Background: Human skeletal muscle fibers are the red, white, and intermediate fibers. They differ in their mitochondrial structure and enzyme activity. Scanning electron microscopy (SEM) was used on specially prepared specimens to determine the distinctive features of mitochondria and sarcoplasmic reticulum (SR) in each fiber type.
Methods: Specimens of human limb muscles were glutaraldehyde fixed, frozen, fractured, and macerated by the aldehyde-osmium-DMSO-osmium procedure to expose large areas of mitochondria and SR. Osmium-hydrazine-impregnated tissues were examined without metal coating by ultra-high-resolution SEM.
Results: In white fibers, paired long, thin mitochondria encircled myofibrils at the I-band level. In red fibers, the paired rows of stubby mitochondria at the I-band level were often connected across the A-band to the next row of mitochondria by a slender mitochondrial stalk. Intermediate fiber mitochondria resembled those in red fibers but were longer and thinner. Intermyofibrillar mitochondrial columns were most common in red fibers. All three muscle types had T-tubules along the A-I junction level, and small periodic terminal cisternae formed triads or dyads. Sarcotubules from terminal cisternae formed continuous three-dimensional networks at the I-band level, but intermittent straight sarcotubules, narrow two-dimensional networks, and some axial tubules traversed the A-band. The subsarcolemmal space had continuous two-dimensional SR at the H-band level and a coarse SR network at the I-band. These two SR networks were connected by single A-band sarcotubules.
Conclusions: Mitochondrial shape and configuration were distinctive for each human skeletal muscle fiber type, but the SR was similar in all muscles examined.