Induction of primordial germ cell-like cells from common marmoset embryonic stem cells by inhibition of WNT and retinoic acid signaling.

Reconstitution of the germ cell lineage using pluripotent stem cells provides a unique platform to deepen our understanding of the mechanisms underlying germ cell development and to produce functional gametes for reproduction. This study aimed to establish a culture system that induces a robust number of primordial germ cell-like cells (PGCLCs) from common marmoset (Callithrix jacchus) embryonic stem cells. The robust induction was achieved by not only activation of the conserved PGC-inducing signals, WNT and BMP4, but also temporal inhibitions of WNT and retinoic acid signals, which prevent mesodermal and neural differentiation, respectively, during PGCLC differentiation. Many of the gene expression and differentiation properties of common marmoset PGCLCs were similar to those of human PGCLCs, making this culture system a reliable and useful primate model. Finally, we identified PDPN and KIT as surface marker proteins by which PGCLCs can be isolated from embryonic stem cells without genetic manipulation. This study will expand the opportunities for research on germ cell development and production of functional gametes to the common marmoset.

Reconstitution of the oocyte transcriptional network with transcription factors.

During female germline development, oocytes become a highly specialized cell type and form a maternal cytoplasmic store of crucial factors. Oocyte growth is triggered at the transition from primordial to primary follicle and is accompanied by dynamic changes in gene expression1, but the gene regulatory network that controls oocyte growth remains unknown. Here we identify a set of transcription factors that are sufficient to trigger oocyte growth. By investigation of the changes in gene expression and functional screening using an in vitro mouse oocyte development system, we identified eight transcription factors, each of which was essential for the transition from primordial to primary follicle. Notably, enforced expression of these transcription factors swiftly converted pluripotent stem cells into oocyte-like cells that were competent for fertilization and subsequent cleavage. These transcription-factor-induced oocyte-like cells were formed without specification of primordial germ cells, epigenetic reprogramming or meiosis, and demonstrate that oocyte growth and lineage-specific de novo DNA methylation are separable from the preceding epigenetic reprogramming in primordial germ cells. This study identifies a core set of transcription factors for orchestrating oocyte growth, and provides an alternative source of ooplasm, which is a unique material for reproductive biology and medicine.

Germ cell-intrinsic effects of sex chromosomes on early oocyte differentiation in mice.

A set of sex chromosomes is required for gametogenesis in both males and females, as represented by sex chromosome disorders causing agametic phenotypes. Although studies using model animals have investigated the functional requirement of sex chromosomes, involvement of these chromosomes in gametogenesis remains elusive. Here, we elicit a germ cell-intrinsic effect of sex chromosomes on oogenesis, using a novel culture system in which oocytes were induced from embryonic stem cells (ESCs) harboring XX, XO or XY. In the culture system, oogenesis using XO and XY ESCs was severely disturbed, with XY ESCs being more strongly affected. The culture system revealed multiple defects in the oogenesis of XO and XY ESCs, such as delayed meiotic entry and progression, and mispairing of the homologous chromosomes. Interestingly, Eif2s3y, a Y-linked gene that promotes proliferation of spermatogonia, had an inhibitory effect on oogenesis. This led us to the concept that male and female gametogenesis appear to be in mutual conflict at an early stage. This study provides a deeper understanding of oogenesis under a sex-reversal condition.

Hypoxia induces the dormant state in oocytes through expression of Foxo3.

In mammals, most immature oocytes remain dormant in the primordial follicles to ensure the longevity of female reproductive life. A precise understanding of mechanisms underlying the dormancy is important for reproductive biology and medicine. In this study, by comparing mouse oogenesis in vivo and in vitro, the latter of which bypasses the primordial follicle stage, we defined the gene-expression profile representing the dormant state of oocytes. Overexpression of constitutively active FOXO3 partially reproduced the dormant state in vitro. Based on further gene-expression analysis, we found that a hypoxic condition efficiently induced the dormant state in vitro. The effect of hypoxia was severely diminished by disruption of the Foxo3 gene and inhibition of hypoxia-inducible factors. Our findings provide insights into the importance of environmental conditions and their effectors for establishing the dormant state.

Reconstitution of mouse oogenesis in a dish from pluripotent stem cells.

This protocol is an extension to: Nat. Protoc. 8, 1513-1524 (2013); doi: 10.1038/nprot.2013.090; published online 11 July 2013Generation of functional oocytes in culture from pluripotent stem cells should provide a useful model system for improving our understanding of the basic mechanisms underlying oogenesis. In addition, it has potential applications as an alternative source of oocytes for reproduction. Using the most advanced mouse model in regard to reproductive engineering and stem cell biology, we previously developed a culture method that produces functional primorial germ cells starting from pluripotent cells in culture and described it in a previous protocol. This Protocol Extension describes an adaptation of this existing Protocol in which oogenesis also occurs in vitro, thus substantially modifying the technique. Oocytes generated from embryonic stem cells (ESCs) or induced pluripotent stem cells give rise to healthy pups. Here, we describe the protocol for oocyte generation in culture. The protocol is mainly composed of three different culture stages: in vitro differentiation (IVDi), in vitro growth (IVG), and in vitro maturation (IVM), which in total take ∼5 weeks. In each culture period, there are several checkpoints that enable the number of oocytes being produced in the culture to be monitored. The basic structure of the culture system should provide a useful tool for clarifying the complicated sequence of oogenesis in mammals.

Reconstitution in vitro of the entire cycle of the mouse female germ line.

The female germ line undergoes a unique sequence of differentiation processes that confers totipotency to the egg. The reconstitution of these events in vitro using pluripotent stem cells is a key achievement in reproductive biology and regenerative medicine. Here we report successful reconstitution in vitro of the entire process of oogenesis from mouse pluripotent stem cells. Fully potent mature oocytes were generated in culture from embryonic stem cells and from induced pluripotent stem cells derived from both embryonic fibroblasts and adult tail tip fibroblasts. Moreover, pluripotent stem cell lines were re-derived from the eggs that were generated in vitro, thereby reconstituting the full female germline cycle in a dish. This culture system will provide a platform for elucidating the molecular mechanisms underlying totipotency and the production of oocytes of other mammalian species in culture.

Sphere-formation culture of testicular germ cells in the common marmoset, a small New World monkey.

Spermatogonia are specialized cells responsible for continuous spermatogenesis and the production of offspring. Because of this biological property, in vitro culture of spermatogonia provides a powerful methodology to advance reproductive biology and engineering. However, methods for culturing primate spermatogonia are poorly established. We have designed a novel method for culturing spermatogonia in the common marmoset (Callithrix jacchus), a small primate. By using our method with a suite of growth factors, adult marmoset testis-derived germ cells could be cultured in the form of a floating sphere for several weeks. Notably, this method could be applied not only to freshly isolated cells but also to cryopreserved cell stocks. The spheres enriched spermatogonia and early spermatocytes, and could be assembled from a C-KIT(+) spermatogonial population. Techniques for culturing spermatogonia could facilitate increased understanding of primate reproduction as well as the preservation of valuable biomaterials from nonhuman primates.

Generation of germ cells in vitro in the era of induced pluripotent stem cells.

Induced pluripotent stem cells (iPSCs) are stem cells that can be artificially generated via "cellular reprogramming" using gene transduction in somatic cells. iPSCs have enormous potential in stem-cell biology as they can give rise to numerous cell lineages, including the three germ layers. An evaluation of germ-line competency by blastocyst injection or tetraploid complementation, however, is critical for determining the developmental potential of mouse iPSCs towards germ cells. Recent studies have demonstrated that primordial germ cells obtained by the in vitro differentiation of iPSCs produce functional gametes as well as healthy offspring. These findings illustrate not only that iPSCs are developmentally similar to embryonic stem cells (ESCs), but also that somatic cells from adult tissues can produce gametes in vitro, that is, if they are reprogrammed into iPSCs. In this review, we discuss past and recent advances in the in vitro differentiation of germ cells using pluripotent stem cells, with an emphasis on ESCs and iPSCs. While this field of research is still at a stage of infancy, it holds great promises for investigating the mechanisms of germ-cell development, especially in humans, and for advancing reproductive and developmental engineering technologies in the future.