The artificial ovary: any new step is a step forward Several advances have come about during the last decade in the field of fertility preservation. Cryopreservation of ovarian cortex and transplantation (COC-T) is probably the most relevant technique for patients who are not eligible to undergo oocyte or embryo vitrification. This procedure minimizes time lapses before oncologic treatments and allows the preservation of fertility in young prepubertal girls for whom controlled ovarian stimulation is not a valid option. Unfortunately, COC-T is not safe when there is an increased risk of reintroducing malignant cells, such as Burkitt's lymphoma or leukemia (1). At present, there is no clinical alternative for these kinds of patients to preserve their fertility, although there are several experimental approaches that aim to provide safe options. All of them focus on in vitro follicular growth and maturation, aiming to obtain mature and fertilizable oocytes without reintroducing the tissue back into the patient (2). Despite much progress, limitations for translation to a clinical setup still remain: viable embryos from in vitro–cultured follicles have never been produced in a species other than the mouse, mainly from cultures of activated follicles (3). Such limitations result from the fact that the development of the follicular unit is a complex and not completely clarified process that requires the participation and interaction of the oocyte itself, the granulosa and theca cells, and the surrounding cells and matrix of the ovarian connective tissue. From activation of primordial follicles to the last steps of oocyte maturation after LH surge, the needs of the developing follicle change in terms of paracrine stimuli, both biochemical and mechanical (2). This evolving situation explains why follicular culture is not so easy, even with the use of multistep systems. In the study published by Luyckx et al. (4), the authors have designed an artificial ovary using a three-dimensional matrix to create a more physiological environment to promote follicular development. They employed highly biocompatible substances such as fibrin and thrombin, supplemented by stromal cells to the matrix in which preantral follicles were encapsulated. These novel results represent another step forward in the development of techniques aiming to diminish the risk of malignant cell reintroduction after ovarian cortex retrieval and retransplantation in oncology patients. Taking into account the aforementioned problems related to safety and the difficulty of culturing follicles in vitro, the present paper has the important potential of translation to the clinical field. There are several key points that could contribute to the success of the present study: [1] the use of fibrin and thrombin to make the matrix biodegradable and therefore adaptable to the further potential needs of the follicle, which are different from those at the moment of isolation. The use of fibrin has been proven to be effective in promoting follicular development when used for in vitro follicle growth in nonhuman primates (5); [2] the presence of somatic ovarian cells within the matrix. Such cells could promote follicular development and growth by being a source of paracrine signals; and [3] the use of an in vivo model together with the biodegradable nature of the matrix would allow for vascular invasion from the host 940

and potential recruitment of immune cells, which would help in the final steps of follicular maturation. The exciting results of the present work open the door to new therapies in the field of fertility preservation. Nevertheless, reasons for caution must be raised: [1] the authors propose to use the artificial ovary in patients at the risk of reintroduction of malignant cells; in a clinical situation, the source for follicles and stromal cells to build the artificial ovary would be the cryopreserved tissue of the patient, therefore, it should be mandatory to prove that such an artificial ovary is free of malignant cells. [2] There are some issues that could limit the present results from becoming a clinical reality: the growth rate and size of a human follicle differs from that of a mouse, so that further research using human tissue and immunodeficient mice would be welcome. [3] The results presented in the paper are encouraging enough to keep working along such a line of research, but they are still basic and the follow-up is too short: oocyte competence must be proven, and fertilization, embryo development, and, of course, healthy offspring should be obtained in murine and nonhuman primates to prove the effectiveness of an artificial ovary. To what extent an in vivo model of follicular growth could contribute to solve the problems faced by the different in vitro approaches should be seen in the next few years. Whatever happens, the more options, the more likely it is that solutions will be found. Cesar Díaz-García, M.D. Sonia Herraiz, Ph.D. Grupo de investigaci on de Medicina Reproductiva, Instituto de Investigaci on Sanitario La Fe and Unidad de Preservaci on  de la Fertilidad, Area de Salud de la Mujer, Hospital Universitario y Politecnico La Fe, Valencia, Spain http://dx.doi.org/10.1016/j.fertnstert.2014.01.057 You can discuss this article with its authors and with other ASRM members at http://fertstertforum.com/diazgarciac-artificial-ovaryfertility-preservation/ Use your smartphone to scan this QR code and connect to the discussion forum for this article now.* * Download a free QR code scanner by searching for “QR scanner” in your smartphone’s app store or app marketplace.

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Meirow D, Hardan I, Dor J, Fridman E, Elizur S, Ra'anani H, et al. Searching for evidence of disease and malignant cell contamination in ovarian tissue stored from hematologic cancer patients. Hum Reprod 2008;23:1007–13. Telfer EE, Zelinski MB. Ovarian follicle culture: advances and challenges for human and nonhuman primates. Fertil Steril 2013;99:1523–33. Spears N, Boland NI, Murray AA, Gosden RG. Mouse oocytes derived from in vitro grown primary ovarian follicles are fertile. Hum Reprod 1994;9:527–32. Luyckx V, Dolmans MM, Vanacker J, Legat C, Fortuneo Moya C, Donnez J, et al. A new step towards the artificial ovary: survival and proliferation of isolated murine follicles after autologous transplantation in a fibrin scaffold. Fertil Steril 2014;101:1149–56. Xu J, Lawson MS, Yeoman RR, Molskness TA, Ting AY, Stouffer RL, et al. Fibrin promotes development and function of macaque primary follicles during encapsulated three-dimensional culture. Hum Reprod 2013;28:2187–200. VOL. 101 NO. 4 / APRIL 2014