Role of Ovarian Stem Cells in Postnatal Oogenesis and Implications in Fertility Preservation.

It is a long-held dogma in reproductive biology that females are born with a set nonrenewable number of oocytes in the ovary. While some animal studies challenged this dogma by demonstrating allegedly postnatal oogenesis and the presence of certain stem cell factor expression germ cell markers, their biological roles are yet to be defined. In this chapter, we revisited this issue revisit this dogma in light of most recent data. The chapter outlines the characteristics of these putative OSCs, including their morphological features, expression of germline markers (e.g., DDX4, DAZL), telomerase activity, and methods for their isolation and culture, acknowledging the ongoing debate and methodological controversies surrounding their identification, particularly the reliability of DDX4-based sorting. We critically evaluate the evidence for their capacity to differentiate into oocyte-like cells in vitro and in vivo, including experiments involving transplantation into animal models. Finally, the chapter explores the profound implications these OSCs hold for fertility preservation, especially for women facing premature ovarian insufficiency, age-related infertility, or gonadotoxic treatments. We discuss potential future applications, such as autologous OSC transplantation to restore ovarian function or in vitro generation of mature oocytes for fertilization. We highlight the significant technical and safety hurdles that must be overcome, including optimizing culture systems, ensuring genomic stability, and validating human functional competency. While the physiological role and definitive existence of functional OSCs in adult human ovaries remain contentious subjects requiring further rigorous investigation using advanced techniques like single-cell analysis and lineage tracing, the potential therapeutic promise continues to drive research in this exciting, albeit controversial, area of reproductive medicine.