Muscle specific differences in the regulation of myogenic differentiation in chickens genetically selected for divergent growth rates

With the human population predicted to reach 9 billion by 2050, increasing food supplies while maintaining adequate standards of animal welfare has become a global priority. In the poultry industry, broilers are genetically selected for greater pectoral but not leg muscularity yield leading to leg disorders and thereby welfare issues. It is known that the pectoralis major of broilers contains more muscle fibres of larger diameters than egg-layers but little is known about the leg gastrocnemius muscle cellular characteristics. As muscle fibre numbers are set by hatch, the molecular regulation of myogenesis was investigated in pectoral (selected) and gastrocnemius (unselected) muscles of chick embryos to help explain diverging post-hatch phenotypes. Results showed that broilers were more active from embryonic day (ED) 8 and heavier from ED12 to 18 than layers. The pectoral muscle of broilers exhibited increased myoblast proliferation on ED15 (raised myonuclei, MyoD and PCNA) followed by increased differentiation from ED16 (raised myogenin, IGF-I) leading to increased muscle fibre hyperplasia and mass by ED18 compared to layers. In the gastrocnemius muscle of broilers, cell proliferation was also raised up to ED15 accompanied by increased PCNA, MyoD and IGF-I mRNAs. However, from ED16, myogenin and IGF-I mRNAs were similar to that of layers and PCNA was reduced leading to similar fibre area, nuclei numbers and muscle mass at ED18. We conclude that genetic selection for enhanced post-hatch pectoral muscle growth has altered the temporal expression of IGF-I and thereby myogenin transcription affecting cellular characteristics and mass by hatch in a muscle specific manner. These observations should help develop intervention strategies aimed at improving leg muscle strength and thereby animal welfare to meet growing consumer demand.