Multiple contact sites and nascent fusion pores form between tightly apposed myotube and myoblast surfaces. (A–C’). Three examples of paired and tightly apposed myotube (MT)–myoblast (MB) surface membranes displaying points of contact. Primed panels represent high magnification views of regions in left panels. Yellow asterisks mark the X-shaped configuration, where both membranes remain intact, whereas red asterisks mark blurred interfaces bordered by membrane loops, presumably representing nascent fusion pores. (D and D’) Myotube–myoblast interface displaying small fusion pores (red arrowheads). (D’) Cytoplasmic continuity can be recognized by apparent flow of dark, round ribosomes through the pores. (E) Tomogram 3D reconstruction of a myotube–myoblast interface displaying multiple cell–cell contacts (green) and small pores (yellow). Membranes are colored pink. STEM was performed on 350–400-nm-thick sections. Also see Video 3. (F and F’). Tomogram 3D reconstruction of a single pore, viewed from two different angles. Also see Video 4. Bars: (A, A’, B, B’, and C’) 100 nm; (C and D) 500 nm; (D’ and E) 200 nm; (F and F’) 25 nm.