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A central dogma of reproductive biology has been that the postnatal ovary harbors a limited supply of oocytes that cannot be replenished or regenerated if lost during injury or disease[1]. However, this has been challenged recently by the surprising observation that embryonic stem cells (ESCs) and bone marrow mesenchymal stem cells (BM-MSCs) can differentiate into female germ cells[2-3]. These discoveries would benefit the study of oogenesis and advance the field of therapeutic cloning by providing an alternative source of oocytes. However, immune rejection and a limited number of cell donors[4-5] constrain the applications of ESCs and BM-MSCs. Hence, there is an acute need to find alternative cell sources for such cell therapies.
One potential alternative source of MSCs appears feasible with the report of the successful culture of human umbilical cord-derived stem cells [6]. Umbilical cord stem cells express low levels of human leukocyte antigen (HLA) and have been used for bone marrow replacement in case HLA-matched donors cannot be found[7]. Mesenchymal precursors have been isolated from umbilical cords and these cells can be induced to differentiate into lipoblasts, osteoblasts, chondroblasts or “neurocyte-like” cells[8-11]. Thus, we postulate that these cells would probably be a potential alternative source of mesenchymal stem cells to generate oocytes.
Here we isolated matrix stem cells from human umbilical cords (UC-MSCs) and observed the expression of some germ cell markers on these cells and oocyte-like structures. Our results support UC-MSCs as a potential alternative source of oocytes.

ESCs and BM-MSCs have been recently demonstrated to be able to differentiate into oocytes[2-3]. Results from this study demonstrated the intrinsic ability of human UC-MSCs to differentiate into female germline.
The mammalian germ lineages are thought to originate from the epiblasts. Primordial germ cells (PGCs) are present in the posterior wall of the yolk sac and migrate to the gonads[14]. It is possible that some of the PGCs are “lost” during migration, and reside in tissues outside of the gonads where they retain their germ-cell potential. Because germ cells and the umbilical cord are both derived from the yolk sac during embryonic development[15-16], it is presumed that more germline stem cells exist in the umbilical cord than in other tissues except the gonads.
Oct4 was found to be restricted to germ cells[17-18]. Ifitm and Stella are the key genes in the development of PGCs[19-21]. It has already been demonstrated that Oct4, Ifitm and Stella express in mouse and human BM[22-24]. In the present study, we detected the expression of germline markers on UC-MSCs from human female donors. Results indicated that there may be PGCs in UC-MSCs, or that some UC-MSCs may reserve the intrinsic potential to generate germ cells, regardless of the stage of development.
The key events we observed during differentiation of UC-MSCs into germ cells included colony formation, follicle-like cell aggregate suspension and oocyte-like cell extrusion, and these events are consistent with those of oocytes derived from mice embryonic and somatic stem cells[2-3, 25]. Previous studies indicated that 5% of FBS and 5% of follicular fluid are suitable to support germ cell differentiation[25]. We showed previously that follicular fluid at concentrations ranging from 5% to 20% induces cell differentiation into germ cells. Furthermore, higher concentrations of follicular fluid result in earlier cell differentiation. The findings, together with the expression of germline cell markers, indicated that UC-MSCs have the intrinsic potential to generate oocytes in vitro. However, it is important to emphasize that development of oocyte is a complex process regulated by many signals in the cell and the microenvironment of the ovary[26]. The oocytes derived in vitro from ESCs were reported to arrest at the early stages of oogenesis[27-28]. No further development of oocytes derived from UC-MSCs was observed in our previous studies. These results suggested that current cell induction methods did not provide all the factors that are required to direct a limited number of early-stage oocytes to undergo meiosis and grow. Hence, further studies should be performed to identify the culture conditions (including culture medium and growth factors) suitable for follicle maturation.
As UC-MSCs are comparable with BM-MSCs[8-11], umbilical cords can serve as a rich source of stem cells. UC-MSCs may be used as an alternative source of these cells in experimental and clinical settings. Furthermore, these cells may be a new source of cells for cellular therapies of infertility. This new source of cells will avoid the ethical and technical issues involved in the use of other cell sources.
In summary, results from this study indicate that UC-MSCs have a potential to differentiate into oocytes, and that umbilical cords represent an important substitute for bone marrow as a human allogeneic cell source for cell therapy of infertility and tissue engineering.
　
Acknowledgments
We thank the staff of the Department of Gynaecology and Obstetrics of Southwest Hospital for their help with umbilical cord collection. The authors also wish to thank Dr. Wang Xu-bu for his assistance in preparing this manuscript. Our work was financially supported by grants from the Scientific Research Division of Southwest Hospital.

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