Cytochalasin B (CB) induces a biphasic retraction is some cell types. The rapid response that peaks in 30 min leads to the "dendritic" condition. Replicating myogenic and fibrogenic cells, as well as postmitotic myoblasts and myotubes, participate in this reaction. This is followed by a slower phase that requires 40 h for stabilization and leads to the fully "absorized" state. Only replicating myogenic and fibrogenic cells participate in this reaction. Postmitotic myoblasts and myotubes do not arborize but round up and float off into the medium. Pretreatment with Colcemid does not block the rapid response to CB, but does block arborization. CB-arborized cells exposed to Colcemid while in the presence of CB develop sufficient tension to pull themselves apart. If CB depolymerizes actin-like filaments, and if such filaments constitute the only contractile system in the cell, then it is difficult to visualize how cells in CB develop such tension. Colcemid induces twisting, birefringent bands in interphase- and metaphase-arrested myogenic and fibrogenic cells, and in postmitotic myotubes. Such bands are more evident when CB-arborized cells are removed from CB and allowed to relax in Colcemid. These birefringent bands assemble in the prescence of cycloheximide, and may constitute 20% of the volume of the cell.

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