Preservation of mitochondrial volume homeostasis at the early stages of age-related synaptic deterioration

A morphometric study on synaptic mitochondria was performed in the frontal (FC) and temporal (TC) cortex of adult and aged monkeys to seek ultrastructural alterations due to age. The overall volume covered by mitochondria (volume density: Vv), the number of mitochondria/mu m(3) of tissue (numeric density: Nv), the average mitochondrial size (average volume: V), and the average mitochondrial shape (average length: Fmax) were calculated. Either in FC and TC, no significant age-related differences were revealed for any of the above-mentioned morphometric parameters. Namely, in FC of aged monkeys, a decrease of Vv (2%) and Nv (6%) was observed, whereas V and Fmax were increased by 5% and 2%, respectively. In TC of aged animals, both Vv and Nv increased by 7%, V decreased by 2%, and Fmax increased by 1%. The above morphometric parameters account for changes in single aspects of mitochondrial ultrastructure; nonetheless, when considered together per experimental group, they provide information regarding the structural rearrangements occurring on discrete populations of organelles. Considering these assumptions, the present findings document a preservation of the mitochondrial volume homeostasis in the brain of aged monkeys. Because our data from a previous investigation on the same animals showed early signs of synaptic deterioration in FC and TC during aging, this seems to be in contrast with the results of the present study. However, the clear age-related preservation of the mitochondrial potential for structural dynamics may be interpreted as a reactive response to early signs of synaptic deterioration.