Simple SummaryThe woody breast myopathy in broiler chickens results in inferior quality breast meat. The severity of woody breast can negatively impact consumers’ acceptance, resulting in significant economic loss to the industry each year. The underlying biological mechanisms that cause woody breast development are not well understood. This study demonstrated that ribonucleotide reductase (an enzyme necessary for DNA synthesis and repair) and genes related to mitochondria function reduced for woody breast, while woody breast increased fibrosis, suggesting a possible pathway for woody breast to impair energy production in the affected muscle. As direct mechanistic connections between ribonucleotide reductase and woody breast have not been established before, further studies are required to provide a better understanding of how ribonucleotide reductase is involved in woody breast development.AbstractThe cellular events leading to the development of the woody breast myopathy in broiler breast muscle are unclear. Affected woody breast muscle exhibits muscle fiber degeneration/regeneration, connective tissue accumulation, and adverse morphological changes in mitochondria. Ribonucleotide reductase (RNR) is an enzyme for the synthesis of dNTP, which is important for mitochondria DNA content (mtDNA). RNR consists of two subunits:RRM1/RRM2. A decrease inRRM2is associated with a decrease in mtDNA and mitochondria proteins, leading to impaired ATP production. The objective of this study was to investigate potential RNR differences between woody breast (WB) and normal (N) breast muscle by examiningRRM2expression and associated pathways. Gene expression and enzyme activities were examined by qPCR and commercial kits. Results showed thatRRM2expression reduced for WB (p= 0.01) and genes related to mitochondria, includingATP6(p= 0.03),COX1(p= 0.001),CYTB(p= 0.07),ND2(p= 0.001) andND4L(p= 0.03). Furthermore,NDUFB7andCOX 14, which are related to mitochondria and ATP synthesis, tended to be reduced in WB. Compared to N,GLUT1reduced for WB (p= 0.05), which is responsible for glucose transport in cells. Consequently,PDK4(p= 0.0001) andPPARG(p= 0.008) increased in WB, suggesting increased fatty acid oxidation. Citric synthase activity and the NAD/NADH ratio (p= 0.02) both reduced for WB, while WB increasedCHRNDexpression (p= 0.001), which is a possible indicator of high reactive oxygen species levels. In conclusion, a reduction inRRM2impaired mitochondria function, potentially ATP synthesis in WB, by increasing fibrosis and the down-regulation of several genes related to mitochondria function.