We have used fluorescence analogue cytochemistry in conjunction with time lapse recording to study the dynamics of alpha-actinin, a major component of the Z line, during myofibrillogenesis. Rhodamine-labeled alpha-actinin microinjected into living cultured chick skeletal myotubes became localized in discrete cellular structures within 1 h and remained specifically associated with structures for up to 4 d, allowing individual identified structures to be followed during development. In the most immature cells used, alpha-actinin was found in diffuse aggregates, some of which displayed sarcomeric periodicity. Aggregates were observed to coalesce into better defined structures (Z bands) that were approximately 1.0-micron wide. Z bands condensed into narrow, more intensely fluorescent Z lines in 4-48 h. During this period, Z lines grew laterally, primarily by the addition of small beads of alpha-actinin to existing Z lines or by the merging of small Z lines. In more mature cells, alpha-actinin added to Z lines without going through a visible intermediary structure. Mean sarcomere length did not change significantly during the stages examined, although the variability of sarcomere length did decrease markedly over time for identified sets of sarcomeres. At early stages, myofibrils frequently shifted position in both the longitudinal and lateral directions. Neighboring myofibrils were frequently associated for one or more sarcomeres sporadically along their length, such that the intervening sarcomeres were often misaligned. Associations between myofibrils were often transitory. Shifts in myofibril location in conjunction with the formation, breaking, and reformation of lateral associations between myofibrils facilitated the alignment of Z lines through a trial and error process.

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