Developing chick myotubes in tissue culture were freeze-fractured to yield complementary replicas of large areas of membrane. Regions of muscle fibers with high concentrations of acetylcholine receptors were identified by binding of fluorescent-labeled alpha-bungarotoxin. Membranes in such regions contained clusters of large (100 A Diam) angular particles, similar in appearance to particles found in postsynaptic membranes of cholinergic synapses. Particles appeared in apposing areas of cytoplasmic and external leaflets but were more prevalent in the cytoplasmic leaflet. The areas of high particle concentration were coextensive with the fluorescence due to bound toxin. Treatment of cultures with tetrodotoxin increased the size of fluorescent spots and areas of high concentration of particles relative to those found in control cultures. In muscle cultures grown in the presence of spinal cord explants, some neurites contacted and innervated nearby myotubes. Intense fluorescence due to binding or alpha-bungarotoxin was present at portions of such neurite-myotube contacts. At these same portions, a high concentration of large angular particles was present in the sarcolemma adjacent to the neurite. In addition, an ordered arrangement of large particles was seen in the cytoplasmic leaflet of the neuronal plasmalemma directly apposing the muscle. The possible significance of these arrangements is discussed. Clusters on myotubes tended to be larger (contain more particles) when they occurred in groups, defined as three or more clusters with an intercluster distance of less than 0.5 micrometers. Clusters were also larger in myotubes treated with tetrodotoxin and in myotubes adjacent to some neurites in nerve-muscle cocultures. Several depressions containing particles similar to those in the clusters were found in the sarcolemma. The implications of these depressions are discussed in light of current theories of incorporation of proteins into cell membranes.

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