FIG. 4.

Electron microscopy of Myc-hFis1-expressing cells confirms mitochondrial fragmentation. (A) Transmission electron microscopy image of an untransfected HeLa cell showing normal tubular mitochondria with cristae. (B) HeLa cells expressing Myc-hFis1 contain numerous small mitochondria throughout the cytoplasm. Mitochondria in these cells have a hollow appearance, with a less electron-dense matrix and markedly reduced cristae (arrowheads). Connected mitochondria were occasionally observed in these cells (arrows). (C and D) Mitochondria connected by a narrow neck, presumably undergoing fission. Electron-dense necks are apparent at the point of constriction (arrows). Again, note the loss of cristae and lightly stained matrices in these mitochondria (asterisks). Bars, 1.0 μm (A and B) and 200 nm (C and D).