DEVELOPMENT OF MUSCLE FIBERS IN THE COMPLEXUS MUSCLE OF NORMAL AND DYSTROPHIC CHICKS

Musculus complexus in the chick hypertrophies prior to hatching, primarily because of lymph infiltration, then regresses in size after hatching. Growth resumes after 1 week of age. The population of β fibers is formed early in myogenesis and is completed prior to completion of a fiber formation. As in the fetal lamb, β fibers serve as structural centers around which a fibers develop. Cytochemical studies of chick muscle show that β fibers initially exhibit adenosine triphosphatase activity which is stable to both acid and alkaline treatment, but with further differentiation the activity is stable only to acid treatment. Only a small percentage of dystrophic chicks from 2 weeks to 3 years of age exhibits signs of muscle fiber degeneration in musculus complexus and then only a few fibers are affected. In contrast, in the pectoralis muscle of 3-year-old dystrophic chicks there are few, if any, fibers remaining which do not exhibit advanced stages of dystrophy. The medial adductor muscle, which has a fiber type composition comparable to that of musculus complexus, also exhibits myopathic changes which are severe, relative to those observed in musculus complexus of dystrophic chicks. Progression of hereditary muscular dystrophy in muscle fibers of the chick appears to be correlated with reduction of aerobic metabolic capacity and accompanying reduction of fiber activity.