Most mitochondrial encounters/visitations do not result in fusion. (A–C) Fluorescence microscopy images of merged (red + green) channels as described for Fig. 3. Dotted lines approximately outline the neuronal processes. Transparent arrows/lines track the same mitochondrion. (A) A mitochondrion (red, not photoactivated) moves past a yellow photoactivated mitochondrion without exchanging contents. The indicated imaging times are relative to the earliest frame shown. (B) Activated (yellow) mitochondria move past stationary/slow unactivated (red) mitochondria without exchanging contents. (C) Two adjacent/overlapping mitochondria (one photoactivated) move at similar velocities without fusing to each other or to a third stationary mitochondrion.