Formation of binucleated cardiac myocytes in rat heart: II. Cytoskeletal organisation.

Neonatal cardiac myocytes continue to undergo nuclear division, but lose their ability to complete cell division between 3 and 4 days of age. To examine cytoskeletal organisation of cardiac myocytes during mitosis, freshly isolated cardiac myocytes from 2-, 4-, 6- and 8-day-old rats were fixed and labeled with anti-tubulin, vinculin, desmin and sarcomeric alpha-actinin antibodies. The central, nuclear region of cardiac myocytes is expanded to form a balloon-like structure when they entered prophase. The organisation of microtubules, vinculin and desmin in mitotic myocytes from 4-, 6- and 8-day-old rats was identical to that in dividing myocytes from 2-day-old animals. Microtubules emanating from the nuclear membrane mainly ran along the longitudinal axis of cardiac myocytes in interphase. Microtubules were disassembled and reorganised into the mitotic spindle during mitosis. Desmin was disassembled, either diffusely distributed in the cytoplasm or formed spotty cytoplasmic aggregates during mitosis. Vinculin was disassembled in prometaphase, diffusely distributed in the cytoplasm and associated with cell membranes. During telophase it concentrated in the equator of mitotic spindles. Sarcomeric alpha-actinin became dispersed in the cytoplasm of mitotic myocytes from 2-day-old rats in prometaphase. It remained diffusely distributed in the cytoplasm and associated with cell membranes until the completion of cytokinesis. However, sarcomeric alpha-actinin was only partially disassembled in 4-, 6- and 8-day-old myocytes. Striations of alpha-actinin with full sarcomere length were observed in the cytoplasm as well as in the region of furrow formation. Thus, incomplete disassembly and presence of myofibrils in the equator region where cleavage furrows from may physically impede the furrowing of sarcolemma driven by the contractile ring, resulting in the formation of binucleated cardiac myocytes.