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 an d surrounded by a common basement membrane. The characteristic localization differentiates them from embryonic and fetal myoblasts, SC are able to lea ve the G(0) phase in which they usually reside in normal adult muscle, ente r 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, regen eration, and hypertrophy. Furthermore, they are targets in denervation atro phy, which results in SC loss in the long run. There is evidence From exper imental 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 hete rogeneity must be taken into account if future therapy strategies for muscu lar dystrophies are designed.