Mammary gland development represents a complex and highly regulated biological process that is critical for mammalian reproductive success. This review provides a comprehensive analysis of the molecular, cellular, and hormonal mechanisms governing mammary gland development across distinct developmental stages: embryonic, pubertal, pregnancy, lactation, and involution. Mammary organogenesis is orchestrated by multiple factors, including conserved genetic pathways, endocrine signaling cascades, and environmental stimuli. Drawing primarily from studies in rodents and dairy ruminants, recent advances in molecular biology have elucidated key signaling pathways, including Wnt, Notch, and TGF-β, which provide fundamental insights into mammary epithelial growth and differentiation. Furthermore, we examine the effects of external factors, including thermal stress, photoperiodic variation, and dry period duration on mammary function and lactational performance. The dry period, in particular, serves a critical role in tissue remodeling and preparation for subsequent lactation cycles. Through systematic review of physiological and molecular dynamics across developmental stages, this article identifies key genetic regulators and cellular mechanisms, offering valuable insights for optimizing milk production and composition in agricultural systems.

The major histocompatibility complex (MHC) represents a critical genetic locus that orchestrates adaptive immune responses through antigen presentation and T cell activation, characterized by exceptional polymorphism and species-specific diversity. In herbivores, MHC polymorphisms constitute fundamental determinants of resistance to parasitic, viral, and bacterial infections while serving as crucial indicators of genetic diversity and breeding value in animal populations. Recent advances in high-throughput sequencing technologies, single-cell omics, and population genetics have substantially advanced understanding of MHC gene structure, allelic polymorphism, evolutionary patterns, and associations with disease susceptibility and vaccine responsiveness. This comprehensive review systematically examines the structural characteristics and regulatory mechanisms of MHC genes, their associations with infectious disease resistance, their role in vaccine responses, and their practical applications in molecular breeding programs, conservation genetics, and immunological research. We identify current technical limitations and outline future research directions to establish a robust theoretical foundation for disease resistance breeding and immune regulation strategies in herbivore populations.