Greater frequency and concentration of satellite cells at the ends of developing skeletal muscle fibers

Abstract

Satellite cells (SCs) are mononuclear myogenic stem cells located immediately beneath the basal laminae of skeletal muscle fibers. Once activated, SCs fuse to the fibers as new myonuclei. Since myonuclei are more concentrated in tapered fiber ends, this study tests the hypothesis that there is a greater frequency (number of SC nuclei over all nuclei within the basal laminae) and a higher concentration of SCs at the ends of skeletal muscle fibers. Chicken pectoralis was used as the experimental model because of its overlapping fibers and the presence of neonatal myosin in the fiber ends. Samples were excised from birds ranging in age from nine to 115 days. Antibodies against Pax7 were used to identify SC nuclei. Applying immunocytochemical techniques and computer image analyses, the frequency of SCs was quantified for different types of fiber profiles (neonatal, transforming and adult) and with regards to fiber size. While the frequency of SCs decreased significantly (P<0.01) with age from about 20% to less than 5%, along the fiber lengths the ends retained the highest frequency ranging between 20-13%. Also, the area of plasmalemma per SC was calculated for each type of fiber profile. There was significantly (P<0.05) less plasmalemma per SC at the ends of the fibers. This study demonstrates that throughout development there is a higher frequency and concentration of SCs at the tapered ends of skeletal muscle fibers. Funds for this study were provided by the Natural Sciences and Engineering Research Council of Canada.