Assembly and remodelling of myofibrils and intercalated discs in cultured neonatal rat heart cells

The reorganization of myofibrils and the re-formation of intercalated discs was examined in neonatal rat cardiac muscle cells during the first 72 h of culture. Rhodamine phalloidin was used to monitor the organizational state of the myofibrils and antibodies to desmoplakin and vinculin were used as markers for the presence of desmosomes and fasciae adherentes, respectively. Tiny punctate desmosomes were observed between muscle cells after 24 h and apparently increased in number and/or size between 24 and 48 h in culture. Fasciae adherentes were not detectable with antibodies to vinculin until after 48 h in culture. Well-defined sarcomeres were restored after 48 h in culture. Once formed the sarcomeric organization of the myofibrils was found to be stable provided they were attached to the sarcolemma via intercalated discs. However, if the myofibrils attached to regions of the membrane that lacked intercalated discs the sarcomeres appeared to break down gradually centripetally. When myofibrils attached to the membrane at the free edges of cells that were not in contact with other muscle cells, the striations stopped abruptly at a considerable distance before the myofibril attached to the membrane. These non-striated terminals elongated between 48 and 72 h and were associated with focal contacts that contained vinculin. Overall the results suggest that cell-cell contact may be critical for the stabilization of normal myofibrillar structure in the heart.