Myogenesis in vitro as seen with the scanning electron microscope.

In this paper, we review our recent observations by scanning electron microscopy (SEM) on the differentiation of the cell surface and cytoplasmic organelles in embryonic chick skeletal muscle cells in vitro. The changes of the surface structures of myoblasts during mitosis were essentially similar to those of other cell types, but the characteristic spindle shape of myoblasts did not change throughout most of this period. Cytoskeletal structures under the sarcolemma were examined by Triton extraction and metal coating. Cells in S, G2 and M possessed a dense, and those in G1 a loose filament network under the membrane. Myotubes possessed a dense network under the sarcolemma. In the fusion area between a myoblast and a myotube, the cytoskeletal domain of the former could be distinguished from the latter because of the mosaic appearance of the subsarcolemmal cytoskeletal network. This network was composed predominantly of 10-13 nm filaments; they were identified as actin filaments because of their decoration with myosin subfragment-1. Triton treatment and thiocarbohydrazide-osmium staining allowed us to visualize myofibrils. They ran in the direction of inferred stress lines brought about by elongation and adhesion of the cells to substrate. Intracellular membranous organelles could be seen by the freeze-polishing and osmium-maceration procedure. Mitochondria exhibited complex irregular branchings. T system tubules ran a tortuous course. Sarcoplasmic reticula with occasional dilatations were connected to each other. The results are of sufficient promise to encourage more extensive analysis of myogenesis by SEM.