Purpose of review: The long-held belief that 'the total number of oocytes present in the mammalian ovary is generated during fetal ovarian development with no additional oocyte formation during reproductive life' has recently been challenged. This review aims to summarize the scientific evidence and discuss the criticism put forth by other investigators in the field. In addition, we will entertain possible future directions, underlining clinical implications of de-novo oocyte formation during adulthood.
Recent findings: An initial report of oocyte generation from mouse stem cells in vitro was followed by a publication by Johnson et al., suggesting that new oocyte formation occurs in adult mice. Their more recent findings point toward bone marrow as a source of germline stem cells that give rise to new oocytes.
Summary: The findings of Johnson et al. strongly suggest the generation of oocytes from stem cells in the adult mouse. Criticism by prominent investigators in the field has been voiced but not yet adequately supported by experimental evidence. Crucially, independent confirmation of the findings of Johnson et al. is also lacking. If proven to occur in human, de-novo oocyte formation from stem cells would have significant implications for fertility preservation.