Oogonial stem cells

In mammals, oogenesis is believed to be primarily prenatal. The existence of oogonial stem cells in mammals is controversial,[7] except for the finding of OSCs in two species of loris[8][9] and three species of bat.[10]

In 2004, considerable evidence was provided for the existence of germline stem cells in adult mouse ovaries capable of generating oocytes to form new follicles.[11][12][13] Questions exist about cell-sorting techniques used to isolate the OSCs,[14] and some researchers prefer the less-conclusive term "female germline stem cells" to "OSCs".[15] New research indicates that oogonial stem cells do not exist in mice and there is no convincing evidence they exist in other mammals.[16][17]

A study published in 2015 reported that the formation of new oocytes from newly-discovered germline stem cells, known as oogonial stem cells, has opened new avenues for the treatment of female infertility.[18][19]

Research by Zuckerman et al. published in 1951, established a central dogma that neo-oogenesis in mammals does not occur postnatally.[20] These conclusions were supported by other researches, such as Peters et al., who investigated DNA synthesis in oocytes during embryonic development. However, since they didn’t study oogenesis postnatally, they could not make any conclusions about postnatal oogenesis.[21] In 1967, Loannou et al., studied proliferation of oogonia and sought to identify whether they were undergoing mitosis. They would be able to show this if there was observance of mitotic activity and whether or not they were contributing to stem cell populations. To do this, they used haematoxylin stains to stain for mitotic divisions. However, these results were inconclusive as they did not have oocyte markers and thus could not say for sure that these cells were a part of the stem cell population.[22] A number of scientists have since then used mathematical models to suggest that, without an oocyte stem cell (OSC) population, the female mammal will not have enough oocytes to complete their reproductive lives due to rate of atresia during the normal cycle is significant. However, in 2004, new research by Jonathan Tilly and colleagues came about to suggest that a new population of stem cells in female mammals does exist, which could possibly be used for personalized therapeutics. Using mouse studies, they were able to detect OSCs that were able to generate new eggs within these mouse ovaries. Tilly et al. used GFP to try to label the OSCs, but they didn’t know exactly where to find these stem cell populations, so it is difficult to say whether somatic cells or stem cells were labeled. This study challenged previously expected notions, as it contradicted the central dogma of oogenesis, and has thus led to a rapid increase in the amount of researching being conducted to suggest whether there does indeed exist oocyte stem cells in the mammalian ovary.[23] Lineage tracing and other studies, following initial observations in Tilly's lab, have found no supporting evidence for oogonial stem cells.[24] [25] [26]