Addition of cytochalasin B to primary muscle cultures allows the physical separation of postmitotic myogenic cells from replicating presumptive myoblasts and replicating fibroblasts. Mononucleated, postmitotic myoblasts proceed without fusion to synthesize myosin and actin and to assemble these proteins into thick and thin filaments. Although sarcomeres oriented in tandem are not evident and A, H, and I bands are atypical in these mononucleated myoblasts, the irregularly scattered clusters of myofilaments are assembled into remarkably normal interdigitating arrays. These scattered clusters of stacked thick and thin filaments permit the cell to contact spontaneously in the presence of cytochalasin B.

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