Division of mitochondria in cultured human fibroblasts.

Ovate mitochondria in cultured human fibroblasts divide by pinching. In the process, as observed by transmission electron microscopy, a deep incisure of the surface membranes separates the organelle into two lobes connected by a slender isthmus. A single element of smooth endoplasmic reticulum (SER) invariably accompanies each incisure, extending deep into the cleft. When the ingrowing membranes meet and fuse with the antipodal membranes, fission occurs. Elongated mitochondria that give no indication of division often are cloaked by a single, continuous cistern of SER.