Quantitative cytochemical mapping of mitochondrial enzymes in rat cerebella

Mitochondrial metabolic competence, defined as the organelle's capacity to provide adequate amounts of ATP in due time, appears to constitute an important determinant in several biological processes and pathological conditions. Thus, the assessment of the metabolic efficiency of the mitochondrial population in a given tissue area or cellular compartment may provide clues to identifying alterations of the cellular bioenergetic machinery, which may constitute a predisposing condition leading to impaired organ and system functions. In the cerebellar cortex of adult rats, the activities of the enzymes cytochrome oxidase (COX) and succinic dehydrogenase (SDH) were, respectively, evidenced by means of the diaminobenzidine and copper ferrocyanide preferential cytochemical techniques. At the electron microscope, the activities of these two key molecules of the respiratory chain were clearly visualised as dark precipitates at the inner mitochondrial membrane sites where COX and SDH are located. By means of the disector method, unbiased mitochondrial samplings were carried out to measure: the number of mitochondria/microm(3) of tissue (numeric density: Nv); the mitochondrial volume fraction/microm(3) of tissue (volume density: Vv) and the average mitochondrial volume (V) both on COX- and SDH-positive organelles in the cerebellar glomeruli and Purkinje cells, respectively. The ratio R (total area of the precipitates due either to COX or SDH activity within the single mitochondrion/area of the same organelle) was also evaluated to get information on the enzyme activity related to mitochondrial size.The documented accumulation of mutant mitochondrial DNA particularly in postmitotic cells results in a marked heteroplasmy (mixtures of normal and mutated genomes) at mitochondrial and cellular levels, thus the cellular potential for energy production is demanded to a mosaic of organelles with different functional capabilities. Assessment of the mitochondrial mosaic outline by means of quantitative cytochemistry of key enzymes of the respiratory chain, such as COX and SDH, may allow for the morphofunctional metabolic mapping of mitochondrial efficiency in discrete cellular or tissue compartments.