Mammalian in vitro gametogenesis

Abstract

Description Reconstituting reproduction in culture Research on in vitro gametogenesis (IVG) aims to reconstitute germ cell development, oogenesis and spermatogenesis, in culture. Saitou and Hayashi review some of the recent developments in mammalian IVG. Advances in methods and culture conditions in mice to generate mature oocytes and spermatocytes from pluripotent stem cells have informed similar studies with nonhuman primate and human cells, but differences among species are clear. IVG has great potential for reproductive medicine, including novel diagnosis and modeling of infertility. The realization of human IVG requires further intensive efforts, but as technical hurtles are overcome, careful consideration must be given to the potential application of methods for reproductive purposes. —BAP A review explains how in vitro gametogenesis advances understanding of basic and applied reproductive biology and medicine. BACKGROUND Mammalian germ cells differentiate into oocytes in females and spermatozoa in males. An oocyte and a spermatozoon fuse to form a zygote and then develop to create a new individual, thereby transmitting their genetic and epigenetic information to the next generation. During development, germ cells undergo epigenetic reprogramming and programming to acquire totipotency upon fertilization, maintain genome information with high fidelity, and, paradoxically, create genome diversity through meiotic recombination. Intensive efforts have been made to explore the molecular and systems-level mechanisms underlying these distinctive functions that germ cells acquire during their development. On the basis of the knowledge obtained through such endeavors, notable progress has been made over the past decade on research aiming at reconstituting germ cell development in vitro using pluripotent stem cells (PSCs), including embryonic stem cells (ESCs) derived from preimplantation embryos and induced PSCs (iPSCs) generated from somatic cells through transcription factor (TF)–induced reprogramming. This process, referred to as in vitro gametogenesis (IVG), is not only providing a basis for further exploration of the mechanisms of germ cell development but also creating prospects for innovative medical applications. ADVANCES Mouse PSCs (mPSCs) have been induced into mouse primordial germ cell–like cells (mPGCLCs), the founding germ cell population. Oogenesis has been reconstituted by culturing mPGCLCs with mouse embryonic ovarian somatic cells [reconstituted ovaries (rOvaries)], and the resultant oocytes have produced offspring. This system has revealed key mechanisms for oocyte development, including the TFs engendering oocyte-like growth in mouse ESCs. The mPGC-to-spermatogonia development has been reconstituted by culturing mPGCLCs with mouse embryonic testicular somatic cells [reconstituted testes (rTestes)], and the resultant spermatogonia have propagated in vitro and contributed to spermatogenesis upon transplantation into testes. Furthermore, the oogenic fate determination pathway has been reconstituted without the use of ovarian somatic cells, revealing the mechanisms for epigenetic reprogramming, oogenic fate determination, and meiotic entry. Human PSCs (hPSCs) have been induced into human PGCLCs (hPGCLCs), and the mechanism for hPGC(LC) specification has been clarified and shown to involve key TFs, their hierarchy of actions, and their regulatory wiring, all of which are divergent from those for mPGC(LC) specification. hPGCLCs have been shown to undergo epigenetic reprogramming and differentiate into early oocytes or prospermatogonia by culturing with mouse embryonic ovarian or testicular somatic cells (xenogeneic rOvaries or rTestes), respectively, establishing a foundation for human IVG. More recently, mPSCs have been induced into mouse embryonic ovarian somatic cell–like cells fully capable of supporting mPGCLC-based oogenesis, paving a way for generating corresponding cells in humans and other species, including endangered species, and hence for robustly promoting their IVG. OUTLOOK The proof-of-concept mouse IVG and the basic framework of human IVG have been established, creating opportunities for exploring the mechanisms of mouse and human germ cell development, including those that have been both technically and ethically difficult to address. IVG technology can be applied to other animals, including endangered species, enabling species preservation. Realizing human IVG will require further advances but will create possibilities for diagnosing and modeling infertility, exploring its remedies, and improving artificial reproductive technologies, thereby advancing reproductive medicine. Human IVG could also be extended for reproductive application, but before that would be deemed permissible, analyses must include appropriate “normality” assessments of the IVG-derived animal models—ideally primate models—and genetic and epigenetic assessments of hPSCs and the resultant gametes. Once all technological concerns have been resolved, it will be crucial to hold society-wide discussions about whether to use IVG-derived gametes for human reproduction, because such an application would change our understanding of human origins and the continuity of life. In vitro gametogenesis. IVG aims to recreate germ cell development in vitro using PSCs, including iPSCs from somatic cells. Human IVG realization requires further efforts and will create new opportunities in reproductive biology research and medicine. ART, assisted reproductive technologies. Germ cells differentiate into sexually dimorphic gametes, oocytes, and spermatozoa, which unite to form new individuals. Accordingly, germ cell development entails intricate regulations of genome functions for genetic and epigenetic inheritance. The past decade has seen considerable advances in in vitro gametogenesis (IVG), which aims to recreate germ cell development from pluripotent stem cells (PSCs) in culture. Mouse PSCs can be induced into functional oocytes and spermatozoa, whereas human PSCs can be induced into early oocytes and prospermatogonia, promoting mechanistic understanding of mammalian germ cell development. The prospect for inducing human gametes with appropriate functions has been heightened, and such advances will create possibilities in reproductive medicine, including modeling infertility to explore remedies. The use of IVG-derived gametes for human reproduction will require careful legal and ethical discussions.