[Intercellular interactions preceding muscle cell fusion]

Membrane interactions of myogenic cells of the 7-8 old chick embryos were investigated by electron microscopy. Tannic acid was used as specific fixative for biological membranes. Three types of specialized contacts can be revealed in fused myogenic cells: the first is like gap junctions, the second represents a pentalamellar structure, and the third is a so far non-described type of contact showing a membrane complex consisting of two parallel membranes arranged at a distance of approximately 15 nm and connected by electron-dense "bridges". The third type contact is revealed only by tannin-fixation. It is suggested that the "bridge" contact precedes the pentalamellar structure. A transition from the first type of contact to the second one is possible. The pentalamellar structure can be considered as an initial phase of fusion.