The distribution of intermediate filament (IF) subunits during maturation of skeletal myotubes in vitro was examined by immunofluorescence, using antibodies against two different types of chick IF subunits: (a) 58-kdalton subunits of fibroblasts (anti-58K), and (b) 55-kdalton subunits of smooth muscle (anti-55K) . Anti-58K bound to a filament network in replicating presumptive myoblasts and fibroblasts, as well as in immature myotubes. The distribution in immature myotubes was in longitudinal filaments throughout the cytoplasm . With maturation, staining of myotubes by anti-58K diminished and eventually disappeared . Anti-55K selectively stained myotubes, and the fluorescence localization underwent a drastic change in distribution with maturation-from dense, longitudinal filaments in immature myotubes to a cross-striated distribution in mature myotubes that was associated with the I-Z region of myofibrils . However, the emergence of a cross-striated anti-55K pattern did not coincide temporally with the emergence of striated myofibrils, but occurred over a period of days thereafter. KEY WORDS intermediate filaments myogenesis " muscle differentiation immunofluorescenceReports from this laboratory and from others have demonstrated that the intermediate or 100-A filament (IF) subunits from different cell types and organisms are not all identical, and can be distinguished immunologically and biochemically (I,2,9, 11a, IIb,21,29) . We demonstrated that antibodies against the 58-kdalton chick fibroblast IF subunit (anti-58K) stained filaments in many different kinds of cultured chick cells, including fibroblasts, chondroblasts, melanocytes, neurons, glia, cardiac and smooth muscle myoblasts, and both replicating presumptive skeletal myoblasts and postmitotic myoblasts and myotubes. Contrastingly, antibodies against the 55-kdalton chick smooth muscle IF subunit (anti-55K) were highly selective and stained only definitive smooth, cardiac, and skeletal muscle cells. Replicating pre-J . CELL BIOLOGY

The protein subunit of 100-A filaments constitutes approximately 50% of the cytoskeleton protein of chick fibroblasts.

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Most theories of determination or differentiation assume that embryonic cells differ from mature cells. Embryonic cells are thought to have metastable control mechanisms. These labile controls are believed to become progressively more stabilized as the cells differentiate. Zygote, blastula, neural plate, limb bud, somite, or ‘stem’ cells are conceived of as undifferentiated, totipotent, or multipotential cells. As such, these cells supposedly have available for activation a larger repertoire of phenotypic programmes than their progeny. A necessary corollary to this view is that the activation of one particular phenotypic programme out of the many available is a function of instructive exogenous inducing molecules.

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