Animal Molecular Breeding 2024, Vol.14, No.5, 318-325 http://animalscipublisher.com/index.php/amb 319 2 Muscle Development Pathways in Broilers 2.1 Myogenesis and its regulation Myogenesis, the formation of muscle tissue, is a highly regulated process involving the activation and differentiation of myogenic precursor cells. Key regulatory genes such as MyoDand myogenin play crucial roles in this process. Studies have shown that the expression of these genes is significantly influenced by various factors, including genetic selection and environmental conditions (Payne et al., 2020). For instance, thermal manipulation during embryogenesis has been found to upregulate MyoD and myogenin, thereby enhancing muscle growth post-hatch (Al-Zghoul and El-Bahr, 2019). Additionally, the expression of myogenic regulatory factors (MRFs) such as MyoD and myogenin is modulated by growth hormone-boosting peptides (GHBP), which can accelerate muscle development in broilers (Ibrahim et al., 2021). 2.2 Key molecular pathways involved in muscle formation Several molecular pathways are integral to muscle formation in broilers. The insulin-like growth factor (IGF) signaling pathway, particularly involving IGF-1 and mTOR, is pivotal for muscle growth and differentiation (Ahmad et al., 2020). The upregulation of IGF-1 and mTOR has been associated with enhanced muscle development, as evidenced by increased muscle fiber size and weight gain in broilers treated with GHBP. Furthermore, the extracellular matrix-receptor interaction and focal adhesion pathways have been identified as significant contributors to muscle development, influencing cellular activities such as migration, assembly, and differentiation (Xue et al., 2017). The differential expression of genes within these pathways, such as MYOD1, IGF2BP2, and FGF2, underscores their importance in regulating early growth and muscle formation in chickens. 2.3 Role of growth factors and hormones Growth factors and hormones are critical regulators of muscle development in broilers (Yalcin et al., 2019). The somatotropic axis, involving growth hormone (GH) and IGF, plays a central role in this process. Modern broiler lines exhibit enhanced expression of IGF1 and IGF2 in breast muscle, contributing to their rapid growth and muscle accretion (Figure 1) (Vaccaro et al., 2022). Additionally, the administration of synthetic GHBP has been shown to modulate the expression of muscle-specific genes and microRNAs (myomiRs), leading to improved muscle growth and feed efficiency. Hormonal regulation also involves the action of IGF-binding proteins (IGFBPs), which modulate the availability and activity of IGFs. For example, IGFBP2 produced in the liver inhibits body growth, while its local production in breast muscle facilitates muscle development (Duan et al., 2010). The interplay between these growth factors and hormones underscores their vital role in optimizing muscle development in broilers. By understanding these pathways and regulatory mechanisms, researchers can develop strategies to enhance muscle growth and improve the overall productivity of broiler chickens. 3 Gene Expression Analysis Techniques 3.1 Overview of gene expression analysis methods Gene expression analysis is a critical tool in understanding the molecular mechanisms underlying various biological processes, including muscle development in broilers. The primary methods used for gene expression analysis include RNA sequencing (RNA-seq) and microarray analysis. RNA-seq provides a comprehensive view of the transcriptome, allowing for the identification of differentially expressed genes (DEGs) and non-coding RNAs, such as microRNAs (miRNAs) and circular RNAs (circRNAs) (Liu et al., 2019). Microarray analysis, on the other hand, is a high-throughput technique that measures the expression levels of thousands of genes simultaneously, providing valuable insights into gene expression patterns and regulatory networks (Kanakachari et al., 2022). 3.2 RNA sequencing and microarray analysis in broiler research RNA sequencing has been extensively used in broiler research to investigate the genetic basis of muscle development. For instance, RNA-seq was employed to study the transcriptome of breast muscle in male Jinghai yellow chickens at different growth stages, revealing thousands of DEGs and their associated biological processes and pathways (Zhang et al., 2021a). Another study utilized RNA-seq to analyze the transcriptome of skeletal muscle in fast- and slow-growing chickens, identifying key circRNAs and their potential regulatory roles in muscle development (Zhang et al., 2022).
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