When ultrathin frozen sections of chicken cardiac muscle were osmicated, dehydrated in ethanol, embedded in ethyl cellulose, and stained with acidic uranyl acetate, filaments of 10-12 nm width were visualized in wide interfibrillar spaces. Immunostaining of the frozen sections for desmin resulted in exclusive labeling of such filaments. These observations indicated that longitudinally oriented networks of intermediate filaments were present in the interfibrillar spaces, in addition to the transversely oriented networks that surround myofibrils at the level of Z band. As in skeletal muscle (Tokuyasu, K. T., A. H. Dutton, and S. J. Singer, 1983, J. Cell Biol. 97:1727-1735), desmin in chicken cardiac muscle is believed to be largely, if not entirely, in the form of intermediate filaments.

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