INTERACTION OF FORCE TRANSMISSION AND SARCOMERE ASSEMBLY AT THE MUSCLE-TENDON JUNCTIONS OF CARP (CYPRINUS-CARPIO) - ULTRASTRUCTURE AND DISTRIBUTION OF TITIN (CONNECTIN) AND ALPHA-ACTININ

At muscle-tendon junctions of red and of white axial muscle fibres of carp, new sarcomeres are found adjacent to existing sarcomeres along t he bundles of actin filaments that connect the myofibrils with the jun ctional sarcolemma. As the filament bundles that transmit force to the junction originate proximal to new sarcomeres, they probably relieve these new sarcomeres from premature loading. In red Fibres, these fila ment bundles are long (up to 20 mu m) and dense, permitting light-micr oscopical immunohistochemistry (double reactions: anti-titin or anti-a -actinin and phalloidin). New sarcomeres have clear I bands; their A b and lengths are similar to those of older sarcomeres and the thick fil aments lie in register. T tubules are found at the distal side of new sarcomeres but terminal Z lines are absent. The late addition of alpha -actinin suggests that alpha-actinin mainly has a stabilizing role in sarcomere formation. The presence of titin in the terminal fibre protr usions is in agreement with its supposed role in sarcomere formation, viz. the integration of thin and thick filaments. The absence of a ter minal Z line from sarcomeres with well-registered A bands suggests tha t this structure is not essential for the anchorage of connective (tit in) filaments.