Skeletal muscle fibers have a strong ability to adapt to environmental alterations by changing their phenotype. The enormous organized arrangement of muscle fibers contributes to a variety of structural properties and functional capabilities. Close to the functional demands in recruited muscle fibers there are changes in response to several stimuli (e.g. mechanical, metabolic, neuronal, hormonal) that lead to load-dependent muscle fiber type specific alterations. Basically structural muscle fiber type delineation is based on specific myosin profiles, especially the myosin heavy chain (MyHC) expression.

The aim of this study was to investigate the intramuscular MyHC gene pattern in single human skeletal muscle fibers with regard to a chimeric arrangement.

Using laser microdissection (P.A.L.M. System, Zeiss, Munich, Germany) single myofibrillar ATPase stained muscle fibers from a primary muscle bundle (human vastus lateralis muscle) were precisely excised. Subsequently, quantitative Real Time-PCR (qRT-PCR) was performed and the MyHC-I, -IIA, -IIX as well as MyHC-embryonic genes were analyzed in single fast twitch fibers (FTF; IIA, IIX) and slow twitch fibers (STF; I).

The analysis of MyHC genes in single muscle fibers shows an intramuscular heterogeneity in fiber type composition. Attention should be paid to the MyHC isoform profile of the fast twitch fibers (IIA/IIX). In contrast to the predominant dogma we could show a conspicuous gene expression of MyHC I. This strong relative amount of MyHC I on mRNA level demonstrates an opposed character of the single fiber than the ATPase staining illustrates.

In conclusion, single muscle fiber-specific gene expression analysis demonstrates the existence of MyHC-gene chimera. The performed histochemical ATPase staining shows crucial opposed results between phenotype and genotype. Remarkable is the strong MyHC I isoform expression in slow twitch fibers. These results demonstrate the various properties of single muscle fibers and their enormous influence on exercise induced muscle plasticity.