Are myonuclei forever?
Bruusgaard J.C., Johansen I.B., Egner I.M., Gundersen K.
J Muscle Res Cell Motil (2009) 30: 324
“Muscle size and force is mainly altered by changing the cross-sectional area of each muscle fibre. Muscle fibres contain multiple nuclei, and it has generally been assumed that each nucleus supports a certain cytoplasmic domain, and that some myonuclei are lost by a selective apoptosis during atrophy, while new nuclei are added from satellite cells during hypertrophy. We have recently used novel in vivo time lapse techniques to demonstrate that the number of myonuclei are constant during inactivity-atrophy. We have now used similar techniques to study overload-hypertrophy. A 53% increase in the number of nuclei was observed after 6–10 days after the overload intervention and this preceded the hypertrophy that commenced after 8 days. The newly added nuclei were persistence even if the fibres were subjected to subsequent denervation for up to 3 months: the number of nuclei remained as high in the severely atrophic, inactive fibres, as in normally innervated, overloaded muscles. Thus, the overload episode induced a permanently elevated number of nuclei. This effect might explain the observations that previously strength trained individuals are easily retrained. This phenomenon has been dubbed ‘‘muscle memory’’, and has previously been attributed to motor learning. Since anabolic steroids also increase the number of myonuclei, our findings might have an impact on the suspension periods of athletes after a doping offence.”