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Dilations of the sarcotubular system and misaligned myofilaments have been reported as early indicators of muscular dystrophy in skeletal muscle. Since the developing tubular component is believed instrumental in initial myofilament alignment during myogenesis, tubular development is evaluated using normal and dystrophic chick embryo skeletal muscle and cultures of normal and dystrophic embryonic pectoral muscle incubated in the presence of horse spleen ferritin. Comparisons of the findings show that periodic tubules are absent from dystrophic somitic muscle and that invaginating tubules from the sarcolemma are found in fewer, randomly located areas of dystrophic pectoral muscle cells. The results indicate that the tubular component is not involved in the bizarre vesiculations seen in mature dystrophic muscle, however, the malalignment of dystrophic myofilaments is probably the result of the poorer development of the T system in this muscle.

Abnormalities have previously been reported in the pectoral muscle of embryos and young chicks from a pure strain of New Hampshire Red chickens homozygous for inherited muscular dystrophy. Fine structural studies of the musculus complexus in normal and dystrophic embryos were undertaken because of a sharp decrease in hatching by the diseased birds. Ultrastructural differences found between the normal and dystrophic embryos included a leached sarcoplasm, swollen and distorted mitochondria and tubular components, a lack of polyribosomes (myosin synthesis), and the formation of pseudostraps during differentiation of the myopathic hatching muscle. These differences may curtail differentiation until a point after the critical hatching time.

Early aspects of in vivo sarcomere organization were studied using normal and dystrophic New Hampshire chick embryos. Brachial somites 17-22 were removed from stages 16 through 28 embryos, processed for electron microscopy, and analyzed. Comparisons of normal and dystrophic material disclosed that thick filaments appeared later in dystrophic myotomal cells. This was correlated with a similar delay in the appearance of long polyribosomes. By stage 28, normal myotomal cells contained well-defined sarcomeres, whereas dystrophic sarcomeres were frequently unorganized, with myofilaments in poor longitudinal alignment. Periodic tubules were poorly developed or lacking.

Growth patterns during myogenesis were observed in vitro using normal and dystrophic New Hampshire Red chick embryos. The pectoral muscles from 6 d old embryos were removed, dissociated and grown in culture. Smaller muscle straps, both in size and number, as well as pseudostraps were seen in cultures from the myopathic embryos. There was a 1 1/2 d delay in the appearance of thick myosin-like myofilaments as well as large polyribosomes actively synthesizing myosin in the dystrophic muscle cells. Myosin may be a necessary constituent during myogenesis for the occurrence of normal fusion and thus myotubes.

New Hampshire chickens, homozygous for inherited muscular dystrophy, display clinical manifestations at an early age. A fine structural examination of embryos from this strain shows marked degenerative changes four days prior to hatching. The Z bands appear to dissolve progressively to the point where finally the myofibrils become uniformly dense with no detectable banding patterns.

In New Hampshire chickens, the primary clinical symptom of dystrophy is limitation of wing motility. Examination of the brachial-level motor unit in chick embryos homozygous for dystrophy reveals abnormalities in both muscular and neural components. Wing motility in these embryos is abnormal as early as six days, and there is a corresponding lack of differentiation of the pectoralis major muscle. The findings suggest that delayed development of brachial-level neuronal pathways is responsible for the decreased wing motility and early degeneration of the pectoral muscle.