Abstract
Oogonia are the proliferating diploid germ cells within the female gonad giving rise to oocytes. Oocytes are the meiotic female germ cell stages. In humans, almost all oogonia enter meiosis between weeks 9 and 22 of prenatal development. As a consequence, neonatal human ovaries generally lack oogonia. The same was found in neonatal ovaries of the rhesus monkey, a representative of the old world monkeys (Catarrhini). In the present study we used the common marmoset monkey (Callithrix jacchus) as a representative of the new world monkeys (Platyrrhini). In the first part of the study we could show that the neonatal marmoset monkey ovary has an extremely immature histological appearance compared with human and rhesus monkey ovaries. It contains numerous oogonia expressing the pluripotency factors OCT4A, SALL4, and LIN28. These cells also express the proliferation marker Ki-67, which has previously been shown in human ovary to be restricted to fetal premeiotic germ cells. Together, these data demonstrate the primitiveness of the neonatal marmoset ovary compared with human. This part of the study may introduce the marmoset monkey as a suitable non-human primate model to experimentally study aspects of primate primitive gonad development, follicle assembly, and germ cell biology in vivo. In the second part of the study we aimed at culturing marmoset female germline stem cells (FGSCs) from neonatal ovaries. We successfully established ovarian cell cultures for more than 20 passages and 5 months. Importantly, comparative transcriptome analysis of the early passages with reference samples including native neonatal ovary and embryonic stem cells revealed a lack of germ cell and pluripotency genes indicating the immediate loss of the typical germ cell populations in culture. However, from passage 4 onwards, the cultured cells produced oocyte-like cells under a specific cell culture condition, while oocyte-like cells did not develop under embryonic stem cell conditions. Oocyte-like cells were free-floating, approximately 50 µm in diameter and strongly expressed several germ cell markers. The cultured ovarian cells did not develop treratoma or ovarian-like tissues after transplantation into immuno-deficient mice. In summary, our novel primate ovarian cell culture initially lacked germ cells, but then recovered germ cell expression and produced oocyte-like cells. This suggests the presence of FGSCs in the cultures. However, their identity needs to be determined, but we hypothesize that FGSCs lack the typical germ cell signature. The presence of functional FGSCs in the postnatal primate ovary of the common marmoset monkey could have important implications for the development of possible novel strategies for the protection and preservation of human female fertility during the treatment of cancer and other conditions using chemo- and radiation therapy.
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