The composition and organization of myofibrils at extra-junctional membrane attachment sites in cultured neonatal rat cardiac muscle cells were analysed by immunofluorescence and electron microscopy. When myofibril terminals attached to the cell membrane via focal contacts at regions of the sarcolemma that lacked intercalated discs, they appeared to be non-striated and resembled thick actin cables. Although the non-striated terminals contained actin, myosin and alpha-actinin, the proteins were not organized into recognizable sarcomeres at the light microscopic level. Analysis of the structure of the terminals in the electron microscope confirmed that the usual sarcomeric organization and attachments to the sarcolemma were markedly modified. The non-striated myofibril terminals differed in structure from both stress fibres in non-muscle cells and stress fibre-like structures present in embryonic heart cells in culture. Non-striated myofibril terminals attached to the cell membrane by lateral contact with extra-junctional electron-dense membrane plaques rather than by insertion by their ends into the fascia adherens. It is proposed that the structure and composition of membrane-attachment points for myofibrils may have an influence on the structure, organization or stability of contractile elements in cardiac muscle.

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