Fig. 3. A schematic illustration about interplay between mitochondrial dynamics and mitophagy.

A As a first line of defense to toxins, mitochondrial fusion works as a “transient response”, by mixing and sharing mitochondrial contents, to dilute and buffer mitochondrial damage (black dots), as well as to recover bad mitochondria back to healthy mitochondria. B When deleterious damage accumulates in the mitochondria, fission results in the segregation of impaired mitochondria from healthy ones. Damaged mitochondria in small sizes are destined for autophagic removal. C In response to acute stress, mitochondria fuse in self-repair mechanism as a first-aid. Simultaneously, stress cumulates on the mitochondria. When the damage reaches a threshold at which “transient response” (fusion) is incompetent to repair, in the case that mitochondrial fission is retarded by mitochondria dysfunction or restrained by genetic ablation of fission machinery, mitochondria undergo autophagic elimination at a hyper-fused state. Fused mitochondria are permissible to mitophagy.