From myofibril to membrane; the transitional junction at the intercalated disc.

Cardiomyocytes are coordinated by linking together at their ends through the intercalated disc. The intercalated disc with its complex folded membrane, encompasses many structural and signalling functions and is thought to play a role in cell growth and sarcomere addition. Its relationship to the contractile myofibrils is central to myocyte function. The myofibrils continue their ordered sarcomeric structure up to the edge of the intercalated disc where there is no terminal Z-disc but, instead a transitional junction. Thin actin-containing filaments from the final half sarcomere extend beyond their normal length through the transitional junction to the folded intercalated disc membrane where tension is transmitted. The peaks of the membrane folds also occur at the transitional level. They are spectrin rich and associated with sarcoplasmic reticulum vesicles. A subset of Z-disc proteins including titin, alpha-actinin and ZASP/cypher/oracle are found in the transitional region while others such as telethonin and FATZ/calsarcin/myozenin are absent. The presence of titin enables ordered sarcomeres to be maintained independently of changes in the amplitude of the membrane folds. The transitional junction is therefore poised to act as a site for a new Z-disc/SR/T-tubule complex and sarcomere addition. The evidence for this is reviewed.