Gene silencing: maturation of mouse fetal germ cells in vitro

Pronuclear transfer rescues poor embryo development of in vitro-grown secondary mouse follicles

STUDY QUESTION

Is pronuclear transfer (PNT) capable of restoring embryo developmental arrest caused by cytoplasmic inferiority of in vitro-grown (IVG) mouse oocytes?

SUMMARY ANSWER

PNT to in vivo matured cytoplasm significantly improved embryo development of IVG mouse oocytes, leading to living, fertile offspring.

WHAT IS KNOWN ALREADY

In vitro follicle culture has been considered as a fertility preservation option for cancer patients. Studies describing the culture of human follicles remain scarce, owing to low availability of tissue. Mouse models have extensively been used to study and optimize follicle culture. Although important achievements have been accomplished, including the production of healthy offspring in mice, IVG oocytes are of inferior quality when compared to in vivo-grown oocytes, likely because of cytoplasmic incompetence.

STUDY DESIGN SIZE DURATION

The study was carried out from September 2020 to February 2022. In total, 120 15-day-old B6D2 mice were used to perform secondary follicle culture and assess the quality of IVG oocytes. In vivo-grown control oocytes were obtained from 85 8- to 12-week-old B6D2 mice, following ovarian stimulation. For sperm collection, four B6D2 males between 10 and 14 weeks old were used. For embryo transfer, 14 8- to 12-week-old CD1 females served as surrogate mothers and 10 CD1 vasectomized males 10-24 weeks old were used to generate pseudo-pregnant females. Finally, for mating, four B6D2 female mice aged 8-10 weeks and two B6D2 male mice aged 10 weeks old were used to confirm the fertility of nuclear transfer (NT)-derived pups.

PARTICIPANTS/MATERIALS SETTING METHODS

Secondary follicles from 15-day-old B6D2 mice were isolated from the ovaries and cultured for 9 days, before a maturation stimulus was given. Following 16-18 h of maturation, oocytes were collected and evaluated on maturation rate, oocyte diameter, activation rate, spindle morphology, calcium-releasing ability, and mitochondrial membrane potential. For every experiment, in vivo-grown oocytes were used as a control for comparison. When cytoplasmic immaturity and poor embryo development were confirmed in IVG oocytes, PNT was performed. For this, the pronuclei from IVG oocytes, created following parthenogenetic activation and IVF, were transferred to the cytoplasm of fertilized, in vivo-grown oocytes. Genetic analysis and embryo transfer of the generated embryos were implemented to confirm the safety of the technique.

MAIN RESULTS AND THE ROLE OF CHANCE

Following 9 days of follicle culture, 703 oocytes were collected, of which 76% showed maturation to the metaphase II stage. Oocyte diameters were significantly lower in IVG oocytes, measuring 67.4 μm versus 73.1 μm in controls (P < 0.001). Spindle morphology did not differ significantly between IVG and control oocytes, but calcium-releasing ability was compromised in the IVG group. An average calcium release of 1.62 arbitrary units was observed in IVG oocytes, significantly lower than 5.74 in control oocytes (P < 0.001). Finally, mitochondrial membrane potential was inferior in IVG compared to the control group, reaching an average value of 0.95 versus 2.27 (P < 0.001). Developmental potential of IVG oocytes was assessed following parthenogenetic activation with strontium chloride (SrCl2). Only 59.4% of IVG oocytes cleaved to two cells and 36.3% reached the blastocyst stage, significantly lower than 89.5% and 88.2% in control oocytes, respectively (P < 0.001 and 0.001). Both PNT and spindle transfer (ST) were explored in pilot experiments with parthenogenetically activated oocytes, as a means to overcome poor embryo development. After the added value of NT was confirmed, we continued with the generation of biparental embryos by PNT. For this purpose, IVG and control oocytes first underwent IVF. Only 15.5% of IVG oocytes were normally fertilized, in contrast to 45.5% in controls (P < 0.001), with resulting failure of blastocyst formation in the IVG group (0 versus 86.2%, P < 0.001). When the pronuclei of IVG zygotes were transferred to the cytoplasm of control zygotes, the blastocyst rate was restored to 86.9%, a similar level as the control. Genetic analysis of PNT embryos revealed a normal chromosomal profile, to a rate of 80%. Finally, the generation of living, fertile offspring from PNT was possible following embryo transfer to surrogate mothers.

LARGE-SCALE DATA

N/A.

LIMITATIONS REASONS FOR CAUTION

Genetic profiles of analysed embryos from PNT originate from groups that are too small to draw concrete conclusions, whilst ST, which would be the preferred NT approach, could not be used for the generation of biparental embryos owing to technical limitations. Even though promising, the use of PNT should be considered as experimental. Furthermore, results were acquired in a mouse model, so validation of the technique in human IVG oocytes needs to be performed to evaluate the clinical relevance of the technology. The genetic profiles from IVG oocytes, which would be the ultimate characterization for chromosomal abnormalities, were not analysed owing to limitations in the reliable analysis of single cells.

WIDER IMPLICATIONS OF THE FINDINGS

PNT has the ability to overcome the poor cytoplasmic quality of IVG mouse oocytes. Considering the low maturation efficiency of human IVG oocytes and potential detrimental effects following long-term in vitro culture, NT could be applied to rescue embryo development and could lead to an increased availability of good quality embryos for transfer.

STUDY FUNDING/COMPETING INTERESTS

A.C. is a holder of FWO (Fonds voor Wetenschappelijk Onderzoek) grants (1S80220N and 1S80222N). B.H. and A.V.S. have been awarded with a special BOF (Bijzonder Onderzoeksfonds), GOA (Geconcerteerde onderzoeksacties) 2018000504 (GOA030-18 BOF) funding. B.H. has been receiving unrestricted educational funding from Ferring Pharmaceuticals (Aalst, Belgium). The authors declare that they have no conflict of interest.

Signalling pathways and mechanistic cues highlighted by transcriptomic analysis of primordial, primary, and secondary ovarian follicles in domestic cat

In vitro growth (IVG) of dormant primordial ovarian follicles aims to produce mature competent oocytes for assisted reproduction. Success is dependent on optimal in vitro conditions complemented with an understanding of oocyte and ovarian follicle development in vivo. Complete IVG has not been achieved in any other mammalian species besides mice. Furthermore, ovarian folliculogenesis remains sparsely understood overall. Here, gene expression patterns were characterised by RNA-sequencing in primordial (PrF), primary (PF), and secondary (SF) ovarian follicles from Felis catus (domestic cat) ovaries. Two major transitions were investigated: PrF-PF and PF-SF. Transcriptional analysis revealed a higher proportion in gene expression changes during the PrF-PF transition. Key influencing factors during this transition included the interaction between the extracellular matrix (ECM) and matrix metalloproteinase (MMPs) along with nuclear components such as, histone HIST1H1T (H1.6). Conserved signalling factors and expression patterns previously described during mammalian ovarian folliculogenesis were observed. Species-specific features during domestic cat ovarian folliculogenesis were also found. The signalling pathway terms "PI3K-Akt", "transforming growth factor-β receptor", "ErbB", and "HIF-1" from the functional annotation analysis were studied. Some results highlighted mechanistic cues potentially involved in PrF development in the domestic cat. Overall, this study provides an insight into regulatory factors and pathways during preantral ovarian folliculogenesis in domestic cat.

In ovo culturing of turkey (Meleagris gallopavo) ovarian tissue to assess graft viability and maturation of prefollicular germ cells and follicles

Biobanking of turkey ovarian tissue appears to be the most cost-effective method for the long-term preservation of female genetics. However, to ensure the successful transplantation of biobanked ovarian tissue for breed or line revival, the transplantation and development of fresh ovarian tissue must be evaluated. To assess transplantability, ovaries from poults 1 to 15 days posthatch (dph) were cultured in ovo in chicken eggs for 6 d and compared with the equivalent fresh tissue. The viability of cultured ovarian tissue was evaluated visually, whereas the level of late-stage apoptosis was measured via the TUNEL assay. In addition, the diameter and density of prefollicular germ cells and follicles (primordial and primary) were measured to assess maturation. Results showed that all cultured grafts (74/74), on surviving chicken chorioallantoic membrane, were viable with low levels (0.8 ± 0.1%) of late-stage apoptosis. The diameter of prefollicular germ cells in cultured ovaries from poults at 5 and 7 dph were larger (P < 0.002) than that of their preculture counterparts but were not able to reach their in vivo size. No significant follicular growth was observed in ovaries cultured in ovo; however, prefollicular germ cell density was over 4-fold greater in ovaries cultured from 7 dph poults (81,030 ± 17,611/mm³) than in their in vivo counterpart (16,463 ± 6,805/mm³). Interestingly, cultured ovaries from all other ages displayed equal or lower (P ≤ 0.05) prefollicular germ cell densities than their in vivo counterparts. Cultured ovaries from poults at 5 and 7 dph also exhibited an increase (P ≤ 0.05) in follicle density compared with their preculture counterparts; whereas, cultured ovaries from 15 dph poults had decreased densities (P < 0.001) compared with their preculture counterparts. This study demonstrated that, although age of ovarian tissue cultured in ovo did not affect the overall viability, 7 dph ovaries appeared to have a better cellular morphology after culturing in ovo than other ages. In addition, we also demonstrated for the first time that avian follicles can form during tissue culturing in ovo.

Efficiently accumulating germ-like stem cells from mouse postnatal ovary by in situ tissue culture

Although recent studies have revealed that germline stem cells (GSCs) exist in the mouse postnatal ovary, how to efficiently obtain GSCs for regenerating neo-oogenesis is still a technical challenge. Here, we report that using in situ tissue culture we can efficiently accumulate large amounts of proliferating germ-like cells from mouse postnatal ovaries. Usually, more than 10,000 germ-like cells can be derived from one ovary by this method, and over 20% of these cells can grow into germ-like cells with self-renewal, which thus can serve as a good cell pool to isolate GSCs by other cell assorting methods such as FACS. This method is simple and time-saving, which should be useful for in future studies on mouse GSCs.

Roadmap of germline development and in vitro gametogenesis from pluripotent stem cells

BACKGROUND

The germ cell lineage is a fundamental component of the metazoan life cycle, ensuring the perpetuation and substantial diversification of genetic information across generations. Recent advances in the understanding of mouse germ cell development have culminated in the ability to reconstitute gametogenesis in vitro, thereby enabling the biochemical and molecular analyses of germ cell specification and subsequent development in mice. Similar advances in reconstituting human germ cells in vitro would provide critical insight into the etiology of various reproductive conditions and disorders, including infertility.

OBJECTIVES

This review presents the mechanisms leading to germ cell development in mammals, particularly in mice and non-human primates, as well as the applicability of these animal models to human germ cell development. The induction methods performed to recapitulate germ cell development in vitro are also discussed in this review, specifically focusing on in vitro gametogenesis from pluripotent stem cells.

MATERIALS AND METHODS

This review compiles the key methods and findings of various references relevant to the above-mentioned topic.

RESULTS

Murine models have provided essential mechanistic insight into the process of germ cell lineage development. However, there are several structural differences between mice and humans during early embryogenesis that hinder the extrapolation of findings made in murine models to what may occur in humans. Recent studies using human or non-human primate embryos and human-induced pluripotent stem cell (hiPSC)-derived germ cells shed light on key cellular and genetic mechanisms governing germ cell development in humans.

DISCUSSION

Utilizing the knowledge obtained from studying germ cell development in different animal models, induction methods established by various laboratories now permit partial reconstitution of human gametogenesis in vitro.

CONCLUSION

In vitro gametogenesis will constitute an emergent new field in human reproductive medicine in the near future, although legal and ethical considerations must be taken into account.

Effects of activin A on the transcriptome of mouse oogenesis in vitro

From the previous research, it has been supported that activin A (ActA) is conducive to ovarian development in vitro. In the present paper, with the aim to identify the molecular pathways through which ActA can influence processes of the fetal and early postnatal oogenesis, we analyzed the transcriptome of embryonic ovaries (12.5 days postcoitum) in vitro cultured with or without ActA for 6 days, as well as the produced oocytes for 28 days, and further compared the gene expression profile with their in vivo counterparts. With the confirmation of designed test, we found that the addition of ActA to the ovary culture tended, generally, to align oocyte gene expression to the in vivo condition, in particular of a number of genes involved in meiosis and epigenetic modifications of histones. In particular, we identified DNA recombination during the oocyte meiotic prophase I and lysine trimethylation of the histone H3K27 during the oocyte growth phase as molecular pathways modulated by ActA.

Gametogenesis: A journey from inception to conception

Gametogenesis, the process of forming mature germ cells, is an integral part of both an individual's and a species' health and well-being. This chapter focuses on critical male and female genetic and epigenetic processes underlying normal gamete formation through their differentiation to fertilization. Finally, we explore how knowledge gained from this field has contributed to progress in areas with great clinical promise, such as in vitro gametogenesis.

Update on oogenesis in vitro

INTRODUCTION

Ovarian tissue is increasingly being collected from cancer patients and cryopreserved for fertility preservation. Alternately to the autologous transplantation, the development of culture systems that support oocyte development from the primordial follicle stage represent a valid strategy to restore fertility. The aim of this study is to review the most recent data regarding oogenesis in vitro and to provide an up-to-date on the contemporary knowledge of follicle growth and development in vitro.

EVIDENCE ACQUISITION

A comprehensive systematic MEDLINE search was performed since February 2018 for English-language reports by using the following terms: "ovary," "animal and human follicle," "in vitro growth and development," "ovarian tissue culture," "fertility preservation," "IVM," "oocyte." Previous published reviews and recent published original articles were preferred in order to meet our study scope.

EVIDENCE SYNTHESIS

Over time, many studies have been conducted with the aim to optimize the characteristics of ovarian tissue culture systems and to better support the three main phases: 1) activation of primordial follicles; 2) isolation and culture of growing preantral follicles; 3) removal from the follicle environment and maturation of oocyte cumulus complexes. While complete oocyte in vitro development has been achieved in mouse, with the production of live offspring, the goal of obtaining oocytes of sufficient quality to support embryo development has not been completely reached into higher mammals despite decades of effort.

CONCLUSIONS

Over the years, many improvements have been made on ovarian tissue cultures with the future purpose that patients will be provided with a greater number of developmentally competent oocytes for fertility preservation.

Whole Mount Immunofluorescence and Follicle Quantification of Cultured Mouse Ovaries

Research in the field of mammalian reproductive biology often involves evaluating the overall health of ovaries and testes. Specifically, in females, ovarian fitness is often assessed by visualizing and quantifying follicles and oocytes. Because the ovary is an opaque three-dimensional tissue, traditional approaches require laboriously slicing the tissue into numerous serial sections in order to visualize cells throughout the entire organ. Furthermore, because quantification by this method typically entails scoring only a subset of the sections separated by the approximate diameter of an oocyte, it is prone to inaccuracy. Here, a protocol is described that instead utilizes whole organ tissue clearing and immunofluorescence staining of mouse ovaries to visualize follicles and oocytes. Compared to more traditional approaches, this protocol is advantageous for visualizing cells within the ovary for numerous reasons: 1) the ovary remains intact throughout sample preparation and processing; 2) small ovaries, which are difficult to section, can be examined with ease; 3) cellular quantification is more readily and accurately achieved; and 4) the whole organ imaged.

The integration of cloning by nuclear transfer in the conservation of animal genetic resources

Cloning mammals from somatic cells by nuclear transfer has the potential to assist with the preservation of genetic diversity. An increasing number of species have been successfully cloned by this approach; however, present methods are inefficient with few cloned embryos resulting in healthy offspring. In those livestock species that have already been cloned, it is clearly feasible to use cloning to preserve endangered breeds (e.g. the last surviving Enderby Island cow). The opportunity exists to recover oocytes from these cloned heifers and use frozen Enderby Island sperm from deceased bulls for in vitro fertilisation and thus, expand the genetic diversity of this breed. Where there exists an adequate understanding of the reproductive biology and embryology of the species concerned and adequate sources of females to supply both recipient oocytes and surrogates to gestate the pregnancies, intra-specific nuclear transfer and embryo transfer can be utilised. However, when these requirements cannot be met, as is common for most endangered species, cloning technology invariably involves the use of inter-species nuclear transfer and embryo transfer. Even in intra-specific cloning the source of oocyte for nuclear transfer is an important consideration. Typically, cloned animals are only genomic copies of the founder if they possess mitochondrial DNA which differs from the original animal. Different maternal lineages of oocytes both within and between breeds significantly affect cloning efficiency and livestock production characteristics. Cloning should not distract conservation efforts from encouraging the use of indigenous livestock breeds with traits of adaptation to local environments, the preservation of wildlife habitats or the use of other forms of assisted reproduction. Whilst it is often difficult to justify cloning in animal conservation at present, the appropriate cryo-preservation of tissues and cells from a wide selection of biodiversity is of paramount importance. This provides an insurance against further losses of genetic variation from dwindling populations, disease epidemics or even possible extinction. It would also complement the gene banking of gametes or embryos and can be performed more easily and cheaply. Future cloning from preserved somatic cells can reintroduce lost genes back into the breeding pool. With greater appreciation of the heritable attributes of traditional livestock breeds there is the desire to identify superior animals within these local populations and the genetic loci involved. Through clonal family performance testing, nuclear transfer can aid the selection of desirable genotypes and then the production of larger numbers of embryos or animals for natural breeding to more widely disseminate the desirable traits. With the identification of alleles conferring desirable attributes, transgenesis could be utilised to both improve traditional and industrial livestock breeds. This further emphasizes the importance of preserving global farm animal genetic resources.

Complete in vitro oogenesis: retrospects and prospects

Precise control of mammalian oogenesis has been a traditional focus of reproductive and developmental biology research. Recently, new reports have introduced the possibility of obtaining functional gametes derived in vitro from stem cells. The potential to produce functional gametes from stem cells has exciting applications for regenerative medicine though still remains challenging. In mammalian females ovulation and fertilization is a privilege reserved for a small number of oocytes. In reality the vast majority of oocytes formed from primordial germ cells (PGCs) will undergo apoptosis, or other forms of cell death. Removal occurs during germ cell cyst breakdown and the establishment of the primordial follicle (PF) pool, during the long dormancy at the PF stage, or through follicular atresia prior to reaching the ovulatory stage. A way to solve this limitation could be to produce large numbers of oocytes, in vitro, from stem cells. However, to recapitulate mammalian oogenesis and produce fertilizable oocytes in vitro is a complex process involving several different cell types, precise follicular cell-oocyte reciprocal interactions, a variety of nutrients and combinations of cytokines, and precise growth factors and hormones depending on the developmental stage. In 2016, two papers published by Morohaku et al. and Hikabe et al. reported in vitro procedures that appear to reproduce efficiently these conditions allowing for the production, completely in a dish, of a relatively large number of oocytes that are fertilizable and capable of giving rise to viable offspring in the mouse. The present article offers a critical overview of these results as well as other previous work performed mainly in mouse attempting to reproduce oogenesis completely in vitro and considers some perspectives for the potential to adapt the methods to produce functional human oocytes.

Development of fertile mouse oocytes from mitotic germ cells in vitro

Mammalian fetal ovaries contain numerous primordial germ cells (PGCs), although few mature oocytes are obtained from females, owing to apoptosis and follicle atresia. The regulatory mechanisms underlying oogenesis/folliculogenesis remain unknown. Development of methods for obtaining mature oocytes from PGCs in fetal ovaries in vitro could contribute to clarifying these mechanisms. The failure of follicle assembly has been found to be the most challenging aspect in conventional culture conditions. Recently, we established novel culture conditions that enable successful follicle assembly, sustaining interactions between the oocyte and somatic cells, and, in turn, promoting oocyte growth and maturation. Mature oocytes were differentiated from PGCs after a 1-month culture period. A hundred mouse offspring were obtained from approximately a thousand mature oocytes, indicating that oocytes that were differentiated from PGCs in vitro acquired totipotency after fertilization. Here we provide a detailed protocol for using this in vitro system. This in vitro system will potentially provide a novel platform for studying oogenesis and preservation of female germ cells.

Gametogenesis from Pluripotent Stem Cells

The germ cell lineage originates early in development and undergoes a series of complex developmental processes that culminate in the generation of fully matured gametes, the spermatozoa and the oocytes. Remarkably, researchers have been recapitulating these developmental pathways using mouse and human pluripotent stem cells (PSCs). With further studies, including those involving non-human primate models, human gametogenesis may be fully reconstituted from PSCs, which would profoundly facilitate our understanding of human germ cell development and infertility. Here we discuss groundbreaking studies that lay the foundation for this achievement, the current state of the field, and challenges for deriving gametes from hPSCs.

Differentiation of Mouse Primordial Germ Cells into Functional Oocytes In Vitro

Various complex molecular events in oogenesis cannot be observed in vivo. As a bioengineering technique for female reproductive tissues, in vitro culture systems for female germ cells have been used to analyze oogenesis and preserve germ cells for over 20 years. Recently, we have established a new methodological approach for the culture of primordial germ cells (PGCs) and successfully obtained offspring. Our PGC culture system will be useful to clarify unresolved mechanisms of fertility and sterility from the beginning of mammalian oogenesis, before meiosis. This review summarizes the history of culture methods for mammalian germ cells, our current in vitro system, and future prospects for the culture of germ cells.

Etoposide damages female germ cells in the developing ovary

BACKGROUND

As with many anti-cancer drugs, the topoisomerase II inhibitor etoposide is considered safe for administration to women in the second and third trimesters of pregnancy, but assessment of effects on the developing fetus have been limited. The purpose of this research was to examine the effect of etoposide on germ cells in the developing ovary. Mouse ovary tissue culture was used as the experimental model, thus allowing us to examine effects of etoposide on all stages of germ cell development in the same way, in vitro.

RESULTS

Fetal ovaries from embryonic day 13.5 CD1 mice or neonatal ovaries from postnatal day 0 CD1 mice were cultured with 50-150 ng ml(-1) or 50-200 ng ml(-1) etoposide respectively, concentrations that are low relative to that in patient serum. When fetal ovaries were treated prior to follicle formation, etoposide resulted in dose-dependent damage, with 150 ng ml(-1) inducing a near-complete absence of healthy follicles. In contrast, treatment of neonatal ovaries, after follicle formation, had no effect on follicle numbers and only a minor effect on follicle health, even at 200 ng ml(-1). The sensitivity of female germ cells to etoposide coincided with topoisomerase IIα expression: in the developing ovary of both mouse and human, topoisomerase IIα was expressed in germ cells only prior to follicle formation.

CONCLUSIONS

Exposure of pre-follicular ovaries, in which topoisomerase IIα expression was germ cell-specific, resulted in a near-complete elimination of germ cells prior to follicle formation, with the remaining germ cells going on to form unhealthy follicles by the end of culture. In contrast, exposure to follicle-enclosed oocytes, which no longer expressed topoisomerase IIα in the germ cells, had no effect on total follicle numbers or health, the only effect seen specific to transitional follicles. Results indicate the potential for adverse effects on fetal ovarian development if etoposide is administered to pregnant women when germ cells are not yet enclosed within ovarian follicles, a process that starts at approximately 17 weeks gestation and is only complete towards the end of pregnancy.

Complete in vitro generation of fertile oocytes from mouse primordial germ cells

Reconstituting gametogenesis in vitro is a key goal for reproductive biology and regenerative medicine. Successful in vitro reconstitution of primordial germ cells and spermatogenesis has recently had a significant effect in the field. However, recapitulation of oogenesis in vitro remains unachieved. Here we demonstrate the first reconstitution, to our knowledge, of the entire process of mammalian oogenesis in vitro from primordial germ cells, using an estrogen-receptor antagonist that promotes normal follicle formation, which in turn is crucial for supporting oocyte growth. The fundamental events in oogenesis (i.e., meiosis, oocyte growth, and genomic imprinting) were reproduced in the culture system. The most rigorous evidence of the recapitulation of oogenesis was the birth of fertile offspring, with a maximum of seven pups obtained from a cultured gonad. Moreover, cryopreserved gonads yielded functional oocytes and offspring in this culture system. Thus, our in vitro system will enable both innovative approaches for a deeper understanding of oogenesis and a new avenue to create and preserve female germ cells.

In vitro differentiation of germ cells from stem cells: a comparison between primordial germ cells and in vitro derived primordial germ cell-like cells

Stem cells are unique cell types capable to proliferate, some of them indefinitely, while maintaining the ability to differentiate into a few or any cell lineages. In 2003, a group headed by Hans R. Schöler reported that oocyte-like cells could be produced from mouse embryonic stem (ES) cells in vitro. After more than 10 years, where have these researches reached? Which are the major successes achieved and the problems still remaining to be solved? Although during the last years, many reviews have been published about these topics, in the present work, we will focus on an aspect that has been little considered so far, namely a strict comparison between the in vitro and in vivo developmental capabilities of the primordial germ cells (PGCs) isolated from the embryo and the PGC-like cells (PGC-LCs) produced in vitro from different types of stem cells in the mouse, the species in which most investigation has been carried out. Actually, the formation and differentiation of PGCs are crucial for both male and female gametogenesis, and the faithful production of PGCs in vitro represents the basis for obtaining functional germ cells.

Generation of mouse functional oocytes in rat by xeno-ectopic transplantation of primordial germ cells

Primordial germ cells (PGCs) are germ cell progenitors in the fetal genital ridge; female PGCs give rise to definitive oocytes that contribute to the next generation. Artificial PGCs have been induced in vitro from pluripotent stem cells and gonad-like tissue has been induced in vivo by cotransplantation of PGCs with PGC-free gonadal cells. To apply these technologies to human infertility treatment or conservation of rare species, PGC transplantation must be established in xenogenic animals. Here, we established a xenogeneic transplantation model by inducing ovary-like tissue from PGCs in xenogenic animals. We transplanted enzymatically dispersed PGCs with PGC-free gonadal cells under the kidney capsule of xenogenic immunodeficient animals. The transplanted cells formed ovary-like tissues under the kidney capsule. These tissues were histologically similar to the normal gonad and expressed the oocyte markers Vasa and Stella. In addition, mouse germinal vesicle-stage oocyte-like cells collected from ovary-like tissue in rats matured to metaphase II via in vitro maturation and gave rise to offspring by intracytoplasmic sperm injection. Our studies show that rat/mouse female PGCs and PGC-free gonadal cells can develop and reconstruct ovary-like tissue containing functional oocytes in an ectopic xenogenic microenvironment.

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.

Reprogramming of round spermatids by the germinal vesicle cytoplasm in mice

The birthrate following round spermatid injection (ROSI) remains low in current and evidence suggests that factors in the germinal vesicle (GV) cytoplasm and certain substances in the GV such as the nucleolus might be responsible for genomic reprogramming and embryonic development. However, little is known whether the reprogramming factors in GV oocyte cytoplasm and/or nucleolus in GV are beneficial to the reprogramming of round spermatids and development of ROSI embryos. Here, round spermatids were treated with GV cytolysates and injected this round spermatid alone or co-injected with GV oocyte nucleolus into mature metaphase II oocytes. Subsequent embryonic development was assessed morphologically and by Oct4 expression in blastocysts. There was no significant difference between experimental groups at the zygote to four-cell development stages. Blastocysts derived from oocytes which were injected with cytolysate treated-round spermatid alone or co-injected with nucleoli injection yielded 63.6% and 70.3% high quality embryos, respectively; comparable to blastocysts derived by intracytoplasmic sperm injection (ICSI), but higher than these oocytes which were co-injected with lysis buffer-treated round spermatids and nucleoli or injected with the lysis buffer-treated round spermatids alone. Furthermore, the proportion of live offspring resulting from oocytes which were co-injected with cytolysate treated-round spermatids and nucleoli or injected with cytolysate treated-round spermatids alone was higher than those were injected with lysis buffer treated-round spermaids, but comparable with the ICSI group. Our results demonstrate that factors from the GV cytoplasm improve round spermatid reprogramming, and while injection of the extra nucleolus does not obviously improve reprogramming its potential contribution, although which cannot be definitively excluded. Thus, some reprogramming factors are evidently present in GV oocyte cytoplasm and could significantly facilitate ROSI technology, while the nucleolus in GV seems also having a potential to improve reprogramming of round spermatids.

Aberrant gene expression and sexually incompatible genomic imprinting in oocytes derived from XY mouse embryonic stem cells in vitro

Mouse embryonic stem cells (ESCs) have the potential to differentiate into germ cells (GCs) in vivo and in vitro. Interestingly, XY ESCs can give rise to both male and female GCs in culture, irrespective of the genetic sex. Recent studies showed that ESC-derived primordial GCs contributed to functional gametogenesis in vivo; however, in vitro differentiation techniques have never succeeded in generating mature oocytes from ESCs due to cryptogenic growth arrest during the preantral follicle stages of development. To address this issue, a mouse ESC line, capable of producing follicle-like structures (FLSs) efficiently, was established to investigate their properties using conventional molecular biological methods. The results revealed that the ESC-derived FLSs were morphologically similar to ovarian primary-to-secondary follicles but never formed an antrum; instead, the FLSs eventually underwent abnormal development or cell death in culture, or formed teratomas when transplanted under the kidney capsule in mice. Gene expression analyses demonstrated that the FLSs lacked transcripts for genes essential to late folliculogenesis, including gonadotropin receptors and steroidogenic enzymes, whereas some other genes were overexpressed in FLSs compared to the adult ovary. The E-Cadherin protein, which is involved in cell-to-cell interactions, was also expressed ectopically. Remarkably, it was seen that oocyte-like cells in the FLSs exhibited androgenetic genomic imprinting, which is ordinarily indicative of male GCs. Although the FLSs did not express male GC marker genes, the DNA methyltransferase, Dnmt3L, was expressed at an abnormally high level. Furthermore, the expression of sex determination factors was ambiguous in FLSs as both male and female determinants were expressed weakly. These data suggest that the developmental dysfunction of the ESC-derived FLSs may be attributable to aberrant gene expression and genomic imprinting, possibly associated with uncertain sex determination in culture.

Epigenetic reprogramming in somatic cells induced by extract from germinal vesicle stage pig oocytes

Genomic reprogramming factors in the cytoplasm of germinal vesicle (GV) stage oocytes have been shown to improve the efficiency of producing cloned mouse offspring through the exposure of nuclei to a GV cytoplasmic extract prior to somatic cell nuclear transfer (SCNT) to enucleated oocytes. Here, we developed an extract of GV stage pig oocytes (GVcyto-extract) to investigate epigenetic reprogramming events in treated somatic cell nuclei. This extract induced differentiation-associated changes in fibroblasts, resulting in cells that exhibit pluripotent stem cell-like characteristics and that redifferentiate into three primary germ cell layers both in vivo and in vitro. The GVcyto-extract treatment induced large numbers of high-quality SCNT-generated blastocysts, with methylation and acetylation of H3-K9 and expression of Oct4 and Nanog at levels similar to in vitro fertilized embryos. Thus, GVcyto-extract could elicit differentiation plasticity in treated fibroblasts, and SCNT-mediated reprogramming reset the epigenetic state in treated cells more efficiently than in untreated cells. In summary, we provide evidence for the generation of stem-like cells from differentiated somatic cells by treatment with porcine GVcyto-extract.

Recovery of functional oocytes from cultured premeiotic germ cells after kidney capsule transplantation

The efficiency of in vitro culture systems for a premeiotic female germ cell is still low, mostly because of our incomplete understanding of the mechanisms controlling oogenesis and the obvious difficulties in reproducing the complex in vivo environment of such a process under in vitro conditions. Here we explored the possibility of recovering the developmental potential of mouse oocytes generated in vitro from premeiotic germ cells by transplantation under a kidney capsule of adult animals. To this aim, mouse embryonic ovaries of 12.5 days postcoitum cultured in vitro in a serum-free medium for 7 or 14 days, were transplanted beneath the kidney capsule of immunodeficient mice and analyzed after 21 (7+21 group) or 14 days (14+14 group). Cultured ovaries before transplantation showed delayed oocyte meiotic progression and follicle development. Interestingly, grafted ovaries of both groups, especially those of the 7+21 group, seemed able to restore the reproductive cycle of recipients. While the almost complete absence of primordial follicles was observed in grafted ovaries, oocytes from these ovaries showed transcript levels of genes associated to oocyte maturation similar to control. Moreover, the developmental stage of follicles and oocytes of the 7+21 group ovaries were comparable to that of 21 days post partum in vivo ovaries, whereas significant developmental delay were found in the 14+14 group ovaries. Nevertheless, oocytes retrieved from transplanted ovaries of both groups matured (around 80%) and were fertilized in vitro (around 20%-45%). Two-cell embryos from the fertilized oocytes developed to hatching blastocysts (about 50%) or gave rise to healthy live offspring (from 6% to 10%) when transplanted in a host mother. In conclusion, our results indicate that premeiotic female germ cells cultured in vitro up to primordial/primary follicle stages preserve their capability to complete oogenesis and can be fertilized and generate live pups after transplantation into a suitable in vivo environment.

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

Reconstitution of female germ cell development in vitro is a key challenge in reproductive biology and medicine. We show here that female (XX) embryonic stem cells and induced pluripotent stem cells in mice are induced into primordial germ cell-like cells (PGCLCs), which, when aggregated with female gonadal somatic cells as reconstituted ovaries, undergo X-reactivation, imprint erasure, and cyst formation, and exhibit meiotic potential. Upon transplantation under mouse ovarian bursa, PGCLCs in the reconstituted ovaries mature into germinal vesicle-stage oocytes, which then contribute to fertile offspring after in vitro maturation and fertilization. Our culture system serves as a robust foundation for the investigation of key properties of female germ cells, including the acquisition of totipotency, and for the reconstitution of whole female germ cell development in vitro.

Growth of mouse oocytes to maturity from premeiotic germ cells in vitro

In the present study, we established an in vitro culture system suitable for generating fertilizable oocytes from premeiotic mouse female germ cells. These results were achieved after first establishing an in vitro culture system allowing immature oocytes from 12–14 day- old mice to reach meiotic maturation through culture onto preantral granulosa cell (PAGC) monolayers in the presence of Activin A (ActA). To generate mature oocytes from premeiotic germ cells, pieces of ovaries from 12.5 days post coitum (dpc) embryos were cultured in medium supplemented with ActA for 28 days and the oocytes formed within the explants were isolated and cocultured onto PAGC monolayers in the presence of ActA for 6–7 days. The oocytes were then subjected to a final meiotic maturation assay to evaluate their capability to undergo germinal vesicle break down (GVBD) and reach the metaphase II (MII) stage. We found that during the first 28 days of culture, a significant number of oocytes within the ovarian explants reached nearly full growth and formed preantral follicle-like structures with the surrounding somatic cells. GSH level and Cx37 expression in the oocytes within the explants were indicative of proper developmental conditions. Moreover, the imprinting of Igf2r and Peg3 genes in these oocytes was correctly established. Further culture onto PAGCs in the presence of ActA allowed about 16% of the oocytes to undergo GVBD, among which 17% reached the MII stage during the final 16–18 hr maturation culture. These MII oocytes showed normal spindle and chromosome assembly and a correct ERK1/2 activity. About 35% of the in vitro matured oocytes were fertilized and 53.44% of them were able to reach the 2-cell stage. Finally, around 7% of the 2-cell embryos developed to the morula/blastocyst stage.

Differential methylation of pluripotency gene promoters in in vitro matured and vitrified, in vivo-matured mouse oocytes

OBJECTIVE

To assess the methylation patterns of four pluripotency gene promoters in mouse oocytes after in vivo maturation, in vitro maturation (IVM), and vitrification followed by IVM.

DESIGN

Experimental study.

SETTING

Research laboratory.

ANIMAL(S)

Three populations of metaphase II mouse oocytes were analyzed after in vivo maturation, IVM, and vitrification followed by IVM (V-IVM). Cumulus cells and blastocyst embryos were controls.

INTERVENTION(S)

The CpG methylation patterns (overall and CpG specific) in the promoters of four pluripotency genes (Oct4, Nanog, Foxd3, and Sox2) were analyzed for each cell type by traditional DNA bisulfite sequencing.

MAIN OUTCOME MEASURE(S)

Differences for overall methylation were evaluated using the Student's t-test and for individual CpG sites by χ2 analysis.

RESULT(S)

Significantly lower levels of overall methylation in promoters of Oct4 (25%) and Sox2 (4.5%) were noted in V-IVM oocytes compared with in vivo-matured oocytes (62.5% and 8.5%, respectively). Cumulus cell promoters were generally hypomethylated at Nanog, Foxd3. and Sox2, but hypermethylated at Oct4.

CONCLUSION(S)

The methylation status of Oct4 and Sox2 promoters of V-IVM mouse oocytes are altered when compared with in vivo-matured oocytes. The biological risk and significance of these changes are unknown and this study indicates caution and that further analyses are warranted.

Effects of in vitro culture of mouse fetal gonads on subsequent ovarian development in vivo and oocyte maturation in vitro

Under organ culture, female fetal gonads in mice cannot develop beyond the preantral follicle stage unless the follicles are individually isolated and cultured again. In this study, we investigated the effect of in vitro culture of female fetal gonads before transplantation on subsequent in vivo development. The gonads derived from female fetuses 12.5 days postcoitum were organ-cultured for 0, 7 and 14 days, and then were grafted underneath the kidney capsules of severe combined immunodeficient mice and recovered at 21, 14 and 7 days post-transplantation, respectively. The histological analysis of the grafts showed that the in vitro culture of the fetal gonads restricted follicular development to the antral follicle stage post-transplantation. In the grafts cultured for 14 days, particularly, no antral follicle was observed. However, the oocytes in these follicles had grown to around 65 microm in diameter and had competence to resume meiosis in vitro. When the fetal gonads were grafted after culture for 7 and 14 days, 13.0% and 6.8% of the oocytes progressed to the metaphase II stage, respectively. These data showed significant differences (P < 0.05) in comparison with the control group (25.3%). Our results indicate that the in vitro culture of female fetal gonads before transplantation affects the subsequent in vivo development of both follicular cells and oocytes, and in vitro oocyte maturation. However, this effect seems to be more severe in terms of follicular development when compared with oocyte growth and maturation.

Transplantation directs oocyte maturation from embryonic stem cells and provides a therapeutic strategy for female infertility

Ten to 15% of couples are infertile, with the most common causes being linked to the production of few or no oocytes or sperm. Yet, our understanding of human germ cell development is poor, at least in part due to the inaccessibility of early stages to genetic and developmental studies. Embryonic stem cells (ESCs) provide an in vitro system to study oocyte development and potentially treat female infertility. However, most studies of ESC differentiation to oocytes have not documented fundamental properties of endogenous development, making it difficult to determine the physiologic relevance of differentiated germ cells. Here, we sought to establish fundamental parameters of oocyte development during ESC differentiation to explore suitability for basic developmental genetic applications using the mouse as a model prior to translating to the human system. We demonstrate a timeline of definitive germ cell differentiation from ESCs in vitro that initially parallels endogenous oocyte development in vivo by single-cell expression profiling and analysis of functional milestones including responsiveness to defined maturation media, shared genetic requirement of Dazl, and entry into meiosis. However, ESC-derived oocyte maturation ultimately fails in vitro. To overcome this obstacle, we transplant ESC-derived oocytes into an ovarian niche to direct their functional maturation and, thereby, present rigorous evidence of oocyte physiologic relevance and a potential therapeutic strategy for infertility.

Instructing an embryonic stem cell-derived oocyte fate: lessons from endogenous oogenesis

Female reproductive potential is limited in the majority of species due to oocyte depletion. Because functional human oocytes are restricted in number and accessibility, a robust system to differentiate oocytes from stem cells would enable a thorough investigation of the genetic, epigenetic, and environmental factors affecting human oocyte development. Also, the differentiation of functional oocytes from stem cells may permit the success of human somatic cell nuclear transfer for reprogramming studies and for the production of patient-specific embryonic stem cells (ESCs). Thus, ESC-derived oocytes could ultimately help to restore fertility in women. Here, we review endogenous and ESC-derived oocyte development, and we discuss the potential and challenges for differentiating functional oocytes from ESCs.

Genetic modification for bimaternal embryo development

Full mammalian development typically requires genomes from both the oocyte and spermatozoon. Biparental reproduction is necessary because of parent-specific epigenetic modification of the genome during gametogenesis; that is, a maternal methylation imprint imposed during the oocyte growth period and a paternal methylation imprint imposed in pregonadal gonocytes. This leads to unequivalent expression of imprinted genes from the maternal and paternal alleles in embryos and individuals. It is possible to hypothesise that the maternal methylation imprint is necessary to prevent parthenogenesis, which extinguishes the opportunity for having descendents, whereas the paternal methylation imprint prevents parthenogenesis, ensuring that a paternal contribution is obligatory for any descendants. To date, there are several lines of direct evidence that the epigenetic modifications that occur during oocyte growth have a decisive effect on mammalian development. Using bimaternal embryos with two sets of maternal genomes, the present paper illustrates how parental methylation imprints are an obstacle to the progression of parthenogenesis.

The in vitro growth and maturation of follicles

The development of technologies to grow oocytes from the most abundant primordial follicles to maturity in vitro holds many attractions for clinical practice, animal production technology and research. The production of fertile oocytes and live offspring has been achieved in mice following the long-term culture of oocytes in primordial follicles from both fresh and cryopreserved ovarian tissue. In contrast, in non-rodent species advances in follicle culture are centred on the growth of isolated preantral follicles. As a functional unit, mammalian preantral follicles are well-suited to culture but primordial and primary follicles do not grow well after isolation from the ovarian stroma. The current challenges for follicle culture are numerous and include: optimisation of culture media and the tailoring of culture environments to match the physiological needs of the cell in vivo; the maintenance of cell-cell communication and signalling during culture; and the evaluation of the epigenetic status, genetic health and fertility of in vitro derived mature oocytes. In large animals and humans, the complete in vitro growth and maturation of oocytes is only likely to be achieved following the development of a multistage strategy that closely mimics the ovary in vivo. In this approach, primordial follicle growth will be initiated in situ by the culture of ovarian cortex. Isolated preantral follicles will then be grown to antral stages before steroidogenic function is induced in the somatic cells. Finally, cytoplasmic and nuclear maturation will be induced in the in vitro derived oocytes with the production of fertile metaphase II gametes.

Protocol for the production of viable bimaternal mouse embryos

A reliable nuclear transfer method was first reported in 1983; it provided definite evidence that parthenogenetic embryos are lethal at early postimplantation in mammals. Subsequently, nuclear transfer has been extensively used as an important and versatile tool for investigating embryo and somatic-cell cloning and nucleo-cytoplasmic interactions. Further development of this technique has enabled the generation of bimaternal embryos containing two haploid sets of maternal genomes from female germ cells of different origins. By using a 2-d nuclear transfer system for oocyte reconstruction, viable mice can be produced solely from maternal genomes, without the participation of the paternal genome. This oocyte reconstruction system, as described in this protocol, could provide valuable guidelines for exploring the potential endowments of gametes and for conferring novel properties to them.

Developmental and Ultrastructual Characteristics of Mouse Oocytes Grown in Vitro from Primordial Germ Cells

The aim of this study was to clarify the developmental and ultrastructual characteristics of oocytes grown in vitro from primordial germ cells. The female genital ridges at 12.5 days post coitus were cultured for 18 days on an insert membrane in Waymouth's MB752/1 medium, supplemented with 15% fetal bovine serum and 1 mM sodium pyruvate; subsequently, the follicles isolated from the tissue were cultured for eight days in Waymouth's medium supplemented with 5 microg/ml insulin, 5 microg/ml transferrin, 5 ng/ml selenium, 10 mIU/ml follicle stimulating hormone, and 100 ng/ml stem cell factor. The primordial germ cells developed in vitro into oocytes of more than 60 microm in diameter. The transmission electron microscopic analysis indicated that the oocytes, which developed in vitro, showed no obvious abnormality in their ultrastructure and had organelles appropriate for the oocyte size. However, a delay in the progressive changes of morphology in some of the organelles during oocyte growth was often found when comparing them to oocytes grown in vivo.

Global mapping of DNA methylation in mouse promoters reveals epigenetic reprogramming of pluripotency genes

DNA methylation patterns are reprogrammed in primordial germ cells and in preimplantation embryos by demethylation and subsequent de novo methylation. It has been suggested that epigenetic reprogramming may be necessary for the embryonic genome to return to a pluripotent state. We have carried out a genome-wide promoter analysis of DNA methylation in mouse embryonic stem (ES) cells, embryonic germ (EG) cells, sperm, trophoblast stem (TS) cells, and primary embryonic fibroblasts (pMEFs). Global clustering analysis shows that methylation patterns of ES cells, EG cells, and sperm are surprisingly similar, suggesting that while the sperm is a highly specialized cell type, its promoter epigenome is already largely reprogrammed and resembles a pluripotent state. Comparisons between pluripotent tissues and pMEFs reveal that a number of pluripotency related genes, including Nanog, Lefty1 and Tdgf1, as well as the nucleosome remodeller Smarcd1, are hypomethylated in stem cells and hypermethylated in differentiated cells. Differences in promoter methylation are associated with significant differences in transcription levels in more than 60% of genes analysed. Our comparative approach to promoter methylation thus identifies gene candidates for the regulation of pluripotency and epigenetic reprogramming. While the sperm genome is, overall, similarly methylated to that of ES and EG cells, there are some key exceptions, including Nanog and Lefty1, that are highly methylated in sperm. Nanog promoter methylation is erased by active and passive demethylation after fertilisation before expression commences in the morula. In ES cells the normally active Nanog promoter is silenced when targeted by de novo methylation. Our study suggests that reprogramming of promoter methylation is one of the key determinants of the epigenetic regulation of pluripotency genes. Epigenetic reprogramming in the germline prior to fertilisation and the reprogramming of key pluripotency genes in the early embryo is thus crucial for transmission of pluripotency.

Epigenetics, brain evolution and behaviour

Molecular modifications to the structure of histone proteins and DNA (chromatin) play a significant role in regulating the transcription of genes without altering their nucleotide sequence. Certain epigenetic modifications to DNA are heritable in the form of genomic imprinting, whereby subsets of genes are silenced according to parent-of-origin. This form of gene regulation is primarily under matrilineal control and has evolved partly to co-ordinate in-utero development with maternal resource availability. Changes to epigenetic mechanisms in post-mitotic neurons may also be activated during development in response to environmental stimuli such as maternal care and social interactions. This results in long-lasting stable, or short-term dynamic, changes to the neuronal phenotype producing long-term behavioural consequences. Use of evolutionary conserved mechanisms have thus been adapted to modify the control of gene expression and embryonic growth of the brain as well as allowing for plastic changes in the post-natal brain in response to external environmental and social cues.

Expression of growth differentiation factor 9, bone morphogenetic protein 15, and anti-Müllerian hormone in cultured mouse primary follicles

Growth differentiation factor 9 (GDF9), bone morphogenetic protein 15 (BMP15), and anti-Müllerian hormone (AMH) play an important role in the primary to secondary follicle transition and follicle activation in vivo. In organ culture of neonatal mouse ovaries, it was observed that significantly fewer primary follicles develop to the secondary stage. The objectives of this study were: (1) to compare ovarian follicular populations between organ-cultured neonatal mouse ovaries and freshly isolated age-matched control ovaries; (2) to quantify RNA levels of Gdf9, Bmp15, and Amh in cultured primary follicles; and (3) to immunolocalize GDF9 and AMH in cultured ovaries. Ovaries from 3-day-old (PND 3) mice were cultured for 7 or 10 days in the absence or presence of FSH. Follicular populations were counted in freshly isolated 13-day-old (PND 13) ovaries and organ-cultured ovaries. Transcripts were quantified in isolated primary follicles using real-time RT-PCR, and protein expressions were localized using immunohistochemistry. The number of secondary follicles in organ-cultured ovaries was significantly lower than in vivo controls. Gdf9 and Bmp15 mRNA expression levels were similar as in controls. Amh mRNA levels were significantly (P<0.05) lower after day 10 of culture in the absence of FSH. GDF9 and AMH proteins were respectively detected in the oocytes and the granulosa cells (GC) beginning at the primary and primordial stages onward. GDF9 and BMP15 production in cultured primary follicles are not different from in vivo controls; hence abnormal early follicular growth was not related to a deficient transcription of these factors.

Growth of follicles of various animals following ovarian grafting under the kidney capsules of immunodeficient mice

Aim:  Various researchers have studied xenografting ovarian tissues into immunodeficient mice to accelerate the follicular growth of several mammalian species. In this study, the authors focused on the following three points in growing follicles in transplanted ovarian tissues under kidney capsules: the effects of the storage conditions of the donor ovarian tissues, the effects of donor age on the survival rates of grafted mouse ovaries, and the methods used to grow the follicles of xenografted bovine ovaries. Methods and Results:  When ovaries stored for 0, 6, 12 or 24 h at 4°C and at room temperature were transplanted under the kidney capsules of immunodeficient mice, fewer mouse and rabbit grafts survived following 24 h storage. The survival rates of bovine grafts were relatively low for all storage times. When mouse ovaries were held for 24 h at 4°C or at room temperature, low-temperature storage effectively improved the survival rates of the grafts. Although the survival rates of grafted genital ridges containing premeiotic germ cells from fetuses and grafted ovaries from mice 0, 10, 20, 40 and 80 days after birth were similar among donors of different ages, the cleavage rate of oocytes following insemination was significantly lower in the grafts from the ovaries of 80-day-old mice. Antral follicles formed in surviving bovine ovarian grafts. Cumulus-oocyte complexes were collected from the grafted ovaries of fetuses and calves, and the oocytes reached the metaphase II stage following culture, but they did not develop to the pronuclear stage after in vitro fertilization. Conclusion:  Our findings provide basic data on xenografting ovarian tissues into immunodeficient mice to accelerate the growth of follicles. (Reprod Med Biol 2008; 7: 45-54).

In vitro development of mouse fetal germ cells into mature oocytes

Little is known about the mechanisms underlying primordial follicular formation and the acquisition of competence to resume meiosis by growing oocytes. It is therefore important to establish an in vitro experimental model that allows one to study such mechanisms. Mouse follicular development has been studied in vitro over the past several years; however, no evidence has been presented showing that mature oocytes can be obtained from mouse fetal germ cells prior to the formation of primordial follicles. In this study, a method has been established to obtain mature oocytes from the mouse fetal germ cells at 16.5 days postcoitum (dpc). From the initiation of primordial follicular formation to the growth of early secondary follicles, ovarian tissues from 16.5 dpc fetal mice were cultured in vitro for 14 days. Subsequently, 678 intact secondary follicles were isolated from 182 mouse fetal ovaries and cultured for 12 days. A total of 141 oocytes inside antral follicles were matured in vitro, and 102 oocytes underwent germinal vesicle breakdown. We found that 97 oocytes were fertilized and 15 embryos were able to form morula-blastocysts. We also analyzed various genomic imprinting markers and showed that the erasure of genomic imprinting markers in the parental generation was also imposed on the oocytes that developed from fetal germ cells. Our results demonstrate that mouse fetal germ cells are able to form primordial follicles with ovarian cells, and that oocytes within the growing follicles are able to mature normally in vitro.

Carbohydrate metabolism by murine ovarian follicles and oocytes grown in vitro

Metabolic markers are potentially valuable for assessment of follicle development in vitro. Carbohydrate metabolism of murine preantral follicles grown to maturity over 13 days in vitro has been measured, and metabolism of resulting oocyte-cumulus complexes (OCCs) and denuded oocytes has been compared with in vivo ovulated control counterparts. Spent follicle culture media were analysed for glucose, lactate and pyruvate concentrations. During follicle in vitro growth, glycolysis accounted for a rise from approximately 24 to 60% of all glucose consumed. Ovulation induction caused a significant increase in glucose uptake and lactate production by in vitro-grown follicles to 71.7+/-1.2 and 96.6+/-4.8 nmoles/day respectively. OCCs grown in vitro had significantly higher rates of glucose consumption and lactate and pyruvate production (110.1+/- 3.5, 191.8+/- 8.9 and 31.7+/- 1.7 pmoles/h respectively) than in vivo ovulated controls (67.4+/- 8.1, 113.9+/- 17.1 and 20.2+/- 4.0 pmoles/h respectively), but a reduced capacity for pyruvate consumption (1.13+/- 0.06 vs 1.49+/- 0.06 pmoles/h by in vivo ovulated oocytes). Metabolism of OCCs was affected by the quality of the original follicle. In vitro-grown oocytes had a reduced cytoplasmic volume when compared with controls (168.3+/- 2.0 vs 199.0+/- 3.2 proportionately respectively) but a similar rate of metabolism per unit volume. Meiotic status influenced metabolism of both OCCs and denuded oocytes. In conclusion, glucose consumption and lactate production by cultured follicles increased in tandem with developmental progression and were stimulated prior to ovulation. Additionally, the metabolic profiles of in vitro produced OCCs and the oocytes within them are affected by long-term exposure to the culture environment.

Long-term effects of in vitro growth of mouse oocytes on their maturation and development

Since very few oocytes grow completely in vivo, in vitro growth (IVG) of ovarian oocytes may provide a new source of functional oocytes. The long-term effects of in vitro maturation (IVM) of oocytes and in vitro culture of fertilized eggs have been reported; however, the effects of IVG of oocytes are unknown. Here in, we report the long-term effects of IVG of oocytes. Ovaries from 1-day-old mice containing non-growing oocytes were cultured for 10 days; the isolated follicles were then cultured for 11 days. Secondary follicles from 10-day-old mice were also cultured for 11 days. The nuclei of oocytes collected from the IVG and Graafiais follicles of adult mice were transferred to enucleated oocytes grown in vivo, respectively. Developmental competence was examined following IVM of the reconstituted oocytes. Chronologically, oocytes of 1-day-old, 10-day-old and adult mice were cultured for 22, 12 and 1 day(s). The result showed that the reconstituted eggs developed into pups at high rates after nuclear transfer and in vitro fertilization (IVF) in all the experimental groups (29-45%). However, the pups from reconstituted eggs containing the nuclei of 22-day cultured oocytes were heavier than the control pups (P<0.05). We concluded that long-term culture of oocytes did not affect their nuclear ability to develop to term; however, fetal growth was affected by the culture duration or culture conditions during the initial phase of follicular growth.

Artificial gametes

In vitro fertilization (IVF) has been an efficient medical treatment for infertility in the past decades. However, conventional IVF approaches may be insufficient when gametes are lacking or non-viable thus precluding a significant number of patients from treatment. Ultimately, creation of artificial gametes may provide an universal solution for all indications. Somatic cell nuclear transfer (SCNT) has provided successful cloning in different animal species indicating that a derived technology may be applicable in infertility treatment procedures. Attempts to produce functional male or female gamete through nuclear transfer have been described through the process called haploidization. Initial successes have been observed, however, significant alterations at spindle construction and chromosomal segregation were also described. Stem cell technology may provide an alternative route to obtain fully functional gametes. Both sperm cells and oocytes were obtained using specific culture conditions for embryo originated stem cell. These two mainstream approaches are presented in the current review. Both of these techniques are involving sophisticated methods and consequently both of them demonstrate technical and ethical challenges. Related questions on (mitotic/meiotic) cell division, genetic/epigenetic alterations and cell renewal are needed to be addressed before clinical application.