Offspring from oocytes derived from in vitro primordial germ cell-like cells in mice

Fate of iPSCs derived from azoospermic and fertile men following xenotransplantation to murine seminiferous tubules

Historically, spontaneous deletions and insertions have provided means to probe germline developmental genetics in Drosophila, mouse and other species. Here, induced pluripotent stem cell (iPSC) lines were derived from infertile men with deletions that encompass three Y chromosome azoospermia factor (AZF) regions and are associated with production of few or no sperm but normal somatic development. AZF-deleted iPSC lines were compromised in germ cell development in vitro. Undifferentiated iPSCs transplanted directly into murine seminiferous tubules differentiated extensively to germ-cell-like cells (GCLCs) that localized near the basement membrane, demonstrated morphology indistinguishable from fetal germ cells, and expressed germ-cell-specific proteins diagnostic of primordial germ cells. Alternatively, all iPSCs that exited tubules formed primitive tumors. iPSCs with AZF deletions produced significantly fewer GCLCs in vivo with distinct defects in gene expression. Findings indicate that xenotransplantation of human iPSCs directs germ cell differentiation in a manner dependent on donor genetic status.

Formation and cultivation of medaka primordial germ cells

Primordial germ cell (PGC) formation is pivotal for fertility. Mammalian PGCs are epigenetically induced without the need for maternal factors and can also be derived in culture from pluripotent stem cells. In egg-laying animals such as Drosophila and zebrafish, PGCs are specified by maternal germ plasm factors without the need for inducing factors. In these organisms, PGC formation and cultivation in vitro from indeterminate embryonic cells have not been possible. Here, we report PGC formation and cultivation in vitro from blastomeres dissociated from midblastula embryos (MBEs) of the fish medaka (Oryzias latipes). PGCs were identified by using germ-cell-specific green fluorescent protein (GFP) expression from a transgene under the control of the vasa promoter. Embryo perturbation was exploited to study PGC formation in vivo, and dissociated MBE cells were cultivated under various conditions to study PGC formation in vitro. Perturbation of somatic development did not prevent PGC formation in live embryos. Dissociated MBE blastomeres formed PGCs in the absence of normal somatic structures and of known inducing factors. Most importantly, under culture conditions conducive to stem cell derivation, some dissociated MBE blastomeres produced GFP-positive PGC-like cells. These GFP-positive cells contained genuine PGCs, as they expressed PGC markers and migrated into the embryonic gonad to generate germline chimeras. Our data thus provide evidence for PGC preformation in medaka and demonstrate, for the first time, that PGC formation and derivation can be obtained in culture from early embryos of medaka as a lower vertebrate model.

Methylated DNA immunoprecipitation and high-throughput sequencing (MeDIP-seq) using low amounts of genomic DNA

DNA modifications, such as methylation and hydroxymethylation, are pivotal players in modulating gene expression, genomic imprinting, X-chromosome inactivation, and silencing repetitive sequences during embryonic development. Aberrant DNA modifications lead to embryonic and postnatal abnormalities and serious human diseases, such as cancer. Comprehensive genome-wide DNA methylation and hydroxymethylation studies provide a way to thoroughly understand normal development and to identify potential epigenetic mutations in human diseases. Here we established a working protocol for methylated DNA immunoprecipitation combined with next-generation sequencing [methylated DNA immunoprecipitation (MeDIP)-seq] for low starting amounts of genomic DNA. By using spike-in control DNA sets with standard cytosine, 5-methylcytosine (5mC), and 5-hydroxymethylcytosine (5hmC), we demonstrate the preferential binding of antibodies to 5mC and 5hmC, respectively. MeDIP-PCRs successfully targeted highly methylated genomic loci with starting genomic DNA as low as 1 ng. The enrichment efficiency declined for constant spiked-in controls but increased for endogenous methylated regions. A MeDIP-seq library was constructed starting with 1 ng of DNA, with the majority of fragments between 250 bp and 600 bp. The MeDIP-seq reads showed higher quality than the Input control. However, after being preprocessed by Cutadapt, MeDIP (97.53%) and Input (94.98%) reads showed comparable alignment rates. SeqMonk visualization tools indicated MeDIP-seq reads were less uniformly distributed across the genome than Input reads. Several commonly known unmethylated and methylated genomic loci showed consistent methylation patterns in the MeDIP-seq data. Thus, we provide proof-of-principle that MeDIP-seq technology is feasible to profile genome-wide DNA methylation in minute DNA samples, such as oocytes, early embryos, and human biopsies.

Stem cells as new agents for the treatment of infertility: current and future perspectives and challenges

Stem cells are undifferentiated cells that are present in the embryonic, fetal, and adult stages of life and give rise to differentiated cells that make up the building blocks of tissue and organs. Due to their unlimited source and high differentiation potential, stem cells are considered as potentially new therapeutic agents for the treatment of infertility. Stem cells could be stimulated in vitro to develop various numbers of specialized cells including male and female gametes suggesting their potential use in reproductive medicine. During past few years a considerable progress in the derivation of male germ cells from pluripotent stem cells has been made. In addition, stem cell-based strategies for ovarian regeneration and oocyte production have been proposed as future clinical therapies for treating infertility in women. In this review, we summarized current knowledge and present future perspectives and challenges regarding the use of stem cells in reproductive medicine.

Perspectives of germ cell development in vitro in mammals

Pluripotent stem cells, such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are able to differentiate into all cell lineages of the embryo proper, including germ cells. This pluripotent property has a huge impact on the fields of regenerative medicine, developmental biology and reproductive engineering. Establishing the germ cell lineage from ESCs/iPSCs is the key biological subject, since it would contribute not only to dissection of the biological processes of germ cell development but also to production of unlimited numbers of functional gametes in vitro. Toward this goal, we recently established a culture system that induces functional mouse primordial germ cells (PGCs), precursors of all germ cells, from mouse ESCs/iPSCs. The successful in vitro production of PGCs arose from the study of pluripotent cell state, the signals inducing PGCs and the technology of transplantation. However, there are many obstacles to be overcome for the robust generation of mature gametes or for application of the culture system to other species, including humans and livestock. In this review, we discuss the requirements for a culture system to generate the germ cell lineage from ESCs/iPSCs.

Human amniotic fluid stem cells possess the potential to differentiate into primordial follicle oocytes in vitro

Previous reports have demonstrated that embryonic stem cells were capable of differentiating into primordial germ cells through the formation of embryoid bodies that subsequently generated oocyte-like cells (OLCs). Such a process could facilitate studies of primordial follicle oocyte development in vitro and regenerative medicine. To investigate the pluripotency of human amniotic fluid stem cells (hAFSCs) and their ability to differentiate into germ cells, we isolated a CD117(+)/CD44(+) hAFSC line that showed fibroblastoid morphology and intrinsically expressed both stem cell markers (OCT4, NANOG, SOX2) and germ cell markers (DAZL, STELLA). To encourage differentiation into OLCs, the hAFSCs were first cultured in a medium supplemented with 5% porcine follicular fluid for 10 days. During the induction period, cell aggregates formed and syntheses of steroid hormones were detected; some OLCs and granulosa cell-like cells could be loosened from the surface of the culture dish. Cell aggregates were collected and replated in oocyte culture medium for an additional 7-10 days. OLCs ranging from 50 to 120 μm presenting zona pellucida were observed in cumulus-oocyte complexes; some OLCs developed spontaneously into multicell structures similar to preimplantation embryos. Approximately 2% of the hAFSCs differentiated to meiotic germ cells that expressed folliculogenesis- and oogenesis-associated markers. Although the in vitro maturation and fertilization potentials are as yet unproven, short-term (<25 days) and high-efficiency (>2%) derivation of OLCs from hAFSCs might provide a new approach to the study of human germ cell development in vitro.

Induced pluripotent stem cell potential in medicine, specifically focused on reproductive medicine

Since 2006, several laboratories have proved that somatic cells can be reprogramed into induced pluripotent stem cells (iPSCs). iPSCs have enormous potential in stem cell biology as they can give rise to numerous cell lineages, including the three germ layers. In this review, we discuss past and recent advances in human iPSCs used for modeling diseases in vitro, screening drugs to test new treatments, and autologous cell and tissue regenerative therapies, with a special focus on reproductive medicine applications. While this latter field of research is still in its infancy, it holds great promise for investigating germ cell development and studying the genetic and physiopathological mechanisms of infertility. A major cause of infertility is the absence of germ cells in the testes, mainly due to genetic background or as a consequence of gonadotoxic treatments. For these patients, no effective fertility restoration strategy has so far been identified. The derivation of germ cells from iPSCs represents an alternative source of stem cells able to differentiate into spermatozoa. Lessons learned from animal models as well as studies on human iPSCs for reproductive purposes are reviewed.

Retinoic acid induces mouse bone marrow-derived CD15⁺, Oct4⁺ and CXCR4⁺ stem cells into male germ-like cells in a two-dimensional cell culture system

We have examined the effect of retinoic acid (RA) on differentiation of bone marrow-derived CD15(+) , Oct4(+) and CXCR4(+) cells into male germ cells. Bone marrow stem cells (BMSCs) were isolated from the femur of 3-4-week-old male C57BL/6 mice. Magnetic-activated cell sorting (MACS) system was used to sort CD15(+) , Oct4(+) and CXCR4(+) cells. RT-PCR was used to follow the expression of pluripotency markers. Sorted CD15(+) , Oct4(+) and CXCR4(+) cells were cultured in an undifferentiated condition on a feeder layer of mitomycin C-inactivated C2C12. The embryoid-like bodies were differentiated into male germ cells by retinoic acid. To identify the expression of male germ specific markers, differentiated cells were analysed by means of reverse transcriptase polymerase chain reaction (RT-PCR) and immunofluorescence staining. RT-PCR and immunofluorescence show that bone marrow-derived CD15(+) , Oct4(+) and CXCR4(+) cells express pluripotency markers, Oct4, Nanog, Rex-1, SOX-2 and AP. The purified CD15(+) , Oct4(+) and CXCR4(+) formed structures like embryoid bodies when plated over a feeder layer; these bodies were alkaline phosphatase positive. When cells were induced by RA, bone marrow-derived CD15(+) , Oct4(+) and CXCR4(+) were positive for Mvh, Dazl, Piwil2, Dppa3 and Stra8, that known molecular markers of male germ cells. Thus RA can induce differentiation of mouse bone marrow-derived CD15(+) , Oct4(+) and CXCR4(+) cells into male germ cells in vitro. Negative results for the gene expression analysis of female germ cells markers, GDF9 and ZP3, confirmed this conclusion.

How to make a primordial germ cell

Primordial germ cells (PGCs) are the precursors of sperm and eggs, which generate a new organism that is capable of creating endless new generations through germ cells. PGCs are specified during early mammalian postimplantation development, and are uniquely programmed for transmission of genetic and epigenetic information to subsequent generations. In this Primer, we summarise the establishment of the fundamental principles of PGC specification during early development and discuss how it is now possible to make mouse PGCs from pluripotent embryonic stem cells, and indeed somatic cells if they are first rendered pluripotent in culture.

Adult somatic cells to the rescue: nuclear reprogramming and the dispensability of gonadal germ cells

With advances in cancer therapies, survival rates in prepubescent patients have steadily increased. However, a number of these surviving patients have been rendered sterile owing to their rigorous oncologic treatment regimens. In addition to cancer treatments, men and women, who are genetically fertile, can become infertile owing to immune suppression treatments, exposure to environmental and industrial toxicants, and injury. Notwithstanding the great emotional burden from an inability to conceive a child with their partner, the financial burdens for testing and treatment are high, and successful treatment of these patients' sterility is rare. Recent advances in pluripotent stem cell differentiation and the generation of patient-specific, induced pluripotent stem cells indicate that stem cell replacement therapies or in vitro differentiation followed by IVF may be on the horizon. Here we discuss these recent advances, their relevance to treating male-factor and female-factor infertility, and what experimental procedures must be carried out in animal models before these exciting new treatments can be used in a clinical setting. The goal of this research is to generate functional gametes from no greater starting material than a mere skin biopsy.

Ethical and legal issues arising in research on inducing human germ cells from pluripotent stem cells

Derivation of eggs or sperm from pluripotent stem cells or direct reprogramming from somatic cells would have huge effects on assisted reproductive technology. Here we discuss important ethical, legal, and social issues that would be raised by the development of such female or male gametes for clinical use.

A mesodermal factor, T, specifies mouse germ cell fate by directly activating germline determinants

Germ cells ensure reproduction and heredity. In mice, primordial germ cells (PGCs), the precursors for spermatozoa and oocytes, are induced in pluripotent epiblast by BMP4 and WNT3, yet the underlying mechanism remains unclear. Here, using an in vitro PGC specification system, we show that WNT3 induces many transcription factors associated with mesoderm in epiblast-like cells through β-CATENIN. Among these, T (BRACHYURY), a classical and conserved mesodermal factor, was essential for robust activation of Blimp1 and Prdm14, two of the germline determinants. T, but not SMAD1 or TCF1, binds distinct regulatory elements of both Blimp1 and Prdm14 and directly upregulates these genes, delineating the downstream PGC program. Without BMP4, a program induced by WNT3 prevents T from activating Blimp1 and Prdm14, demonstrating a permissive role of BMP4 in PGC specification. These findings establish the key signaling mechanism for, and a fundamental role of a mesodermal factor in, mammalian PGC specification.

Fate of induced pluripotent stem cells following transplantation to murine seminiferous tubules

Studies of human germ cell development are limited in large part by inaccessibility of germ cells during development. Moreover, although several studies have reported differentiation of mouse and human germ cells from pluripotent stem cells (PSCs) in vitro, differentiation of human germ cells from PSCs in vivo has not been reported. Here, we tested whether mRNA reprogramming in combination with xeno-transplantation may provide a viable system to probe the genetics of human germ cell development via use of induced pluripotent stem cells (iPSCs). For this purpose, we derived integration-free iPSCs via mRNA-based reprogramming with OCT3/4, SOX2, KLF4 and cMYC alone (OSKM) or in combination with the germ cell-specific mRNA, VASA (OSKMV). All iPSC lines met classic criteria of pluripotency. Moreover, global gene expression profiling did not distinguish large differences between undifferentiated OSKM and OSKMV iPSCs; however, some differences were observed in expression of pluripotency factors and germ cell-specific genes, and in epigenetic profiles and in vitro differentiation studies. In contrast, transplantation of undifferentiated iPSCs directly into the seminiferous tubules of germ cell-depleted immunodeficient mice revealed divergent fates of iPSCs produced with different factors. Transplantation resulted in morphologically and immunohistochemically recognizable germ cells in vivo, particularly in the case of OSKMV cells. Significantly, OSKMV cells also did not form tumors while OSKM cells that remained outside the seminiferous tubule proliferated extensively and formed tumors. Results indicate that mRNA reprogramming in combination with transplantation may contribute to tools for genetic analysis of human germ cell development.

Generation of thyroid follicular cells from pluripotent stem cells: potential for regenerative medicine

Nearly 12% of the population in the United States will be afflicted with a thyroid related disorder during their lifetime. Common treatment approaches are tailored to the specific disorder and include surgery, radioactive iodine ablation, antithyroid drugs, thyroid hormone replacement, external beam radiation, and chemotherapy. Regenerative medicine endeavors to combat disease by replacing or regenerating damaged, diseased, or dysfunctional body parts. A series of achievements in pluripotent stem cell research have transformed regenerative medicine in many ways by demonstrating "repair" of a number of body parts in mice, of which, the thyroid has now been inducted into this special group. Seminal work in pluripotent cells, namely embryonic stem cells and induced pluripotent stem cells, have made possible their path to becoming key tools and biological building blocks for cell-based regenerative medicine to combat the gamut of human diseases, including those affecting the thyroid.

Somatic cells, stem cells, and induced pluripotent stem cells: how do they now contribute to conservation?

More than a decade has now passed since the birth of the first endangered species produced from an adult somatic cell reprogrammed by somatic cell nuclear transfer. At that time, advances made in domestic and laboratory animal species provided the necessary foundation for attempting cutting-edge technologies on threatened and endangered species. In addition to nuclear transfer, spermatogonial stem cell transplantation and induction of pluripotent stem cells have also been explored. Although many basic scientific questions have been answered and more than 30 wild species have been investigated, very few successes have been reported. The majority of studies document numerous obstacles that still need to be overcome to produce viable gametes or embryos for healthy offspring production. This chapter provides an overview of somatic cell and stem cell technologies in different taxa (mammals, fishes, birds, reptiles and amphibians) and evaluates the potential and impact of these approaches for animal species conservation.

Recreating the female reproductive tract in vitro using iPSC technology in a linked microfluidics environment

The female reproductive tract produces hormones for reproductive function and cardiovascular, bone and sexual health; the tract supplies a finite number of gametes, and it supports fetal development. Diseases that affect each of the female reproductive tract organs, along with treatments that have direct, deleterious effects on the reproductive tract (for example, chemotherapeutics), are understudied due to the lack of model systems that phenocopy in vivo function. This review describes a path toward developing female reproductive tract mimics. The models use isolated primary support cells cultured onto a biological scaffold and within a microfluidic system to create a niche and support the desired differentiation of epithelia, germ and somatic cells from patient-derived induced pluripotent stem cells. Improving our fund of knowledge about reproductive tract biology and creating reproductive organs for patients who have lost gonadal, uterine or vaginal/ cervical function is a major step forward in women's health and an important advancement in personalized medicine.

Haploid genomes illustrate epigenetic constraints and gene dosage effects in mammals

Sequencing projects have revealed the information of many animal genomes and thereby enabled the exploration of genome evolution. Insights into how genomes have been repeatedly modified provide a basis for understanding evolutionary innovation and the ever increasing complexity of animal developmental programs. Animal genomes are diploid in most cases, suggesting that redundant information in two copies of the genome increases evolutionary fitness. Genomes are well adapted to a diploid state. Changes of ploidy can be accommodated early in development but they rarely permit successful development into adulthood. In mammals, epigenetic mechanisms including imprinting and X inactivation restrict haploid development. These restrictions are relaxed in an early phase of development suggesting that dosage regulation appears less critical. Here we review the recent literature on haploid genomes and dosage effects and try to embed recent findings in an evolutionary perspective.

Fertility preservation in women

In women, ∼10% of cancers occur in those <45 years old. Chemotherapy, radiotherapy and bone marrow transplantation can cure >90% of girls and young women with diseases that require such treatments. However, these treatments can result in premature ovarian failure, depending on the follicular reserve, the age of the patient and the type and dose of drugs used. This article discusses the different fertility preservation strategies: medical therapy before chemotherapy; ovarian transposition; embryo cryopreservation; oocyte vitrification; and ovarian tissue cryopreservation. The indications, results and risks of these options are discussed. Whether medical therapy should be used to protect the gonads during chemotherapy remains a source of debate. Fertility preservation needs to be completed before chemotherapy and/or irradiation is started and might take 2-3 weeks with established techniques such as embryo or oocyte cryopreservation. Further studies are needed in patients with cancer to confirm the excellent outcomes obtained in patients without cancer or in egg donation programmes. For prepubertal girls or cases where immediate therapy is required, cryopreservation of ovarian tissue is the only available option. Finally, possible future approaches are reviewed, including in vitro maturation of nonantral follicles, the artificial ovary, oogonial stem cells and drugs to prevent follicle loss.

European stem cell research in legal shackles

Advances in stem cell biology have raised legal challenges to the patentability of stem cells and any derived technologies and processes. In 1999, Oliver Brüstle was granted a patent for the generation and therapeutic use of neural cells derived from human embryonic stem cells (hESCs). The patent was challenged and put before the European Court of Justice, which ruled that inventions involving the prior destruction of human embryos cannot be patented. The legal maneuvering around this case demonstrates that the future of stem cell-based patents in Europe remains unsettled. Furthermore, owing to the European Court's broad definition of hESC as 'any cell that is capable of commencing development into a human being,' novel technologies that could eliminate the need for hESCs, such as induced pluripotent stem cells (iPSCs), are at risk of being included under the same ruling. Advances in the in vitro development of germ cells from pluripotent stem cells may one day provide a direct developmental path from iPSC to oocyte and sperm, and, according to the European Court's reasoning, legally equate iPSCs with human embryos. In this review, we will briefly discuss the Brüstle v Greenpeace case and the implications of the European Court of Justice's ruling. We will identify potential risks for stem cell research and future therapeutics resulting from the broad legal definition of the human embryo. Finally, we will broach the current legal landscape, as this broad definition has also created great uncertainty about the status of human iPSCs.

Ovarian stem cells--potential roles in infertility treatment and fertility preservation

One of the principal beliefs in reproductive biology is that women have a finite ovarian reserve, which is fixed from the time they are born. This theory has been questioned recently by the discovery of ovarian stem cells which are purported to have the ability to form new oocytes under specific conditions post-natally. Almost a decade after their discovery, ovarian, or oogonial, stem cells (OSCs) have been isolated in mice and humans but remain the subject of much debate. Studies in mice have shown that these cells can be cultured to a mature oocyte stage in vitro, and when injected into germ-cell depleted ovary they can form follicles and have resulted in the birth of healthy offspring. There are few data from human OSCs but this finding would open the door to novel fertility preservation strategies for women with both age-related and premature ovarian insufficiency (POI). As the number of girls and young women surviving cancer increases worldwide, POI secondary to gonadotoxic treatments, such as chemotherapy, is becoming more common. The ideal fertility preservation approach would prevent delays in commencing life-saving treatment and avoid transplanting malignant cells back into a woman after treatment: OSCs may offer one route to achieving this. This review summarises our current understanding of OSCs and discusses their potential clinical application in infertility treatment and fertility preservation.

Characterization of embryonic stem-like cells derived from HEK293T cells through miR302/367 expression and their potentiality to differentiate into germ-like cells

Human induced pluripotent stem (iPS) cells have great value for regenerative medicine, but are facing problems of low efficiency. MicroRNAs are a recently discovered class of 19-25 nt small RNAs that negatively target mRNAs. miR302/367 cluster has been demonstrated to reprogram mouse and human somatic cells to iPS cells without exogenous transcription factors, however, the repetition and differentiation potentiality of miR302/367-induced pluripotent stem (mirPS) cells need to be improved. Here, we showed overexpression of miR302/367 cluster reprogrammed human embryonic kidney 293T cells into mirPS cells in serum-free N2B27-based medium. The mirPS cells had similar morphology with embryonic stem cells, and expressed pluripotent markers including Oct4, Sox2, Klf4, and Nanog. In addition, through formation of embryoid bodies, various cells and tissues from three germ layers could be determined. Moreover, we examined the potential of mirPS cells differentiating into germ cells both in vitro and in vivo. Taken together, these data might provide a new source of cells and technique for the investigation of the mechanisms underlying reprogramming and pluripotency.

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.

Technological overview of iPS induction from human adult somatic cells

The unlimited proliferation capacity of embryonic stem cells (ESCs) combined with their pluripotent differentiation potential in various lineages raised great interest in both the scientific community and the public at large with hope for future prospects of regenerative medicine. However, since ESCs are derived from human embryos, their use is associated with significant ethical issues preventing broad studies and therapeutic applications. To get around this bottleneck, Takahashi and Yamanaka have recently achieved the conversion of adult somatic cells into ES-like cells via the forced expression of four transcription factors: Oct3/4, Sox2, Klf4 and c-Myc. This first demonstration attracted public attention and opened a new field of stem cells research with both cognitive - such as disease modeling - and therapeutic prospects. This pioneer work just received the 2012 Nobel Prize in Physiology or Medicine. Many methods have been reported since 2006, for the generation of induced pluripotent stem (iPS) cells. Most strategies currently under use are based on gene delivery via gamma-retroviral or lentiviral vectors; some experiments have also been successful using plasmids or transposons- based systems and few with adenovirus. However, most experiments involve integration in the host cell genome with an identified risk for insertional mutagenesis and oncogenic transformation. To circumvent such risks which are deemed incompatible with therapeutic prospects, significant progress has been made with transgene-free reprogramming methods based on e.g.: sendai virus or direct mRNA or protein delivery to achieve conversion of adult cells into iPS. In this review we aim to cover current knowledge relating to both delivery systems and combinations of inducing factors including chemicals which are used to generate human iPS cells. Finally, genetic instability resulting from the reprogramming process is also being considered as a safety bottleneck for future clinical translation and stem cell-therapy prospects based on iPS.

DNA methylation profiles provide a viable index for porcine pluripotent stem cells

Porcine induced pluripotent stem cells (iPSCs) provide useful information for translational research. The quality of iPSCs can be assessed by their ability to differentiate into various cell types after chimera formation. However, analysis of chimera formation in pigs is a labor-intensive and costly process, necessitating a simple evaluation method for porcine iPSCs. Our previous study identified mouse embryonic stem cell (ESC)-specific hypomethylated loci (EShypo-T-DMRs), and, in this study, 36 genes selected from these were used to evaluate porcine iPSC lines. Based on the methylation profiles of the 36 genes, the iPSC line, Porco Rosso-4, was found closest to mouse pluripotent stem cells among 5 porcine iPSCs. Moreover, Porco Rosso-4 more efficiently contributed to the inner cell mass (ICM) of blastocysts than the iPSC line showing the lowest reprogramming of the 36 genes (Porco Rosso-622-14), indicating that the DNA methylation profile correlates with efficiency of ICM contribution. Furthermore, factors known to enhance iPSC quality (serum-free medium with PD0325901 and CHIR99021) improved the methylation status at the 36 genes. Thus, the DNA methylation profile of these 36 genes is a viable index for evaluation of porcine iPSCs. genesis 51:763–776. © 2013 Wiley Periodicals, Inc.

Influence of activin A supplementation during human embryonic stem cell derivation on germ cell differentiation potential

Human embryonic stem cells (hESCs) are more similar to "primed" mouse epiblast stem cells (mEpiSCs). mEpiSCs, which are derived in Activin A, show an increased propensity to form primordial germ cell (PGC)-like cells in response to bone morphogenic protein 4 (BMP4). Hence, we hypothesized that hESCs derived in the presence of Activin A may be more competent in differentiating towards PGC-like cells after supplementation with BMP4 compared to standard hESC lines. We were able to successfully derive two hESC lines in the presence of Activin A, which were pluripotent and showed higher base levels of STELLA and cKIT compared to standard hESC lines derived without Activin A addition. Furthermore, upon differentiation as embryoid bodies in the presence of BMP4, we observed upregulation of VASA at day 7, both at the transcript and protein level compared to standard hESC lines, which appeared to take longer time for PGC specification. Unlike other hESC lines, nuclear pSMAD2/3 presence confirmed that Activin signalling was switched on in Activin A-derived hESC lines. They were also responsive to BMP4 based on nuclear detection of pSMAD1/5/8 and showed endodermal differentiation as a result of GATA-6 expression. Hence, our results provide novel insights into the impact of hESC derivation in the presence of Activin A and its subsequent influence on germ cell differentiation potential in vitro.

Seeking the origin of female germline stem cells in the mammalian ovary

The function of female germline stem cells (FGSCs, also called oogonial stem cells) in the adult mammalian ovary is currently debated in the scientific community. As the evidence to support or discard the possible crucial role of this new class of germ cells in mammals has been extensively discussed, in this review, we wonder which could be their origin. We will assume that FGSCs are present in the post-natal ovaries and speculate as to what origin and characteristics such cells could have. We believe that the definition of these features might shed light on future experimental approaches that could clarify the ongoing debate.

Induction of mouse germ-cell fate by transcription factors in vitro

The germ-cell lineage ensures the continuity of life through the generation of male and female gametes, which unite to form a totipotent zygote. We have previously demonstrated that, by using cytokines, embryonic stem cells and induced pluripotent stem cells can be induced into epiblast-like cells (EpiLCs) and then into primordial germ cell (PGC)-like cells with the capacity for both spermatogenesis and oogenesis, creating an opportunity for understanding and regulating mammalian germ-cell development in both sexes in vitro. Here we show that, without cytokines, simultaneous overexpression of three transcription factors, Blimp1 (also known as Prdm1), Prdm14 and Tfap2c (also known as AP2γ), directs EpiLCs, but not embryonic stem cells, swiftly and efficiently into a PGC state. Notably, Prdm14 alone, but not Blimp1 or Tfap2c, suffices for the induction of the PGC state in EpiLCs. The transcription-factor-induced PGC state, irrespective of the transcription factors used, reconstitutes key transcriptome and epigenetic reprogramming in PGCs, but bypasses a mesodermal program that accompanies PGC or PGC-like-cell specification by cytokines including bone morphogenetic protein 4. Notably, the transcription-factor-induced PGC-like cells contribute to spermatogenesis and fertile offspring. Our findings provide a new insight into the transcriptional logic for PGC specification, and create a foundation for the transcription-factor-based reconstitution and regulation of mammalian gametogenesis.

Induced pluripotent stem cells in medicine and biology

Differentiated cells can be reprogrammed to pluripotency and other cell fates by treatment with defined factors. The discovery of induced pluripotent stem cells (iPSCs) has opened up unprecedented opportunities in the pharmaceutical industry, in the clinic and in laboratories. In particular, the medical applications of human iPSCs in disease modeling and stem cell therapy have been progressing rapidly. The ability to induce cell fate conversion is attractive not only for these applications, but also for basic research fields, such as development, cancer, epigenetics and aging.

Differentiation of newborn mouse skin derived stem cells into germ-like cells in vitro

Studying germ cell formation and differentiation has traditionally been very difficult due to low cell numbers and their location deep within developing embryos. The availability of a "closed" in vitro based system could prove invaluable for our understanding of gametogenesis. The formation of oocyte-like cells (OLCs) from somatic stem cells, isolated from newborn mouse skin, has been demonstrated and can be visualized in this video protocol. The resulting OLCs express various markers consistent with oocytes such as Oct4 , Vasa , Bmp15, and Scp3. However, they remain unable to undergo maturation or fertilization due to a failure to complete meiosis. This protocol will provide a system that is useful for studying the early stage formation and differentiation of germ cells into more mature gametes. During early differentiation the number of cells expressing Oct4 (potential germ-like cells) reaches ~5%, however currently the formation of OLCs remains relatively inefficient. The protocol is relatively straight forward though special care should be taken to ensure the starting cell population is healthy and at an early passage.

Generation of eggs from mouse embryonic stem cells and induced pluripotent stem cells

Oogenesis is an integrated process through which an egg acquires the potential for totipotency, a fundamental condition for creating new individuals. Reconstitution of oogenesis in a culture that generates eggs with proper function from pluripotent stem cells (PSCs) is therefore one of the key goals in basic biology as well as in reproductive medicine. Here we describe a stepwise protocol for the generation of eggs from mouse PSCs, such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). ESCs and iPSCs are first induced into primordial germ cell-like cells (PGCLCs) that are in turn aggregated with somatic cells of female embryonic gonads, the precursors for adult ovaries. Induction of PGCLCs followed by aggregation with the somatic cells takes up to 8 d. The aggregations are then transplanted under the ovarian bursa, in which PGCLCs grow into germinal vesicle (GV) oocytes in ∼1 month. The PGCLC-derived GV oocytes can be matured into eggs in 1 d by in vitro maturation (IVM), and they can be fertilized with spermatozoa by in vitro fertilization (IVF) to obtain healthy and fertile offspring. This method provides an initial step toward reconstitution of the entire process of oogenesis in vitro.

Oocyte differentiation is genetically dissociable from meiosis in mice

Oogenesis is the process by which ovarian germ cells undertake meiosis and differentiate to become eggs. In mice, Stra8 is required for the chromosomal events of meiosis to occur, but its role in differentiation remains unknown. Here we report Stra8-deficient ovarian germ cells that grow and differentiate into oocyte-like cells that synthesize zonae pellucidae, organize surrounding somatic cells into follicles, are ovulated in response to hormonal stimulation, undergo asymmetric cell division to produce a polar body and cleave to form two-cell embryos upon fertilization. These events occur without premeiotic chromosomal replication, sister chromatid cohesion, synapsis or recombination. Thus, oocyte growth and differentiation are genetically dissociable from the chromosomal events of meiosis. These findings open to study the independent contributions of meiosis and oocyte differentiation to the making of a functional egg.

The mammalian germline as a pluripotency cycle

Naive pluripotency refers to the capacity of single cells in regulative embryos to engender all somatic and germline cell types. Only germ cells - conventionally considered to be unipotent - can naturally re-acquire pluripotency, by cycling through fertilisation. Furthermore, primordial germ cells express, and appear to be functionally dependent upon, transcription factors that characterise the pluripotent state. We hypothesise that germ cells require pluripotency factors to control a de-restricted epigenome. Consequently, they harbour latent potential, as manifested in teratocarcinogenesis or direct conversion into pluripotent stem cells in vitro. Thus, we suggest that there exists an unbroken cycle of pluripotency, naive in the early epiblast and latent in the germline, that is sustained by a shared transcription factor network.

Hormonal regulation of ovarian bursa fluid in mice and involvement of aquaporins

In rodent species, the ovary and the end of oviduct are encapsulated by a thin membrane called ovarian bursa. The biological functions of ovarian bursa remain unexplored despite its structural arrangement in facilitating oocytes transport into oviduct. In the present study, we observed a rapid fluid accumulation and reabsorption within the ovarian bursa after ovarian stimulation (PMSG-primed hCG injection), suggesting that the ovarian bursa might play an active role in regulating local fluid homeostasis around the timing of ovulation. We hypothesized that the aquaporin proteins, which are specialized channels for water transport, might be involved in this process. By screening the expression of aquaporin family members (Aqp1-9) in the ovarian tissue and isolated ovarian bursa (0, 1, 2 and 5 h after hCG injection), we found that AQP2 and AQP5 mRNA showed dynamic changes after hCG treatment, showing upregulation at 1-2 h followed by gradually decrease at 5 h, which is closely related with the intra-bursa fluid dynamics. Further immunofluorescence examinations of AQP2 and AQP5 in the ovarian bursa revealed that AQP2 is specifically localized in the outer layer (peritoneal side) while AQP5 localized in the inner layer (ovarian side) of the bursa, such cell type specific and spatial-temporal expressions of AQP2 and 5 support our hypothesis that they might be involved in efficient water transport through ovarian bursa under ovulation related hormonal regulation. The physiological significance of aquaporin-mediated water transport in the context of ovarian bursa still awaits further clarification.

Isolation, characterization and propagation of mitotically active germ cells from adult mouse and human ovaries

Accruing evidence indicates that production of new oocytes (oogenesis) and their enclosure by somatic cells (folliculogenesis) are processes not limited to the perinatal period in mammals. Endpoints ranging from oocyte counts to genetic lineage tracing and transplantation experiments support a paradigm shift in reproductive biology involving active renewal of oocyte-containing follicles during postnatal life. The recent purification of mitotically active oocyte progenitor cells, termed female germline stem cells (fGSCs) or oogonial stem cells (OSCs), from mouse and human ovaries opens up new avenues for research into the biology and clinical utility of these cells. Here we detail methods for the isolation of mouse and human OSCs from adult ovarian tissue, cultivation of the cells after purification, and characterization of the cells before and after ex vivo expansion. The latter methods include analysis of germ cell-specific markers and in vitro oogenesis, as well as the use of intraovarian transplantation to test the oocyte-forming potential of OSCs in vivo.