The generation of fiber diversity during myogenesis.

Adult muscle is composed of different fiber types distinguished by their speed of contraction and metabolism. The generation of these differences is related both to the sequence in which muscle fibers form and to differences between the myogenic cells involved. Fibers form in two successive waves (primary and secondary) whose time of appearance can be correlated with the existence of successive populations of myogenic cells (embryonic and fetal). The differences between fibers arise through an interplay between heritable cellular commitment, where cells are preprogrammed to produce particular types of fiber and influences from the limb environment. The techniques of genetically marking cells and clonal analysis in vivo and in vitro are starting to reveal the relationship between these different influences. Although the process of myogenesis is similar in birds and mammals it is likely that cell autonomous behaviour plays a more important role during avian development as compared to mammals. The identification of muscle specific transcription factors has provided some clues to the mechanisms by which development is controlled but the expression of relatively few of these has been correlated with the sequence of events seen in myogenesis.