Ultrastructural and electrophysiological correlates of cell coupling and cytoplasmic fusion during myogenesis in vitro

An electrophysiological monitoring strategy involving iontophoretic application of acetylcholine and intracellular recording has been employed in an investigation of the time course of cell fusion during myogenesis in vitro. Pairs of closely apposed, acetylcholine-sensitive rat or chick myogenic cells were continuously monitored. The onset of high-efficiency electrical coupling between members of such pairs corresponded to the moment at which cytoplasmic continuity was established. Ultrastructural analysis of the serial section record of two rat myotubes, fixed 2–3 min after fusion began, demonstrated the rapid disappearance of surface membranes in the fusion area at an average rate > 1 μm² of membrane per second. Ten pairs of acetylcholine-sensitive cells were also observed to form lower-efficiency electrical connections. Such cells were not fused but were associated via specialized close membrane appositions resembling gap junctions. These membrane appositions apparently persist through the early events of cell fusion, for their remnants were found in recently fused cells. Possible roles of electrical coupling and of these close junctions in the fusion process are considered.