When the research contained within this thesis was commenced, there had only been one report of OSCs being isolated in humans (White et al., 2012). Since then, one other group has reported that they have also been successful using the same protocol as White et al., although with differing molecular findings (Hernandez et al., 2015). Utilising the same DDX4-selecting FACS protocol as in their mouse studies, White et al. demonstrated that a rare population of DDX4-positive cells could be isolated from human ovarian cortex, comprising 1.7% ± 0.6% of the ovarian cortex (White et al., 2012). Of note, the group have subsequently reported that cells have been found in women in their 50s, indicating they are not solely a phenomenon of young women (unpublished data cited within (Woods and Tilly, 2012)). The cells expressed germ cell markers (including DDX4, IFITM3, C-KIT, PRDM1), spontaneously generated larger OLCs during in vitro culture which expressed the oocyte markers Y-box binding protein 2 (YBX2) and LIM homeobox protein 8 (LHX8) at the protein level and some cells exhibited haploid status (White et al., 2012). The putative OSCs were reported to undergo oogenesis both in vitro and in vivo with detection of the differentiated OSCs performed by transfecting them with a GFP-containing retrovirus beforehand. In vitro, aggregates of putative OSCs and dissociated ovarian cortex produced large GFP-positive cells surrounded by GFP-negative cells in follicle-like structures within 3 days of culture. Xenografting of GFP-OSC injected human ovarian cortex into immunodeficient mice revealed GFP-positive oocytes surrounded by non-GFP cells in primordial or primary follicles within 7 days of transplantation (White et al., 2012). More mature follicles were not generated and both ethical and legal reasons currently prevent assessment of the fertilisation capabilities of any mature oocytes.

Hernandez et al. used an identical isolation protocol, but in contrast to the above study the size of the DDX4-positive population was larger and more variable, comprising 4.5 – 24% of ovarian cells (Hernandez et al., 2015). Furthermore, and consistent with the group’s mouse and rhesus macaque monkey findings in the same paper, although the cells expressed germ cell markers (PRDM1, IFITM3, DPPA3), they did not appear to express DDX4, either using PCR or mass spectrometry, despite this being the protein the cells were selected for. The cells appeared to generate OLCs spontaneously during in vitro culture, but no further oogeneic assessments were performed (Hernandez et al., 2015).