Abstract
Skeletal muscle aging is accompanied by mass reduction and functional decline, as a result of multiple factors, such as protein expression, morphology of organelles, metabolic equilibria, and neural communication. Skeletal muscles are formed by multiple fibers that express different Myosin Heavy Chains (MyHCs) and have different metabolic properties and different blood supply, with the purpose to adapt their contraction to the functional need. The fine interplay between the different fibers composing a muscle and its architectural organization determine its functional properties. Immunohistochemical and histochemical analyses of the skeletal muscle tissue, besides evidencing morphological characteristics, allow for the precise determination of protein expression and metabolic properties, providing essential information at the single-fiber level. Aiming to gain further knowledge on the influence of aging on skeletal muscles, we investigated the expression of the MyHCs, the Succinate Dehydrogenase (SDH) activity, and the presence of capillaries and Tubular Aggregates (TAs) in the tibialis anterior muscles of physiologically aging C57BL/6J mice aged 8 (adult), 18 (middle aged), and 24 months (old). We observed an increase of type-IIB fast-contracting fibers, an increase of the oxidative capacity of type-IIX and -IIA fibers, a general decrease of the capillarization, and the onset of TAs in type-IIB fibers. These data suggest that aging entails a selective modification of the muscle fiber profiles.