Abstract
We have studied the modifications that occur in plasma membrane during the process of rat myoblast fusion in vitro . Fusion was controlled by modifying the Ca 2+ concentration in the medium. The first step leading to cell fusion is membrane apposition which is revealed by tannic acid fixation, as a pentalaminar structure formed by the two adjacent plasma membranes. Freeze-fracture replicas did not show any evidence, at this stage, of plasma membrane modification. The second step, membrane fusion, appeared in freeze-fracture as a continuous passage of the fracture plane from E to P face, and vice versa, of the plasma membranes of the adjacent myoblasts. Finally membrane fission appeared in the same areas as small intracytoplasmatic bridges between two myoblasts. The enlargement of the fission points was due to a remodeling of the plasma membrane architecture by endocytotic vesicles. Number, size, and organization of intramembrane particles did not change in any steps of membrane apposition, fusion, and fission. When myoblast fusion was inhibited by lowering Ca 2+ concentration, membrane fission was the most affected step. Reversal of inhibition, by replacing the normal Ca 2+ complement, produced a rapid membrane fusion and fission among adjacent myoblasts. However cells that have been inhibited for more than three days partially loose the ability of fusion after Ca 2+ replacement. In these cells the plasma membrane showed numerous small particle-free areas, but the rare fissions did not occur in these special areas of the membrane.
Published Version
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