Vertebrate Myogenesis : Stem Cells and Precursors

Muscle development of vertebrates has been a paradigm of cell differentiation for many years. Three types of muscle are found in the vertebrate body: skele- tal, heart and smooth muscle, and there has been a gradient of concern about these different muscle types in the sequence they are mentioned here. Skeletal muscle has received much attention because it can be induced to differentiate in vitro and because of the clinical relevance of myopathies. The discovery of the muscle-specific members of the bHLH and MADS families of transcription factors must be regarded as a breakthrough not only in muscle research and have opened new insights into the genetic control of differentiation. Conse- quently, the effects of gene-targeting of the MyoD-related (myfs) and MEF transcription factors soon became objects of investigation. Along with the genetic control of skeletal and heart muscle development, the temporal-spatial appearance of cells fated to become myocytes has been of foremost interest. The source of all skeletal muscle of the trunk is the paraxial mesoderm, which gives rise to metameric entities, the somites. The somite can be regarded as a turntable of mesodermal cell fates, chondrocytes, fibroblasts, angioblasts of these deriva- and skeletal muscle precursors. The coordinated development tives is tightly controlled by local tissue interactions between embryonic struc- tures such as the neural tube, the notochord, the lateral plate and the ectoderm.