Commentaries on Viewpoint: "Muscle memory" not mediated by myonuclear number? Secondary analysis of human detraining data - Increased myonuclear density may require longer period of disuse in human muscles

Changes of number and density of myonuclei attracted considerable attention since the fundamental study by Gundersen and coworkers which gave a solid experimen- tal basis to the constant nuclear domain size hypothesis (1). In the attempt to clarify the question about the possible contribution of myonuclei to the phenomenon of muscle memory, Psilander et al. (4) showed a lack of significant variation of myonuclei in skeletal muscle fibers during training and detraining. The Viewpoint recently published by Murach et al. (2) focused the attention on what happens during the detraining phase and gives the opportunity to reanalyze the data of Psilander et al. (4), which are publicly available, showing that, among the 11 subjects who in- creased myonuclear number (normalized to fiber length) after training, 7 underwent a loss and 4 an accretion of myonuclei number during the detraining phase. Our data recently published on a group of oldest-old individuals confined to wheelchair for the last two years suggest that the issue of myonuclear adaptation in disuse could be more complicated (3). Surprisingly, we observed in vastus late- ralis muscle of immobilized individuals a greater myonu- clear density (myonuclei/mm fiber) compared with age- matched subjects still able to walk. It is unclear whether this is due to the persistence of myonuclei in atrophic fibers or to a fiber type shift, as in those subjects we observed a slow-to-fast transition, or to other mechanisms with a com- pensatory effect.