Postnatal alterations in skeletal muscle fiber composition of myostatin-knockout chickens

In a previous study, we identified a hypermuscular phenotype attributed to both muscle fiber hyperplasia and hypertrophy in myostatin (MSTN)-knockout (KO) chickens generated by clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR-Cas9) technology. In mammals, MSTN deletion results in increased production of fast glycolytic fibers, accompanied by decreased production of slow oxidative fibers. However, the effects of MSTN deletion on muscle fiber types have rarely been studied in avian species such as chickens. In this study, we analyzed muscle fibers in chickens at various ages to investigate whether MSTN deletion alters muscle fiber composition after hatching. Immunofluorescence and histochemical staining, including ATPase and nicotinamide adenine dinucleotide hydrogen–tetrazolium reductase (NADH–TR) staining, were performed on biceps femoris (BF) tissues of wild-type (WT) and MSTN-KO chickens at hatching and at 10 and 18 weeks. BF muscle fibers were classified into three types (1, 2A, and 2B). MSTN-KO chickens produced more muscle fibers than did WT chickens at all ages. Fiber hypertrophy was not observed in MSTN-KO chickens at hatching, but was detected in all fiber types at 10 and 18 weeks. Compared to WT chickens, there was a significant decrease in Type 1 fibers and an increase in Type 2B fibers in the BF tissues of MSTN-KO chickens, whereas the amount of Type 2A fibers remained unchanged. Furthermore, MSTN deletion led to upregulated expression of genes in the glycolytic pathway and MyoD in skeletal muscles. Thus, MSTN deletion altered muscle fiber composition in chickens, inducing Type 2B fiber dominance and hyperplasia during the embryonic stage, whereas muscle hypertrophy due to nutritional uptake became predominant after hatching.