Ultrastructural Examination of Type I and Type II Fibers from Human Vastus Lateralis Muscle

Abstract

<p>It is known that human skeletal muscle has several fiber types, each with a different metabolic and physiological profile. It is not known whether there are different ultrastructural characteristics within each fiber type which may account for the different contractile properties. The purpose of this study was to examine the relationship between fiber type and ultrastructural characteristics.</p> <p>Needle biopsies were obtained from the right vastus lateralis of five healthy males. According to the method described by Alway et al (1980), the tissue was prepared for electron microscopy and fiber types were identified based on their myoglobin content. Three non-overlapping electron micrographs were taken from the interior of each fiber under a Philips 300 electron microscope, and 50 type I and 50 type II fibers were examined from each subject. The constituents which were studied included: myofibrillar protein, mitochondria, cytoplasm, lipid and sarcoplasmic reticulum (SR). Volume densities were calculated for each constituent by means of a l68-point counting technique from electron micrographs projected onto a screen.</p> <p>The results indicated that type I fibers were significantly greater with respect to mitochondrial volume density and lipid volume. However type II fibers had significantly greater myofibrillar volume densities and sarcoplasmic reticulum volume density (by approximately twofold). Fiber areas were also greater in type II than type I fibers while myosin packing density did not differ between fiber types.</p> <p>It was concluded that SR and myofibrillar protein ultrastructural differences between fiber types may be important factors in determining twitch contraction properties and tension production for type I and II fibers. Finally, differences in mitochondrial volume density may be important in determining fatiguability of fiber types.</p>