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Serial cross-sections of muscle from a patient with Kearns-Sayre syndrome

(A) The modified Gomori trichrome stain shows fibers with purplish patches representing abnormal accumulations of mitochondria. These are known as “ragged-red fibers” (RRF). (B) The succinate dehydrogenase (SDH) stain better reveals the abnormal proliferation of mitochondria as excessively stained fibers corresponding to RRF. These fibers are labeled “ragged-blue.” (C) The cytochrome c oxidase (COX) stain shows complete or partial loss of the normal brown color in ragged-red or ragged-blue fibers. These are COX-negative or COX-deficient fibers. (D) When the SDH and COX stainings are superimposed, COX-negative and COX-deficient fibers appear blue, while in normal fibers the brown COX stain prevails. (Contributed by Dr. Salvatore DiMauro.)

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