Muscle cultures treated with cytochalasin B yield mono-and oligonucleated cells of two kinds: (i) arborized, replicating precursor myogenic cells and fibroblasts; and (ii) round, post-mitotic, terminally differentiating myoblasts and myotubes. The arb6rized cells do not bind fluorescein-labeled antibody against myosin, do not contract rhythmically, and do not display hexagonally stacked thick and thin filaments. The round, mononucleated myoblasts and round, oligonucleated myotubes bind the fluorescein-labeled antibody against myosin, contract rhythmically, and display clusters of hexagonallystacked thick and thin filaments. When cytochalasin B is removed and replaced by colcemide, the arborized cells, but not the post-mitotic muscle cells, acquire a radial symmetry and are induced to assemble massive, meandering cables that may occupy over 25% of the cell volume. These tortuous cables are positively birefringent and consist exclusively of enormous numbers of 100-A, intermediate-sized filaments.After 3 days in vitro primary cultures of embryonic chick muscle contain immature, post-mitotic, multinucleated myotubes, post-mitotic mononucleated myoblasts, replicating myogenic cells, replicating fibrogenic cells, and replicating mesenchyme cells (1,2,40). The addition of cytochalasin B to these cultures promptly blocks fusion of myoblasts into myotubes. Cytochalasin B also rapidly blocks locomotion of all replicating cells, induces them to "retract," and gradually transforms them into fully arborized cells (3-6). Cytochalasin B does not arborize immature myotubes, but it does cause them to retract from the clot, and subsequently the myotubes degenerate. Because cytocholasin B blocks, not the initiation, but the completion, of cytokinesis (6, 7), these cultures show an increase in oligonucleated cells with time. Cytocholasin B does not significantly impair either the initiation or continued synthesis of myosin, actin, or tropomyosin, for by day 6 the post-mitotic cells generated in these cultures contract spontaneously (3, 6).The addition of colcemide to 3-day-old muscle cultures results in the metaphase-arrest of many mononucleated myogenic and fibrogenic cells. Colcemide fragments the elongated, immature myotubes into multinucleated myosacs and dismantles their microtubules. The myosacs are rich in "lakes" of i00-A, intermediate-sized filaments, and it has been proposed that there is an inverse correlation between the microtubules in myotubes and the 100-A filaments in myosacs (6,8). Colcemide has no detectable effect on the synthesis of contractile proteins, their organization into hexagonally-stacked thick and thin filaments, or their interaction required for spontaneous contractions (9). We wish to report that muscle cultures exposed to cytochalasin B gen-513 erate large numbers of post-mitotic, mononucleated myoblasts rich in scattered stacks of thick and thin filaments. Although Z. H, and I bands are atypical, these scattered stacks of thick and thin filaments permit the cell to contract spontaneously. W...
