Incorporation of microinjected biotin-labelled actin into nascent myofibrils of cardiac myocytes: an immunoelectron microscopic study.

Incorporation of microinjected biotin-labelled actin into nascent myofibrils of cultured cardiac muscle cells was investigated by immunogold electron microscopy. At the proximal parts of myofibrils, gold labelling was first found (at about 4 min after injection) around the A-band level. This observation suggests that polymerization of actin or the addition of newly-formed actin filaments occurs preferentially in association with myosin filaments to increase the myofibrillar girth. The distal terminals of developing myofibrils were also labelled at about 4 min after injection. This rapid incorporation of actin subunits at the myofibrillar ends suggests the continued reorganization and/or de novo formation of myofibrils at these positions. Along the extending direction of the myofibrillar terminals, gold particles were arranged in rows on the inner surface of the sarcolemma. These rows of particles continued to become longer with incubation. It appears that actin subunits are added at the membrane-associated ends of pre-existing actin filaments to increase the length of myofibrils.