Stretch-induced growth in chicken wing muscles: myofibrillar proliferation

The patigialis muscle (PAT) in the wing of the chicken can be induced to grow rapidly in length and in diameter by passively stretching the muscle with a spring-loaded aluminum bar (Holly et al., Am. J. Physiol. 238 (Cell Physiol. 7): C62-C71, 1980). Rates of DNA, RNA, and protein synthesis are accelerated. Sarcomere length falls from 3.19 micrometers after 1 day of stretch to only 10% above control values at 7 days of stretch. Myofibrils are wavy and misaligned. Electron microscopy of cross-sectioned muscles shows that the fraction of cell volume occupied by myofibrils remains constant throughout the experimental period, even though cross-sectional area of the muscle fibers increases by 55%. The mean diameter of myofibrils in stretched muscle increases by more than 25%. The number of splitting myofibrils increases from 15% before stretching to 45% after 1 wk of stretch. Splits appear to originate in the center of the I band, and then progress to the A band and the periphery of the myofibril. Elements of the sarcotubular system develop quickly at the origin of the fractures. It is concluded that rapid growth of the myofibril is required for initiation of splitting. Neither neurally mediated active tension nor muscle contraction are required.