T system abnormalities in differentiating skeletal fibers of dystrophic chickens

Ultrastructural T system abnormalities are common in mature skeletal fibers from dystrophic chickens, but little is known of their development. Muscle strips from dystrophic and normal chickens aged 1 day through 4 weeks were examined in ultrathin plastic sections. At hatching, triads were sparse, with no apparent abnormalities. At 1 week, triads were more common; in some dystrophic fibers the T tubules were swollen, ballooned beyond the triad, or assembled asymmetrically, with or without supernumerary (greater than 2) junctional sarcoplasmic reticulum components. At 12 weeks, many dystrophic fibers also displayed degenerative changes, i.e., myofibrillar disintegration and cytoplasmic dispersion. At 4 weeks normal triad assembly was complete, T system abnormalities were widespread in dystrophic fibers. Fiber degeneration had progressed to breakdown and replacement. Thus, derangement of the T system (which parallels onset of muscle dysfunction) superimposed on triad assembly suggests that compromised excitation-contraction coupling is an early event in the chicken dystrophy.