Accurate measurements of mitochondrial mass require a probe that will accumulate in mitochondria regardless of the mitochondrial membrane potential, a property displayed by several of our MitoTracker and MitoFluor dyes — MitoFluor Green (M7502), MitoFluor Red 589 (M22424), MitoTracker Red 580 (M22425) and MitoTracker Deep Red 633 (M22426). Mitochondrial uptake of nonyl acridine orange (NAO, A1372) and MitoTracker Green FM (M7514) has also been reported to be independent of mitochondrial membrane potential,^1,2 although studies have shown that this may not be true for all cell types.^3,4 Mitochondrial staining by nonyl acridine orange is attributed to binding of cardiolipin in the inner mitochondrial membrane,⁵ suggesting that the observed fluorescence signal may not correlate with the "mass" of the entire mitochondria but instead may measure the amount of inner membrane present. Indeed, one study has shown that mitochondrial mass can increase while the mitochondrial volume remains unchanged.⁶

Mitochondrial mass varies with cell type and can be useful in separating populations of healthy cells (see Figure 1). The mitochondrial mass of a cell does change with time, as demonstrated by the loss of mitochondrial mass in studies of aging cells.⁷ Changes in mitochondrial mass are also observed in apoptotic cells, but are not necessarily associated with the loss of mitochondrial membrane potential.^6,8.