Fusion pore expansion leads to cytoplasmic continuity between fusing myoblasts and myotubes. (A and A’). Myotube (MT)–myoblast (MB) interface displaying expanded fusion pores (red arrowheads). A’ is a higher magnification view of a portion of A. Cytoplasmic continuity is clearly discernible, and paired membranes between pores form an expanded enclosure. (B) Myotube–myoblast pair at an advanced phase of fusion. The cytoplasm is uniform in appearance, and the cell–cell interface appears as a series of vesicle-like structures (red arrowheads). n, nucleus. Bars: (A and B) 500 nm; (A’) 200 nm.