A. SDH staining of skeletal muscle biopsies from individuals IV:1 from Family 1a (a), V:6 from Family 1b (b) and IV:2 from Family 2 (c). B. Transmission electron microscopy of the same muscle specimens as in A. Both SDH histoenzymatic staining (A) and ultrastructural evaluation (B) demonstrate mitochondrial proliferation as highlighted respectively by subsarcolemmal increase of SDH reaction and corresponding accumulation of mitochondria. C. Assessment of mtDNA content per cell, presented as column with mean ± SD (n = 3; **p<0.001), confirms the activation of mitochondrial biogenesis as shown by the significant increase in LHON samples. D. The evaluation of citrate synthase activity, presented as scatter plot with mean ± SD, parallels again the results observed in C, with an increased mean value in the LHON samples. E. Evaluations of Complex I, Complex II+III, Complex III, Complex IV activities, presented as scatter plot with mean ± SD, reveal an increase of all activities in LHON samples.