Muscle fibers form from the fusion of myoblasts (muscle stem cells) into multi-nucleated fibers (called myotubes). In the early development of an embryo these myoblasts will proliferate if enough fibroblast growth factor is present. When the FGF runs out, the myoblasts cease division and secrete fibronectin onto their extracellular matrix.

The second stage involves the alignment of the myoblasts into the myotubes. The identity of the myoblasts in this stage is not crucial. Studies have shown that even rat and chick myoblasts have aligned!

The third stage is the actual cell fusion itself. In this stage, calcium ions are critical for development. Fusion is mediated by a set of metalloproteinases called meltrins. Myocyte Enhance Factors (MEFs) promote myogenesis. Serum response factor (SRF) plays a central role during myogenesis, being required for the expression of striated alpha-actin genes. Expression of skeletal alpha-actin is also regulated by the Androgen Receptor; steroids can thereby regulate myogenesis .

References

• Gilbert, Scott F. Developmental Biology, Sixth Edition - Myogenesis - The Development of Muscle