The gastrocnemius muscle (GM) of young (3 months) and aged (12 months) female wildtype
C57/BL6 mice was examined by light and electron microscopy, looking for the presence
of structural changes at early stage of the ageing process. Morphometrical parameters
including body and gastrocnemius weights, number and type of muscle fibers, cross section
area (CSA), perimeter, and feret's diameter of single muscle fiber, were measured. Moreover,
lengths of the sarcomere, A-band, I-band, H-zone, and number and CSA of intermyofibrillar
mitochondria (IFM), were also determined. The results provide evidence that 12 month-old
mice had significant changes on skeletal muscle structure, beginning with the reduction of
gastrocnemius weight to body weight ratio, compatible with an early loss of skeletal muscle
function and strength. Moreover, light microscopy revealed increased muscle fibers size, with
a significant increase on their CSA, perimeter, and diameter of both type I and type II muscle
fibers, and a reduction in the percentage of muscle area occupied by type II fibers. Enhanced
connective tissue infiltrations, and the presence of centrally nucleated muscle fibers, were
also found in aged mice. These changes may underlie an attempt to compensate the loss of
muscle mass and muscle fibers number. Furthermore, electron microscopy discovered a
significant age-dependent increase in the length of sarcomeres, I and H bands, and reduction
on the overlapped actin/myosin length, supporting contractile force loss with age. Electron
microscopy also showed an increased number and CSA of IFM with age, which may reveal
more endurance at 12 months of age. Together, mice at early stage of aging already show
significant changes in gastrocnemius muscle morphology and ultrastructure that are
suggestive of the onset of sarcopenia.