FIGURE 2 | Cytochrome c oxidase deficiency in mitochondrial DNA-associated disease and ageing.

Transverse tissue sections that are reacted for both cytochrome c oxidase (COX) and succinate dehydrogenase (SDH) activities sequentially, with COX-positive cells shown in brown and COX-deficient cells shown in blue. a | Skeletal muscle from a patient with a heteroplasmic mitochondrial tRNA point mutation. The section shows a typical 'mosaic' pattern of COX activity, with many muscle fibres harbouring levels of mutated mtDNA that are above the crucial threshold to produce a functional enzyme complex. b | Cardiac tissue (left ventricle) from a patient with a homoplasmic tRNA mutation that causes hypertrophic cardiomyopathy, which demonstrates an absence of COX in most cells. c | A section of cerebellum from a patient with an mtDNA rearrangement that highlights the presence of COX-deficient neurons. d,e | Tissues that show COX deficiency that is due to clonal expansion of somatic mtDNA mutations within single cells — a phenomenon that is seen in both post-mitotic cells (d; extraocular muscles) and rapidly dividing cells (e; colonic crypt) in ageing humans.

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