Uncoupled mitochondria quickly shorten along their long axis to form indented spheroids, instead of rings, in a fission-independent manner.

Loss of mitochondrial membrane potential (ΔΨm) triggers dramatic structural changes in mitochondria from a tubular to globular shape, referred to as mitochondrial fragmentation; the resulting globular mitochondria are called swelled or ring/doughnut mitochondria. We evaluated the early period of structural changes during the ΔΨm loss-induced transformation after carbonyl cyanide m-chlorophenyl hydrazine (CCCP) administration using a newly developed correlative microscopic method combined with fluorescence microscopic live imaging and volume electron microscopy. We found that most mitochondria changed from a tubular shape to a globular shape without fusion or fission and typically showed ring shapes within 10 min after CCCP exposure. In contrast, most ring mitochondria did not have a true through hole; rather, they had various indents, and 47% showed stomatocyte shapes with vase-shaped cavities, which is the most stable physical structure without any structural support if the long tubular shape shortens into a sphere. Our results suggested that loss of ΔΨm triggered collapse of mitochondrial structural support mechanisms.