[Mitochondrial diseases].
Mitochondrial diseases are very complex. Their description, recent but still in progress, makes all classifications risky. In the first part of this article we present the original character of mitochondria, which is due to their functional structure aimed to produce energy, the respiratory chain being fundamental for the phosphorylation/oxidation and ATP production processes. Beside Mendelian transmission, mitochondria have their own DNA which codes for proteins that are few but play an essential role; the nuclear DNA probably has a regulatory effect. For these reasons, explorations of mitochondrial functioning, mainly by morphological, biochemical and genetic techniques, are specific. In the second, clinical part, we analyse the whole range of symptoms and syndromes which includes purely muscular lesions, predominantly nervous lesions and multisystemic lesions. Some syndromes can be individualized on a biochemical basis, whereas other are individualized on a genetic basis involving mainly mutations of mitochondrial DNA. Finally, we merely list lesions the origin of which might be mitochondrial but remains unknown.