have a sparse accumulation of fibronectin; the antibody-antigen reaction indicates its presence on cell membranes, especially where several cells are in proximity. Undifferentiated cells in high density cultures have two forms of fibronectin localization-a diffuse staining on the membrane and a dense staining on an extracellular filamentous matrix. This matrix is composed of filaments ranging from 20-25 nm in diameter which occur singly or coalesce to form bundles. The filaments in this matrix are also observed to have dense globules scattered along their length. These filaments, which are at least in part composed of fibronectin, also react with concanavalin A, as do certain plasma membrane components. In contrast to the observations seen in undifferentiated cells, differentiated cells or myotubes have a diffuse membrane staining with antifibronectin antibodies, and the filamentous form is usually absent.
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We wish to thank S. Gentry, R. Scott, J. lrke and S. Palm for technical assistance, and C. Furcht for assistance in the preparation of this manuscript. This work was supported by a Basil O’Connor starter research grant from the National Foundation March of Dimes, the NIH and the Leukemia Task Force to L.T.F. and the University of Wisconsin Graduate School, and an institutional grant to the University of Wisconsin from the American Cancer Society to D.F.M. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be