FINE STRUCTURE OF MUSCLE IN HUuMAN DISUSE ATROPHY: SIGNIFICANCE OF PROXIMAL MUSCLE INVOLVEMENT IN MUSCLE DISORDERS*

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Abstract
The universal occurrence of skeletal musculature weakness on disuse, however produced, and the paucity of published reports on fine structural changes in human disuse atrophy, promted this investigation. The quadriceps muscle of a leg immobilized in plaster cast (for fracture) and of the opposite non-immobilized limb was biopsied in 4 adult males, after periods of immobilization from 50-75 days. The 8 muscle specimens were examined for histopathological changes and muscle fiber diameters were measured by micrometry from paraffin sections. The histograms revealed a larger proportion of small fibers (< 20 .mu.m) and a smaller proportion of large fibers (> 40 .mu.m) in the immobilized limb compared to the opposite. Light microscopy showed only atrophic changes. EM confirmed this; atrophy of few to several muscle fibers was seen in loss of myofibrils, collapse and folding of the basement membrane and prominence of glycogen or muscle nuclei. The atrophic change was more severe in the immobilized limbs but was noticeable in all the non-immobilized limbs. Degenerative changes, especially disorganization and breakdown of myofibrils, and fragmentation of plasma membrane, were seen in occasional atrophied muscle fibers, again more frequently in the immobilized limb. Lipofuscin was often found in muscle fibers and occasionally in endothelial cells of intramuscular blood vessels; to the latter showed prominent pinocytotic vesicles or thickened basement membrane. Both atrophy and degeneration of fibers of proximal muscles can occur as non-specific consequences of limb disuse in man and degeneration is a later and more severe change. Muscles even of the non-immobilized leg are subjected to disuse atrophy during bed-rest; the proximal muscles in man seem to have a natural susceptibility to atrophy and degeneration in any muscular disorder.