Indirect evidence that monkeys may undergo post-natal neo-oogenesis was provided in the middle of the 20th century, when investigations into follicular atresia in the rhesus macaque monkey (Macaca mulatta; an Old World monkey) were published (Vermande-Van Eck, 1956). Just as Johnson et al. described in their mouse studies (Johnson et al., 2004), Vermande-Van Eck determined that the rate of atresia was such that the ovarian reserve should be exhausted within 2 years (Vermande-Van Eck, 1956). In reality, rhesus macaque monkeys do not go through the menopause until approximately 25 years old (Walker and Herndon, 2008). From her observations, Vermande-Van Eck calculated that the average lifespan of a rhesus macaque oocyte was 6 months, with a maximum lifespan of approximately 2 years, thus she concluded that to allow for a reproductive lifespan of more than two decades, oogenesis must continue in adulthood (Vermande-Van Eck, 1956).

This line of evidence has been supported by the reported isolation of OSCs from the same species of monkey (Hernandez et al., 2015). Using the published FACS protocol utilising DDX4 for cell selection (White et al., 2012, Woods and Tilly, 2013), a DDX4-positive population of cells comprising 2.5 – 50.6% of ovarian cells was isolated from 17 different monkeys (Hernandez et al., 2015). After in vitro expansion, these putative OSCs were fluorescently tagged with GFP using lentiviral transduction before autologous injection was performed. Four months later, the monkey underwent ovarian hyperstimulation and 5 oocytes were aspirated (including 4 MII oocytes and 1 immature oocyte), with one MII oocyte expressing GFP as confirmed by fluorescence microscopy and polymerase chain reaction (PCR) (Wolff et al., 2013, Wolff et al., 2014). This was the first demonstration of an OSC-derived mature oocyte in primates, and fertilisation and early embryo development (but not pregnancy) has subsequently been reported (Wolff, 2016).

Recently, the possibility that post-natal neo-oogenesis may also occur in New World monkeys has been raised (Fereydouni et al., 2014, Fereydouni et al., 2016). Research in neonatal common marmoset monkeys (Callithrix jacchus) has demonstrated that they possess primitive ovaries containing many oogonia which retain a pluripotent molecular mRNA signature (POU5F1 and LIN28) despite being post-natal (Fereydouni et al., 2014). Immunohistochemistry demonstrated pre-meiotic, POU5F1- and LIN28-positive germ cells as well as more mature, POU5F1- and LIN28-negative germ cells and by the time the monkey was 1 year old, no POU5F1 or LIN28 expression was detectable. Cells from dissociated neonatal ovaries could be cultured long-term on feeder layers, did not produce teratomas and spontaneously produced OLCs which expressed germ cell markers (including DDX4 and DPPA3) and a marker of meiotic entry (SYCP3) (Fereydouni et al., 2016). However, in contrast to previous studies into OSCs and the findings in whole neonatal ovaries, these cultured cells did not express markers of OSCs until later passages (e.g. LIN28, DPPA3, PRDM1, DDX4) or, in some cases, did not express them at all (e.g. POU5F1, NANOG), suggesting that they are not OSCs, as defined by previous groups. The authors hypothesised that this incongruity may be due to the fact that the cultured cells contained OSE cells with stem cell capabilities and the pluripotency marker-expressing oogonia did not survive in the culture environment (Fereydouni et al., 2016). Investigation into the in vitro oogeneic potential of adult marmoset ovarian cells was not performed.