High resolution scanning electron microscopy of frog sartorius muscle.

A field emission-type scanning electron microscope (SEM) was used to study the three-dimensional ultrastructure of frog sartorius muscles. Various preparative procedures were tested to seeks better specimen preparation for high resolution SEM observation. Procedures should be chosen depending on the information desired. The cell surface and intracellular organization of muscle fibers were best visualized when the tissues were fixed with tannic acid-OsO4 and torn after critical point drying. The basal lamina appeared as a continuous felt-like layer, onto which fine filamentous materials adhered. The true outer surface of the sarcolemma was not seen, whereas the true inner surface was occasionally exposed and exhibited numerous caveolae, membraneous fragments and fine filaments attached to its surface. In freeze-fractured and dried tissues, the cleaved sarcolemma showed numerous apertures of caveolae and T-system tubules. Inside the cell, the myofibrils showed a typical branding pattern of the sarcomere. Thick myofilaments were regularly beaded except for the pseudo-H-zone. Around the myofibrils the sarcoplasmic reticulum and T-system were also clearly observed. The results are discussed with special reference to the usefulness and limitation of the high resolution SEM in studying the ultrastructure of cells and tissues.