Muscle satellite cell heterogeneity: in vitro and in vivo evidences for populations that fuse differently

During development, muscle growth results from the proliferation of satellite cells (SC) and their fusion with fibers. Several studies revealed heterogeneity of SC population notably based on the proliferation rate. Here, we examined the SC characteristics of turkey skeletal muscles in terms of proliferation and more specifically fusion, to define if the ability of these cells to fuse may represent a distinct characteristic between them and could be directly associated with their proliferation properties. Freshly extracted SC were plated in clonal condition and their proliferation rate was assessed 11 days later. To investigate the SC fusion behavior, in vitro and in vivo approaches were developed. Highly and slowly proliferative SC were initially labeled with a nuclear beta-galactosidase (beta-Gal) activity and co-cultured with differentiated primary cultures. After 5 days, distribution of beta-Gal positive (beta-Gal+) nuclei was examined. Also, the two labeled SC types were transplanted into different muscles in autologous model. One week later, number of beta-Gal+ nuclei per fiber and diameter of fibers displaying beta-Gal+ nuclei were determined. In vitro, we showed that SC from turkey skeletal muscle are present as a heterogeneous population in terms of proliferation. Examination of their fusion properties in vitro as well as in vivo revealed that highly proliferative SC exclusively exhibited fusion with differentiated myotubes or myofibers, whereas slowly proliferative SC mainly fused together. Collectively, these data demonstrate for the first time that SC with different proliferation rate also intrinsically differ in their fusion potential, suggesting distinct roles for these sub-populations in muscle growth.