Cellular and molecular regulations of oocyte selection and activation in mammals.

Oocytes, a uniquely pivotal cell population, play a central role in species continuity. In mammals, oogenesis involves distinct processes characterized by sequential rounds of selection, arrest, and activation to produce a limited number of mature eggs, fitting their high-survival yet high-cost fertility. During the embryonic phase, oocytes undergo intensive selection via cytoplasmic and organelle enrichment, accompanied by the onset and arrest of meiosis, thereby establishing primordial follicles (PFs) as a finite reproductive reserve. Subsequently, the majority of primary oocytes enter a dormant state and are gradually recruited through a process termed follicle activation, essential for maintaining orderly fertility. Following activation, oocytes undergo rapid growth, experiencing cycles of arrest and activation regulated by endocrine and paracrine signals, ultimately forming fertilizable eggs. Over the past two decades, advancements in genetically modified animal models, high-resolution imaging, and omics technologies have significantly enhanced our understanding of the cellular and molecular mechanisms that govern mammalian oogenesis. These advances offer profound insights into the regulatory mechanisms of mammalian reproduction and associated female infertility disorders. In this chapter, we provide an overview of current knowledge in mammalian oogenesis, with a particular emphasis on oocyte selection and activation in vivo.