###### Abstract  
Direct reprogramming of somatic cells to myoblasts and myotubes holds great potential for muscle development, disease modeling and regenerative medicine. According to recent studies, direct conversion of fibroblasts to myoblasts was performed by using a transcription factor, myoblast determination protein (MyoD), which belongs to a family of myogenic regulatory factors. Therefore, MyoD is considered to be a key driver in the generation of induced myoblasts. In this study, we compared the direct phenotypic conversion of bovine dermal fibroblasts (BDFs) into myoblasts and myotubes by supplementing a transcription factor, bovine MyoD (bMyoD), in the form of recombinant protein or the bMyoD gene, through retroviral vectors. As a result, the delivery of the bMyoD gene to BDFs was more efficient for inducing reprogramming, resulting in direct conversion to myoblasts and myotubes, when compared with protein delivery. BDFs cultured with retrovirus encoding bMyoD increased myogenic gene expression, such as MyoG, MYH3 and MYMK. In addition, the cells expressed myoblast or myotube-specific marker proteins, MyoG and Desmin, respectively. Our findings provide an informative tool for the myogenesis of domestic-animal-derived somatic cells via transgenic technology. By using this method, a new era of regenerative medicine and cultured meat is expected. direct conversion; bovine MyoD (bMyoD); bovine dermal fibroblasts (BDFs); bovine myoblasts; bovine myotubes +
Footnote †: journal: Applied  
recruitment, expansion, and differentiation in the muscle stem cells, resulting in muscle fiber regeneration for damaged tissues [7]. In detail, the pro-inflammatory cytokines, such as TNF-\(\alpha\) and IL-1\(\beta\), and the anti-inflammatory cytokine represented as IL-10, enhance not only the proliferation but also differentiation of myoblasts. Then, cell signaling pathways (e.g., p38 MAPK or NF-B) are initiated and activated by those cytokines. Cytokines, such as TNF-\(\alpha\), may regulate muscle cells during early phases of muscle regeneration through their effects on pro-inflammatory macrophages and also influence muscle cells more directly by binding their receptors on the cells in the later phases. However, serious physical damage, such as from car accidents, explosive injuries, and iatrogenic disorders, can cause volumetric muscle loss (VML) [8]. It could induce insufficient muscle stem cell recruitment and, thus, serious damage to the endogenous self-repair system, resulting in non-functional scarring. Currently, therapy of the VML is considered to be a major challenge, since the conventional treatment method, injection of autonomous muscle cells, has limitations in practical treatment, due to the unstable supply of muscle tissues and significant donor site morbidity [9]. Therefore, it is essential to have efficient techniques to solve the problems of conventional methods. In the field of therapeutic approaches for tissue regeneration, effective treatments for the damaged skeletal muscles include clinical applications with multi-potent stem cells, such as SCs, myoblast or muscle-derived stem cells (MDSCs), and mesenchymal stem cells (MSCs) [10; 11; 12]. Therefore, it has become more significant to regulate differentiation of the multi-potential stem cells, using biochemical factors, represented as growth factors or other biomolecules [13; 14], and biophysical factors, such as mechanical or electrical stimulations [15; 16; 17; 18; 19]. Moreover, using biochemical factors combined with biophysical factors has been suggested as a useful method for the control of stem cell differentiation. Growth factors can be treated by systematic injection methods, bio-scaffold, and drug delivery systems [20; 21; 22]. Despite these improved methods, it is still difficult and inefficient to regulate stem cell fate into intended images delicately. In addition, the use of stem cells cannot be free from high cost and supply problems [23].