Myogenesis

Myogenesis is the formation of muscular tissue, particularly during embryonic development. Myogenesis is the formation of muscular tissue, particularly during embryonic development. Muscle fibers generally form the fusion of myoblasts into multi-nucleated fibers called myotubes. In the early development of an embryo, myoblasts can either proliferate, or differentiate into a myotube. What controls this choice in vivo is generally unclear. If placed in cell culture, most myoblasts will proliferate if enough fibroblast growth factor (FGF) or another growth factor is present in the medium surrounding the cells. When the growth factor runs out, the myoblasts cease division and undergo terminal differentiation into myotubes. Myoblast differentiation proceeds in stages. The first stage, involves cell cycle exit and the commencement of expression of certain genes. The second stage of differentiation involves the alignment of the myoblasts with one another. Studies have shown that even rat and chick myoblasts can recognise and align with one another, suggesting evolutionary conservation of the mechanisms involved. The third stage is the actual cell fusion itself. In this stage, the presence of calcium ions is critical. In mice, fusion is aided by a set of metalloproteinases called meltrins and a variety of other proteins still under investigation. Fusion involves recruitment of actin to the plasma membrane, followed by close apposition and creation of a pore that subsequently rapidly widens.