Categorisation of female germline stem cells based on their morphology and marker expression profiles

In 2004, researchers found a population of mitotically active germ stem cells in adult mice capable of differentiating into oocyte-like cells, which, by contradicting the long-existing dogma on the non-existence of postnatal oogenesis, caused a shift in fertility research. Further studies conducted in mice, human and other species have continuously confirmed the existence of these female germline stem cells. Given that these cells represent an extremely small portion of the total ovarian cell population, their isolation has been exceptionally challenging, with some studies being unable to identify these cells. Moreover, studies that identified these stem cells have observed a variety of sizes, morphologies and expression profiles. Thus, there is a lack of consensus on the characteristics of female germline stem cells, including their terminology, as different researchers have called them 'female germline stem cells', 'oogonial stem cells', 'egg precursor cells', 'very small embryonic-like stem cells' or other terms. This review summarises the findings on these stem cells from the ovaries of a range of species, with the aim of proposing a standardised terminology based on their intrinsic characteristics. We hope that this terminology will be unanimously used in future fertility studies.