A network of transverse and longitudinal intermediate filaments is associated with sarcomeres of adult vertebrate skeletal muscle.

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Abstract
An extensive network of transverse and longitudinal filamentous bridges was revealed when small myofibril bundles, prepared from Triton-EGTA[ethyleneglycol-bis(.beta.-aminoethylether)-N,N,N'',N''-tetraacetic acid]-treated rabbit skeletal muscles, were extracted with KI to remove the majority of thin and thick filaments. Transmission and scanning electron microscopic studies of these salt-resistant cytoskeletal residues indicated the following: small bundles of short transverse filaments connect adjacent myofibrils by forming Z-Z and M-M bridges, and parallel, continuous longitudinal filaments connect the peripheries of successive Z-disks and ensheath the sarcomere. These transverse and longitudinal filaments have the characteristic morphology of intermediate filaments; 2 rings of tightly interwoven and tangled filaments, connected laterally by short filaments, encircle each Z disk. This double-ring also encircles a web-like meshwork which penetrates the sarcomeric space. From the peripheries of these rings, transverse and longitudinal intermediate filaments emerge and a massive amount of material translocated and accumulated near Z disks during KI extraction. The residues were fairly resistant to solubilization by urea and SDS [sodium dodecyl sulfate], and complete dissolution was achieved only with guanidinium chloride. SDS-PAGE [polyacrylamide gel electrophoresis] indicated that the residues consisted mainly of titin, nebulin and variable amounts of residual myosin and actin. Desmin represented only a few percent of total residual proteins; however, it may be a major component of the intermediate filament network. The intermediate filament should be considered an integral sarcomeric component that may play important cytoskeletal roles in muscle structure and mechanics.