Formation of myofibrils in spreading chick cardiac myocytes

Cardiac myocytes were isolated from 5–6‐day‐old chick embryos and allowed to spread in culture. The distribution of alpha‐actinin in the cells was followed for five days in culture by exposing permeabilized cells to rhodamine‐labeled alpha‐actinin and also by injecting the labeled alpha‐actinin into living myocytes. In addition to labeling the Z bands of sarcomeres, the added alpha‐actinin also labeled small particles that were usually arranged periodically in linear arrays with a spacing between particles of 0.3–2.0 μm. Actin was localized between the particles of alpha‐actinin by means of fluorescein‐labeled heavy meromyosin. The punctate localization of alpha‐actinin was prominent in pseudopods, behind ruffles, and at the periphery of spreading cells. Long rows of particles of alpha‐actinin were often parallel to one another with the alpha‐actinin particles in register. These linear arrays appeared to merge laterally to form strands with broader concentrations of alpha‐actinin. Other linear arrays were parallel to myofibrils in the cell and some extended outward from the ends of myofibrils. We conclude that during spreading of cardiac myocytes, myofibrils form at the cell periphery behind the extending margins of the cell, and that the aggregates of alpha‐actinin found in these areas are nascent Z bands in the forming myofibrils.