In recent studies, the formation of germ cells from ES cells in a cell-autonomous manner when LIF is depleted from the medium, and the generation of post-meiotic cells from ES-derived germ cells in long-term culture have been elucidated. These remarkable results indicate the possibility of producing gametes from ES cells in culture. Together with in vitro maturation of gametes (eggs and sperm), ES (or iPS)-derived germ cell research has the potential both to reveal the basic mechanisms of gamete biology and to overcome clinical problems. For example, ES-derived germ cells purified from various culture stages enable us to explore molecular mechanisms such as gene functions and epigenetic modifications, which are essential for gametogenesis and fertility in humans as well as in experimental animals. Also, in vitro gametes from pluripotent stem cells will allow us to develop new ways to treat infertility, and advances in this field may realize a novel system of the alternation of generations in vitro without formation of individual bodies. Thus, it seems that ES cell- or iPS cell-derived gamete research is of incalculable scientific value.

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Primordial germ cells (PGCs), the precursors of the germ cells, arise from pluripotent epiblast cells during gastrulation in the mouse embryo. During early PGC development, there are a series of cellular events, each of which is important in the acquisition of totipotency. Pluripotent stem cells, such as embryonic stem (ES) cells, are a potential source of PGCs in vitro. Accumulating evidence suggests that ES cells can differentiate into PGCs via either embryoid body formation or monolayer cell culture. However, it remains unclear whether these in vitro-derived PGCs properly accomplish all developmental processes during PGC specification in vivo. This article reviews current studies of PGC specification both in vivo and in vitro, shelding light on the significance of PGC specification, and provides a guideline for verification in the process of PGC production in vitro.

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Mammalian spermatogenesis is a classic adult stem cell system. Identification of a crucial self renewal factor, glial cell line derived neurotrophic factor (GDNF), has provided new opportunities in cell culture systems of testicular cells. Spermatogonia can be maintained for years in culture with GDNF. Transplantation experiments of cultured spermatogonia have shown that they are able to reconstitute all the germ cells with proper programming in the testes, suggesting that the stem cell population of spermatogonia is maintained in vitro for at least, a couple years. Recently, it has also been reported that cultured spermatogonia have the potency to convert to multi-potent stem cells. The rapid progress of in vitro culture systems of spermatogonia makes it possible to apply the culture systems not only to the study of male infertility but also to regenerative medicine.

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In placental mammals, including mice and humans, critical functions of the maternal and paternal genomes are derived from the striking differences in epigenetic modification by DNA methylation between oocytes and sperms. This phenomenon of epigenetic modification is called genomic imprinting. Parental-origin-specific monoallelic gene expression of imprinted genes is regulated by DNA methylation in the differentially methylated region (DMR), and epigenetic modification is independently imposed during oogenesis and spermatogenesis. The majority of imprinted genes are believed to be epigenetically modified during oogenesis. In this paper, I review the DNA methylation imprints during mouse oogenesis.

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Cumulus cells directly surround oocyte to form cumulus oocyte complex (COC). Oocyte secreted factors act on cumulus cells to regulate the specific functions. Since oocytes have less glycolytic activity, the energy sources, such as piruvate or amino acid, are transferred from cumulus cells to oocyte via gap junctional communications, which are required for oocyte growth. After the LH surge, granulosa cells produce EGF-like factors that act on cumulus cells to induce cumulus expansion and oocyte maturation. EGF receptor expressed on cumulus cells up-regulates the ERK1/2-dependent pathway. The signaling pathway plays important roles in oocyte meiotic maturation and cumulus expansion (accumulation of hyaluronan-rich matrix within cumulus cell layers). During the fertilization process, the hyaluronan-rich matrix is broken down and the small hyaluronan fragments stimulate cumulus cells via the toll-like receptor (TLR) pathway to secrete the chemokine family. The secret chemokines act on sperm to induce sperm capacitation. The TLRs expressed on cumulus cells of ovulated COCs are also involved in innate immune functions that recognize bacteria in order to protect the oocyte from infection. Since cumulus cells regulate oocyte growth, oocyte maturation and the fertilization process, a study clarifying the role of cumulus cells might contribute to the technical development of in vitro growth or in vitro maturation of oocytes, and the regeneration of oocytes from stem cells.

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Development of immature ovarian follicles in vitro is a feasible strategy for preservation of biologically valuable genetic sources and infertility treatments. Especially, ovarian tissue banking is an effective method for protecting against infertility caused by strong cancer therapy. Grafting, however, carries a risk of malignant cell recurrence. It is necessary to develop culture methods for immature follicles from young women who have recovered from cancer, but such systems are not currently well developed. It is difficult to induce normal growth in vitro in early stage follicles. In this article, the current status of ovarian immature follicle growth culture as well as multi-step culture methods is reviewed.

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Among infertile men the degree of spermatogenesis varies from a barely compromised sperm number in the ejaculate to complete absence-the condition known as azoospermia. The scarcity of gametes often represents a major hindrance to overcoming spermatogenic failure through the use of assisted fertilization techniques. Therefore, we attempted to replicate the fertilizing spermatozoon by 'male genome cloning' in order to empower an individual spermatozoon. Spermatozoa injected into mature ooplasts were capable of maintaining and replicating a haploid male genome as haploid androgenotes. Isolated nuclei from such androgenotes entered into syngamy with female PNs following nuclear transfer fertilization. By utilizing mouse 8-cell stage androgenotes, we were able to obtain 6 blastocysts per single spermatozoon, and consequently live offspring. Thus, we were able to enhance the reproductive performance of single spermatozoa, indicating a possible future application of male gamete genome profiling prior to fertilization.

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Preimplantation diagnosis (PGD) has been performed for sex-linked inherited diseases since 1990, and it has also been utilised in Japan. In other countries, Preimplantation Genetic Screening (PGS) has been introduced to reduce pregnancies with aneuploidies. It is useful for diagnosing inherited diseases and reciprocal translocation of early embryos. However, its indication and effectiveness for cases of habitual abortion, advanced maternal age, and repetitive reproductive failure should continue to be a matter of concern.

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Retroviruses infect the host, from which new genes are often acquired, and subsequently use the function of these genes. It is well known that one of retroviruses, Rous sarcoma virus (RSV) acquired an oncogene, src, from the host. If a retrovirus uses the function of a gene from the host, a host organelle could also acquire new genes from retroviruses. Vertebrate animals possess many genomic sequences that might have been acquired from retroviral genes (Endogenous Retrovirus, ERV). Evidence so far accumulated indicates that the expression of genes, which might have been acquired from retroviruses, is very high in the placenta. It is therefore speculated that retroviral genes are actively involved in placental formation and, possibly, functioning. This review proposes a working hypothesis based on recent findings and developments associated with syncytins of retroviral origins and their functions in trophoblast differentiation/placental formation.

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To determine an effective in vitro culture method of rabbit immature ovarian follicles, preantral follicles (200 to 299 μm in diameter) were cultured by the whole follicle culture method and by the open type culture method. The use of tuna matrix collagen (MC) as a medium material to maintain 3-dimensional structure of the follicles was also investigated for the open type culture method. In the whole follicle culture method, the diameter of the preantral follicles increased from 252.8 ± 2.7 μm to 395.6 ± 9.6 μm. However, culture for oocyte maturation was not possible because most of the oocytes had degenerated. In the open culture method, in which oocyte-granulosa cell complexes (OGs) collected from preantral follicles were cultured, the rates of the oocytes that resumed meiosis were 1.6%, 5.4%, and 73.7% in 0, 0.3, and 3 mg/ml of MC supplementation, respectively. Morphologically, the addition of MC was found to show a beneficial effect for maintenance of the three-dimensional structure of the follicles. In conclusion, open type culture with MC supplementation seems to be a most appropriate culture method for rabbit immature follicles.

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