SATELLITE CELLS AND MYONUCLEI IN YOUNG AND ELDERLY MUSCLES: EFFECT OF TRAINING 1649

Abstract

A loss of myonuclei in rat skeletal muscles accompanies atrophy induced by inactivity. Correspondingly, satellite cells must contribute nuclei for normal muscle growth to occur. This study asked: 1) Do skeletal muscle fibers in elderly men hypertrophy with training? 2) Do myonuclear numbers increase to accommodate this hypertrophy? 3) Is the capacity to incorporate satellite cells diminished in the elderly? Data came from two previous studies that involved training the vastus lateralis muscle: (1) with weights (n=10, age=64.0±5.1 yrs) and 2) with a rowing or cycle ergometer (n=11, age=22.1±2.3 yrs). Immunohistochemistry for anti-dystrophin antibody and hematoxylin staining on quadriceps frozen sections discriminated myo- from satellite cell nuclei. The fibers of the elderly enlarged by 32%, and the young by 26%. The area associated with each myonucleus increased by 31% in the elderly, but only 8.5% in the young. These values matched estimations of nuclei per mm of fiber length, which increased by only 12% in the elderly compared to 24% in the young. Counts of satellite cells and myonuclei of control young and elderly men using electron microscopy showed 46% more satellite cells in young than in the elderly muscle fibers. These results suggest that not only are there fewer satellite cells in the elderly, but they have less capacity to be activated and incorporated as myonuclei into the fiber. The elderly muscles can hypertrophy, but each nucleus serves a larger volume of sarcoplasm. In contrast, hypertrophy in young muscles results in a proportional increase in number of myonuclei.