Mitochondria are dynamic organelles capable of significant changes of their ultrastructural features according to the tissue-specific energy demands. In human biopsies of vastus lateralis and anterior tibialis muscles from young (25.0 ± 4.4 years), middle-aged (50.4 ± 7.5 years) and old (75.5 ± 3.9 years) healthy volunteers, we carried out a morphometric study on subsarcolemmal and intermyofibrillar mitochondria to assess whether age-related alterations of the morphology of these organelles contribute to the muscle performance decay in aging, By computer-assisted methods, we measured: the average area (MAA), the longer diameter (Dmax) and the ratio perimeter to area (pleomorphic index: Plei) of mitochondria. No significant age-related ultrastructural differences were found either in subsarcolemmal or intermyofibrillar organelles. However, in middle-aged as well as in the old group of patients vs. the young one, MAA and Dmax showed a clear trend to decrease, while Plei showed a marked, age-related tendency to increase. Higher percentages of less pleomorphic organelles were found in the youngest group of patients and this was particularly evident in the subsarcolemmal mitochondrial population. In addition to reporting on discrete aspects of mitochondrial ultrastructure, MAA, Dmax and Plei are closely related to each other and provide a reliable index of the muscle mitochondria adaptive response to age. Thus, we interpret our results as indicating a substantial preservation of muscle mitochondrial ultrastructure during aging.
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