= Mitochondrial fusion
<Mitochondrial fusion> is the joining of previously separate <mitochondria>, allowing their internal contents to mix. Because each <mitochondrion> has two boundary <cell membranes>, complete <mitochondrial fusion> requires joining both the outer and the <inner mitochondrial membrane>. Together with <mitochondrial fission-fusion cycle>, it makes <mitochondria> a changing network rather than a fixed collection of bean-shaped objects.
A useful experiment distinguishes joining from mere contact. <Hsiuchen Chen> and colleagues mixed mouse <cells> carrying differently coloured fluorescent <proteins> targeted to the mitochondrial interior, then fused the <cells> so both labelled populations shared one <cytoplasm>. Appearance of both colours within the same <mitochondria> reported content mixing. Loss of either <mitofusin> Mfn1 or Mfn2 reduced this mixing and produced fragmented <mitochondria>; restoring the missing <protein> rescued the defect. <Mitochondrial fusion> therefore has a measurable molecular requirement and permits exchange within the mitochondrial population.<Mitofusins Mfn1 and Mfn2 coordinately regulate mitochondrial fusion and are essential for embryonic development by Hsiuchen Chen and others>[Results, Mitochondrial fusion is severely reduced in mutant cells; polyethylene-glycol cell-fusion assay and rescue experiments]{ref}
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