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Functional and structural adaptations of skeletal muscle to microgravity

Robert H. Fitts^1,*, Danny R. Riley² and Jeffrey J. Widrick³

¹ Department of Biology, Marquette University, Milwaukee, WI 53201, USA,
² Department of Cellular Biology and Anatomy, Medical College of Wisconsin, Milwaukee, WI 53226, USA and
³ Department of Exercise and Sport Sciences, Oregon State University, Corvallis, OR 97331, USA

View larger version (179K): [in a new window]   Fig. 1. Cross section electron micrographs of soleus muscle fibers obtained pre-flight (A) and post-flight (B) from astronaut B of the 17-day STS-78 shuttle flight. The micrographs show the spaceflight-induced atrophy of the myofibrils and the rounding of the mitochondria. The I-bands of representative myofibrils are labeled (I). The amount of glycogen-like particles (the black dots surrounding the fibrils) is similar in both fibers. Scale bar, 0.5µm.

View larger version (15K): [in a new window]   Fig. 2. Relationship between fiber diameter and peak Ca2+-activated isometric force (P0) for subject B of the 17-day STS-78 shuttle flight. Each symbol represents the results from a single soleus fiber. Type I fibers, shaded circles; type IIa fibers, filled triangles; type I/IIa fibers, open triangles (redrawn from) (Widrick et al., 1999).

View larger version (17K): [in a new window]   Fig. 3. Schematic representation of the in vivo status of thin-filament packing density and spacing in half a sarcomere from a normal pre-flight muscle and in half a sarcomere from an atrophic muscle after a 17-day space flight in humans. Subsequent to atrophy after space flight, short thin filaments increased in number by 9%, and 17% of the thin filaments were lost. These changes summed to produce a 26% decrease in thin filament density in the overlap A-band region (redrawn) (from Riley et al., 2000).

View larger version (16K): [in a new window]   Fig. 4. Force–velocity (F-V) and force–power (F-P) relationships of pre- and post-flight soleus fibers from subject B of the 17-day STS-78 shuttle flight. Continuous lines represent composite pre-flight force–velocity and force–power relationships. Broken lines represent post-flight force–velocity and force–power relationships (redrawn from)(Widrick et al., 1999). FL, fiber length.

View larger version (135K): [in a new window]   Fig. 5. Toluidine-Blue-stained, longitudinal semithin section of adductor longus muscle fibers from a rat orbited in microgravity for 2 weeks in Cosmos biosatellite mission 2044 and returned to terrestrial gravity loading for approximately 10h before tissue fixation. The central muscle fiber exhibits large and small lighter-stained foci of sarcomere eccentric-like contraction damage, representing myofilament disruption. Sarcomere lesions were detected in 44% of the fibers examined. Scale bar, 42µm.

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