Precise quantification of skeletal muscle fibers reveals the physiological basis for growth rate discrepancies in broilers1

Skeletal muscle is composed of multinucleated muscle fibers, which play a crucial role in determining the quality of meat products in livestock. Quantifying the total number of muscle fibers ( TNM ) is essential for understanding muscle composition, yet remains challenging in poultry, particularly due to the size of the livestock that complicates the preparation of tissue sections for analysis and renders the counting process laborious. Our previous study developed an automatic muscle fiber quantification tool powered by deep learning, named MyoV, which has addressed this bottleneck. This study aimed to employ the tool for the accurate quantification of the TNM in the pectoral muscles of slow-growing ( SL ), medium-growing ( ML ), and fast-growing ( FL ) broilers. Results showed that FL exhibited higher growth performance compared to ML and SL from embryonic to rearing stages. Processing of whole slide images of pectoral muscle revealed significantly higher TNM in FL and ML than in SL ( P < 0.01). The TNM of FL, ML and SL were 693,568.00 ± 54,169.80, 652,122.00 ± 65,822.60 and 539,778.57±40,722.94 at 7 days of age ( D7 ), respectively. And the TNM at D35 were 663,014.93±58,801.11, 645,784.76±80,204.34 and 507,280.29±98,092.16 of FL, ML and SL. Differences in cross-sectional area ( CSA ) of muscle fibers among the three groups were consistent with TNM results. Correlation analysis showed a correlation coefficient of 0.73-0.89 between body weight ( BW ) and TNM and a correlation coefficient of 0.78-0.87 between BW and CSA. These findings directly indicate that the number of muscle fibers in broilers is an important foundation for their rapid growth and development. This study precisely quantifies the muscle fiber number of important skeletal muscle in poultry for the first time, providing the direct evidence for the physiological basis of rapid development in broilers and offering important data support for further in-depth researches on muscle fiber development.