Influence of sex- and fiber type on the satellite cell pool in human skeletal muscle.

The repair, remodeling and regeneration of myofibers is dependent on satellite cells (SCs), although, the distribution of SCs in different fiber types of human muscle remains inconclusive. There is also a paucity of research comparing muscle fiber characteristics in a sex-specific manner. Therefore, the aim of this study was to investigate fiber type-specific SC content in men and women. Muscle biopsies from vastus lateralis were collected from 64 young (mean age 27 ± 5), moderately trained men (n=34) and women (n=30). SCs were identified by Pax7-staining together with immunofluorescent analyses of fiber type composition, fiber size and myonuclei content. In a mixed population, comparable number of SCs were associated to type I and type II fibers (0.07 ± 0.02 vs 0.07 ± 0.02 SCs per fiber, respectively). However, unlike men, women displayed a fiber type-specific distribution, with SC content being lower in type II than type I fibers (P=0.041). Sex-based differences were found specifically for type II fibers, where women displayed lower SC content compared to men (P<0.001). In addition, positive correlations (r-values between 0.36-0.56) were found between SC content and type I and type II fiber size in men (P=0.03 and P<0.01, respectively), whereas similar relationships could not be detected in women. Sex-based differences were also noted for fiber type composition and fiber size, but not for myonuclei content. We hereby provide evidence for sex-based differences present at the myocellular level, which may have important implications when studying exercise- and training induced myogenic responses in skeletal muscle.