Morphological observations in skeletal muscle from patients with a mitochondrial myopathy

Mitochondrial metabolic dysfunction is considered to be the cause of certain congenital myopathies and a number of multisystem disorders in humans. The morphological hallmark of these diseases is the ‘ragged red’ fibre, which shows abnormally intensive oxidative enzyme reactions. Electron microscopy reveals that the numerically increased mitochondria in these fibres are often markedly enlarged and possess aberrant configurations of cristae. The mitochondrial matrix often contains lipid‐like inclusions or shows vacuolation. The most characteristic mitochondrial abnormality is the occurrence of highly ordered inclusions in the intracristal or intermembrane space. These inclusions appear to be true crystals, composed of proteinaceous material. It is argued that the activity of accumulation of proteins in the mitochondria is related to the nuclear and nucleolar hypertrophy noticeable in the ragged red fibres. Since protein crystals in mitochondria in particular occur when an increased capillary density around the ragged red fibres is present, it is suggested that oxygen free radicals and lipid peroxidation processes are involved in the ragged red fibre pathology.