Figure 12:

During the determination phase of myogenesis, stem cells divide and are committed to myoblasts, a process requiring expression of the regulatory factors MyoD and Myf-5. Subsequently, myoblast that are non-proliferative fuse to form primary myotubes, which eventually become muscle fibers that express MyHC_SLOW. Primary myotubes that have further fusion of non-proliferative myoblasts become secondary myotubes. These myotubes develop into muscle fibers capable of expressing MyHC_SLOW or any of the fast MyHC isoforms.