Growth of fiber types and apparent fiber number in skeletal muscle of broiler- and layer-type chickens.

Proportions of myofiber types, radial growth (diameter) of myofibers and apparent myofiber number were measured at the midsection of the sartorius muscle of broiler- and layer-type chickens. Broiler birds grew mor rapidly than layer birds so that in comparisons at equal age, broilers had heavier body and muscle weights, larger diameter type I and II myofibers and greater apparent myofiber number. The proportion of type II red myofibers decreased and that of type II white myofibers increased during growth. These changes occurred at a younger age in broiler-type birds. At equal body weights, however, broiler- and layer-type birds had similar proportions of the various myofiber types, which indicates that development of myofiber types is affected by functional demands on skeletal muscle related to increasing body weight. At equal body weights, broilers had larger diameter type II myofibers than layers and had a more rapid rate of type II myofiber radial hypertrophy during growth. In contrast, layer-type birds had larger type I myofibers than did broilers and the rate of radial growth was similar between breeds. Apparent myofiber number per unit of body weight increased during growth at a similar rate in the two types of birds but broilers had greater numbers of myofibers. It was concluded that more rapid growth and greater muscularity of broiler-type birds are caused by more rapid myofiber hypertrophy and the presence of more myofibers. It is suggested that selection for growth and muscularity favors factors that promote selective radial hypertrophy of type II myofibers as seen in broiler-type chickens.