The effect of immobilization on skeletal muscle fibre and myofibril growth in the developing chick latissimus dorsi posterior muscle was investigated. Immobilization, immediately upon hatching, results in a 25 % decrease in mean myofibre cross-sectional area within 24 h. Longer periods of immobilization further retard normal myofibre growth until by day 27 after hatching the mean myofibre cross-sectional area was 70 % less than normal. The variation in individual myofibre cross-sectional areas

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... measured as the coefficient of variation, CV = standard deviation/mean x 100) was greater in immobilized muscles. The larger immobilized myofibres showed atypical myofibril and sarcotubular complex organization at the ultrastructural level of observation whereas the smaller immobilized myofibres appeared normal, except for their size. In 1-to 27-day-old chicks, freeing the immobilized muscles for 24 h resulted in myofibril reorganization and sarcotubular proliferation but little myofibre growth. Freeing the immobilized muscles for 72 h resulted in a nearly complete recovery, in chicks up to 18 days post-hatching, of both cross-sectional myofibre area and ultrastrucrural morphology. Myofibres immobilized for longer than 18 days and subsequently freed for 72 h develop a normal myofibrillar and sarcotubular morphology but remain 23 to 29 % smaller in crosssectional area than myofibres from control muscles. Normal myofibril growth and organization are severely retarded in the immobilized myofibres. The evidence presented here suggests that sarcomere shortening causes growing myofibrils to rupture. The ruptured myofibrils are filled with proliferating elements of the sarcotubular complex which, in turn, completely subdivide (split) the myofibril.