A comparative electron microscope study of visceral muscle fibers in Cambarus,Drosophila and Lumbricus

Summary The arrangement of myofilaments in the striated visceral muscle fibers of two arthropods (crayfish and fruitfly) and in the unstriated visceral fibers of one annelid (earthworm) was studied comparatively. Transverse sections through the A bands of arthropod visceral fibers indicate that each thick myofilament is surrounded by approximately 12 thin filaments. The myofilaments are less organized in the visceral fibers of the earthworm than in muscle fibers of the crayfish and fruitfly. The thick myofilaments of the earthworm are composed of subunits, 20–30 Å in diameter. The presence of two distinct sets of myofilaments in these slowly contracting striated and unstriated visceral muscle fibers suggests that contraction is accomplished via a sliding filament mechanism.In crayfish visceral fibers the sarcolemma invaginates at irregular intervals to form a long and unbranched tubular system at any level in the sarcomere. Dyads formed by the apposition of T and SR membranes are observed frequently. The distribution of the T and SR systems in the visceral fibers of the fruitfly and the earthworm is markedly reduced and dyads are infrequently observed. The reduced T and SR systems may be related to the slow contraction of these fibers. Transport of specific substances across the sarcolemma could initiate contraction or relaxation in these fibers.This study was supported by a training grant GM-00582-06 from the U.S. Public Health Service.