Electron tomography allows us to see the 3D structure of mitochondria. (see our publication: von Ahsen et al., 2000; from a collaboration with Guy Perkins and Mark Ellisman at UC San Diego).

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An Untreated Mitochondrion (click to view movie)
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A mitochondrion treated with the pro-apoptotic protein tBid. Note that the structure is essentially the same as the untreated mitochondrion.
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A mitochondrion treated with calcium. Note the swollen matrix.