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

Superovulation treatment of immature female rabbits increases the number of ovulated oocytes that can in vitro develop into blastocytes

To clarify the efficiency of superovulation in immature female rabbits, immature female rabbits were superovulated with pregnant mare serum gonadotropin, and the number of recovered oocytes, their maturity, and their ability to develop into blastocysts under in vitro fertilization and culture were examined in this study. More than 80 oocytes were recovered from 12-14-week-old immature female rabbits. In particular, an average of more than 100 oocytes were recovered from 13-week-old immature female rabbits. The number of oocytes in immature female rabbits was significantly-approximately 4 times-higher than in mature female rabbits. To compare oocyte maturity, oocytes from immature and mature female rabbits were compared by brilliant cresyl blue staining and mitochondrial distribution analysis. The proportion of mature oocytes detected by brilliant cresyl blue staining was higher in oocytes from mature female rabbits than in oocytes from immature female rabbits, but there were no significant differences in active mitochondrial distribution, fertilization rate, or blastocyst development rate. These results indicate that superovulation with immature female rabbits may be a useful technique for the collection of many oocytes.

In vitro gametogenesis: Towards competent oocytes: Limitations and future improvements for generating oocytes from pluripotent stem cells in culture

Production of oocytes from pluripotent cell cultures in a dish represents a new paradigm in stem cell and developmental biology and has implications for how we think about life. The spark of life for the next generation occurs at fertilization when sperm and oocyte fuse. In animals, gametes are the only cells that transmit their genomes to the next generation. Oocytes contain in addition a large cytoplasm with factors that direct embryonic development. Reconstitution of mouse oocyte and embryonic development in culture provides experimental opportunities and facilitates an unprecedented understanding of molecular mechanisms. However, the application of in vitro gametogenesis to reproductive medicine or infertility treatment remains challenging. One significant concern is the quality of in vitro-derived oocytes. Here, we review the current understanding and identify limitations in generating oocytes in vitro. From this basis, we explore opportunities for future improvements of the in vitro approach to generating high-quality oocytes.

Combination of FSH and testosterone could enhance activation of primordial follicles and growth of activated follicles in 1-day-old mice ovaries in vitro cultured for 12 days

Treatment with follicle-stimulating hormone (FSH) and testosterone (T2) and their combination have been observed to be influential on ovarian follicles of 1-day-old mice ovaries cultured for 8 days. Given that extension of the culture period could positively impact the development of follicles in cultured ovaries, the present study was conducted to evaluate the main and interaction effects of FSH by T2 on the development of ovarian follicles in 1-day-old mice ovaries cultured for 12 days. One-day-old mice ovaries were initially cultured with base medium for 4 days; thereafter, different hormonal treatments were added to the culture media, and the culture was continued for 8 additional days until day 12. Ovaries were collected for histological and molecular assessments on day 12. The greatest activation of primordial follicles and progression of activated follicles to the preantral stage was detected in ovaries treated with the combination of FSH and T2 (P < 0.05). This positive effect on the morphology of ovarian follicles was accompanied by upregulation of Pi3k, Gdf9, Bmp15, Cx37 and Fshr in the ovaries cultured with the combination of FSH and T2 (P < 0.05). Nonetheless, treatment with FSH and T2 led to a diminished proportion of intact follicles (P < 0.05), even though Bax/Bcl2 gene expression ratio, as an apoptotic index, was less in hormone-treated ovaries (P < 0.05). In conclusion, the combination of FSH and T2 could improve the activation of primordial follicles and the growth of activated follicles towards the preantral stage. This positive effect of FSH plus T2 appeared to be at least partly mediated through the upregulation of Pi3k and oocyte-derived growth factors including Gdf9 and Bmp15.

A tug-of-war between germ cell motility and intercellular bridges controls germline cyst formation in mice

Gametes in many species develop in cysts-clusters of germ cells formed by incomplete cytokinesis-that remain connected through intercellular bridges (ICBs). These connections enable sharing of cytoplasmic components between germ cells and, in the female germ line, enrich select cells in the cyst to become the oocyte(s). In mice, germline cysts of variable sizes are generated during embryonic development, thought to result from cyst fractures. Studies of fixed samples failed to capture fracture events, and thus, the mechanism remained elusive. Here, we use high-resolution live imaging of germ cells within their native tissue environment to visualize germline cyst dynamics. With this novel approach, we reveal a striking motile phenotype of gonad-resident germ cells and show that this randomly oriented cell-autonomous motile behavior during cyst formation underlies fracture events. Conversely, we show that stabilized ICBs help resist excessive fracturing. Additionally, we find that motility and thus fracture rates gradually decrease during development in a sex-dependent manner, completely ceasing by the end of cyst-forming divisions. These results lead to a model where the opposing activities of developmentally regulated cell motility and stable ICBs give rise to cysts of variable sizes. We corroborate these results by developing a model that uses experimentally measured fracture rates to simulate cyst formation and fracture and show that it can reproduce experimentally measured cyst sizes in both male and female. Understanding how variable cysts form will enable further studies of mammalian oocyte selection and establishment of the ovarian reserve.

Effect and mechanisms of cyclophosphamide-induced ovarian toxicity on the quality of primordial follicles with respect to age at treatment initiation

Chemotherapy-induced ovarian toxicity in patients with cancer significantly affects future fertility depending on the age of initiation of treatment. However, the mechanisms underlying the age-related depletion of the ovarian reserve are not well understood. We investigated the effects of chemotherapy on pre- and postpubertal ovarian reserves in a mouse model. Juvenile (3-week-old) and adult (8-week-old) mice were injected with vehicle or cyclophosphamide (CPA;100 mg/kg). We assessed the short-term effects at 24 h and 72 h after injection and the long-term effects at 10 and 12 weeks of age by counting the follicles. The number of primordial follicles in the juvenile group was significantly reduced by CPA treatment compared with that in the adult group. To elucidate the mechanisms of this depletion, we performed immunostaining for γH2AX, cleaved PARP1, and FOXO3 at 24 h post-treatment. CPA-treated juvenile mice had a significantly higher proportion of γH2AX-positive primordial follicles, indicating double-strand DNA breaks. By contrast, 4-hydroperoxy CPA, an activated analog of CPA, induced γH2AX-positive primordial follicles in both groups in vitro, suggesting age-dependent differences in humoral ovarian microenvironment. Moreover, the level of cleaved PARP1 was specifically elevated in CPA-treated juvenile mice. However, primordial follicle activation was unaffected in the CPA-treated groups, as assessed by FOXO3 translocation. In conclusion, our findings suggest that ovaries in juveniles are more susceptible to DNA damage and subsequent apoptosis, leading to a higher rate of primordial follicle depletion. Therefore, it is crucial to recognize that cancer treatment, especially in children, can exert a substantial influence on future fertility.

In vitro gametogenesis (IVG): reflections from a workshop

Key ideas from a workshop convened by the National Academies of Sciences, Engineering, and Medicine to discuss developments in IVG (National Academies of Sciences, Engineering, and Medicine 2023) chaired by Dr Eli Y. Adashi (former Dean of Medicine and Biological Sciences at Brown University). The authors are solely responsible for the content of this paper, which does not necessarily represent the views of the National Academies of Sciences, Engineering, and Medicine.

Luteinizing Hormone Receptor Mutation (LHRN316S) Causes Abnormal Follicular Development Revealed by Follicle Single-Cell Analysis and CRISPR/Cas9

Abnormal interaction between granulosa cells and oocytes causes disordered development of ovarian follicles. However, the interactions between oocytes and cumulus granulosa cells (CGs), oocytes and mural granulosa cells (MGs), and CGs and MGs remain to be fully explored. Using single-cell RNA-sequencing (scRNA-seq), we determined the transcriptional profiles of oocytes, CGs and MGs in antral follicles. Analysis of scRNA-seq data revealed that CGs may regulate follicular development through the BMP15-KITL-KIT-PI3K-ARF6 pathway with elevated expression of luteinizing hormone receptor (LHR). Because internalization of the LHR is regulated by Arf6, we constructed LHRN316S mice by CRISPR/Cas9 to further explore mechanisms of follicular development and novel treatment strategies for female infertility. Ovaries of LHRN316S mice exhibited reduced numbers of corpora lutea and ovulation. The LHRN316S mice had a reduced rate of oocyte maturation in vitro and decreased serum progesterone levels. Mating LHRN316S female mice with ICR wild type male mice revealed that the infertility rate of LHRN316S mice was 21.4% (3/14). Litter sizes from LHRN316S mice were smaller than those from control wild type female mice. The oocytes from LHRN316S mice had an increased rate of maturation in vitro after progesterone administration in vitro. Furthermore, progesterone treated LHRN316S mice produced offspring numbers per litter equivalent to WT mice. These findings provide key insights into cellular interactions in ovarian follicles and provide important clues for infertility treatment.

In vitro production of viable eggs from undeveloped oocytes in mouse preantral follicles by reconstructing granulosa cell-oocyte complexes†

In vitro culture of ungrown oocytes in preantral follicles is one of the intriguing subjects being pursued to produce viable eggs in assisted reproductive technology. Previous studies have succeeded in obtaining mature eggs after in vitro culture of preantral follicles, while denuded undeveloped oocytes, which are obtained occasionally when collecting preantral follicles, seem to be almost useless. Moreover, methods to culture them efficiently to produce viable eggs have not been established yet. The present study was conducted to demonstrate in vitro culture of mouse denuded undeveloped oocytes by reconstructing granulosa cell-oocyte complexes, and to analyze cellular communication in reconstructed granulosa cell-oocyte complexes. Single denuded undeveloped oocytes were aggregated with 1 × 104 granulosa cells in wells with U-shaped bottoms in a low-binding cell culture plate for 8 days under either 20% or 5% O2, and then the reconstructed granulosa cell-oocyte complexes formed were cultured on a collagen-coated culture membrane insert for 4 days under 5% O2. At day 8 of culture, the rates of reconstructed granulosa cell-oocyte complexes formation were significantly higher in the culture group under 5% O2 (64.9%) than that under 20% O2 (42.3%; P < 0.001); furthermore, the formation of transzonal projections was observed. After maturation and fertilization, we produced matured eggs and blastocysts at higher rates (>90% and 61.9%, respectively) in the group cultured under 5% O2. After transferring 126 two- to four-cell stage embryos, six live pups were obtained. This is the first report that demonstrates production of viable eggs after in vitro culture of denuded undeveloped oocytes from preantral follicles by reconstruction of granulosa cell-oocyte complexes.

Improved low-invasive mRNA electroporation method into immature mouse oocytes visualizes protein dynamics during development†

One of the major causes of oocyte quality deterioration, chromosome segregation abnormalities manifest mainly during meiosis I, which occurs before and during ovulation. However, currently, there is a technical limitation in the introduction of mRNA into premature oocytes without impairing embryonic developmental ability. In this study, we established a low-invasive electroporation (EP) method to introduce mRNA into pre-ovulatory, germinal vesicle (GV) mouse oocytes in an easier manner than the traditional microinjection method. The EP method with an optimized impedance value resulted in the efficient introduction of mRNAs encoding enhanced green fluorescent protein (EGFP) into the GV oocytes surrounded by cumulus cells at a survival rate of 95.0%. Furthermore, the introduction of histone H2B-EGFP mRNA into the GV oocytes labeled most of the oocytes without affecting the blastocyst development rate, indicating the feasibility of the visualization of oocyte chromosomal dynamics that enable us to assay chromosomal integrity in oocyte maturation and cell count in embryonic development. The establishment of this EP method offers extensive assays to select pre-implantation embryos and enables the surveying of essential factors for mammalian oocyte quality determination.

Advancing fertility preservation in prepubertal mice: Efficacy of ovarian tissue culture and in vitro growth in mature oocyte development

AIM

This study aimed to evaluate the ovarian tissue culture and in vitro follicle growth as safer alternatives to cryopreservation for generating in vitro fertilization (IVF)-ready mature oocytes from prepubertal mice without the risk of cancer cell contamination.

METHODS

Ovaries from prepubertal B6D2F1 mice were cultured in α-minimum essential medium supplemented with an estrogen receptor antagonist, ICI 182780. Culture duration was investigated to identify the optimal timeframe for follicle growth and oocyte maturation. Follicles were isolated mechanically or using 1 mg/mL collagenase and cultured in Matrigel matrix or polyvinylpyrrolidone. Oocyte development at metaphase II was induced by in vitro maturation, followed by IVF.

RESULTS

The optimal culture duration was 2-4 days, and tissues cultured beyond this period showed significant follicular degeneration. ICI 182780 supplementation resulted in the recovery of 20.5 follicles per ovary compared with 9.5 follicles in non-supplemented cultures (p < 0.05). Of the 452 isolated follicles, 237 (52.4%) showed growth, 150 (33.2%) underwent germinal vesicle breakdown, and 18 (4.0%) reached metaphase II. However, none of the metaphase II oocytes were successfully fertilized after IVF. Matrigel demonstrated a significantly higher in vitro maturation rate compared with polyvinylpyrrolidone in a comparative analysis of culture matrices (p < 0.001).

CONCLUSIONS

This study highlighted ovarian tissue culture and in vitro growth as effective strategies for producing mature oocytes from prepubertal mice. Further studies are required to overcome fertilization hurdles and understand the mechanisms that improve post-IVF embryo viability.

Secondary follicles enable efficient germline mtDNA base editing at hard-to-edit site

Efficient germline mtDNA editing is required to construct disease-related animal models and future gene therapy. Recently, the DddA-derived cytosine base editors (DdCBEs) have made mitochondrial genome (mtDNA) precise editing possible. However, there still exist challenges for editing some mtDNA sites in germline via zygote injection, probably due to the suspended mtDNA replication during preimplantation development. Here, we introduce a germline mtDNA base editing strategy: injecting DdCBEs into oocytes of secondary follicles, at which stage mtDNA replicates actively. With this method, we successfully observed efficient G-to-A conversion at a hard-to-edit site and also obtained live animal models. In addition, for those editable sites, this strategy can greatly improve the base editing efficiency up to 3-fold, which is more than that in zygotes. More important, editing in secondary follicles did not increase more the risk of off-target effects than that in zygotes. This strategy provides an option to efficiently manipulate mtDNA sites in germline, especially for hard-to-edit sites.

Generation of mouse and rat xenogeneic ovaries in vitro for production of mouse oocyte

The system forming ovarian follicles is developed to investigate in vitro folliculogenesis in a confined environment to obtain functional oocytes. Several studies have reported the successful generation of fully functional oocytes using mouse-induced pluripotent stem cells (iPSCs) and mouse female germline stem cells (fGSCs) as sources of stem cells for in vitro gametogenesis models. In addition, human oogonia have been generated through heterologous co-culture of differentiated human primordial germ cell-like cells (hPGCLCs) with mouse germline somatic cells, although oocyte formation remains challenging. Thus, studies on in vitro ovarian formation in other species are utilized as an introductory approach for in vitro mammalian gametogenesis by understanding the differences in culture systems between species and underlying mechanisms. In this study, we optimized the method of the entire oogenesis process from rat embryonic gonads. We identified well-maturated MII oocytes from rat gonads using our constructed method. Moreover, we generated the first successful in vitro reconstitution of xenogeneic follicles from mouse primordial germ cells (PGCs) and rat somatic cells. We also established an appropriate culture medium and incubation period for xenogeneic follicles. This method will be helpful in studies of xenogeneic follicular development and oocyte generation.

In Vitro Growth of Mammalian Follicles and Oocytes

Mammalian ovaries contain a large number of immature follicles, most of which are destined to degenerate before ovulation [...].

Current Fertility Preservation Steps in Young Women Suffering from Cancer and Future Perspectives

Childhood cancer incidence, especially in high-income countries, has led to a focus on preserving fertility in this vulnerable population. The common treatments, such as radiation and certain chemotherapeutic agents, though effective, pose a risk to fertility. For adult women, established techniques like embryo and egg freezing are standard, requiring ovarian stimulation. However, for prepubescent girls, ovarian tissue freezing has become the primary option, eliminating the need for hormonal preparation. This review describes the beginning, evolution, and current situation of the fertility preservation options for this young population. A total of 75 studies were included, covering the steps in the current fertility preservation protocols: (i) ovarian tissue extraction, (ii) the freezing method, and (iii) thawing and transplantation. Cryopreservation and the subsequent transplantation of ovarian tissue have resulted in successful fertility restoration, with over 200 recorded live births, including cases involving ovarian tissue cryopreserved from prepubescent girls. Despite promising results, challenges persist, such as follicular loss during transplantation, which is attributed to ischemic and oxidative damage. Optimizing ovarian tissue-freezing processes and exploring alternatives to transplantation, like in vitro systems for follicles to establish maturation, are essential to mitigating associated risks. Further research is required in fertility preservation techniques to enhance clinical outcomes in the future. Ovarian tissue cryopreservation appears to be a method with specific benefits, indications, and risks, which can be an important tool in terms of preserving fertility in younger women.

Using Embryo Models to Understand the Development and Progression of Embryonic Lineages: A Focus on Primordial Germ Cell Development

BACKGROUND

Recapitulating mammalian cell type differentiation in vitro promises to improve our understanding of how these processes happen in vivo, while bringing additional prospects for biomedical applications. The establishment of stem cell-derived embryo models and embryonic organoids, which have experienced explosive growth over the last few years, opens new avenues for research due to their scale, reproducibility, and accessibility. Embryo models mimic various developmental stages, exhibit different degrees of complexity, and can be established across species. Since embryo models exhibit multiple lineages organized spatially and temporally, they are likely to provide cellular niches that, to some degree, recapitulate the embryonic setting and enable "co-development" between cell types and neighbouring populations. One example where this is already apparent is in the case of primordial germ cell-like cells (PGCLCs).

SUMMARY

While directed differentiation protocols enable the efficient generation of high PGCLC numbers, embryo models provide an attractive alternative as they enable the study of interactions of PGCLCs with neighbouring cells, alongside the regulatory molecular and biophysical mechanisms of PGC competency. Additionally, some embryo models can recapitulate post-specification stages of PGC development (including migration or gametogenesis), mimicking the inductive signals pushing PGCLCs to mature and differentiate and enabling the study of PGCLC development across stages. Therefore, in vitro models may allow us to address questions of cell type differentiation, and PGC development specifically, that have hitherto been out of reach with existing systems.

KEY MESSAGE

This review evaluates the current advances in stem cell-based embryo models, with a focus on their potential to model cell type-specific differentiation in general and in particular to address open questions in PGC development and gametogenesis.

Making human eggs in a dish: are we close?

In the space of 50 years, we have seen incredible achievements in human reproductive medicine. With these leaps forward, it is no wonder that there is a major interest in women's reproductive health research, including extension of reproductive lifespan. Substantial effort is currently being made to address this challenge, including from the commercial sector. In vitro gametogenesis (IVG) in mice is a spectacular breakthrough and has the potential to offer hope to women with intractable infertility. However, with such lofty goals, some reflection may be called for: mastering all of the techniques required for complete and safe IVG in women is likely to be extraordinarily difficult.

Analysis of Meiotic Progression by Ex Vivo Culture of Mouse Embryonic Ovaries

Oogenesis is the central process required to produce viable oocytes in female mammals. It is initiated during embryonic development, and it involves the specification of primordial germ cells (PGCs) and progresses through the activation of the meiotic program, reaching a crucial phase in prophase I before pausing at diplotene around the time of birth. The significance of meiosis, particularly the prophase I stage, cannot be overstated, as it plays a pivotal role in ensuring the formation of healthy gametes, a prerequisite for successful reproduction. While research has explored meiosis across various organisms, understanding how environmental factors, including radiation, drugs, endocrine disruptors, reproductive age, or diet, influence this complex developmental process remains incomplete. In this chapter, we describe an ex vivo culture method to investigate meiotic prophase I and beyond and the disruption of oogenesis by external factors. Using this methodology, it is possible to evaluate the effects of individual xenobiotics by administering chemicals at specific points during oogenesis. This culture technique was optimized to study the effects of two selected endocrine disruptors (vinclozolin and MEHP), demonstrating that vinclozolin exposure delayed meiotic differentiation and MEHP exposure reduced follicle size. This approach also opens avenues for future applications, involving the exploration of established or novel pharmaceutical substances and their influence on essential events during prophase I, such as homologous recombination and chromosome segregation. These processes collectively dictate the ultimate fitness of oocytes, with potential implications for factors relevant to the reproductive age and fertility.

Epigenetic regulation limits competence of pluripotent stem cell-derived oocytes

Recent studies have reported the differentiation of pluripotent cells into oocytes in vitro. However, the developmental competence of in vitro-generated oocytes remains low. Here, we perform a comprehensive comparison of mouse germ cell development in vitro over all culture steps versus in vivo with the goal to understand mechanisms underlying poor oocyte quality. We show that the in vitro differentiation of primordial germ cells to growing oocytes and subsequent follicle growth is critical for competence for preimplantation development. Systematic transcriptome analysis of single oocytes that were subjected to different culture steps identifies genes that are normally upregulated during oocyte growth to be susceptible for misregulation during in vitro oogenesis. Many misregulated genes are Polycomb targets. Deregulation of Polycomb repression is therefore a key cause and the earliest defect known in in vitro oocyte differentiation. Conversely, structurally normal in vitro-derived oocytes fail at zygotic genome activation and show abnormal acquisition of 5-hydroxymethylcytosine on maternal chromosomes. Our data identify epigenetic regulation at an early stage of oogenesis limiting developmental competence and suggest opportunities for future improvements.

Optimisation of hormonal treatment to improve follicular development in one-day-old mice ovaries cultured under in vitro condition

CONTEXT

Base medium containing knock-out serum replacement (KSR) has been found to support formation and maintenance of follicles in one-day-old mice ovaries, but has not been shown to properly support activation and growth of primordial follicles.

AIMS

The present study was conducted to tailor the hormonal content of base medium containing KSR to enhance development of primordial follicles in neonatal ovaries.

METHODS

One-day-old mice ovaries were initially cultured with base medium for four days, and then, different hormonal treatments were added to the culture media and the culture was proceeded for four additional days until day eight. Ovaries were collected for histological and molecular assessments on days four and eight.

KEY RESULTS

In experiment I, the main and interactive effects of FSH and testosterone were investigated and FSH promoted activation of primordial follicles and development of primary and preantral follicles, and upregulated genes of phosphoinositide 3-kinase (Pi3k ), KIT ligand (Kitl ), growth differentiation factor 9 (Gdf9 ) and follicle stimulating hormone receptor (Fshr ) (P Bmp15 ), Connexin-43 (Cx43 ) and luteinising hormone and choriogonadotropin receptor (Lhcgr ) (P P Lhcgr (P P >0.05).

CONCLUSIONS

Supplementation of culture medium containing KSR with gonadotropins, particularly hMG, could improve follicular growth and expression of factors regulating follicular development.

IMPLICATIONS

This study was a step forward in formulating an optimal medium for development of follicles in cultured one-day-old mice ovaries.

The effect of ACVR1B/TGFBR1/ACVR1C signaling inhibition on oocyte and granulosa cell development during in vitro growth culture

The signals of the transforming growth factor β (TGF-β) superfamily play a critical role in follicular development in mammals. ACVR1B/TGFBR1/ACVR1C receptors mediate the signaling of several TGF-β superfamily ligands in granulosa cells. Although the requirement for ACVR1B/TGFBR1/ACVR1C receptor signaling in follicular development has been confirmed using mutant mouse models, the detailed roles of the signaling in granulosa cell and oocyte development have not been clearly defined. In the present study, we examined the requirement for ACVR1B/TGFBR1/ACVR1C receptor signaling in granulosa cells using an in vitro growth culture of oocyte-granulosa cell complexes (OGCs) and SB431542, a potent inhibitor of the receptor signaling. Although cumulus-oocyte complexes isolated from the control OGCs were able to undergo cumulus expansion, those isolated from OGCs grown with the inhibitor were not competent, even in the presence of in vivo-grown oocytes. The diameter of the oocytes in the SB431542-treated OGCs was comparable with that of the control; however, these oocytes were not competent for complete meiotic maturation or preimplantation development. Therefore, ACVR1B/TGFBR1/ACVR1C receptor signaling is not required for oocytes to increase their volume but is essential for the normal development of cumulus cells and oocyte developmental competence.

The neonatal southern white rhinoceros ovary contains oogonia in germ cell nests

The northern white rhinoceros is functionally extinct with only two females left. Establishing methods to culture ovarian tissues, follicles, and oocytes to generate eggs will support conservation efforts using in vitro embryo production. To the best of our knowledge, this is the first description of the structure and molecular signature of any rhinoceros, more specifically, we describe the neonatal and adult southern white rhinoceros (Ceratotherium simum simum) ovary; the closest relation of the northern white rhinoceros. Interestingly, all ovaries contain follicles despite advanced age. Analysis of the neonate reveals a population of cells molecularly characterised as mitotically active, pluripotent with germ cell properties. These results indicate that unusually, the neonatal ovary still contains oogonia in germ cell nests at birth, providing an opportunity for fertility preservation. Therefore, utilising ovaries from stillborn and adult rhinoceros can provide cells for advanced assisted reproductive technologies and investigating the neonatal ovaries of other endangered species is crucial for conservation.

Key mechanisms and in vitro reconstitution of fetal oocyte development in mammals

During fetal oocyte development in mammals, germ cells progress through meiotic prophase I to form primordial follicles with pregranulosa cells. The primordial follicles remain dormant until oogenesis resumes during puberty. Studies in mice have elucidated mechanisms governing oogenesis, leading to the successful induction of functional oocytes from mouse pluripotent stem cells in vitro. Based on the in vivo/in vitro knowledge in mice and the histological and transcriptomic evidence for fetal oocyte development in humans and primates, human/primate oocyte-like cells corresponding to the early stage of oocytes in vivo have been successfully induced in vitro. Here, we discuss recent advances in our understanding of the mechanisms of fetal oocyte development in mammals, as well as in in vitro oogenesis.

Facilitation of Ovarian Response by Mechanical Force-Latest Insight on Fertility Improvement in Women with Poor Ovarian Response or Primary Ovarian Insufficiency

The decline in fertility in aging women, especially those with poor ovarian response (POR) or primary ovarian insufficiency (POI), is a major concern for modern IVF centers. Fertility treatments have traditionally relied on gonadotropin- and steroid-hormone-based IVF practices, but these methods have limitations, especially for women with aging ovaries. Researchers have been motivated to explore alternative approaches. Ovarian aging is a complicated process, and the deterioration of oocytes, follicular cells, the extracellular matrix (ECM), and the stromal compartment can all contribute to declining fertility. Adjunct interventions that involve the use of hormones, steroids, and cofactors and gamete engineering are two major research areas aimed to improve fertility in aging women. Additionally, mechanical procedures including the In Vitro Activation (IVA) procedure, which combines pharmacological activators and fragmentation of ovarian strips, and the Whole Ovary Laparoscopic Incision (WOLI) procedure that solely relies on mechanical manipulation in vivo have shown promising results in improving follicle growth and fertility in women with POR and POI. Advances in the use of mechanical procedures have brought exciting opportunities to improve fertility outcomes in aging women with POR or POI. While the lack of a comprehensive understanding of the molecular mechanisms that lead to fertility decline in aging women remains a major challenge for further improvement of mechanical-manipulation-based approaches, recent progress has provided a better view of how these procedures promote folliculogenesis in the fibrotic and avascular aging ovaries. In this review, we first provide a brief overview of the potential mechanisms that contribute to ovarian aging in POI and POR patients, followed by a discussion of measures that aim to improve ovarian folliculogenesis in aging women. At last, we discuss the likely mechanisms that contribute to the outcomes of IVA and WOLI procedures and potential future directions.

Gastruloid-derived primordial germ cell-like cells develop dynamically within integrated tissues

Primordial germ cells (PGCs) are the early embryonic precursors of gametes - sperm and egg cells. PGC-like cells (PGCLCs) can currently be derived in vitro from pluripotent cells exposed to signalling cocktails and aggregated into large embryonic bodies, but these do not recapitulate the native embryonic environment during PGC formation. Here, we show that mouse gastruloids, a three-dimensional in vitro model of gastrulation, contain a population of gastruloid-derived PGCLCs (Gld-PGCLCs) that resemble early PGCs in vivo. Importantly, the conserved organisation of mouse gastruloids leads to coordinated spatial and temporal localisation of Gld-PGCLCs relative to surrounding somatic cells, even in the absence of specific exogenous PGC-specific signalling or extra-embryonic tissues. In gastruloids, self-organised interactions between cells and tissues, including the endodermal epithelium, enables the specification and subsequent maturation of a pool of Gld-PGCLCs. As such, mouse gastruloids represent a new source of PGCLCs in vitro and, owing to their inherent co-development, serve as a novel model to study the dynamics of PGC development within integrated tissue environments.

In vitro oogenesis from murine premeiotic germ cells using a new three-dimensional culture system

A faithful reconstitution of the complete process of oogenesis in vitro is helpful for understanding the molecular mechanisms, genetics, and epigenetic changes related to gametogenesis; it can also be useful for clinical drug screening, disease research, and regenerative medicine. To this end, given the consensus that murine female germ cells initiate meiosis at E13.5, substantial works have reported the successful generation of fertile oocytes using E12.5 female gonads as starting materials. Nevertheless, our data demonstrated that murine germ cells at E12.5 have heterogeneously initiated a meiotic transcriptional program based on a measurement of pre-mRNAs (unspliced) and mature mRNAs (spliced) at a single-cell level. Therefore, to establish a platform that faithfully recapitulates the entire process in vitro (from premeiotic murine germ cells to fully developed oocytes), we here report a novel three-dimensional organoid culture (3-DOC) system, which successfully induced fully developed oocytes from E11.5 premeiotic female germ cells (oogonia). Compared with 2D culture and other 3D culture methods, this new culture system is more cost-effective and can create high-quality oocytes similar to in vivo oocytes. In summary, our new culture platform provides an experimental model for future research in regenerative medicine and reproductive biology.

From in vivo to in vitro: exploring the key molecular and cellular aspects of human female gametogenesis

Human oogenesis is a highly complex and not yet fully understood process due to ethical and technological barriers that limit studies in the field. In this context, replicating female gametogenesis in vitro would not only provide a solution for some infertility problems, but also be an excellent study model to better understand the biological mechanisms that determine the formation of the female germline. In this review, we explore the main cellular and molecular aspects involved in human oogenesis and folliculogenesis in vivo, from the specification of primordial germ cells (PGCs) to the formation of the mature oocyte. We also sought to describe the important bidirectional relationship between the germ cell and the follicular somatic cells. Finally, we address the main advances and different methodologies used in the search for obtaining cells of the female germline in vitro.

Prospects for fertility preservation: the ovarian organ function reconstruction techniques for oogenesis, growth and maturation in vitro

Today, fertility preservation is receiving more attention than ever. Cryopreservation, which preserves ovarian tissue to preserve fertility in young women and reduce the risk of infertility, is currently the most widely practiced. Transplantation, however, is less feasible for women with blood-borne leukemia or cancers with a high risk of ovarian metastasis because of the risk of cancer recurrence. In addition to cryopreservation and re-implantation of embryos, in vitro ovarian organ reconstruction techniques have been considered as an alternative strategy for fertility preservation. In vitro culture of oocytes in vitro Culture, female germ cells induction from pluripotent stem cells (PSC) in vitro, artificial ovary construction, and ovaria-related organoids construction have provided new solutions for fertility preservation, which will therefore maximize the potential for all patients undergoing fertility preservation. In this review, we discussed and thought about the latest ovarian organ function reconstruction techniques in vitro to provide new ideas for future ovarian disease research and fertility preservation of patients with cancer and premature ovarian failure.

State of the art in assisted reproductive technologies for patients with advanced maternal age

According to the World Health Organization, the female reproductive age lasts up to 49 years, but problems with the realization of women's reproductive rights may arise much earlier. Significant numbers of factors affect the state of reproductive health: socioeconomic, ecological, lifestyle features, the level of medical literacy, and the state of the organization and medical care quality. Among the reasons for fertility decline in advanced reproductive age are the loss of cellular receptors for gonadotropins, an increase in the threshold of sensitivity of the hypothalamic-pituitary system to the action of hormones and their metabolites, and many others. Furthermore, negative changes accumulate in the oocyte genome, reducing the possibility of fertilization, normal development and implantation of the embryo and healthy offspring birth. Another theory of ageing causing changes in oocytes is the mitochondrial free radical theory of ageing. Taking into account all these age-related changes in gametogenesis, this review considers modern technologies aimed at the preservation and realization of female fertility. Among the existing approaches, two main ones can be distinguished: methods allowing the preservation of reproductive cells at a younger age using ART intervention and cryobanking, as well as methods aimed at improving the basic functional state of advanced-age women's oocytes and embryos.

Relationship between Amino Acid Metabolism and Bovine In Vitro Follicle Activation and Growth

The amino acid metabolism of bovine follicles during in vitro growth (IVG) was evaluated to identify potential indicators of health during culture. The bovine ovarian cortex was sliced, prepared as strips, and cultured for 6 days. Tissue samples were examined histologically before and after 6 days of culture, and the degree of follicle activation was classified as either high or low based on the number of growing secondary follicles present (high: 7~11; low: 0~1). In a separate experiment, secondary follicles (diameter range: 100~200 μm) were manually isolated and cultured, and their growth was monitored for 6 days. Cultured follicles were classified as growth or degenerate based on diameter change during culture (growth: +60.5~74.1 μm; degenerate: -28~15.2 μm). Free amino acids and their metabolites were measured in the spent culture medium from each group. In cultured ovarian cortical strips, the concentration of α-aminoadipic acid was significantly higher in the low activation group than in the high group (p < 0.05), while those of methionine, lysine, and arginine were higher in the high activation group. In cultured isolated secondary follicles, concentrations of methionine, tyrosine, histidine, and hydroxyproline were higher in the degenerate group (p ≤ 0.05). In conclusion, amino acid metabolism has the potential to serve as an indicator of primordial follicle activation and subsequent growth rate during bovine IVG.

Rescue of oocytes recovered from postmortem mouse ovaries

It is well known that the survivability of gametes of postmortem carcass was decreased as time passes after death. In this study, it was examined whether cytoplasmic replacement rescues the survivability of germinal vesicle stage (GV) oocytes of postmortem carcass in the mouse. Reactive oxygen species (ROS) levels and mitochondria numbers in GV oocytes of the dead mice stored at 4 degrees were significantly impaired after 44 h postmortem compared to the control (0 h). However, when kayoplasts of GV oocytes of postmortem carcass was transferred to recipient ooplasts (GV transfer), proportion of in vitro maturation (IVM), normal spindle morphology, in vitro and in vivo developmental ability after in vitro fertilization (IVF) of reconstituted oocytes was improved. Moreover, secondary follicle oocytes of postmortem carcass were developed, matured and fertilized in vitro and developed to go to term, when GV transfer was conducted at the GV phase. Thus, transfer of GV karyoplast recovered from postmortem carcass, which viability was decreased, into fresh GV recipient ooplasm, rescues survivability of reconstituted oocytes. It suggested the effective use of oocytes of dead animals in the mouse and this achievement must apply to other rare animal species, especially animals under control by human.

Cycloastragenol activation of telomerase improves β-Klotho protein level and attenuates age-related malfunctioning in ovarian tissues

Age-related deterioration in the reproductive capacity of women is directly related to the poor developmental potential of ovarian follicles. Although telomerase plays a key role in female fertility, TERT-targeting therapeutic strategies for age-related female infertility have yet to be investigated. This study elucidated the effect of Telomerase activation on mice ovaries and more specifically on Klb (β-Klotho) gene expression, which is linked to ageing, female hormonal regulation, and cyclicity. The homology-based 3D model of hTERT was used to predict its binding mode of Cycloastragenol (CAG) using molecular docking and molecular dynamics simulations. Based on docking score, simulation behavior, and interaction with hTERT residues it was observed that CAG could bind with the hTERT model. CAG treatment to primary cultured mouse granulosa cells and activation of telomerase was examined via telomerase activity assay (Mouse TE (telomerase) ELISA Kit) and telomere length by quantitative fluorescence in situ hybridization. CAG mediated telomerase also significantly improved β-Klotho protein level in the aged granulosa cells. To demonstrate that β-Klotho is telomerase dependent, the TERT was knocked down via siRNA in granulosa cells and protein level of β-Klotho was examined. Furthermore, CAG-mediated telomerase activation significantly enhanced the level of Klb and recovered ovarian follicles in the D-galactose (D-gal)-induced ovarian ageing mouse model. Moreover, Doxorubicin-induced ovarian damage, which changes ovarian hormones, and inhibit follicular growth was successfully neutralized by CAG activated telomerase and its recovery of β-Klotho level. In conclusion, TERT dependent β-Klotho regulation in ovarian tissues is one of the mechanisms, which can overcome female infertility.

Oocyte aging in comparison to stem cells in mice

To maintain homeostasis, many tissues contain stem cells that can self-renew and differentiate. Based on these functions, stem cells can reconstitute the tissue even after injury. In reproductive organs, testes have spermatogonial stem cells that generate sperm in men throughout their lifetime. However, in the ovary, oocytes enter meiosis at the embryonic stage and maintain sustainable oogenesis in the absence of stem cells. After birth, oocytes are maintained in a dormant state in the primordial follicle, which is the most premature follicle in the ovary, and some are activated to form mature oocytes. Thus, regulation of dormancy and activation of primordial follicles is critical for a sustainable ovulatory cycle and is directly related to the female reproductive cycle. However, oocyte storage is insufficient to maintain a lifelong ovulation cycle. Therefore, the ovary is one of the earliest organs to be involved in aging. Although stem cells are capable of proliferation, they typically exhibit slow cycling or dormancy. Therefore, there are some supposed similarities with oocytes in primordial follicles, not only in their steady state but also during aging. This review aims to summarise the sustainability of oogenesis and aging phenotypes compared to tissue stem cells. Finally, it focuses on the recent breakthroughs in vitro culture and discusses future prospects.

Reconstitution of reproductive organ system that produces functional oocytes

Reproductive organs have unique developmental and functional properties that enable them to manage both germ cell development and the endocrine system in a sex-dependent manner. Proper reconstitution of the reproductive organs, therefore, will contribute to a deeper understanding of the mechanisms underlying germ cell development and sex-determination. However, reproductive organs have not yet been systematically reconstituted from pluripotent stem cells. This is largely due to technical problems in the reconstitution of the germ cell and somatic cell lineages, which have very different developmental trajectories. Accordingly, faithful construction of reproductive organoids requires that the reconstitution and evaluation of these two different cell lineages be performed separately. Here, we update the state-of-the-art in the reconstitution of reproductive organoids that produce functional oocytes.

Stem Cell-Based Therapeutic Strategies for Premature Ovarian Insufficiency and Infertility: A Focus on Aging

Reproductive aging is on the rise globally and inseparable from the entire aging process. An extreme form of reproductive aging is premature ovarian insufficiency (POI), which to date has mostly been of idiopathic etiology, thus hampering further clinical applications and associated with enormous socioeconomic and personal costs. In the field of reproduction, the important functional role of inflammation-induced ovarian deterioration and therapeutic strategies to prevent ovarian aging and increase its function are current research hotspots. This review discusses the general pathophysiology and relative causes of POI and comprehensively describes the association between the aging features of POI and infertility. Next, various preclinical studies of stem cell therapies with potential for POI treatment and their molecular mechanisms are described, with particular emphasis on the use of human induced pluripotent stem cell (hiPSC) technology in the current scenario. Finally, the progress made in the development of hiPSC technology as a POI research tool for engineering more mature and functional organoids suitable as an alternative therapy to restore infertility provides new insights into therapeutic vulnerability, and perspectives on this exciting research on stem cells and the derived exosomes towards more effective POI diagnosis and treatment are also discussed.

Transcription-replication conflicts in primordial germ cells necessitate the Fanconi anemia pathway to safeguard genome stability

Germ cells are capable of preserving their genetic information with high fidelity. We report that rapidly dividing mouse primordial germ cells (PGCs) are faced with high levels of endogenous replication stress due to frequent occurrence of transcription–replication conflicts (TRCs). Thus, PGCs have an increased requirement for the replication-coupled Fanconi anemia (FA) pathway to counteract TRC-induced replication stress, enabling their rapid proliferation to establish a sufficient reproductive reserve. This work provides insights into the unique genome feature of developing PGCs and helps to explain the reproductive defects in FA individuals.

Preserving a high degree of genome integrity and stability in germ cells is of utmost importance for reproduction and species propagation. However, the regulatory mechanisms of maintaining genome stability in the developing primordial germ cells (PGCs), in which rapid proliferation is coupled with global hypertranscription, remain largely unknown. Here, we find that mouse PGCs encounter a constitutively high frequency of transcription–replication conflicts (TRCs), which lead to R-loop accumulation and impose endogenous replication stress on PGCs. We further demonstrate that the Fanconi anemia (FA) pathway is activated by TRCs and has a central role in the coordination between replication and transcription in the rapidly proliferating PGCs, as disabling the FA pathway leads to TRC and R-loop accumulation, replication fork destabilization, increased DNA damage, dramatic loss of mitotically dividing mouse PGCs, and consequent sterility of both sexes. Overall, our findings uncover the unique source and resolving mechanism of endogenous replication stress during PGC proliferation, provide a biological explanation for reproductive defects in individuals with FA, and improve our understanding of the monitoring strategies for genome stability during germ cell development.

Ex vivo reconstitution of fetal oocyte development in humans and cynomolgus monkeys

In vitro oogenesis is key to elucidating the mechanism of human female germ-cell development and its anomalies. Accordingly, pluripotent stem cells have been induced into primordial germ cell-like cells and into oogonia with epigenetic reprogramming, yet further reconstitutions remain a challenge. Here, we demonstrate ex vivo reconstitution of fetal oocyte development in both humans and cynomolgus monkeys (Macaca fascicularis). With an optimized culture of fetal ovary reaggregates over three months, human and monkey oogonia enter and complete the first meiotic prophase to differentiate into diplotene oocytes that form primordial follicles, the source for oogenesis in adults. The cytological and transcriptomic progressions of fetal oocyte development in vitro closely recapitulate those in vivo. A comparison of single-cell transcriptomes among humans, monkeys, and mice unravels primate-specific and conserved programs driving fetal oocyte development, the former including a distinct transcriptomic transformation upon oogonia-to-oocyte transition and the latter including two active X chromosomes with little X-chromosome upregulation. Our study provides a critical step forward for realizing human in vitro oogenesis and uncovers salient characteristics of fetal oocyte development in primates.

Early Gonadal Development and Sex Determination in Mammal

Sex determination is crucial for the transmission of genetic information through generations. In mammal, this process is primarily regulated by an antagonistic network of sex-related genes beginning in embryonic development and continuing throughout life. Nonetheless, abnormal expression of these sex-related genes will lead to reproductive organ and germline abnormalities, resulting in disorders of sex development (DSD) and infertility. On the other hand, it is possible to predetermine the sex of animal offspring by artificially regulating sex-related gene expression, a recent research hotspot. In this paper, we reviewed recent research that has improved our understanding of the mechanisms underlying the development of the gonad and primordial germ cells (PGCs), progenitors of the germline, to provide new directions for the treatment of DSD and infertility, both of which involve manipulating the sex ratio of livestock offspring.

Temperature sensitivity of DNA double-strand break repair underpins heat-induced meiotic failure in mouse spermatogenesis

Mammalian spermatogenesis is a heat-vulnerable process that occurs at low temperatures, and elevated testicular temperatures cause male infertility. However, the current reliance on in vivo assays limits their potential to detail temperature dependence and destructive processes. Using ex vivo cultures of mouse testis explants at different controlled temperatures, we found that spermatogenesis failed at multiple steps, showing sharp temperature dependencies. At 38 °C (body core temperature), meiotic prophase I is damaged, showing increased DNA double-strand breaks (DSBs) and compromised DSB repair. Such damaged spermatocytes cause asynapsis between homologous chromosomes and are eliminated by apoptosis at the meiotic checkpoint. At 37 °C, some spermatocytes survive to the late pachytene stage, retaining high levels of unrepaired DSBs but do not complete meiosis with compromised crossover formation. These findings provide insight into the mechanisms and significance of heat vulnerability in mammalian spermatogenesis.

Understanding the Underlying Molecular Mechanisms of Meiotic Arrest during In Vitro Spermatogenesis in Rat Prepubertal Testicular Tissue

In vitro spermatogenesis appears to be a promising approach to restore the fertility of childhood cancer survivors. The rat model has proven to be challenging, since germ cell maturation is arrested in organotypic cultures. Here, we report that, despite a meiotic entry, abnormal synaptonemal complexes were found in spermatocytes, and in vitro matured rat prepubertal testicular tissues displayed an immature phenotype. RNA-sequencing analyses highlighted up to 600 differentially expressed genes between in vitro and in vivo conditions, including genes involved in blood-testis barrier (BTB) formation and steroidogenesis. BTB integrity, the expression of two steroidogenic enzymes, and androgen receptors were indeed altered in vitro. Moreover, most of the top 10 predicted upstream regulators of deregulated genes were involved in inflammatory processes or immune cell recruitment. However, none of the three anti-inflammatory molecules tested in this study promoted meiotic progression. By analysing for the first time in vitro matured rat prepubertal testicular tissues at the molecular level, we uncovered the deregulation of several genes and revealed that defective BTB function, altered steroidogenic pathway, and probably inflammation, could be at the origin of meiotic arrest.

Artificial Oocyte: Development and Potential Application

Millions of people around the world suffer from infertility, with the number of infertile couples and individuals increasing every year. Assisted reproductive technologies (ART) have been widely developed in recent years; however, some patients are unable to benefit from these technologies due to their lack of functional germ cells. Therefore, the development of alternative methods seems necessary. One of these methods is to create artificial oocytes. Oocytes can be generated in vitro from the ovary, fetal gonad, germline stem cells (GSCs), ovarian stem cells, or pluripotent stem cells (PSCs). This approach has raised new hopes in both basic research and medical applications. In this article, we looked at the principle of oocyte development, the landmark studies that enhanced our understanding of the cellular and molecular mechanisms that govern oogenesis in vivo, as well as the mechanisms underlying in vitro generation of functional oocytes from different sources of mouse and human stem cells. In addition, we introduced next-generation ART using somatic cells with artificial oocytes. Finally, we provided an overview of the reproductive application of in vitro oogenesis and its use in human fertility.

Therapeutic Potential of In Vitro-Derived Oocytes for the Restoration and Treatment of Female Fertility

Considerable progress has been made with the development of culture systems for the in vitro growth and maturation (IVGM) of oocytes from the earliest-staged primordial follicles and from the more advanced secondary follicles in rodents, ruminants, nonhuman primates, and humans. Successful oocyte production in vitro depends on the development of a dynamic culture strategy that replicates the follicular microenvironment required for oocyte activation and to support oocyte growth and maturation in vivo while enabling the coordinated and timely acquisition of oocyte developmental competence. Significant heterogeneity exists between the culture protocols used for different stages of follicle development and for different species. To date, the fertile potential of IVGM oocytes derived from primordial follicles has been realized only in mice. Although many technical challenges remain, significant advances have been made, and there is an increasing consensus that complete IVGM will require a dynamic, multiphase culture approach. The production of healthy offspring from in vitro-produced oocytes in a secondary large animal species is a vital next step before IVGM can be tested for therapeutic use in humans.

Generation of Miniaturized Ovaries by In Vitro Culture from Mouse Gonads

The incidence of infertility among individuals of reproductive age has been growing due to genetic and environmental factors, and considerable research efforts are focused on solving this issue. Ovarian development is an overly complex process in the body, involving the interaction between primordial germ cells and gonad somatic cells. However, follicles located in the center of the in vitro ovary are poorly formed owing to ovarian complexity, nutrient deficiency, and signaling deficiency. In the present study, we optimized methods for dissociating gonads and culture conditions for the in vitro generation of miniaturized ovaries. The gonads from embryos were dissociated into cell masses and cultured on a Transwell-COL membrane for 3–5 weeks. Approximately 12 follicles were present per in vitro ovary. We observed that miniaturized ovaries successfully matured to MII oocytes in vitro from 150 to 100 µm gonad masses. This method will be useful for investigating follicle development and oocyte production.

Comparative analysis of vertebrates reveals that mouse primordial oocytes do not contain a Balbiani body

Oocytes spend the majority of their lifetime in a primordial state. The cellular and molecular biology of primordial oocytes is largely unexplored; yet, studying these is necessary to understand the mechanisms through which oocytes maintain cellular fitness for decades, and why they eventually fail with age. Here, we develop enabling methods for live-imaging based comparative characterization of Xenopus, mouse and human primordial oocytes. We show that primordial oocytes in all three vertebrate species contain active mitochondria, Golgi apparatus and lysosomes. We further demonstrate that human and Xenopus oocytes have a Balbiani body characterized by a dense accumulation of mitochondria in their cytoplasm. However, despite previous reports, we did not find a Balbiani body in mouse oocytes. Instead, we demonstrate what was previously used as a marker for the Balbiani body in mouse primordial oocytes is in fact a ring-shaped Golgi apparatus that is not functionally associated with oocyte dormancy. Our work provides the first insights into the organisation of the cytoplasm in mammalian primordial oocytes, and clarifies relative advantages and limitations of choosing different model organisms for studying oocyte dormancy.

Insights into in vivo follicle formation: a review of in vitro systems

In vitro systems capable of reconstituting the process of mouse oogenesis are now being established to help develop further understanding of the mechanisms underlying oocyte/follicle development and differentiation. These systems could also help increase the production of useful livestock or genetically modified animals, and aid in identifying the causes of infertility in humans. Recently, we revealed, using an in vitro system for recapitulating oogenesis, that the activation of the estrogen signaling pathway induces abnormal follicle formation, that blocking estrogen-induced expression of anti-Müllerian hormone is crucial for normal follicle formation, and that the production of α-fetoprotein in fetal liver tissue is involved in normal in vivo follicle formation. In mouse fetuses, follicle formation is not carried out by factors within the ovaries but is instead orchestrated by distal endocrine factors. This review outlines findings from genetics, endocrinology, and in vitro studies regarding the factors that can affect the formation of primordial follicles in mammals.

In vitro production of viable eggs from isolated mouse primary follicles by successive culture

To produce viable eggs from single primary follicles in vitro, primary follicles containing oocytes (average 39.0 ± 0.2 µm in diameter) were isolated from the ovaries of 1-week-old mice, and cultured in combination with culture membranes for the first 8 days up to the secondary follicle stage, followed by the next 12 days to the later stages. After culture with a combination of first and second culture membranes using high and low adhesion characteristics, the average oocyte diameters of the surviving follicles increased by almost two-fold in all four groups. Further, the oocyte maturation rate was the highest (74.1%) in the culture group with low adhesion with collagenase and high adhesion. In this culture group, when the O₂ concentration was changed from 20% in the first culture to 5% in the second culture, the cleavage rate increased to 47.5%, which was comparable to the level of the in vivo control (34.6%). Finally, 39 embryos at the 2- to 8-cell stages were transferred into the oviducts of three pseudopregnant females, and eight live pups (20.5%) were obtained. Of the eight pups, six survived for at least six months and were fertile. The present study shows successive in vitro cultures of single isolated primary follicles for the production of viable eggs. We believe that this culture system, with a combination of culture membranes under controlled O₂ conditions, is applicable to other mammalian species, including humans.

Optimal conditions for mouse follicle culture

Mammalian ovaries contain a large number of immature follicles. Follicular culture can contribute to the production of fertile oocytes from latent immature follicles, providing a useful tool for exploring the developmental competencies and related factors that oocytes acquire during growth. However, the potential of oocytes produced by follicular culture is limited. Herein, the optimal follicular culture conditions for the addition of polyvinylpyrrolidone to the medium and oxygen concentration were investigated. Polyvinylpyrrolidone with a high molecular weight (≥ 360,000) and a 7% oxygen concentration were found to increase the blastocyst formation rate by more than 20% compared with conventional culture conditions. Although the developmental ability of oocytes produced by follicular culture remained inferior to that of in vivo-derived oocytes, these findings may pave the way for enhanced production of fertile oocytes in vitro and for studying the process of full developmental potency acquisition by oocytes.

Follicle Rescue From Prepubertal Ovaries After Recent Treatment With Cyclophosphamide-An Experimental Culture System Using Mice to Achieve Mature Oocytes for Fertility Preservation

Ovarian tissue cryopreservation is the only feasible method for fertility preservation in prepubertal girls that will undergo gonadotoxic chemotherapy. To date, the only clinical use of cryopreserved tissue is by a later tissue retransplantation to the patient. Clinical challenges in fertility preservation of very young patients with cancer include time constraints that do not allow to retrieve the tissue for cryopreservation before starting chemotherapy and the preclusion of future ovarian tissue transplantation due to the risk of reintroduction of malignant cells in patients with systemic diseases. To overcome these two challenges, we investigated using an experimental model the feasibility of retrieving secondary follicles from ovaries of prepubertal mice after cyclophosphamide (CPA) treatment in increasing doses of 50, 75, and 100 mg/kg. The follicles were thereafter cultured and matured in vitro. The main outcomes included the efficiency of the method in terms of obtained matured oocytes and the safety of these potentially fertility preservative procedures in terms of analyses of oocyte competence regarding normality of the spindle and chromosome configurations. Our findings demonstrated that it was feasible to isolate and culture secondary follicles and to obtain mature oocytes from prepubertal mice ovaries recently treated with CPA. The efficiency of this method was highly demonstrated in the 100 mg/kg CPA group, with near 90% follicle survival rate after 12 days' culture, similarly to control. Around 80% of the follicles met the criteria to put into maturation, and more than 40% of them achieved metaphase II, with normal spindle and chromosome configurations observed. Suboptimal results were obtained in the 50 and 75 mg/kg CPA groups. These paradoxical findings towards CPA dose might probably reflect a more difficult selection of damaged growing follicles from ovaries recently treated with lower doses of CPA and a hampered ability to identify and discard those with reduced viability for the culture.

Mammalian in vitro gametogenesis

[Figure: see text].

Germ Cell Derivation from Pluripotent Stem Cells for Understanding In Vitro Gametogenesis

Assisted reproductive technologies (ARTs) have developed considerably in recent years; however, they cannot rectify germ cell aplasia, such as non-obstructive azoospermia (NOA) and oocyte maturation failure syndrome. In vitro gametogenesis is a promising technology to overcome infertility, particularly germ cell aplasia. Early germ cells, such as primordial germ cells, can be relatively easily derived from pluripotent stem cells (PSCs); however, further progression to post-meiotic germ cells usually requires a gonadal niche and signals from gonadal somatic cells. Here, we review the recent advances in in vitro male and female germ cell derivation from PSCs and discuss how this technique is used to understand the biological mechanism of gamete development and gain insight into its application in infertility.