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Abstract
Commercial genetic selection has produced broilers with high body weights, fast growth rates, and low feed efficiency. Endocrine regulation of growth and metabolism is likely associated with improved broiler performance, but specific effects of the three main hormonal axes regulating these processes are not well understood in birds. The objectives of this research were to (1) identify effects of commercial genetic selection on adrenocorticotropic, thyrotropic, and somatotropic hormonal axes, (2) determine if developmental changes in the somatotropic axis contribute to improved broiler performance, and (3) investigate if thyroid hormones (THs) regulate somatotropic axis activity in muscle cells. Somatotropic, corticotropic, and thyrotropic gene expression was measured in breast muscle and liver of modern Ross 308 and legacy Athens-Canadian Random Bred broilers during embryogenesis and from post-hatch day (d) 10-40. A detailed investigation of developmental somatotropic gene expression was also conducted in Ross 308 broilers from mid-embryogenesis through d21. Circulating hormones were measured post-hatch. In a third study, Quail Muscle Clone 7 (QM7) cells were cultured as undifferentiated myoblasts or differentiated myotubes and treated with triiodothyronine (T3) or thyroxine (T4). A decrease in hormones that control basal metabolism and the stress response, as well as a reduction in expression of adrenocorticotropic and thyrotropic signaling genes, was observed in modern broilers. Though circulating insulin-like growth factors (IGFs) were not different, IGF binding proteins (IGFBPs) were typically upregulated in the liver of modern broilers and downregulated in breast muscle, suggesting that circulating IGFBPs are growth promotive but inhibit muscle development locally. In modern broiler muscle, IGFBPs were largely highest during embryogenesis and lowest post-hatch, suggesting they promote embryonic growth but restrict growth after hatch. THs regulated expression of select IGFBPs in QM7 cells, and cells were more responsive to T3 than T4. Further, undifferentiated cells are likely more responsive to somatotropic and thyrotropic hormonal signaling based on their gene expression profiles. These data suggest that somatotropic and thyrotropic hormonal signaling are important regulators of broiler growth and development, and alterations in their activities as well as crosstalk between these axes contribute to rapid and efficient muscle growth in modern broilers.