Satellite cells as players and targets in normal and diseased muscle.

Satellite cells (SC), also termed adult myoblasts, are mononuclear cells of myogenic lineage. They are attached to the muscle fiber plasma membrane and surrounded by a common basement membrane. The characteristic localization differentiates them from embryonic and fetal myoblasts. SC are able to leave the G0 phase in which they usually reside in normal adult muscle, enter the mitotic cycle, and differentiate to express muscle-specific proteins. The activation of SC into the mitotic compartment and their progression to the differentiative compartment are processes which are tightly regulated by myogenic regulatory factors of the MyoD family and the cyclin-dependent kinases and their inhibitors. Those factors are in turn regulated by growth factors and innervation. SC are key features in muscle fiber growth, regeneration, and hypertrophy. Furthermore, they are targets in denervation atrophy, which results in SC loss in the long run. There is evidence from experimental work that SC age, and that they are a heterogeneous cell population in terms of mitotic cycle duration and have the ability to differentiate. Little is known of SC in human diseased muscle beyond their number. SC heterogeneity must be taken into account if future therapy strategies for muscular dystrophies are designed.