Ragged red or ragged blue fibers.

Fibers called ragged red fibers are generally considered the morphological characteristic of mitochondrial encephalomyopathies. These fibers appear red in the modified Gomori trichrome (Tri) stain due to subsarcolemmal and interfibrillar increase in mitochondrial number and volume. Other accepted morphological abnormalities include partial cytochrome c oxidase deficiency and subsarcolemmal increase in succinate dehydrogenase and NADH tetrazolium reductase stain. We were interested to see which of these abnormalities would be the most specific for mitochondrial cytopathies such as Kearns-Sayre syndrome and chronic progressive external ophthalmoplegia. We analyzed five patients and found 74 fibers compatible with mitochondrial abnormalities as defined above. The modified Gomori Tri stain turned out to be the most specific and reliable technique.