Initial characterisation of adult human ovarian cell populations isolated by DDX4 expression and aldehyde dehydrogenase activity

Zuogui Pills alleviate cyclophosphamide-induced ovarian aging by reducing oxidative stress and restoring the stemness of oogonial stem cells through the Nrf2/HO-1 signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Zuogui Pill (ZGP) is a traditional herbal formula of Chinese Medicine with a long history of use in alleviating ovarian aging.

AIM OF THE STUDY

To examine the impact of ZGP on oxidative stress and the stemness of oogonial stem cells (OSCs) in cyclophosphamide (CTX)-induced ovarian aging, as well as its molecular mechanisms involving the nuclear factor erythroid 2-related factor 2 (Nrf2, NFE2L2)/heme oxygenase-1 (HO-1, Hmox1) pathway.

MATERIALS AND METHODS

Female Sprague-Dawley (SD) rats were randomly divided into seven groups: control, model (CTX), estradiol valerate (EV, 0.103 mg/kg), ZGP-L (low dose Zuogui Pill, 1.851 g/kg), ZGP-H (high dose Zuogui Pill, 3.702 g/kg), ML385 (30 mg/kg), and ML385+ZGP-L. After CTX modeling, the EV, ZGP-L, ZGP-H, and ML385+ZGP-L groups were treated by gavage for 8 weeks, while the ML385 and ML385+ZGP-L groups were administered the Nrf2 antagonist ML385 twice a week. OSCs were isolated after modeling and then treated with drug serum containing 10% ZGP or 10 μM ML385. The general conditions of the rats, including body weight, ovarian weight/body weight ratio, and estrous cycle, were observed. Ovarian ultrastructure, follicle and corpus luteum counts were assessed via hematoxylin and eosin (H&E) staining. Serum hormone levels were measured using enzyme-linked immunosorbent assay (ELISA). Nrf2/HO-1 pathway, stem cell, germ cell, and cell cycle biomarkers were analyzed by qPCR and Western blot. Cell viability was assessed by cell counting kit-8 (CCK-8) assay. Oxidative stress biomarkers were evaluated using flow cytometry and assay kits. Immunofluorescence was employed to detect and locate OSCs in the ovary, quantify the average fluorescence intensity, and identify OSCs.

RESULTS

After ZGP treatment, rats with CTX-induced ovarian aging exhibited improved general condition, increased body weight, higher total ovarian weight to body weight ratio, and a restoration of the estrous cycle similar to the control group. Serum levels of estradiol (E2) and follicle stimulating hormone (FSH), two sex hormones, were also improved. Ovarian ultrastructure and follicle count at all stages showed improvement. Moreover, the viability and proliferation capacity of OSCs were enhanced following ZGP intervention. The Nrf2/HO-1 pathway was found to be down-regulated in CTX-induced aging ovarian OSCs. However, ZGP reversed this effect by activating the expression of Nrf2, HO-1, and NAD(P)H oxidoreductase 1 (NQO1), increasing the activity of antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GSH-PX), and reducing the accumulation of malonaldehyde (MDA) and reactive oxygen species (ROS), thus restoring resistance to oxidative stress. Additionally, ZGP improved the cell cycle of OSCs, up-regulated the expression of Cyclin D1 and Cyclin E1, restored cell stemness, promoted proliferation, enhanced the expression of cell stemness markers octamer-binding transcription factor 4 (Oct4) and mouse VASA homolog (MVH), and down-regulated the expression of P21, thereby inhibiting apoptosis. The therapeutic effects of ZGP against oxidative stress and restoration of cell stemness were attenuated following inhibition of the Nrf2 signaling pathway using ML385.

CONCLUSIONS

ZGP protected against CTX-induced ovarian aging by restoring normal ovarian function, alleviating oxidative stress in aging OSCs, promoting OSCs proliferation, and restoring their stemness in rats, possibly through regulating the Nrf2/HO-1 pathway.

Ovarian microenvironment: challenges and opportunities in protecting against chemotherapy-associated ovarian damage

BACKGROUND

Chemotherapy-associated ovarian damage (CAOD) is one of the most feared short- and long-term side effects of anticancer treatment in premenopausal women. Accumulating detailed data show that different chemotherapy regimens can lead to disturbance of ovarian hormone levels, reduced or lost fertility, and an increased risk of early menopause. Previous studies have often focused on the direct effects of chemotherapeutic drugs on ovarian follicles, such as direct DNA damage-mediated apoptotic death and primordial follicle burnout. Emerging evidence has revealed an imbalance in the ovarian microenvironment during chemotherapy. The ovarian microenvironment provides nutritional support and transportation of signals that stimulate the growth and development of follicles, ovulation, and corpus luteum formation. The close interaction between the ovarian microenvironment and follicles can determine ovarian function. Therefore, designing novel and precise strategies to manipulate the ovarian microenvironment may be a new strategy to protect ovarian function during chemotherapy.

OBJECTIVE AND RATIONALE

This review details the changes that occur in the ovarian microenvironment during chemotherapy and emphasizes the importance of developing new therapeutics that protect ovarian function by targeting the ovarian microenvironment during chemotherapy.

SEARCH METHODS

A comprehensive review of the literature was performed by searching PubMed up to April 2024. Search terms included 'ovarian microenvironment' (ovarian extracellular matrix, ovarian stromal cells, ovarian interstitial, ovarian blood vessels, ovarian lymphatic vessels, ovarian macrophages, ovarian lymphocytes, ovarian immune cytokines, ovarian oxidative stress, ovarian reactive oxygen species, ovarian senescence cells, ovarian senescence-associated secretory phenotypes, ovarian oogonial stem cells, ovarian stem cells), terms related to ovarian function (reproductive health, fertility, infertility, fecundity, ovarian reserve, ovarian function, menopause, decreased ovarian reserve, premature ovarian insufficiency/failure), and terms related to chemotherapy (cyclophosphamide, lfosfamide, chlormethine, chlorambucil, busulfan, melphalan, procarbazine, cisplatin, doxorubicin, carboplatin, taxane, paclitaxel, docetaxel, 5-fluorouraci, vincristine, methotrexate, dactinomycin, bleomycin, mercaptopurine).

OUTCOMES

The ovarian microenvironment shows great changes during chemotherapy, inducing extracellular matrix deposition and stromal fibrosis, angiogenesis disorders, immune microenvironment disturbance, oxidative stress imbalances, ovarian stem cell exhaustion, and cell senescence, thereby lowering the quantity and quality of ovarian follicles. Several methods targeting the ovarian microenvironment have been adopted to prevent and treat CAOD, such as stem cell therapy and the use of free radical scavengers, senolytherapies, immunomodulators, and proangiogenic factors.

WIDER IMPLICATIONS

Ovarian function is determined by its 'seeds' (follicles) and 'soil' (ovarian microenvironment). The ovarian microenvironment has been reported to play a vital role in CAOD and targeting the ovarian microenvironment may present potential therapeutic approaches for CAOD. However, the relation between the ovarian microenvironment, its regulatory networks, and CAOD needs to be further studied. A better understanding of these issues could be helpful in explaining the pathogenesis of CAOD and creating innovative strategies for counteracting the effects exerted on ovarian function. Our aim is that this narrative review of CAOD will stimulate more research in this important field.

REGISTRATION NUMBER

Not applicable.

Reconstruction of ovarian follicular-like structure by recellularization of a cell-free human ovarian scaffold with mouse fetal ovarian cells

The present study assessed the supportive roles of the decellularized human ovarian tissue in homing of mouse fetal ovarian cells into the scaffold as well as the formation of the follicular-like structure. The human ovarian cortical tissues were decellularized by three freeze-thaw cycles and then, treated with Triton X-100 for 15 h and 0.5% sodium dodecyl sulfate for 72 h. After isolation and preparation of mouse fetal ovarian cells (19 dpc) they were seeded into the decellularized scaffolds and cultured for 7 days, then using a light microscope, laser confocal scanning microscope, and scanning electron microscope these scaffolds were studied. Analysis of gene expression related to oocyte and follicular cells such as Ddx4, Nobox, Gdf9, and Connexin37 was assessed by real-time RT-PCR and the DDX4 and GDF9 proteins were detected by immunohistochemistry. The result showed that the human ovarian tissue was decellularized properly and the tissue elements and integrity were well preserved. After 7 days of in vitro culture, the fetal ovarian cells attached and penetrated into different sites and depths of the scaffold. The formed organoid within the scaffold showed large round, small polyhedral, and elongated spindle cells similar to the follicle structure. The molecular analysis and immunohistochemistry were confirmed an increase in the expression of genes and proteins related to oocyte and follicular cells in these reconstructed structures. In conclusion, the recellularization of human ovarian scaffolds by mouse fetal ovarian cells could support the follicular-like structure formation and it provides an in vitro model for follicle reconstitution and offers an alternative approach for clinical usage.

Making a good egg: human oocyte health, aging, and in vitro development

Mammalian eggs (oocytes) are formed during fetal life and establish associations with somatic cells to form primordial follicles that create a store of germ cells (the primordial pool). The size of this pool is influenced by key events during the formation of germ cells and by factors that influence the subsequent activation of follicle growth. These regulatory pathways must ensure that the reserve of oocytes within primordial follicles in humans lasts for up to 50 years, yet only approximately 0.1% will ever be ovulated with the rest undergoing degeneration. This review outlines the mechanisms and regulatory pathways that govern the processes of oocyte and follicle formation and later growth, within the ovarian stroma, through to ovulation with particular reference to human oocytes/follicles. In addition, the effects of aging on female reproductive capacity through changes in oocyte number and quality are emphasized, with both the cellular mechanisms and clinical implications discussed. Finally, the details of current developments in culture systems that support all stages of follicle growth to generate mature oocytes in vitro and emerging prospects for making new oocytes from stem cells are outlined.

Minimal Infiltrative Disease Identification in Cryopreserved Ovarian Tissue of Girls with Cancer for Future Use: A Systematic Review

BACKGROUND

Ovarian tissue cryopreservation and transplantation are the only available fertility techniques for prepubertal girls with cancer. Though autotransplantation carries a risk of reintroducing malignant cells, it can be avoided by identifying minimal infiltrative disease (MID) within ovarian tissue.

METHODS

A broad search for peer-reviewed articles in the PubMed database was conducted in accordance with PRISMA guidelines up to March 2023. Search terms included 'minimal residual disease', 'cryopreservation', 'ovarian', 'cancer' and synonyms.

RESULTS

Out of 542 identified records, 17 were included. Ovarian tissues of at least 115 girls were evaluated and categorized as: hematological malignancies (n = 56; 48.7%), solid tumors (n = 42; 36.5%) and tumors of the central nervous system (n = 17; 14.8%). In ovarian tissue of 25 patients (21.7%), MID was detected using RT-qPCR, FISH or multicolor flow cytometry: 16 of them (64%) being ALL (IgH rearrangements with/without TRG, BCL-ABL1, EA2-PBX1, TEL-AML1 fusion transcripts), 3 (12%) Ewing sarcoma (EWS-FLI1 fusion transcript, EWSR1 rearrangements), 3 (12%) CML (BCR-ABL1 fusion transcript, FLT3) and 3 (12%) AML (leukemia-associated immunophenotypes, BCR-ABL1 fusion transcript) patients.

CONCLUSION

While the majority of malignancies were found to have a low risk of containing malignant cells in ovarian tissue, further studies are needed to ensure safe implementation of future fertility restoration in clinical practice.

Current progress on in vitro differentiation of ovarian follicles from pluripotent stem cells

Mammalian female reproduction requires a functional ovary. Competence of the ovary is determined by the quality of its basic unit-ovarian follicles. A normal follicle consists of an oocyte enclosed within ovarian follicular cells. In humans and mice, the ovarian follicles are formed at the foetal and the early neonatal stage respectively, and their renewal at the adult stage is controversial. Extensive research emerges recently to produce ovarian follicles in-vitro from different species. Previous reports demonstrated the differentiation of mouse and human pluripotent stem cells into germline cells, termed primordial germ cell-like cells (PGCLCs). The germ cell-specific gene expressions and epigenetic features including global DNA demethylation and histone modifications of the pluripotent stem cells-derived PGCLCs were extensively characterized. The PGCLCs hold potential for forming ovarian follicles or organoids upon cocultured with ovarian somatic cells. Intriguingly, the oocytes isolated from the organoids could be fertilized in-vitro. Based on the knowledge of in-vivo derived pre-granulosa cells, the generation of these cells from pluripotent stem cells termed foetal ovarian somatic cell-like cells was also reported recently. Despite successful in-vitro folliculogenesis from pluripotent stem cells, the efficiency remains low, mainly due to the lack of information on the interaction between PGCLCs and pre-granulosa cells. The establishment of in-vitro pluripotent stem cell-based models paves the way for understanding the critical signalling pathways and molecules during folliculogenesis. This article aims to review the developmental events during in-vivo follicular development and discuss the current progress of generation of PGCLCs, pre-granulosa and theca cells in-vitro.

Metformin promotes female germline stem cell proliferation by upregulating Gata-binding protein 2 with histone β-hydroxybutyrylation

BACKGROUND

Metformin as a first-line clinical anti-diabetic agent prolongs the lifespan of model animals and promotes cell proliferation. However, the molecular mechanisms underlying the proliferative phenotype, especially in epigenetics, have rarely been reported. The aim of this study was to investigate the physiological effects of metformin on female germline stem cells (FGSCs) in vivo and in vitro, uncover β-hydroxybutyrylation epigenetic modification roles of metformin and identify the mechanism of histone H2B Lys5 β-hydroxybutyrylation (H2BK5bhb) in Gata-binding protein 2 (Gata2)-mediated proliferation promotion of FGSCs.

METHODS

The physiological effects of metformin were evaluated by intraperitoneal injection and histomorphology. The phenotype and mechanism studies were explored by cell counting, cell viability, cell proliferation assay and protein modification omics, transcriptomics, chromatin immunoprecipitation sequencing in FGSCs in vitro.

RESULTS

We found that metformin treatment increased the number of FGSCs, promoted follicular development in mouse ovaries and enhanced the proliferative activity of FGSCs in vitro. Quantitative omics analysis of protein modifications revealed that H2BK5bhb was increased after metformin treatment of FGSCs. In combination with H2BK5bhb chromatin immunoprecipitation and transcriptome sequencing, we found that Gata2 might be a target gene for metformin to regulate FGSC development. Subsequent experiments showed that Gata2 promoted FGSC proliferation.

CONCLUSION

Our results provide novel mechanistic understanding of metformin in FGSCs by combining histone epigenetics and phenotypic analyses, which highlight the role of the metformin-H2BK5bhb-Gata2 pathway in cell fate determination and regulation.

An evaluation of the effects on the ovaries of hyperbaric oxygen therapy in a rat model of premature ovarian failure created with cyclophosphamide

Objective

To evaluate hyperbaric oxygen therapy (HBO) based on ovarian histology, total antioxidant status (TAS), total oxidant status (TOS), and anti-müllerian hormone (AMH), in the ovarian insufiency (POI) model created with cyclophosphamide (CYP).

Materials and Methods

The rats were separated into 3 groups of the control group (n=6), the CYP group (n=6), and the CYP+HBO group (n=6). The rats in the CYP group and the CYP+HBO group were injected intraperitoneally with 200 mg/kg CYP on day 1, followed by 8 mg/kg/day for 14 days to create POI. From the 15^th day onwards, the rats in the CYP+HBO group were placed in a hyperbaric cabin and exposed to 100% oxygen at 2.4 atm pressure for one h, and were then returned to their cages at the end of the hour.

Results

A statistically significant decrease was determined in the primordial and primary follicle counts in the CYP group compared with the control group (p<0.05). In the CYP+HBO group, a statistically significant increase was determined in the primordial and primary follicle counts (p<0.05). The serum AMH levels were seen to be significantly decreased in the CYP group compared with both the control group and the CYP+HBO groups. The HBO was seen to decrease TOS and increase TAS.

Conclusion

HBO could be an alternative treatment to minimize the effect of ovarian follicle loss caused by CYP, which is used for treating tumors that commonly occur in young females of reproductive age.

Does the ovarian surface epithelium differentiate into primordial follicle and primary follicle precursor structures?

Purpose: The aim of this study is to investigate the differentiation capacity of ovarian surface epithelial cells both in cell culture conditions and in ovarian tissue sections. Materials and Methods: The ovaries of two prepubertal (4 weeks old) female rats were divided into small pieces and explant cell culture was created. Ovarian surface epithelium proliferating together with ovarian stromal cells in mixed cell culture was isolated and reproduced. In addition, ovarian surface epithelium was examined in histological sections of ovarian tissue and images were taken under the microscope. Results: The morphological appearance of the ovarian surface epithelium was found to be cobblestone. In the count performed under phase contrast microscopy, it was observed that 2x106 and 3x106 cells were grown in the culture dishes, respectively. Primordial follicle-like structures were observed in some areas of the petri dishes. On the histological sections, primordial and primary follicle precursor structures were observed on the basement membrane. Conclusion: Showing oocyte markers (Gdf-9, C-Mos, Zpc, Stella) and germ cell markers (Dazl, Vasa, Blimp1, Fragilis) both in cell cultures and in histological sections can give us valuable information in terms of monitoring the differentiation capacity of these cells.

The Continued Absence of Functional Germline Stem Cells in Adult Ovaries

Ovaries are central to development, fertility, and reproduction of women. A particularly interesting feature of ovaries is their accelerated aging compared to other tissues, leading to loss of function far before other organs senesce. The limited pool of ovarian follicles is generated before birth and once exhausted, menopause will inevitably commence around the age of 50 years marking the end of fertility. Yet, there are reports suggesting the presence of germline stem cells and neo-oogenesis in adult human ovaries. These observations have fueled a long debate, created experimental fertility treatments, and opened business opportunities. Our recent analysis of cell types in the ovarian cortex of women of fertile age could not find evidence of germline stem cells. Like before, our work has been met with critique suggesting methodological shortcomings. We agree that excellence starts with methods and welcome discussion on the pros and cons of different protocols. In this commentary, we discuss the recent re-interpretation of our work.

Non-neutral clonal selection and its potential role in mammalian germline stem cell dysfunction with advancing age

The concept of natural selection, or "survival of the fittest", refers to an evolutionary process in nature whereby traits emerge in individuals of a population through random gene alterations that enable those individuals to better adapt to changing environmental conditions. This genetic variance allows certain members of the population to gain an advantage over others in the same population to survive and reproduce in greater numbers under new environmental pressures, with the perpetuation of those advantageous traits in future progeny. Here we present that the behavior of adult stem cells in a tissue over time can, in many respects, be viewed in the same manner as evolution, with each stem cell clone being representative of an individual within a population. As stem cells divide or are subjected to cumulative oxidative damage over the lifespan of the organism, random genetic alterations are introduced into each clone that create variance in the population. These changes may occur in parallel to, or in response to, aging-associated changes in microenvironmental cues perceived by the stem cell population. While many of these alterations will be neutral or silent in terms of affecting cell function, a small fraction of these changes will enable certain clones to respond differently to shifts in microenvironmental conditions that arise with advancing age. In some cases, the same advantageous genetic changes that support survival and expansion of certain clones over others in the population (viz. non-neutral competition) could be detrimental to the downstream function of the differentiated stem cell descendants. In the context of the germline, such a situation would be devastating to successful propagation of the species across generations. However, even within a single generation, the “evolution” of stem cell lineages in the body over time can manifest into aging-related organ dysfunction and failure, as well as lead to chronic inflammation, hyperplasia, and cancer. Increased research efforts to evaluate stem cells within a population as individual entities will improve our understanding of how organisms age and how certain diseases develop, which in turn may open new opportunities for clinical detection and management of diverse pathologies.

Female germline stem cells: aging and anti-aging

The delay of ovarian aging and the fertility preservation of cancer patients are the eternal themes in the field of reproductive medicine. Acting as the pacemaker of female physiological aging, ovary is also considered as the principle player of cancer, cardiovascular diseases, cerebrovascular diseases, neurodegenerative diseases and etc. However, its aging mechanism and preventive measures are still unclear. Some researchers attempt to activate endogenous ovarian female germline stem cells (FGSCs) to restore ovarian function, as the most promising approach. FGSCs are stem cells in the adult ovaries that can be infinitely self-renewing and have the potential of committed differention. This review aims to elucidate FGSCs aging mechanism from multiple perspectives such as niches, immune disorder, chronic inflammation and oxidative stress. Therefore, the rebuilding nichs of FGSCs, regulation of immune dysfunction, anti-inflammation and oxidative stress remission are expected to restore or replenish FGSCs, ultimately to delay ovarian aging.

Workflow Optimization for Identification of Female Germline or Oogonial Stem Cells in Human Ovarian Cortex Using Single-Cell RNA Sequence Analysis

In 2004, the identification of female germline or oogonial stem cells (OSCs) that can support post-natal oogenesis in ovaries of adult mice sparked a major paradigm shift in reproductive biology. Although these findings have been independently verified, and further extended to include identification of OSCs in adult ovaries of many species ranging from pigs and cows to non-human primates and humans, a recent study rooted in single-cell RNA sequence analysis (scRNA-seq) of adult human ovarian cortical tissue claimed that OSCs do not exist, and that other groups working with OSCs following isolation by magnetic-assisted or fluorescence-activated cell sorting have mistaken perivascular cells (PVCs) for germ cells. Here we report that rare germ lineage cells with a gene expression profile matched to OSCs but distinct from that of other cells, including oocytes and PVCs, can be identified in adult human ovarian cortical tissue by scRNA-seq after optimization of analytical workflow parameters. Deeper cell-by-cell expression profiling also uncovered evidence of germ cells undergoing meiosis-I in adult human ovaries. Lastly, we show that, if not properly controlled for, PVCs can be inadvertently isolated during flow cytometry protocols designed to sort OSCs because of inherently high cellular autofluorescence. However, human PVCs and human germ cells segregate into distinct clusters following scRNA-seq due to non-overlapping gene expression profiles, which would preclude the mistaken identification and use of PVCs as OSCs during functional characterization studies.

Progress in germline stem cell transplantation in mammals and the potential usage

Germline stem cells (GSCs) are germ cells with the capacities of self-renewal and differentiation into functional gametes, and are able to migrate to their niche and reconstitute the fertility of recipients after transplantation. Therefore, GSCs transplantation is a promising technique for fertility recovery in the clinic, protection of rare animals and livestock breeding. Though this novel technique faces tremendous challenges, numerous achievements have been made after several decades' endeavor. This review summarizes the current knowledge of GSCs transplantation and its utilization in mammals, and discusses the application prospect in reproductive medicine and animal science.

Artificial Ovary for Young Female Breast Cancer Patients

In recent decades, there has been increasing attention toward the quality of life of breast cancer (BC) survivors. Meeting the growing expectations of fertility preservation and the generation of biological offspring remains a great challenge for these patients. Conventional strategies for fertility preservation such as oocyte and embryo cryopreservation are not suitable for prepubertal cancer patients or in patients who need immediate cancer therapy. Ovarian tissue cryopreservation (OTC) before anticancer therapy and autotransplantation is an alternative option for these specific indications but has a risk of retransplantation malignant cells. An emerging strategy to resolve these issues is by constructing an artificial ovary combined with stem cells, which can support follicle proliferation and ensure sex hormone secretion. This promising technique can meet both demands of improving the quality of life and meanwhile fulfilling their expectation of biological offspring without the risk of cancer recurrence.

PI3K/PTEN/AKT Signaling Pathways in Germ Cell Development and Their Involvement in Germ Cell Tumors and Ovarian Dysfunctions

Several studies indicate that the PI3K/PTEN/AKT signaling pathways are critical regulators of ovarian function including the formation of the germ cell precursors, termed primordial germ cells, and the follicular pool maintenance. This article reviews the current state of knowledge of the functional role of the PI3K/PTEN/AKT pathways during primordial germ cell development and the dynamics of the ovarian primordial follicle reserve and how dysregulation of these signaling pathways may contribute to the development of some types of germ cell tumors and ovarian dysfunctions.

The safety of VASApos presumptive adult ovarian stem cells

Isolation and characterization of presumptive human adult ovarian stem cells (OSC) has broken the long standing dogma of the absence of postnatal neo-oogenesis. Human adult OSC have been immunosorted by antibodies reacting against the RNA helicase VASA and have been reported to engraft into appropriate stem cell niches to promote neo-oogenesis. Analysis of published research, however, questions some of the findings on isolation, characterization, in-vitro self-renewal and clinical safety of the presumptive human adult OSC. In the present study, human VASA^pos embryo-fetal primordial germ cells and presumptive adult OSC are shown to share several pluripotency and early germ cell markers not ascertained in the initial characterization of adult OSC. A new hypothesis is made that the restoration of fertility claimed to result from presumptive human adult OSC may be attributed instead to VASA^pos embryo-fetal primordial germ cell remnants in the adult ovary, or alternatively to earlier VASA^neg germ cells generated by in-vitro de-differentiation of the presumptive OSC. The suggested hypotheses have extensive implications for the practice and safety of adult OSC in the development of new treatments aimed at rescuing the ovarian reserve.

Stem Cells in Adult Mice Ovaries Form Germ Cell Nests, Undergo Meiosis, Neo-oogenesis and Follicle Assembly on Regular Basis During Estrus Cycle

Very small embryonic-like (VSELs) and ovarian (OSCs) stem cells are located in adult mammalian ovary surface epithelium (OSE). OSCs can expand long-term and differentiate into oocyte-like structures in vitro and have resulted in birth of fertile pups. Lineage tracing studies have provided evidence to suggest OSCs differentiation into oocytes in vivo. But how these stem cells function under normal physiological conditions has not yet been well worked out. Besides studying STRA-8 and SCP-3 expression in enzymatically isolated OSE cells smears, mice were injected BrdU to track mitosis, meiosis and follicle assembly. H&E stained OSE cells during late diestrus and proestrus showed VSELs undergoing asymmetrical cell divisions to give rise to slightly bigger OSCs which in turn underwent symmetrical cell divisions followed by clonal expansion (rapid expansion with incomplete cytokinesis) during early estrus to form germ cell nests (GCN). OCT-4, SSEA-1, MVH and DAZL positive cells in GCN expressed Erα, Erβ and FSHR, were interconnected by ring canals (TEX-14), showed mitochondrial aggregation (Cytochrome C) and Balbiani Body (TRAL). Apoptosis in 'nurse' cells was marked by PARP and putative oocytes were clearly visualized. BrdU was detected in cells undergoing mitosis/meiosis and also in an oocyte of secondary follicle. FACS sorted, green fluorescent protein (GFP) positive VSELs upon transplantation resulted in GFP positive GCN suggesting crucial role for VSELs in adult ovaries. Results suggest that various events described during oogenesis and follicle assembly in fetal ovaries are recapitulated on regular basis in adult ovary and result in the formation of follicles.

Enhanced method to select human oogonial stem cells for fertility research

Alternative methods to obtain mature oocytes are still needed for women with premature ovarian failure (POF). Oogonial stem cells (OSCs), found in adult ovaries, have provided insight into potential paths to treating infertility. Previously, the DDX4 antibody marker alone was utilized to isolate OSCs; however, extensive debate over its location in OSCs versus resulting oocytes (transmembrane or intracytoplasmic) has raised doubt about the identity of these cells. Separate groups, however, have efficiently isolated OSCs using another antibody marker Ifitm3 which is consistently recognized to be transmembrane in location. We hypothesized that by using anti-DDX4 and anti-IFITM3 antibodies, in combination, with MACS, we would improve the yield of isolated OSCs versus using anti-DDX4 antibodies alone. Our study supports earlier findings of OSCs in ovaries during the entire female lifespan: from reproductive age through post-menopausal age. MACS sorting ovarian cells using a the two-marker combination yielded a ~ twofold higher percentage of OSCs from a given mass of ovarian tissue compared to existing single marker methods while minimizing the debate surrounding germline marker selection. During in vitro culture, isolated cells retained the germline phenotype expression of DDX4 and IFITM3 as confirmed by gene expression analysis, demonstrated characteristic germline stem cell self-assembly into embryoid bodies, and formed > 40 µm "oocyte-like" structures that expressed the early oocyte markers GDF9, DAZL, and ZP1. This enhanced and novel method is clinically significant as it could be utilized in the future to more efficiently produce mature, secondary oocytes, for use with IVF/ICSI to treat POF patients.

Very small embryonic-like stem cells (VSELs) regenerate whereas mesenchymal stromal cells (MSCs) rejuvenate diseased reproductive tissues

Compared to embryonic and induced pluripotent stem cells, mesenchymal stem/stromal cells (MSCs) have made their presence felt with good therapeutic promise and safety profile. Transplanting MSCs has successfully helped to reverse infertility and resulted in live births in animal models and also in humans. But the underlying mechanism for their therapeutic potential is not yet clear. MSCs are not pluripotent and hence lack plasticity to differentiate into multiple adult cell types. They rather act as 'paracrine providers' to the tissue-resident stem cells since similar beneficial effects are also observed when their secretome (microvesicles or exosomes) is transplanted. Cytokines, growth factors, signaling lipids, mRNAs, and miRNAs secreted by MSCs enables tissue-resident stem cells to undergo differentiation into specific cell types. Tissue-resident stem cells include pluripotent, very small embryonic-like stem cells (VSELs) and progenitors [spermatogonial (SSCs), ovarian (OSCs) and endometrial (EnSCs) stem cells in testes, ovary and uterus respectively] which function in a subtle manner to maintain life-long tissue homeostasis and regenerate damaged (non-functional) reproductive tissues by differentiating into sperm, oocytes and endometrial epithelial cells respectively. Similar to restoring spermatogenesis, primordial follicles numbers are increased upon transplanting MSCs. Published literature suggests that MSCs do not differentiate into epithelial cells in the endometrium. Nuclear OCT-4 positive VSELs and cytoplasmic OCT-4, AXIN2 and KERATIN-19 positive epithelial progenitors have a greater role during endometrial regeneration. We propose, transplantation of MSCs simply provides growth factors/cytokines essential for the tissue-resident stem/progenitor cells to undergo differentiation into sperm, eggs and endometrial epithelial cells in the reproductive tissues.

Germ cells of the mammalian female: A limited or renewable resource?

In many non-mammalian organisms, a population of germ-line stem cells supports continuing production of gametes during most or all the life of the individual, and germ-line stem cells are also present and functional in male mammals. Traditionally, however, they have been thought not to exist in female mammals, who instead generate all their germ cells during fetal life. Over the last several years, this dogma has been challenged by several reports, while supported by others. We describe and compare these conflicting studies with the aim of understanding how they came to opposing conclusions. We first consider studies that, by examining marker-gene expression, the fate of genetically marked cells, and consequences of depleting the oocyte population, addressed whether ovaries of post-natal females contain oogonial stem cells (OSC) that give rise to new oocytes. We next discuss whether ovaries contain cells that, even if inactive under physiological conditions, nonetheless possess OSC properties that can be revealed through cell-culture. We then examine studies of whether cells harvested after long-term culture of cells obtained from ovaries can, following transplantation into ovaries of recipient females, give rise to oocytes and offspring. Finally, we note studies where somatic cells have been re-programmed to acquire a female germ-cell fate. We conclude that the weight of evidence strongly supports the traditional interpretation that germ-line stem cells do not exist post-natally in female mammals. However, the ability to generate germ cells from somatic cells in vitro establishes a method to generate new gametes from cells of post-natal mammalian females.

Potential roles of experimental reproductive technologies in infertile women with diminished ovarian reserve

In assisted reproductive technology treatment, diminished ovarian reserve (DOR) is a condition of utmost clinical and scientific relevance because of its negative influence on patient outcomes. The current methods of infertility treatment may be unsuitable for many women with DOR, which support the need for development of additional approaches to achieve fertility restoration. Various techniques have been tried to improve the quality and increase the quantity of oocytes in DOR patients, including mitochondrial transfer, activation of primordial follicles, in vitro culture of follicles, and regeneration of oocytes from various stem cells. Herein, we review the science behind these experimental reproductive technologies and their potential use to date in clinical studies for infertility treatment in women with DOR.

Two alternative methods for the retrieval of somatic cell populations from the mouse ovary

Many modern techniques employed to uncover the molecular fundamentals underlying biological processes require dissociated cells as their starting point/substrate. Investigations into ovarian endocrinology or folliculogenesis, therefore, necessitate robust protocols for dissociating the ovary into its constituent cell populations. While in the mouse, methods to obtain individual, mature follicles are well-established, the separation and isolation of single cells of all types from early mouse follicles, including somatic cells, has been more challenging. Herein we present two methods for the isolation of somatic cells in the ovary. These methods are suitable for a range of applications relating to the study of folliculogenesis and mouse ovarian development. First, an enzymatic dissociation utilising collagenase and a temporary, primary cell culture step using neonatal mouse ovaries which yields large quantities of granulosa cells from primordial, activating, and primary follicles. Second, a rapid papain dissociation resulting in a high viability single cell suspension of ovarian somatic cells in less than an hour, which can be applied from embryonic to adult ovarian samples. Collectively these protocols can be applied to a broad array of investigations with unique advantages and benefits pertaining to both.

Creation of a Bioengineered Ovary: Isolation of Female Germline Stem Cells for the Repopulation of a Decellularized Ovarian Bioscaffold

Ovarian failure is the most common cause of infertility and affects about 1% of young women. One innovative strategy to restore ovarian function may be represented by the development of a bioprosthetic ovary, obtained through the combination of tissue engineering and regenerative medicine.We here describe the two main steps required for bioengineering the ovary and for its ex vivo functional reassembling. The first step aims at producing a 3D bioscaffold, which mimics the natural ovarian milieu in vitro. This is obtained with a whole organ decellularization technique that allows the maintenance of microarchitecture and biological signals of the original tissue. The second step involves the use of magnetic activated cell sorting (MACS) to isolate purified female germline stem cells (FGSCs). These cells are able to differentiate in ovarian adult mature cells, when subjected to specific stimuli, and can be used them to repopulate ovarian decellularized bioscaffolds. The combination of the two techniques represents a powerful tool for in vitro recreation of a bioengineered ovary that may constitute a promising solution for hormone and fertility function restoring. In addition, the procedures here described allow for the creation of a suitable 3D platform with useful applications both in toxicological and transplantation studies.

Prospects of Germline Nuclear Transfer in Women With Diminished Ovarian Reserve

Diminished ovarian reserve (DOR) is associated with a reduced quantity and quality of the retrieved oocytes, usually leading to poor reproductive outcomes which remain a great challenge for assisted reproduction technology (ART). Women with DOR often have to seek for oocyte donation, precluding genetically related offspring. Germline nuclear transfer (NT) is a novel technology in ART that involves the transfer of the nuclear genome from an affected oocyte/zygote of the patient to the cytoplast of an enucleated donor oocyte/zygote. Therefore, it offers opportunities for the generation of genetically related embryos. Currently, although NT is clinically applied only in women with serious mitochondrial DNA disorders, this technology has also been proposed to overcome certain forms of female infertility, such as advanced maternal age and embryo developmental arrest. In this review, we are proposing the NT technology as a future treatment option for DOR patients. Strikingly, the application of different NT strategies will result in an increase of the total number of available reconstituted embryos for DOR patients.

Human pluripotent stem cell-derived DDX4 and KRT-8 positive cells participate in ovarian follicle-like structure formation

Understanding the mechanisms of human pluripotent stem cells (hPSCs) specification, development and differentiation to gametes are useful for elucidating the causes of infertility and potential treatment. This study aims to examine whether hPSCs can be induced to DDX4 extracellularly expressing primordial germ cell-like cells (DDX4^ec PGCLCs) and further into ovarian follicle stage in a combined in vitro and in vivo model. The transcriptional signatures show that these DDX4^ec PGCLCs are characteristic of PGCs and express ovarian folliculogenesis markers. We also verify that keratin (KRT)-8 is highly expressed in the DDX4^ec PGCLCs and plays a crucial role in germ cell migration. By co-culturing DDX4^ec PGCLCs with human granulosa cells (GCs), these cells are further induced into ovarian follicle-like structures in a xenograft mice model. This approach can in the future design practical strategies for treating germ cell-associated issues of infertility.

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hPSC-derived DDX4 PGCLCs participate ovarian follicle-like structure formation

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Human granulosa cells as a niche environment are participating folliculogenesis

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Keratin 8 plays an essential role in primordial germ cell migration

Premature ovarian insufficiency: an International Menopause Society White Paper

The aim of this International Menopause Society White Paper on premature ovarian insufficiency (POI) is to provide the latest information regarding this distressing condition. The impact of POI has far-reaching consequences due to its impact on general, psychological, and sexual quality of life, fertility prospects, and long-term bone, cardiovascular, and cognitive health. Progress in fully understanding the etiology, diagnosis, and optimal management options has been slow thus far due to the complexity of the condition and fragmented research. Recent advances in epidemiological and genetic research have improved our understanding of this condition and randomized prospective trials are being planned to determine the intervention strategies, which will optimize quality of life and long-term well-being. The International Menopause Society has commissioned a number of experts at the forefront of their specialty to define the state of the art in the understanding of this condition, to advise on practical management strategies, and to propose future research strategies. It is hoped that a global task force will subsequently be convened in order to formulate a consensus statement across key societies, to accelerate date collection and analysis of a global POI registry, and to facilitate progress in the key defined areas of research.

Inhibition of PTEN activates bovine non-growing follicles in vitro but increases DNA damage and reduces DNA repair response

STUDY QUESTION

Does ovarian follicle activation by phosphatase homologue of chromosome-10 (PTEN) inhibition affect DNA damage and repair in bovine oocytes and granulosa cells?

SUMMARY ANSWER

PTEN inhibition promotes bovine non-growing follicle activation but results in increased DNA damage and impaired DNA repair capacity in ovarian follicles in vitro.

WHAT IS KNOWN ALREADY

Inhibition of PTEN is known to activate primordial follicles but may compromise further developmental potential. In breast cancer cells, PTEN inhibition represses nuclear translocation of breast cancer susceptibility 1 (BRCA1) and Rad51; this impairs DNA repair resulting in an accumulation of damaged DNA, which contributes to cell senescence.

STUDY DESIGN, SIZE, DURATION

Bovine ovarian tissue fragments were exposed to control medium alone or containing either 1 or 10 μM bpv(HOpic), a pharmacological inhibitor of PTEN, in vitro for 24 h. A sub-group of tissue fragments were collected for Western blot analysis after bpv(HOpic) exposure. The remainder were incubated in control medium for a further 5 days and then analysed histologically and by immunohistochemistry to detect DNA damage and repair pathways.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Bovine ovaries were obtained from abattoir-slaughtered heifers. Tissue fragments were exposed to either control medium alone or medium containing either 1 μM or 10 μM bpv(HOpic) for 24 h. Tissue fragments collected after 24 h were subjected to Akt quantification by Western blotting (six to nine fragments per group per experiment). Follicle stage and morphology were classified in remaining fragments. Immunohistochemical analysis included nuclear exclusion of FOXO3 as a marker of follicle activation, γH2AX as a marker of DNA damage, meiotic recombination 11 (MRE11), ataxia telangiectasia mutated (ATM), Rad51, breast cancer susceptibility 1 (BRCA1) and breast cancer susceptibility 2 (BRCA2) as DNA repair factors. A total of 29 550 follicles from three independent experiments were analysed.

MAIN RESULTS AND THE ROLE OF CHANCE

Tissue fragments exposed to bpv(HOpic) had increased Akt phosphorylation at serine 473 (pAkt/Akt ratio, 2.25- and 6.23-fold higher in 1 and 10 μM bpv(HOpic) respectively compared to control, P < 0.05). These tissue fragments contained a significantly higher proportion of growing follicles compared to control (78.6% in 1 μM and 88.7% in 10 μM versus 70.5% in control; P < 0.001). The proportion of morphologically healthy follicles did not differ significantly between 1 μM bpv(HOpic) and control (P < 0.001) but follicle health was lower in 10 μM compared to 1 μM and control in all follicle types (P < 0.05). DNA damage in oocytes, indicated by expression of γH2AX, increased following exposure to 1 μM bpv(HOpic) (non-growing, 83%; primary follicles, 76%) and 10 μM (non-growing, 77%; primary, 84%) compared to control (non-growing, 30% and primary, 59%) (P < 0.05 for all groups). A significant reduction in expression of DNA repair proteins MRE11, ATM and Rad51 was observed in oocytes of non-growing and primary follicles of treatment groups (primary follicles in controls versus 10 μM bpv(HOpic): MRE, 68% versus 47%; ATM, 47% versus 18%; Rad51, 48% versus 24%), P < 0.05 for all groups. Higher dose bpv(HOpic) also resulted in lower expression of BRCA1 compared to control and 1 μM bpv(HOpic) (P < 0.001) in non-growing and primary follicles. BRCA2 expression was increased in oocytes of primary follicles in 1 μM bpv(HOpic) (36%) compared to control (20%, P = 0.010) with a marked decrease in 10 μM (1%, P ≤ 0.001). Granulosa cells of primary and secondary follicles in bpv(HOpic) groups showed more DNA damage compared to control (P < 0.05). However, bpv(HOpic) did not impact granulosa cell DNA repair capacity in secondary follicles, but BRCA1 declined significantly in higher dose bpv(HOpic).

LARGE-SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This study focuses on non-growing follicle activation after 6 days culture and may not reflect DNA damage and repair capacity in later stages of oocyte and follicle growth.

WIDER IMPLICATIONS OF THE FINDINGS

In vitro activation of follicle growth may compromise the bidirectional signalling between oocyte and granulosa cells necessary for optimal oocyte and follicle health. This large animal model may be useful in optimising follicle activation protocols with a view to transfer for clinical application.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by Indonesia endowment fund for education. No competing interest.

TRIAL REGISTRATION NUMBER

Not applicable.

Estrogen regulation of germline stem cell differentiation as a mechanism contributing to female reproductive aging

Progressive loss of ovarian estrogen (E2) production is a hallmark feature of, if not a driving force behind, reproductive aging and the menopause. Recent genetic studies in mice have shown that female germline or oogonial stem cells (OSCs) contribute to maintenance of adult ovarian function and fertility under physiological conditions through support of de-novo oogenesis. Here we show that mouse OSCs express E2 receptor-α (ERα). In the presence of E2, ERα interacts with the stimulated by retinoic acid gene 8 (Stra8) promoter to drive Stra8 expression followed by oogenesis. Treatment of mice with E2 in vivo increases Stra8 expression and oogenesis, and these effects are nullified by ERα (Esr1), but not ERβ (Esr2), gene disruption. Although mice lacking ERα are born with a normal quota of oocytes, ERα-deficient females develop premature ovarian insufficiency in adulthood due to impaired oogenesis. Lastly, mice treated with reversible ER antagonists show a loss of Stra8 expression and oocyte numbers; however, both endpoints rebound to control levels after ceasing drug treatment. These findings establish a key physiological role for E2-ERα signaling in promoting OSC differentiation as a potential mechanism to maintain adequate numbers of ovarian follicles during reproductive life.

Single-cell analysis of human ovarian cortex identifies distinct cell populations but no oogonial stem cells

The human ovary orchestrates sex hormone production and undergoes monthly structural changes to release mature oocytes. The outer lining of the ovary (cortex) has a key role in defining fertility in women as it harbors the ovarian reserve. It has been postulated that putative oogonial stem cells exist in the ovarian cortex and that these can be captured by DDX4 antibody isolation. Here, we report single-cell transcriptomes and cell surface antigen profiles of over 24,000 cells from high quality ovarian cortex samples from 21 patients. Our data identify transcriptional profiles of six main cell types; oocytes, granulosa cells, immune cells, endothelial cells, perivascular cells, and stromal cells. Cells captured by DDX4 antibody are perivascular cells, not oogonial stem cells. Our data do not support the existence of germline stem cells in adult human ovaries, thereby reinforcing the dogma of a limited ovarian reserve.

In Vitro Generation of Oocyte Like Cells and Their In Vivo Efficacy: How Far We have been Succeeded

In the last few decades, stem cell therapy has grown as a boon for many pathological complications including female reproductive disorders. In this review, a brief description of available strategies that are related to stem cell-based in vitro oocyte-like cell (OLC) development are given. We have tried to cover all the aspects and latest updates of the in vitro OLC developmental methodologies, marker profiling, available disease models, and in vivo efficacies, with a special focus on mesenchymal stem cells (MSCs), induced pluripotent stem cells (iPSCs), and embryonic stem cells (ESCs) usage. The differentiation abilities of both the ovarian and non-ovarian stem cell sources under various induction conditions have shown different effects on morphological alterations, proliferation- and size-associated developments, hormonal secretions under gonadotropic stimulations, and their neo-oogenesis or folliculogenesis abilities after in vivo transplantations. The attainment of characters like oocyte-like morphology, size expansion, and meiosis initiation have been found to be major obstacles during in vitro oogenesis. A number of reports have either lacked in vivo studies or have shown their functional incapability to produce viable and healthy offspring. Though researchers have gained many valuable insights regarding in vitro gametogenesis, still there are many things to do to make stem cell-derived OLCs fully functional.

Stem Cells, Self-Renewal, and Lineage Commitment in the Endocrine System

The endocrine system coordinates a wide array of body functions mainly through secretion of hormones and their actions on target tissues. Over the last decades, a collective effort between developmental biologists, geneticists, and stem cell biologists has generated a wealth of knowledge related to the contribution of stem/progenitor cells to both organogenesis and self-renewal of endocrine organs. This review provides an up-to-date and comprehensive overview of the role of tissue stem cells in the development and self-renewal of endocrine organs. Pathways governing crucial steps in both development and stemness maintenance, and that are known to be frequently altered in a wide array of endocrine disorders, including cancer, are also described. Crucially, this plethora of information is being channeled into the development of potential new cell-based treatment modalities for endocrine-related illnesses, some of which have made it through clinical trials.

Mitochondria and Female Germline Stem Cells-A Mitochondrial DNA Perspective

Mitochondria and mitochondrial DNA have important roles to play in development. In primordial germ cells, they progress from small numbers to populate the maturing oocyte with high numbers to support post-fertilization events. These processes take place under the control of significant changes in DNA methylation and other epigenetic modifiers, as well as changes to the DNA methylation status of the nuclear-encoded mitochondrial DNA replication factors. Consequently, the differentiating germ cell requires significant synchrony between the two genomes in order to ensure that they are fit for purpose. In this review, I examine these processes in the context of female germline stem cells that are isolated from the ovary and those derived from embryonic stem cells and reprogrammed somatic cells. Although our knowledge is limited in this respect, I provide predictions based on other cellular systems of what is expected and provide insight into how these cells could be used in clinical medicine.

Similar Population of CD133+ and DDX4+ VSEL-Like Stem Cells Sorted from Human Embryonic Stem Cell, Ovarian, and Ovarian Cancer Ascites Cell Cultures: The Real Embryonic Stem Cells?

A population of small stem cells with diameters of up to 5 μm resembling very small embryonic-like stem cells (VSELs) were sorted from human embryonic stem cell (hESC) cultures using magnetic-activated cell sorting (MACS) based on the expression of a stem-cell-related marker prominin-1 (CD133). These VSEL-like stem cells had nuclei that almost filled the whole cell volume and expressed stem-cell-related markers (CD133, SSEA-4) and markers of germinal lineage (DDX4/VASA, PRDM14). They were comparable to similar populations of small stem cells sorted from cell cultures of normal ovaries and were the predominant cells in ascites of recurrent ovarian cancer. The sorted populations of CD133+ VSEL-like stem cells were quiescent in vitro, except for ascites, and were highly activated after exposure to valproic acid and follicle-stimulating hormone (FSH), indicating a new tool to study these cells in vitro. These VSEL-like stem cells spontaneously formed clusters resembling tumour-like structures or grew into larger, oocyte-like cells and were differentiated in vitro into adipogenic, osteogenic and neural lineages after sorting. We propose the population of VSEL-like stem cells from hESC cultures as potential original embryonic stem cells, which are present in the human embryo, persist in adult human ovaries from the embryonic period of life and are involved in cancer manifestation.

Ddx4+ Oogonial Stem Cells in Postmenopausal Women's Ovaries: A Controversial, Undefined Role

Recent studies support the existence of oogonial stem cells (OSCs) in the ovarian cortex of different mammals, including women.These cells are characterized by small size, membrane expression of DEAD(Asp-Glu-Ala-Asp)-box polypeptide-4 (Ddx4), and stemness properties (such as self-renewal and clonal expansion) as well as the ability to differentiate in vitro into oocyte-like cells. However, the discovery of OSCs contrasts with the popular theory that there is a numerically defined oocyte pool for female fertility which undergoes exhaustion with menopause. Indeed, in the ovarian cortex of postmenopausal women OSCs have been detected that possess both viability and capability to differentiate into oocytes, which is similar to those observed in younger patients. The pathophysiological role of this cell population in aged women is still debated since OSCs, under appropriate stimuli, differentiate into somatic cells, and the occurrence of Ddx4⁺ cells in ovarian tumor samples also suggests their potential involvement in carcinogenesis. Although further investigation into these observations is needed to clarify OSC function in ovary physiology, clinical investigators and researchers studying female infertility are presently focusing on OSCs as a novel opportunity to restore ovarian reserve in both young women undergoing early ovarian failure and cancer survivors experiencing iatrogenic menopause.

Extracellular Localisation of the C-Terminus of DDX4 Confirmed by Immunocytochemistry and Fluorescence-Activated Cell Sorting

Putative oogonial stem cells (OSCs) have been isolated by fluorescence-activated cell sorting (FACS) from adult human ovarian tissue using an antibody against DEAD-box helicase 4 (DDX4). DDX4 has been reported to be germ cell specific within the gonads and localised intracellularly. White et al. (2012) hypothesised that the C-terminus of DDX4 is localised on the surface of putative OSCs but is internalised during the process of oogenesis. This hypothesis is controversial since it is assumed that RNA helicases function intracellularly with no extracellular expression. To determine whether the C-terminus of DDX4 could be expressed on the cell surface, we generated a novel expression construct to express full-length DDX4 as a DsRed2 fusion protein with unique C- and N-terminal epitope tags. DDX4 and the C-terminal myc tag were detected at the cell surface by immunocytochemistry and FACS of non-permeabilised human embryonic kidney HEK 293T cells transfected with the DDX4 construct. DDX4 mRNA expression was detected in the DDX4-positive sorted cells by RT-PCR. This study clearly demonstrates that the C-terminus of DDX4 can be expressed on the cell surface despite its lack of a conventional membrane-targeting or secretory sequence. These results validate the use of antibody-based FACS to isolate DDX4-positive putative OSCs.

Female Fertility Preservation through Stem Cell-based Ovarian Tissue Reconstitution In Vitro and Ovarian Regeneration In Vivo

Historically, approaches designed to offer women diagnosed with cancer the prospects of having a genetically matched child after completion of their cytotoxic treatments focused on the existing oocyte population as the sole resource available for clinical management of infertility. In this regard, elective oocyte and embryo cryopreservation, as well as autologous ovarian cortical tissue grafting posttreatment, have gained widespread support as options for young girls and reproductive-age women who are faced with cancer to consider. In addition, the use of ovarian protective therapies, including gonadotropin-releasing hormone agonists and sphingosine-1-phosphate analogs, has been put forth as an alternative way to preserve fertility by shielding existing oocytes in the ovaries in vivo from the side-effect damage caused by radiotherapy and many chemotherapeutic regimens. This viewpoint changed with the publication of now numerous reports that adult ovaries of many mammalian species, including humans, contain a rare population of oocyte-producing germ cells-referred to as female germline or oogonial stem cells (OSCs). This new line of study has fueled research into the prospects of generating new oocytes, rather than working with existing oocytes, as a novel approach to sustain or restore fertility in female cancer survivors. Here, we overview the history of work from laboratories around the world focused on improving our understanding of the biology of OSCs and how these cells may be used to reconstitute "artificial" ovarian tissue in vitro or to regenerate damaged ovarian tissue in vivo as future fertility-preservation options.

Extracellular matrix signaling activates differentiation of adult ovary-derived oogonial stem cells in a species-specific manner

OBJECTIVE

To test if ovarian microenvironmental cues affect oogonial stem cell (OSC) function in a species-specific manner.

DESIGN

Animal and human study.

SETTING

Research laboratory.

PATIENT(S)/ANIMAL(S)

Human ovarian cells obtained from cryopreserved ovarian cortical tissue of reproductive-age women, and ovarian cells and tissues from female C57BL/6 mice.

INTERVENTION(S)

Mouse ovarian tissue, mouse OSCs (mOSCs) and human OSCs (hOSCs) were analyzed for extracellular matrix (ECM) protein expression, and OSCs isolated from adult mouse and human ovaries were cultured in the absence or presence of ECM proteins without or with an integrin signaling inhibitor.

MAIN OUTCOME MEASURE(S)

Gene expression and in vitro derived (IVD) oocyte formation.

RESULT(S)

Culture of mOSCs on a collagen-based ECM significantly elevated the rate of differentiation of the cells into IVD oocytes. Mouse OSCs expressed many integrins, including Arg-Gly-Asp (RGD)-binding subunits, and ECM-mediated increases in mOSC differentiation were blocked by addition of integrin-antagonizing RGD peptides. In comparison, hOSCs expressed a different pattern of integrin subunits compared with mOSCs, and hOSCs were unresponsive to a collagen-based ECM; however, hOSCs exhibited increased differentiation into IVD oocytes when cultured on laminin.

CONCLUSION(S)

These data, along with in silico analysis of ECM protein profiles in human ovaries, indicate that ovarian ECM-based niche components function in a species-specific manner to control OSC differentiation.

Implications and Current Limitations of Oogenesis from Female Germline or Oogonial Stem Cells in Adult Mammalian Ovaries

A now large body of evidence supports the existence of mitotically active germ cells in postnatal ovaries of diverse mammalian species, including humans. This opens the possibility that adult stem cells naturally committed to a germline fate could be leveraged for the production of female gametes outside of the body. The functional properties of these cells, referred to as female germline or oogonial stem cells (OSCs), in ovaries of women have recently been tested in various ways, including a very recent investigation of the differentiation capacity of human OSCs at a single cell level. The exciting insights gained from these experiments, coupled with other data derived from intraovarian transplantation and genetic tracing analyses in animal models that have established the capacity of OSCs to generate healthy eggs, embryos and offspring, should drive constructive discussions in this relatively new field to further exploring the value of these cells to the study, and potential management, of human female fertility. Here, we provide a brief history of the discovery and characterization of OSCs in mammals, as well as of the in-vivo significance of postnatal oogenesis to adult ovarian function. We then highlight several key observations made recently on the biology of OSCs, and integrate this information into a broader discussion of the potential value and limitations of these adult stem cells to achieving a greater understanding of human female gametogenesis in vivo and in vitro.

The existence and potential of germline stem cells in the adult mammalian ovary

It has long been accepted that the complement of follicles within the ovary is formed before birth in humans, or shortly after birth in rodents, and that no follicles are formed thereafter. This follows entry of all oogonia into meiosis in fetal life, with no remaining germ stem cells in the ovary, in contrast to the presence of spermatogonia in the testis. This has been brought back into debate in recent years, following the demonstration of isolation of cells expressing both germline and stem markers from the postnatal ovary in several species, including humans. We describe these cells as putative ovarian stem cells. Isolation of these cells is challenging, adding to the debate as to their existence, and the validity of DDX4 as the main marker used for their isolation has also to be questioned. While different groups have used varying techniques and indeed terminology to describe these cells, the body of evidence regarding their initial characterization after isolation is growing. There remain very limited data regarding their developmental potential, but the demonstration of the production of functional oocytes from induced pluripotent stem cells and the advances in ovarian follicle culture techniques provide a basis for such studies.

Being a good egg in the 21st century

INTRODUCTION

Women are increasingly having children at a later age, but this can conflict with declining fertility in the later 30's and thereafter.

AREAS OF AGREEMENT

Declining egg quality and quantity with age are well-established, although egg quality can only be surmised from reproductive success or failure.

AREAS OF CONTROVERSY

Whether increasing the number of eggs that can be obtained from ovarian stimulation is of value, and whether there are precursor cells within the adult ovary that could become mature eggs.

GROWING POINTS

There is increasing use of donated eggs by older women to enhance their chances of conception. The storage of frozen eggs for potential use later in life is also becoming more common.

AREAS TIMELY FOR DEVELOPING RESEARCH

Understanding of growth initiation of follicles and development of an artificial ovary may lead to the ability to affect fertility and reproductive lifespan.

Oocytes from stem cells

Folliculogenesis describes the process of activating an oocyte-containing primordial follicle from the ovarian reserve and its development to the mature ovulatory stage. This process is highly complex and is controlled by extra- and intra-ovarian signaling events. Oocyte competence and capacity for fertilization to support a viable pregnancy are acquired during folliculogenesis. Cancer and cancer-based therapies can negatively affect this process, compromising fertility. Currently, preservation of fertility in these patients remains limited to surrogacy, oocyte freezing, oocyte donation, or in vitro maturation (IVM). Recent reports of stem cells being used to produce fully competent oocytes and subsequently healthy offspring in mice have opened up a novel avenue for fertility preservation. However, translating these findings into human health first relies on enhancing our understanding of follicle growth and mimicking its intricacies in vitro. Indeed, the future of oocytes from stem cells in humans comes with many possibilities but currently faces several technical and ethical obstacles.