Figure 1. Schematic representation of SC myogenic progression.

Upon activation, SCs (initially Pax7+/MyoD−) can enter the myogenic program by either inducing Myf5 or MyoD expression. Most of the cells subsequently express both determination genes and terminally differentiate by inducing Myogenin (Mgn+) expression and then fusing to form multinucleated fibers. A fraction of the SCs self-renew by down-regulating MyoD. Dashed lines represent possible but not proven connections. A fraction of SCs possibly differentiate without entering the cell cycle (see text). The newly formed fiber on the right is depicted as having centrally placed nuclei to reflect the process that occurs during muscle regeneration as opposed to homeostatic turnover of mature myonuclei by SCs which occurs normally at a very low level.