Distribution of Cell Surface Saccharides and Fibronectin on Cultured Chick Myotubes: Relationship to Acetylcholine Receptor Clusters

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Abstract
This study examines the distribution of glycoconjugates on the surface of cultured chick myotubes with a battery of lectins labeled with ferritin or horseradish peroxidase. In addition, the distribution of a specific glycoprotein, fibronectin, is investigated by immunocytochemistry. Particular attention is paid to the localization of these substances in specialized patches on the cell surface previously shown to contain a high density of acetylcholine receptors as demonstrated with horseradish peroxidase labeled α-bungarotoxin. The specialized patches are found to bind a greater amount of concanavalin A, ricin agglutinin I, and soy bean agglutinin and a lesser amount of wheat germ agglutinin than the general myotube surface. Limulus lectin is distributed over the entire cell surface while other lectins do not bind to any sites. The surface patches contain a high density of acetylcholine receptors as shown by double labeling with ferritin-labeled lectins and peroxidase labeled α-bungarotoxin. Fibronectin occurs in high concentration at the surface patches and is present over other regions of the cell surface as well. These results reveal differential patterns of distribution of glycoconjugates and fibronectin over the myotube surface. These regional differences may be related to the distribution of acetylcholine receptors or to recognition and attachment by the innervating nerve.