Satellite cells were isolated from human quadriceps and masseter muscles and the phenotype of these cells examined in vitro. The expression of the different isoforms of the myosin heavy chains (embryonic, fetal, fast and slow) and light chain isoforms was used to assay myotube diversification. We found that fused cultures of human satellite cells express adult fast and slow MHCs in addition to the embryonic and fetal isoforms. Only the four fast light chains (MLC1emb, MLC1F, MLC2F and MLC3F) were synthesized. No slow MLCs were ever detected in these cultures. In order to determine if the human satellite cells were committed to distinct fast and slow myogenic lineages, a clonal analysis was carried out on both cell populations. All myogenic clones expressed fast and slow MHCs, suggesting that there is no evidence for different fast and slow satellite cell lineages in human skeletal muscle.
