Organization of filaments underneath the plasma membrane of developing chicken skeletal muscle cells in vitro revealed by the freeze-dry and rotary replica method.

Cytoskeletal organization and its association with plasma membranes in embryonic chick skeletal muscle cells in vitro was studied by the freeze-drying and rotary-shadowing method of physically ruptured cells. The cytoskeletal filaments underlying the plasma membranes were sparse in myogenic cells at the stage when cells exhibited great lipid fluidity in plasma membranes (fusion competent mononucleated myoblasts and recently fused young myotubes). Myotubes at more advanced stages of development possessed a highly interconnected dense filamentous network just underneath the cell membrane. This subsarcolemmal network was composed predominantly of 8-10 nm filaments; they were identified as actin filaments because of their decoration with myosin subfragment-1. Fine fibrils having a diameter of 3-5 nm were found on the protoplasmic surface of the plasmalemma at both the early and advanced stages of development. They were associated with the subsarcolemmal cytoskeletal filaments. Short 2-5 nm cross-linking filaments were occasionally seen between filaments in the subsarcolemmal network. We conclude that, although the subsarcolemmal cytoskeletal network contains many actin filaments, this domain appears to play some role in preserving the cell shape in the form of the membrane skeleton rather than membrane mobility.