Figure 1.

Model for changes in expression of myogenic regulatory factors through the myogenic program. In quiescence, satellite cells reside between basal lamina and sarcolemma as satellite stem cells (Pax7⁺/Myf5⁻) and/or as committed satellite cells (Pax7⁺/Myf5⁺). On activation, committed satellite cells upregulate myoblast determination protein (MyoD). Satellite progeny then follow one of two fates. They either enter the cell cycle to proliferate as myoblast and differentiate by down regulating Pax7 and up regulating myogenin and MRF4 or down regulate MyoD and self renew to give rise to Pax7⁺ satellite cells. Syndecan-4 and glypican-1 are regulators of expression of myogenic regulatory factors during satellite cell proliferation and differentiation.