During embryogenesis, pluripotent stem cells become determined myoblasts. Subsequent to withdrawal from the cell cycle, myoblasts enter into a complex program of differentiation, which finally leads to the formation of pluricellular myotubes which become muscle fibers. In the course of muscle formation during development, a population of mononucleated cells arise which do not fuse with the developing myotubes but remains at the peripheral region of the myofibers under the basal lamina. These cells, called satellite cells or adult myoblasts, first described by Mauro (Mauro 1961), are involved in the supply of nuclei into the growing fiber (Moss and LeBlond 1971). In adults, they also provide nuclei to regenerating myofibers after damage-induced myolysis of the muscle fiber (Bischoff 1975. Schultz 1984). In culture, myoblasts proliferate and leave the cell cycle to become postmitotic myoblasts. They then undergo myogenic differentiation i.e. a coordinated expression of the proteins of the so-called myogenic program, to produce muscle phenotype. This myogenic program comprises the synthesis of muscle-specific contractile proteins and enzymes of the glycolytic pathway (Bischoff and Holtzer 1959) (fig 1). It also involves the regulation of a protease-antiprotease-protease receptor system described in muscles. In particular, plasminogene activator system seems to play a prominent role in myoblast motility and their fusion (Barlovatz-Meimon et al. 1990, Le Moigne et al. 1990, Quax et al. 1992). In vitro, myogenic differentiation is achieved by fetal myoblasts or from satellite cell population grown in primary cultures. A large number of cell lines have also been developed which are able to synthesize proteins of the myogenic program and, for most of them, to fuse into multinucleated structures.
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