Research during 1952-1959 consolidated the view that hormonal regulation is the central mechanism driving mammary gland development and function, encompassing pubertal development, pregnancy, and lactation as hormonally coordinated states. The field adopted in vivo endocrine manipulation alongside innovative organ culture and in vitro epithelial culture approaches, enabling controlled dissection of differentiation, growth, and milk synthesis within a unified endocrine framework. This period thereby established the methodological and conceptual foundation for a hormone-centric paradigm in mammary biology.