Changes in myosin distribution in dedifferentiating and redifferentiating smooth muscle cells in tissue culture

Isolated smooth muscle cells and fibroblasts from the newborn guinea-pig vas deferens were grown in culture. In the first 2 days, all cells characterized as smooth muscle by phase-contrast microscopy reacted intensely with fluoresceinated antibodies against smooth muscle myosin. The fluorescence was in the form of particles (termed here "myosin aggregates"), which were often aligned to give the cell a striated appearance. After 3-5 days, coarse fluorescent fibrils were also visible. These were termed "attachment fibrils" ("A-fibrils") since they were thought to represent myosin in microfilament bundles. Between 6 and days in culture, the smooth muscle cells began to dedifferentiate morphologically. At this time, the "myosin aggregates" became clumped and less intensely fluorescent. "A-fibrils" also decreased in fluorescence intensity. By 8 days in culture, the dedifferentiated cells had undergone intense proliferation and gave only a minimal reaction with myosin antibodies. However, when a confluent monolayer of cells formed on day 9 or 10, they immediately began to redifferentiate ultrastructurally and to regain immunofluorescence in both "myosin aggregates" and "A-fibrils". Throughout the entire culture period, cells characterized as fibroblasts by phase contrast microscopy gave only a weak reaction with fluoresceinated antibodies to myosin showing "A-fibrils" but no "myosin aggregates".