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• _4 value "One of the hallmarks of regenerating myofibers is the centrally located position of the myonuclei; upon maturing, muscle fiber nuclei are located along the cell periphery [4]. Notably, repeated cycles of injury and regeneration do not appear to deplete satellite cell numbers, suggesting that these cells have the ability to self-renew [2]. Satellite cells were initially identified in frog leg muscles by electron microscopy [1], and subsequently have been identified in all higher vertebrates. In humans and mice, these quiescent [18], non-fibrillar, mononuclear cells are most plentiful at birth (estimated at 32% of sublaminar nuclei) [19]. The frequency declines post-natally, stabilizing to between 1 to 5% of skeletal muscle nuclei in adult mice [2]. Satellite cell frequency varies in different muscles, likely as a function of variation in fiber type composition (i.e. slow oxidative, fast oxidative, or fast glycolytic fibers). For example, the mouse soleus muscle, which is predominantly made up of slow oxidative fibers, has a higher number of satellite cells than the extensor digitorum longus (EDL) muscle, which primarily contains fast glycolytic fibers. Additionally, the absolute numbers of satellite cells increases in the soleus but not the EDL between 1 and 12 months of age, although the proportion of satellite cells decreases in both muscle types with increasing age [20]. In humans, the proportion of satellite cells in skeletal muscles also decreases with age, which may explain the decreased efficiency of muscle regeneration in older subjects [21]. Satellite cells from aged muscle also display reduced proliferative and fusion capacity, as well as a tendency to accumulate fat, all of which likely contribute to deteriorating regeneration capability [22,23]. That endurance training can offset the decline in satellite cell number with age suggests that poorer regeneration is not simply a result of limited replicative potential of older satellite cells [24]. Several signals and growth factors have been implicated in promotion of satellite cell activation and proliferation (Figure 1). For example, the Notch signaling pathway, which is activated upon muscle injury, regulates satellite cell transition from quiescence to proliferation in single fiber cultures, thereby expanding the myoblast population in injured muscle [25]." provenance.
• _4 wasQuotedFrom 14614776 provenance.