It has been shown that changes in the nuclear number in myofibers are synchronized with myofiber size. Therefore, under some conditions, the myonuclear number is thought to be a determinant factor of myofiber size. However, we have clearly shown that denervation-induced fiber atrophy occurs without any decrease in myonuclear number, indicating that the myonuclear number is not always an important determinant factor of myofiber size. However, this was an event found under experimental conditions. In the present study, we examined the morphological features of single myofibers under normal conditions throughout the lifespan of normal mice. We discovered that the C/N ratio (cell volume/nucleus) greatly increases during the growth period and clearly decreases during the aging period. From 5 weeks to 6 months old, the myofibers undergo fiber hypertrophy accompanied by a decrease in myonuclear number. In muscle at 18 months, we found no correlation between myonuclear number and fiber cross-sectional area. These results suggest that, even under normal physiological conditions, the myonuclear number is not always a determinant factor of the myofiber size.
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