The morphofunctional features of synaptic mitochondria positive to the activity of cytochrome oxidase (COX) were investigated in the cerebellar cortex of adult and old monkeys to seek alterations of the energy metabolism specifically occurring at the neuronal synaptic compartment with advancing age. Numeric density (Nv), volume density (Vv) as well as average volume (V) and average length (Fmax) were the mitochondrial ultrastructural parameters measured by computer-assisted morphometric methods. The ratio (R): area of the COX cytochemical precipitate/area of the mitochondrion was semiautomatically calculated and considered as an estimation of the mitochondrial metabolic competence (MMC), i.e. the capacity of single organelles to provide adequate amounts of adenosinetriphosphate. No age-related significant difference was found in any of the ultrastructural parameters taken into account, but a significant decrease of R was envisaged in old animals. The quartile distribution of the COX-positive organelles according to their respective cross sectional area showed that there is no significant difference of R comparing the data of small (I quartile), medium-sized (II quartile) and large (III quartile) mitochondria, while a significant decrease of R owing to age was clearly apparent when oversized mitochondria (IV quartile) were compared. Although these data document an age-related preservation of the morphological features of COX-positive mitochondria in the monkey cerebellum, from the functional standpoint they must be interpreted by taking into account also the significant decrease of R in old animals. Since COX is the terminal enzyme of the mitochondrial respiratory chain, the estimation of its activity is currently considered a reliable MMC index, thus our findings by matching preferential cytochemistry and morphometry, support the idea that the specific functional impairment of enlarged synaptic mitochondria may seriously affect information processing and cell-to-cell communication at synaptic junctional areas with advancing age.
mitochondrial metabolic competence