Structural change in muscles of the dystrophic chicken. II. Progression of the histopathology in the pectoralis muscle.

Dystrophic (D) and normal (N) chicken pectoral muscle was analysed for histopathological differences from the embryo (day 20) through to the mature adult stage. A variety of indices of structural changes were used, to express the progression quantitatively. At the embryonic stage, fibroblast numbers (but not satellite cell numbers) are increased in D. D shows fibre hypertrophy discernible from about 4 days after hatching. Other abnormalities of fibre size and shape appear in D from about 2 weeks onwards. Nuclei numbers and location change characteristically in D, compared to N, from about 4 days, and these differences subsequently become very prominent. Fibre splitting and phagocytosis become significant in D from about 2 weeks onwards and increase noticeably thereafter. The later stages in D, with gross loss of muscle fibres, are also described, in the pectoral and posterior latissimus dorsi muscles. The analyses show that some of the structural changes in D precede the discernible difference in fibre type composition which develops between D and N. The changes in the muscle used provide a basis for assessing the effects of drug treatments at various stages, including the early post-hatch period.