There are two phases of myogenesis, primary myogenesis, and secondary myogenesis. The primary phase is occurs during the embryonic stage and the secondary phase occurs in the fetal stage. The main role of the primary phase is the development of the myoblasts.
In the primary stage, myogenic progenitor cells are derived from pools of mesenchymal stem cells (MSC) (Yan et al. 2013). The myogenic progenitor cells are converted into myoblasts. Myoblasts are templates for the secondary phase when myotubes are developed, and therefore the number of myoblasts impacts the number of myotubes. Nutrition of the dam plays a key role in the number of myogenic progenitor cells that are produced during this phase; poor nutrition reduces the number of myoblasts produced (Yan et al. 2013). The correct diet is necessary to reach maximum proliferation of myogenic progenitor cells.
Primary myogenesis produces muscle fibres, they are referred to as primary muscle fibers. In the second trimester, the primary fibres develop into myofibers with peripheral nuclei (Albrecht et al. 2013.).
By four and a half months of gestation, the primary muscles fibers are covered by the secondary muscle fibers (Albrecht et al. 2013.). The muscle fibres that develop during the primary phase do not impact the size and number of muscle fibres in an animal because they are produced in minimal numbers (Yan et al. 2013).During the secondary phase of myogenesis, the majority of muscle fibres are developed develop.
The muscle fibres are developed by the myoblasts line up with each other, become fused, and develop into immature muscle fibers, called myotubes (Yan et al. 2013). At six months of gestation, most of the fibers are developed and organized into muscle fiber bundles (Albrecht et al.
2013). Muscle development is controlled by two signalling pathways. Negative regulation is through the myostatatin-Smad3 pathway, reducing the myotube size. The development of muscle is positively regulated by insulin-like growth factor 1– phosphoinositide-3-kinase–Akt/protein kinase B–mammalian target of rapamycin pathway (IGF1–PI3K–Akt/PKB–mTOR) (Schiaffino et al.
2013). The process of myogenesis is activated by the proteins Wingless and Int (Wnt). During the primary phase, paired box genes (Pax) 3 and 7 in MSC activate myogenic regulatory factors (MRFs). The MRFs allow the MSC to commit to myogenic progenitor cells. Myogenin, MRF-4, Myo-D and Myf-5 are the MRFs that have been identified in the myogenesis process (Yan et al. 2013).
MRF-4 is the first MRF that is expressed, followed by Myo-D and Myf-5. Myo-D and Myf-5 are the regulatory factors that convert myogenic progenitor cells to myoblasts. Myogenin is expressed in secondary myogenesis, it fuses the myoblasts together to form myotubes. The majority of muscle fiber development occurs during the prenatal stage; new muscle fibres are produced postnatally when there is an injury to existing muscle fibre. In the postnatal stage, the size of the muscle fibres is increased, and this relies on satellite cells (Yan et al. 2013). Satellite cells are located between the sarcolemma and basal lamina layers of the muscle, and are composed of postnatal MSC. The MSC are dormant mononucleated cells that proliferate when stimulated by growth factors.
The MSC develop into myoblasts with a portion remaining as daughter cells (Yan et al. 2013). The myoblasts are fused to the current muscle fibres and nuclei, leading to increased size of muscle fibers.