Developmental reorganization of the skeletal framework and its surface lamina in fusing muscle cells.

The skeletal framework of cells, composed of internal structural fibers, microtrabeculae, and the surface lamina, is revealed with great clarity after extraction with detergent. When muscle cells fuse to form a multinucleated myotube, their skeletal framework reorganizes extensively. When myoblasts prepare to fuse, the previously continuous surface lamina develops numerous lacunae unique to this stage. The retention of iodinated surface proteins suggests that the lacunae are not formed by the extraction of lamina proteins. The lacunae appear to correspond to extensive patches that do not bind concanavalin A and are probably regions of lipid bilayer devoid of glycoproteins. The lacunae appear to be related to fusion and disappear rapidly after the multinucleated myotube is formed. When muscle cells fuse, their internal structural networks must interconnect to form the framework of the myotube. Transmission electron microscopy of skeletal framework whole mounts shows that proliferating myoblasts have well developed and highly interconnected internal networks. Immediately before fusion, these networks are extensively reorganized and destabilized. After fusion, a stable, extensively cross-linked internal structure is reformed, but with a morphology characteristic of the myotube. Muscle cells therefore undergo extensive reorganization both on the surface and internally at the time of fusion.