The formation of muscle fibers from myoblasts is called myogenesis and begins during embryonic development. The mesoderm, an embryonic cell layer, gives rise to muscle progenitor cells or MPCs. MPCs mature into myoblasts – mononucleated cells that can differentiate and ultimately form the muscle fiber. Myoblasts continuously proliferate in the abundance of growth factors. As the myoblasts internalize these molecules, the growth factor concentration drops, causing the myoblasts to stop dividing. Additionally, transcription factors such as the myoblast determination protein or MyoD halt the cell cycle and activate the expression of muscle-specific genes. The myoblasts then align with each other and fuse at fusion pores to form myotubes, in the presence of cell adhesion molecules or CAMs, calcium ions, and fusion proteins called meltrins. Once fused, the myotubes contract and mature into multinucleated skeletal muscle fibers.