Ovarian surface epithelium in patients with severe ovarian infertility: a potential source of cells expressing markers of pluripotent/multipotent stem cells

Identification and characterization of multipotential stem cells in immortalized normal ovarian surface epithelial cells

The ovarian surface epithelium (OSE) is a single layer of squamous-to-cuboidal epithelial cells that experience repetitive ovulatory rupture and subsequent repair. However, the characteristics of human immortalized ovarian surface epithelial cells (IOSE80) remain elusive. This study aims to determine whether IOSE80 cells have the characteristics of stem cell proliferation and multilineage differentiation and their application in regenerative medicine. IOSE80 cells are sequenced by high-throughput transcriptome analysis, and 5 sets of public data are used to compare the differences between IOSE80 cells and bone marrow mesenchymal stem cells, pluripotent stem cells, and oocytes in transcriptome profiling. The IOSE80 cells present a cobblestone-like monolayer and express the epithelial cell marker KRT18; the stem cell markers IFITM3, ALDH1A1, and VIM; lowly express stem cell marker LGR5 and germ cell markers DDX4 and DAZL. In addition, the GO terms "regulation of stem cell proliferation", "epithelial cell proliferation", etc., are significantly enriched ( P<0.05). IOSE80 cells have the potential to act as mesenchymal stem cells to differentiate into adipocytes with lipid droplets, osteoblasts, and chondroblasts in vitro. IOSE80 cells express pluripotent stem cell markers, including OCT4, SSEA4, TRA-1-60, and TRA-1-81, and they can be induced into three germ layers in vitro. IOSE80 cells also form oocyte-like cells in vitro and in vivo. In addition, IOSE80 cells exhibit robust proliferation, migration, and ovarian repair functions after in vivo transplantation. This study demonstrates that IOSE80 cells have the characteristics of pluripotent/multipotent stem cells, indicating their important role in tissue engineering and regenerative medicine.

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.

Combination of Estradiol with Leukemia Inhibitory Factor Stimulates Granulosa Cells Differentiation into Oocyte-Like Cells

Purpose: Previous studies have documented that cumulus granulosa cells (GCs) can trans-differentiation into different non-ovarian cells, showing their multipotentiality to repopulate the injured cells in ovarian tissue. The current experiment is aimed to assess the differentiation capacity of human cumulus GCs toward the oocyte-like phenotype in vitro.

Methods: GCs were isolated from healthy female volunteers subjected to in vitro fertilization or intra-cytoplasmic sperm injection (IVF-ICSI). The effect of different media supplemented with leukemia inhibitory factors (LIFs), 5 ng/mL estradiol, and 0.005 IU/mL follicle-stimulating hormone (FSH) were investigated to the differentiation of GCs toward oocyte-like phenotype via monitoring the expression of Oct3/4 and GATA-4 using flow cytometry analysis. The expression of genes such as FIGLA, NOBOX, and SYCP3 was measured by real-time polymerase chain reaction (PCR) assay. We also assess morphological adaptation by using bright-field microscopic imaging.

Results: Exposure of GCs to LIFs increased the number of cells expressing stemness factor Oct3/4 coincided with the suppression of GATA-4 after 7 days (P < 0.05). We found that the transcript level of all genes FIGLA, Nobox, and SYCP-3 decreased in cells after treatment with a FSH (P < 0.05). According to our data, the incubation of GCs with estradiol increased the expression of genes related to the oocyte-like phenotype.

Conclusion: Our finding revealed that the combination of LIFs and estradiol could induce the GCs’ oogenesis capacity and thereby is possibly suggested as a therapeutic strategy during the occurrence of gynecological disorders.

Human Granulosa Cells-Stemness Properties, Molecular Cross-Talk and Follicular Angiogenesis

The ovarian follicle is the basic functional unit of the ovary, comprising theca cells and granulosa cells (GCs). Two different types of GCs, mural GCs and cumulus cells (CCs), serve different functions during folliculogenesis. Mural GCs produce oestrogen during the follicular phase and progesterone after ovulation, while CCs surround the oocyte tightly and form the cumulus oophurus and corona radiata inner cell layer. CCs are also engaged in bi-directional metabolite exchange with the oocyte, as they form gap-junctions, which are crucial for both the oocyte’s proper maturation and GC proliferation. However, the function of both GCs and CCs is dependent on proper follicular angiogenesis. Aside from participating in complex molecular interplay with the oocyte, the ovarian follicular cells exhibit stem-like properties, characteristic of mesenchymal stem cells (MSCs). Both GCs and CCs remain under the influence of various miRNAs, and some of them may contribute to polycystic ovary syndrome (PCOS) or premature ovarian insufficiency (POI) occurrence. Considering increasing female fertility problems worldwide, it is of interest to develop new strategies enhancing assisted reproductive techniques. Therefore, it is important to carefully consider GCs as ovarian stem cells in terms of the cellular features and molecular pathways involved in their development and interactions as well as outline their possible application in translational medicine.

Regenerative Medicine Approaches in Bioengineering Female Reproductive Tissues

Diseases, disorders, and dysfunctions of the female reproductive tract tissues can result in either infertility and/or hormonal imbalance. Current treatment options are limited and often do not result in tissue function restoration, requiring alternative therapeutic approaches. Regenerative medicine offers potential new therapies through the bioengineering of female reproductive tissues. This review focuses on some of the current technologies that could address the restoration of functional female reproductive tissues, including the use of stem cells, biomaterial scaffolds, bio-printing, and bio-fabrication of tissues or organoids. The use of these approaches could also be used to address issues in infertility. Strategies such as cell-based hormone replacement therapy could provide a more natural means of restoring normal ovarian physiology. Engineering of reproductive tissues and organs could serve as a powerful tool for correcting developmental anomalies. Organ-on-a-chip technologies could be used to perform drug screening for personalized medicine approaches and scientific investigations of the complex physiological interactions between the female reproductive tissues and other organ systems. While some of these technologies have already been developed, others have not been translated for clinical application. The continuous evolution of biomaterials and techniques, advances in bioprinting, along with emerging ideas for new approaches, shows a promising future for treating female reproductive tract-related disorders and dysfunctions.

Forecasting early onset diminished ovarian reserve for young reproductive age women

PURPOSE

To investigate the biological networks associated with DOR in young women and the subsequent molecular impact on preimplantation embryos.

METHODS

Whole peripheral blood was collected from patients: young women presenting with diminished ovarian reserve (DOR) and age-matched young women with normal ovarian reserve. Maternal exome sequencing was performed on the NovaSEQ 6000 and sequencing validation was completed using Taqman® SNP Genotyping Assays. Blastocyst global methylome and transcriptome sequencing were also analyzed.

RESULTS

Exome sequencing revealed 730 significant DNA variants observed exclusively in the young DOR patients. Bioinformatic analysis revealed a significant impact to the Glucocorticoid receptor (GR) signaling pathway and each young DOR female had an average of 6.2 deleterious DNA variants within this pathway. Additional stratification based on patient age resulted in a cut-off at 31 years for young DOR discrimination. Embryonic global methylome sequencing resulted in only a very small number of total CpG sites with methylation alterations (1,775; 0.015% of total) in the DOR group. Additionally, there was no co-localization between these limited number of altered CpG sites and significant variants, genes, or pathways. RNA sequencing also resulted in no biologically significant transcription changes between DOR blastocysts and controls.

CONCLUSION

GR signaling DNA variants were observed in women with early-onset DOR potentially compromising oocyte production and quality. However, no significant downstream effects on biological processes appear to impact the resulting blastocyst. The ability to forecast premature DOR for young women may allow for earlier identification and clinical intervention for this patient population.

Lin28 gene and mammalian puberty

Lin28a and Lin28b, homologs of the Caenorhabditis elegans Lin28 gene, play important roles in cell pluripotency, reprogramming, and tumorigenicity. Recently, genome-wide association and transgenic studies showed that Lin28a and/or Lin28b gene were involved in the onset of mammalian puberty, the stage representing the attainment of reproduction capacity; however, the detailed mechanism of these genes in mammalian puberty remains largely unknown. The present paper reviews the research progress on the roles of Lin28a/b genes in the onset of mammalian puberty by analyzing the results coming from gene expression patterns, mutations, and transgenic studies, and put forward possible pathways for further studies on their roles in animal reproduction.

Hedgehog pathway inhibition causes primary follicle atresia and decreases female germline stem cell proliferation capacity or stemness

Background

Follicle depletion is one of the causes of premature ovarian failure (POF) and primary ovarian insufficiency (POI). Hence, maintenance of a certain number of female germline stem cells (FGSCs) is optimal to produce oocytes and replenish the primordial follicle pool. The mechanism that regulates proliferation or stemness of FGSCs could contribute to restoring ovarian function, but it remains uncharacterized in postnatal mammalian ovaries. This study aims to investigate the mechanism by which inhibiting the activity of the hedgehog (Hh) signaling pathway regulates follicle development and FGSC proliferation.

Methods and results

To understand the role of the Hh pathway in ovarian aging, we measured Hh signaling activity at different reproductive ages and the correlation between them in physiological and pathological mice. Furthermore, we evaluated the follicle number and development and the changes in FGSC proliferation or stemness after blocking the Hh pathway in vitro and in vivo. In addition, we aimed to explain one of the mechanisms for the FGSC phenotype changes induced by treatment with the Hh pathway-specific inhibitor GANT61 via oxidative stress and apoptosis. The results show that the activity of Hh signaling is decreased in the ovaries in physiological aging and POF models, which is consistent with the trend of expression levels of the germline stem cell markers Mvh and Oct4. In vitro, blocking the Hh pathway causes follicular developmental disorders and depletes ovarian germ cells and FGSCs after treating ovaries with GANT61. The proliferation or stemness of cultured primary FGSCs is reduced when Hh activity is blocked. Our results show that the antioxidative enzyme level and the ratio of Bcl-2/Bax decrease, the expression level of caspase 3 increases, the mitochondrial membrane potential is abnormal, and ROS accumulate in this system.

Conclusions

We observed that the inhibition of the Hh signaling pathway with GANT61 could reduce primordial follicle number and decrease FGSC reproductive capacity or stemness through oxidative damage and apoptosis.

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1299-5) contains supplementary material, which is available to authorized users.

Differentiation of Mouse Ovarian Stem Cells Toward Oocyte-Like Structure by Coculture with Granulosa Cells

An increasing body of evidence has confirmed existence and function of ovarian stem cells (OSCs). In this study, a novel approach on differentiation of OSCs into oocyte-like cells (OLCs) has been addressed. Recently, different methods have been recruited to isolate and describe aspects of OSCs, but newer and more convenient strategies in isolation are still growing. Herein, a morphology-based method was used to isolate OSCs. Cell suspension of mouse neonatal ovaries was cultured and formed colonies were harvested mechanically and cultivated on mouse embryonic fibroblasts. For differentiation induction, colonies transferred on inactive granulosa cells. Results showed that cells in colonies were positive for alkaline phosphatase activity and reverse transcription-polymerase chain reaction (RT-PCR) confirmed the pluripotency characteristics of cells. Immunofluorescence revealed a positive signal for OCT4, DAZL, MVH, and SSEA1 in colonies as well. Results of RT-PCR and immunofluorescence confirmed that some OLCs were generated within the germ stem cell (GSCs) colonies. The applicability of morphological selection for isolation of GSCs was verified. This method is easier and more economic than other techniques. Our results demonstrate that granulosa cells were effective in inducing the differentiation of OSCs into OLCs through direct cell-to-cell contacts.

Mesenchymal-like stem cells in canine ovary show high differentiation potential

OBJECTIVES

Recent studies have reported the existence of stem cells in ovarian tissue that show enhanced proliferative and differentiation potential compared to other adult tissues. Based on this evidence, we hypothesized that ovarian tissue contained mesenchymal-like stem cells (MSC) that could be isolated using a novel rapid plastic adhesion technique.

MATERIALS AND METHODS

We established MSC lines derived from ovarian and adipose tissue based on their ability to rapidly adhere to plastic culture dishes in the first 3 hours after plating and studied their potentiality in terms of molecular markers and differentiation capacity.

RESULTS

Morphological and kinetic properties of in vitro cultured ovarian MSC were similar to adipose-derived MSC, and both reached senescence after similar passage numbers. Ovarian-derived MSC expressed mesenchymal (CD90 and CD44) but not haematopoietic markers (CD34 and CD45), indicating similarity to adipose-derived MSC. Moreover, ovarian-derived MSC expressed NANOG, TERT, SOX2, OCT4 and showed extensive capacity to differentiate not only into adipogenic, osteogenic and chondrogenic tissue but also towards neurogenic and endodermal lineages and even precursors of primordial germ cells.

CONCLUSION

These results show for the first time the derivation of ovarian cells with the molecular properties of MSC as well as wide differentiation potential. Canine ovarian tissue is accessible, expandable, multipotent and has high plasticity, holding promise for applications in regenerative medicine.

Neo-oogenesis in mammals

Recently, the existence of a mechanism for neo-oogenesis in the ovaries of adult mammals has generated much controversy within reproductive biology. This mechanism, which proposes that the ovary has cells capable of renewing the follicular reserve, has been described for various species of mammals. The first evidence was found in prosimians and humans. However, these findings were not considered relevant because the predominant dogma for reproductive biology at the time was that of Zuckerman. This dogma states that female mammals are born with finite numbers of oocytes that decline throughout postnatal life. Currently, the concept of neo-oogenesis has gained momentum due to the discovery of cells with mitotic activity in adult ovaries of various mammalian species (mice, humans, rhesus monkeys, domestic animals such as pigs, and wild animals such as bats). Despite these reports, the concept of neo-oogenesis has not been widely accepted by the scientific community, generating much criticism and speculation about its accuracy because it has been impossible to reproduce some evidence. This controversy has led to the creation of two positions: one in favour of neo-oogenesis and the other against it. Various animal models have been used in support of both camps, including both classic laboratory animals and domestic and wild animals. The aim of this review is to critically present the current literature on the subject and to evaluate the arguments pro and contra neo-oogenesis in mammals.

Advances in improving fertility in women through stem cell-based clinical platforms

INTRODUCTION

Due to their regenerative ability, stem cells are looked at as a promising tool for improving infertility treatments in women. As the main limiting factor in female fertility is represented by the decrease of ovarian reserve, the main goals of stem cell-based clinical platforms would be to obtain in vitro or in vivo neo-oogenesis. Refractory endometrial factor infertility also represents an obstacle for female reproduction for which stem cells might provide novel treatment strategies. Areas covered: A systematic search of the literature was performed on MEDLINE/PubMed database to identify relevant articles using stem-cell based clinical or research platforms in the field of female infertility. Expert opinion: In vitro oogenesis has not so far developed beyond the stage of oocyte-like cells whose normal progression to mature oocytes and ability to be fertilized was not proved. Extensive epigenetic programming of gamete precursors and the complex interactions between somatic and germ cells required for human oogenesis likely represent the main obstacles in stem-cell-based neo-oogenesis. Also resuming oogenesis in vivo in adulthood still appears a distant hypothesis, as there is still a lack of consensus about the existence and functionality of adult ovarian stem cells.

The Hippo Signaling Pathway Regulates Ovarian Function via the Proliferation of Ovarian Germline Stem Cells

OBJECTIVE

To improve the separation, identification and cultivation of ovarian germline stem cells (OGSCs), to clarify the relationship between the Hippo signaling pathway effector YAP1 and the proliferation and differentiation of OGSCs in vitro and to identify the major contribution of Hippo signaling to ovarian function.

METHODS

Two-step enzymatic separation processes and magnetic separation were used to isolate and identify OGSCs by determining the expression of Mvh, Oct4, Nanog, Fragilis and Stella markers. Then, YAP1, as the main effector molecule in the Hippo signaling pathway, was chosen as the target gene of the study. Lentivirus containing overexpressed YAP1 or a YAP1-targeted shRNA was transduced into OGSCs. The effects of modulating the Hippo signaling pathway on the proliferation, differentiation, reproduction and endocrine function of ovaries were observed by microinjecting the lentiviral vectors with overexpressed YAP1 or YAP1 shRNA into infertile mouse models or natural mice of reproductive age.

RESULTS

(1) The specific expression of Mvh, Oct4, Nanog, Fragilis and Stella markers was observed in isolated stem cells. Thus, the isolated cells were preliminarily identified as OGSCs. (2) The co-expression of LATS2, MST1, YAP1 and MVH was observed in isolated OGSCs. Mvh and Oct4 expression levels were significantly increased in OGSCs overexpressing YAP1 compared to GFP controls. Consistently, Mvh and Oct4 levels were significantly decreased in cells expressing YAP1-targeted shRNA. (3) After 14-75 days of YAP1 overexpression in infertile mouse models, we detected follicle regeneration in ovaries, the activation of primordial follicles and increased birth rate, accompanied by increasing levels of E2 and FSH. (4) However, we detected decreasing follicles in ovaries, lower birth rate, and decreasing E2 and FSH in serum from healthy mice of reproductive age following YAP1 shRNA expression.

CONCLUSION

Methods for the isolation, identification and culture of OGSCs were successfully established. Further results indicate that isolated OGSCs can specifically recognize Hippo signaling molecules and that manipulation of YAP1 expression can be used to regulate the proliferation and differentiation of OGSCs, as well as ovarian function in mice. This study suggests that the Hippo signaling pathway may represent a new molecular target for the regulation of mouse ovarian functional remodeling.

Endogenous, very small embryonic-like stem cells: critical review, therapeutic potential and a look ahead

BACKGROUND

Both pluripotent very small embryonic-like stem cells (VSELs) and induced pluripotent stem (iPS) cells were reported in 2006. In 2012, a Nobel Prize was awarded for iPS technology whereas even today the very existence of VSELs is not well accepted. The underlying reason is that VSELs exist in low numbers, remain dormant under homeostatic conditions, are very small in size and do not pellet down at 250-280g. The VSELs maintain life-long tissue homeostasis, serve as a backup pool for adult stem cells and are mobilized under stress conditions. An imbalance in VSELs function (uncontrolled proliferation) may result in cancer.

SEARCH METHODS

The electronic database 'Medline/Pubmed' was systematically searched with the subject heading term 'very small embryonic-like stem cells'.

OBJECTIVE AND RATIONALE

The most primitive stem cells that undergo asymmetric cell divisions to self-renew and give rise to progenitors still remain elusive in the hematopoietic system and testes, while the presence of stem cells in ovary is still being debated. We propose to review the available literature on VSELs, the methods of their isolation and characterization, their ontogeny, how they compare with embryonic stem (ES) cells, primordial germ cells (PGCs) and iPS cells, and their role in maintaining tissue homeostasis. The review includes a look ahead on how VSELs will result in paradigm shifts in basic reproductive biology.

OUTCOMES

Adult tissue-specific stem cells including hematopoietic, spermatogonial, ovarian and mesenchymal stem cells have good proliferation potential and are indeed committed progenitors (with cytoplasmic OCT-4), which arise by asymmetric cell divisions of pluripotent VSELs (with nuclear OCT-4). VSELs are the most primitive stem cells and postulated to be an overlapping population with the PGCs. Rather than migrating only to the gonads, PGCs migrate and survive in various adult body organs throughout life as VSELs. VSELs express both pluripotent and PGC-specific markers and are epigenetically and developmentally more mature compared with ES cells obtained from the inner cell mass of a blastocyst-stage embryo. As a result, VSELs readily differentiate into three embryonic germ layers and spontaneously give rise to both sperm and oocytes in vitro. Like PGCs, VSELs do not divide readily in culture, nor produce teratoma or integrate in the developing embryo. But this property of being relatively quiescent allows endogenous VSELs to survive various kinds of toxic insults. VSELs that survive oncotherapy can be targeted to induce endogenous regeneration of non-functional gonads. Transplanting healthy niche (mesenchymal) cells have resulted in improved gonadal function and live births.

WIDER IMPLICATIONS

Being quiescent, VSELs possibly do not accumulate genomic (nuclear or mitochondrial) mutations and thus may be ideal endogenous, pluripotent stem cell candidates for regenerative and reproductive medicine. The presence of VSELs in adult gonads and the fact that they survive oncotherapy may obviate the need to bank gonadal tissue for fertility preservation prior to oncotherapy. VSELs and their ability to undergo spermatogenesis/neo-oogenesis in the presence of a healthy niche will help identify newer strategies toward fertility restoration in cancer survivors, delaying menopause and also enabling aged mothers to have better quality eggs.

Small putative NANOG, SOX2, and SSEA-4-positive stem cells resembling very small embryonic-like stem cells in sections of ovarian tissue in patients with ovarian cancer

BACKGROUND

In previous studies it has been found that in cell cultures of human adult ovaries there is a population of small stem cells with diameters of 2-4 μm, which are present mainly in the ovarian surface epithelium and are comparable to very small embryonic-like stem cells (VSELs) from bone marrow. These cells are not observed by histopathologists in the ovarian tissue due to their small size and unknown clinical significance. Because these cells express a degree of pluripotency, they might be involved in the manifestation of ovarian cancer. Therefore we studied the ovarian tissue sections in women with borderline ovarian cancer and serous ovarian carcinoma to perhaps identify the small putative stem cells in situ.

METHODS

In 27 women with borderline ovarian cancer and 20 women with high-grade serous ovarian carcinoma the ovarian tissue sections were stained, per standard practice, with eosin and hematoxylin staining and on NANOG, SSEA-4 and SOX2 markers, related to pluripotency, using immunohistochemistry. We focused on the presence and localization of small putative stem cells with diameters of up to 5 μm and with the nuclei spread over nearly the full cell volume.

RESULTS

In ovarian sections of both borderline ovarian cancer and serous ovarian carcinoma patients we were able to identify the presence of small round cells complying with the above criteria. Some of these small cells were NANOG-positive, were located among epithelial cells in the ovarian surface epithelium and as a single cell or groups of cells/clusters in typical "chambers", were found only in the presence of ovarian cancer and not in healthy ovaries and are comparable to those in fetal ovaries. We envision that these small cells could be related to NANOG-positive tumor-like structures and oocyte-like cells in similar "chambers" found in sections of cancerous ovaries, which could support their stemness and pluripotency. Further immunohistochemistry revealed a similar population of SSEA-4 and SOX2-positive cells.

CONCLUSIONS

We may conclude that putative small stem cells expressing markers, related to pluripotency, are present in the ovarian tissue sections of women with borderline ovarian cancer and high-grade serous ovarian carcinoma thus indicating their potential involvement in ovarian cancer.

The Controversy, Challenges, and Potential Benefits of Putative Female Germline Stem Cells Research in Mammals

The conventional view is that female mammals lose their ability to generate new germ cells after birth. However, in recent years, researchers have successfully isolated and cultured a type of germ cell from postnatal ovaries in a variety of mammalian species that have the abilities of self-proliferation and differentiation into oocytes, and this finding indicates that putative germline stem cells maybe exist in the postnatal mammalian ovaries. Herein, we review the research history and discovery of putative female germline stem cells, the concept that putative germline stem cells exist in the postnatal mammalian ovary, and the research progress, challenge, and application of putative germline stem cells in recent years.

VSELs may obviate cryobanking of gonadal tissue in cancer patients for fertility preservation

Background

Infertility is an undesirable side effect and gonadal tissue banking is advocated in young cancer patients who are unable to preserve embryos or gametes prior to oncotherapy to achieve biological parenthood later on. Banking gonadal tissue is challenging and protocols to mature gametes in vitro are not yet clinically established. Transplanting ovarian cortical tissue at hetero-or orthotopic sites in women and bone marrow transplantation (BMT) in both men and women has resulted in spontaneous recovery of fertility, pregnancy and live births. Various studies in humans and mice suggest that genetic origin of offspring after BMT is similar to transplanted patient and not the donor. Thus the source of oocytes/sperm which result in spontaneous pregnancies still remains contentious.

Findings

Very small embryonic-like stem cells (VSELs) have been reported in adult human testis and ovary, in azoospermic testicular biopsies from survivors of childhood cancer and also in women with premature ovarian failure and menopause. VSELs survive chemotherapy because of their quiescent nature and can be detected in chemoablated mice gonads at protein and mRNA level and also by flow cytometry. Surviving VSELs spontaneously differentiate into oocyte-like structures and sperm when inhibitory factors are overcome in vitro. Transplantation of mesenchymal cells (isolated from different sources) has led to regeneration of chemoablated mouse gonads and also live births. Spermatogenesis is also restored from endogenous stem cells on inter-tubular transplantation of Sertoli cells in chemoablated mouse testis.

Conclusions

Endogenous VSELs (which survive oncotherapy) can possibly regenerate non-functional gonads in cancer survivors when exposed to a healthy niche in vitro or in vivo (by way of transplanting mesenchymal cells which secrete trophic factors required for endogenous VSELs to differentiate into gametes). Presence of VSELs can also explain spontaneous pregnancies after BMT and cortical tissue transplantation (at heterotopic or orthotopic sites). This understanding once verified and accepted by the scientific community could obviate the need to remove whole ovary or testicular biopsy for cryopreservation prior to oncotherapy.

Improvement in Isolation and Identification of Mouse Oogonial Stem Cells

Female germline stem cells (FGSCs) or oogonial stem cells (OSCs) have the capacity to generate newborn oocytes and thus open a new door to fight ovarian aging and female infertility. However, the production and identification of OSCs are difficult for investigators. Rare amount of these cells in the ovary results in the failure of the acquisition of OSCs. Furthermore, the oocyte formation by OSCs in vivo was usually confirmed using tissue sections by immunofluorescence or immunohistochemistry in previous studies. STO or MEF feeder cells are derived from mouse, not human. In our study, we modified the protocol. The cells were digested from ovaries and cultured for 2-3 days and then were purified by magnetic-activated cell sorting (MACS). The ovaries and fetus of mice injected with EGFP-positive OSCs were prepared and put on the slides to directly visualize oocyte and progeny formation under microscope. Additionally, the human umbilical cord mesenchymal stem cells (hUC-MSCs) were also used as feeder cells to support the proliferation of OSCs. The results showed that all the modified procedures can significantly improve and facilitate the generation and characterization of OSCs, and hUC-MSCs as feeder will be useful for isolation and proliferation of human OSCs avoiding contamination from mouse.

Novel Action of FSH on Stem Cells in Adult Mammalian Ovary Induces Postnatal Oogenesis and Primordial Follicle Assembly

Adult mammalian ovary has been under the scanner for more than a decade now since it was proposed to harbor stem cells that undergo postnatal oogenesis during reproductive period like spermatogenesis in testis. Stem cells are located in the ovary surface epithelium and exist in adult and menopausal ovary as well as in ovary with premature failure. Stem cells comprise two distinct populations including spherical, very small embryonic-like stem cells (VSELs which express nuclear OCT-4 and other pluripotent and primordial germ cells specific markers) and slightly bigger ovarian germ stem cells (OGSCs with cytoplasmic OCT-4 which are equivalent to spermatogonial stem cells in the testes). These stem cells have the ability to spontaneously differentiate into oocyte-like structures in vitro and on exposure to a younger healthy niche. Bone marrow may be an alternative source of these stem cells. The stem cells express FSHR and respond to FSH by undergoing self-renewal, clonal expansion, and initiating neo-oogenesis and primordial follicle assembly. VSELs are relatively quiescent and were recently reported to survive chemotherapy and initiate oogenesis in mice when exposed to FSH. This emerging understanding and further research in the field will help evolving novel strategies to manage ovarian pathologies and also towards oncofertility.

Postnatal oogenesis in humans: a review of recent findings

In spite of generally accepted dogma that the total number of follicles and oocytes is established in human ovaries during the fetal period of life rather than forming de novo in adult ovaries, some new evidence in the field challenges this understanding. Several studies have shown that different populations of stem cells, such as germinal stem cells and small round stem cells with diameters of 2 to 4 μm, that resembled very small embryonic-like stem cells and expressed several genes related to primordial germ cells, pluripotency, and germinal lineage are present in adult human ovaries and originate in ovarian surface epithelium. These small stem cells were pushed into the germinal direction of development and formed primitive oocyte-like cells in vitro. Moreover, oocyte-like cells were also formed in vitro from embryonic stem cells and induced pluripotent stem cells. This indicates that postnatal oogenesis is not excluded. It is further supported by the occurrence of mesenchymal stem cells that can restore the function of sterilized ovaries and lead to the formation of new follicles and oocytes in animal models. Both oogenesis in vitro and transplantation of stem cell-derived "oocytes" into the ovarian niche to direct their natural maturation represent a big challenge for reproductive biomedicine in the treatment of female infertility in the future and needs to be explored and interpreted with caution, but it is still very important for clinical practice in the field of reproductive medicine.

Stem cells, progenitor cells, and lineage decisions in the ovary

Exploring stem cells in the mammalian ovary has unleashed a Pandora's box of new insights and questions. Recent evidence supports the existence of stem cells of a number of the different cell types within the ovary. The evidence for a stem cell model producing mural granulosa cells and cumulus cells is strong, despite a limited number of reports. The recent identification of a precursor granulosa cell, the gonadal ridge epithelial-like cell, is exciting and novel. The identification of female germline (oogonial) stem cells is still very new and is currently limited to just a few species. Their origins and physiological roles, if any, are unknown, and their potential to produce oocytes and contribute to follicle formation in vivo lacks robust evidence. The precursor of thecal cells remains elusive, and more compelling data are needed. Similarly, claims of very small embryonic-like cells are also preliminary. Surface epithelial cells originating from gonadal ridge epithelial-like cells and from the mesonephric epithelium at the hilum of the ovary have also been proposed. Another important issue is the role of the stroma in guiding the formation of the ovary, ovigerous cords, follicles, and surface epithelium. Immune cells may also play key roles in developmental patterning, given their critical roles in corpora lutea formation and regression. Thus, while the cellular biology of the ovary is extremely important for its major endocrine and fertility roles, there is much still to be discovered. This review draws together the current evidence and perspectives on this topic.

Lin-28 regulates oogenesis and muscle formation in Drosophila melanogaster

Understanding the control of stem cell (SC) differentiation is important to comprehend developmental processes as well as to develop clinical applications. Lin28 is a conserved molecule that is involved in SC maintenance and differentiation by regulating let-7 miRNA maturation. However, little is known about the in vivo function of Lin28. Here, we report critical roles for lin-28 during oogenesis. We found that let-7 maturation was increased in lin-28 null mutant fly ovaries. We showed that lin-28 null mutant female flies displayed reduced fecundity, due to defects in egg chamber formation. More specifically, we demonstrated that in mutant ovaries, the egg chambers fuse during early oogenesis resulting in abnormal late egg chambers. We also showed that this phenotype is the combined result of impaired germline SC differentiation and follicle SC differentiation. We suggest a model in which these multiple oogenesis defects result from a misregulation of the ecdysone signaling network, through the fine-tuning of Abrupt and Fasciclin2 expression. Our results give a better understanding of the evolutionarily conserved role of lin-28 on GSC maintenance and differentiation.

Post-natal oogenesis: a concept for controversy that intensified during the last decade

For decades, scientists have considered that female mammals are born with a lifetime reserve of oocytes in the ovary, irrevocably fated to decline after birth. However, controversy in the matter of the possible presence of oocytes and granulosa cells that originate from stem cells in the adult mammalian ovaries has been expanded. The restricted supply of oocytes in adult female mammals has been disputed in recent years by supporters of neo-oogenesis, who claim that germline stem cells (GSCs) exist in the ovarian surface epithelium (OSE) or the bone marrow (BM). Differentiation of ovarian stem cells (OSCs) into oocytes, fibroblast-like cells, granulosa phenotype, neural and mesenchymal type cells and generation of germ cells from OSCs under the contribution of an OSC niche that consists of immune system-related cells and hormonal signalling has been claimed. Although these arguments have met with intense suspicion, their confirmation would necessitate the revision of the current classic knowledge of female reproductive biology.

Immunohistochemical evaluation of cell proliferation and apoptosis markers in ovarian surface epithelial cells of cladribine-treated rats

Cladribine has been used in the treatment of hairy cell leukemia for about 30 years. In addition, the number of indications for the application of 2-CdA is constantly increasing. The treatment with cladribine, of younger persons and even children, appears to be a major factor stimulating the more exact recognition of its activities. However, till now, little has been known about the impact of cladribine on the reproductive system. The aim of the study was to evaluate the immunohistochemical expression of cell proliferation and apoptosis markers in ovarian surface epithelial (OSE) cells. In our study, ten rats were placed into two equal groups. The study group received daily subcutaneous injections of cladribine in a dose of 0.10 mg/kg of weight/day for one cycle lasting 7 days. The control group received only saline injections. The rats were sacrificed 24 h after the last injection, and their ovaries were extracted. The sections were immunohistochemically stained with cell proliferation marker Ki-67 and the apoptosis marker caspase 3. The expressions of the markers were evaluated using a light microscope. An analysis was made using an image analysis system and the CellAD software. The results were then statistically explored by way of the Mann-Whitney U test. The proliferative index (Ki-67) of ovarian surface epithelial cells was significantly lower in the study group than in the control group (p < 0.05). These results suggest that cladribine treatment has a potential to inhibit the OSE cell proliferation in rats. The apoptosis marker demonstrated a significant increase after the cladribine treatment. These suggest that cladribine induces apoptosis in OSE cells.

Induction of estrogen-sensitive epithelial cells derived from human-induced pluripotent stem cells to repair ovarian function in a chemotherapy-induced mouse model of premature ovarian failure

The incidence of premature ovarian failure (POF), a condition causing amenorrhea and hypergonadotropic hypoestrogenism in women before the age of 40, has been increasing in recent years. As an irreversible pathological change, improved treatment strategies for this disease are urgently needed. In this study, a type of microRNA (miR-17-3p) was used to guide the differentiation of human-induced pluripotent stem (iPS) cells into hormone-sensitive ovarian epithelial (OSE)-like cells in vitro. To prevent their morphological transformation into fibroblast-like cells, MiR-17-3p, a microRNA that suppresses vimentin expression, was transfected into human iPS cells. Subsequently, these cells were successfully induced into OSE-like cells in vitro after treatment with estrogen and cell growth factors. Compared with controls, iPS cells transfected with miR-17-3p expressed higher levels of epithelial markers (cytokeratin 7, AE1, AE3, and E-cadherin) and estrogen receptors (ERα and ERβ) while levels of mesenchymal markers (fibronectin, vimentin, and N-cadherin) lowered after the induction. The human iPS cell-derived OSE-like cells were then injected into cyclophosphamide-induced POF model mice to determine their potential benefit as grafts to repair ovarian tissues. The OSE-like cells survived within POF mouse ovaries for at least 14 days in vivo. Compared with the negative controls, expressions of cytokeratin 7 and ERβ proteins were elevated while fibronectin and vimentin levels in ovarian tissues were downregulated in the OSE-like cell transplantation group. Moreover, the ovarian weight and plasma E2 level increased over time in the transplantation with OSE-like cells, compared with control groups. Hence, we can draw the conclusion that iPS cells can be induced to differentiate into OSE-like cells in vitro.

Plasticity of granulosa cells: on the crossroad of stemness and transdifferentiation potential

The ovarian follicle represents the basic functional unit of the ovary and consists of an oocyte, which is surrounded by granulosa cells (GCs). GCs play an important role in the growth and development of the follicle. They are subject to increased attention since it has recently been shown that the subpopulation of GCs within the growing follicle possesses exceptionally plasticity showing stem cell characteristics. In assisted reproduction programs, oocytes are retrieved from patients together with GCs, which are currently discarded daily, but could be an interesting subject to be researched and potentially used in regenerative medicine in the future. Isolated GCs expressed stem cell markers such as OCT-4, NANOG and SOX-2, showed high telomerase activity, and were in vitro differentiated into other cell types, otherwise not present within ovarian follicles. Recently another phenomenon demonstrated in GCs is transdifferentiation, which could explain many ovarian pathological conditions. Possible applications in regenerative medicine are also given.

Isolation, characterization and differentiation of cells expressing pluripotent/multipotent markers from adult human ovaries

Pluripotent stem cells are still generally accepted not to exist in adult human ovaries, although increasing studies confirm the presence of pluripotent/multipotent stem cells in adult mammalian ovaries, including those of humans. The aim of this study is to isolate, characterize and differentiate in vitro stem cells that originate from the adult human ovarian cortex and that express markers of pluripotency/multipotency. After enzymatic degradation of small ovarian cortex biopsies retrieved from 18 women, ovarian cell cultures were successfully established from 17 and the formation of cell colonies was observed. The presence of cells/colonies expressing some markers of pluripotency (alkaline phosphatase, surface antigen SSEA-4, OCT4, SOX-2, NANOG, LIN28, STELLA), germinal lineage (DDX4/VASA) and multipotency (M-CAM/CD146, Thy-1/CD90, STRO-1) was confirmed by various methods. Stem cells from the cultures, including small round SSEA-4-positive cells with diameters of up to 4 μm, showed a relatively high degree of plasticity. We were able to differentiate them in vitro into various types of somatic cells of all three germ layers. However, these cells did not form teratoma when injected into immunodeficient mice. Our results thus show that ovarian tissue is a potential source of stem cells with a pluripotent/multipotent character for safe application in regenerative medicine.

Small SSEA-4-positive cells from human ovarian cell cultures: related to embryonic stem cells and germinal lineage?

Background

It has already been found that very small embyronic-like stem cells (VSELs) are present in adult human tissues and organs. The aim of this study was to find if there exists any similar population of cells in cell cultures of reproductive tissues and embryonic stem cells, and if these cells have any relation to pluripotency and germinal lineage.

Methods and results

Here we report that a population of small SSEA-4-positive cells with diameters of up to 4 μm was isolated by fluorescence-activated cell sorting (FACS) from the human ovarian cell cultures after enzymatic degradation of adult cortex tissues. These small cells – putative ovarian stem cells – were also observed during cell culturing of up to 6 months and more. In general, small putative ovarian stem cells, isolated by FACS, showed a relatively low gene expression profile when compared to human embryonic stem cells (hESCs) and human adult fibroblasts; this may reflect the quiescent state of these cells. In spite of that, small putative ovarian stem cells expressed several genes related to primordial germ cells (PGCs), pluripotency and germinal lineage, including VASA. The PGC-related gene PRDM1 was strongly expressed in small putative ovarian stem cells; in both hESCs and fibroblasts it was significantly down-regulated. In addition, putative ovarian stem cells expressed other PGC-related genes, such as PRDM14 and DPPA3. Most of the pluripotency and germinal lineage-related genes were up-regulated in hESCs (except VASA). When compared to fibroblasts, there were several pluripotency-related genes, which were up-regulated in small putative ovarian stem cells. Similar populations of small cells were also isolated by FACS from human testicular and hESC cultures.

Conclusions

Our results confirm the potential embryonic-like character of small putative stem cells isolated from human adult ovaries and their possible relation to germinal lineage.

Expression of pluripotency and oocyte-related genes in single putative stem cells from human adult ovarian surface epithelium cultured in vitro in the presence of follicular fluid

The aim of this study was to trigger the expression of genes related to oocytes in putative ovarian stem cells scraped from the ovarian surface epithelium of women with premature ovarian failure and cultured in vitro in the presence of follicular fluid, rich in substances for oocyte growth and maturation. Ovarian surface epithelium was scraped and cell cultures were set up by scrapings in five women with nonfunctional ovaries and with no naturally present mature follicles or oocytes. In the presence of donated follicular fluid putative stem cells grew and developed into primitive oocyte-like cells. A detailed single-cell gene expression profiling was performed to elucidate their genetic status in comparison to human embryonic stem cells, oocytes, and somatic fibroblasts. The ovarian cell cultures depleted/converted reproductive hormones from the culture medium. Estradiol alone or together with other substances may be involved in development of these primitive oocyte-like cells. The majority of primitive oocyte-like cells was mononuclear and expressed several genes related to pluripotency and oocytes, including genes related to meiosis, although they did not express some important oocyte-specific genes. Our work reveals the presence of putative stem cells in the ovarian surface epithelium of women with premature ovarian failure.

Gene expression profiling of human oocytes developed and matured in vivo or in vitro

The quality of the human oocyte determines the success of fertilization and affects the consequent embryo development, pregnancy and birth; it therefore serves as a basis for human reproduction and fertility. The possibility to evaluate oocyte quality in the in vitro fertilization programme is very limited. The only criterion which is commonly used to evaluate oocyte quality is its morphology. There is a mass of oocytes in the in vitro fertilization programme which are not fertilized in spite of normal morphology. In the past, several attempts focused on oocyte gene expression profiling by different approaches. The results elucidated groups of genes related to the human oocyte. It was confirmed that some factors, such as oocyte in vitro maturation, are detectable at the molecular level of human oocytes and their polar bodies in terms of gene expression profile. Furthermore, the first genetic evaluations of oocyte-like cells developed in vitro from human stem cells of different origin were performed showing that these cells express some genes related to oocytes. All these findings provide some new knowledge and clearer insights into oocyte quality and oogenesis that might be introduced into clinical practice in the future.

Isolation of small SSEA-4-positive putative stem cells from the ovarian surface epithelium of adult human ovaries by two different methods

The adult ovarian surface epithelium has already been proposed as a source of stem cells and germinal cells in the literature, therefore it has been termed the “germinal epithelium”. At present more studies have confirmed the presence of stem cells expressing markers of pluripotency in adult mammalian ovaries, including humans. The aim of this study was to isolate a population of stem cells, based on the expression of pluripotency-related stage-specific embryonic antigen-4 (SSEA-4) from adult human ovarian surface epithelium by two different methods: magnetic-activated cell sorting and fluorescence-activated cell sorting. Both methods made it possible to isolate a similar, relatively homogenous population of small, SSEA-4-positive cells with diameters of up to 4 μm from the suspension of cells retrieved by brushing of the ovarian cortex biopsies in reproductive-age and postmenopausal women and in women with premature ovarian failure. The immunocytochemistry and genetic analyses revealed that these small cells—putative stem cells—expressed some primordial germ cell and pluripotency-related markers and might be related to the in vitro development of oocyte-like cells expressing some oocyte-specific transcription factors in the presence of donated follicular fluid with substances important for oocyte growth and development. The stemness of these cells needs to be further researched.

Cells with stem cell characteristics in somatic compartments of the ovary

Antral follicular growth in the ovary is characterized by rapid expansion of granulosa cells accompanied by a rising complexity of their functionality. Within two weeks the number of human granulosa cells increases from less than 500,000 to more than 50 millions cells per follicle and differentiates into groups of cells with a variety of specialized functions involved in steroidogenesis, nursing the oocyte, and forming a functional syncitium. Both the rapid proliferation and different specialized functions of the granulosa cells can only be explained through the involvement of stem cells. However, luteinizing granulosa cells were believed to be terminally differentiated cells. Only recently, stem and progenitor cells with FSH-receptor activity were identified in populations of luteinizing granulosa cells obtained during oocyte collected for assisted reproduction. In the presence of the leukaemia-inhibiting factor (LIF), it was possible to culture a subpopulation of the luteinizing granulosa cells over prolonged time periods. Furthermore, when embedded in a matrix consisting of collagen type I, these cells continued to express the FSH receptor over prolonged time periods, developed globular formations that surrogated as follicle-like structures, providing a promising tool for reproductive biology.

Germline cells in ovarian surface epithelium of mammalians: a promising notion

It is a long held doctrine in reproductive biology that women are born with a finite number of oocytes and there is no oogenesis during the postnatal period. However, recent evidence challenges this by showing the presence of germ line stem cells in the human ovarian surface epithelium (OSE), which can serve as a source of germ cells, and differentiate into oocyte like structures. Postnatal renewal of oocytes may have enormous therapeutic potential especially in women facing the risk of premature ovarian failure idiopathically or iatrogenically after exposure to gonadotoxic chemotherapy and radiation for cancer therapy.This article reviews current knowledge on germ line stem cells in human OSE.

Technical challenges and limitations of current mouse models of ovarian cancer

The development of genetically engineered models (GEM) of epithelial ovarian cancer (EOC) has been very successful, with well validated models representing high grade and low grade serous adenocarcinomas and endometrioid carcinoma (EC). Most of these models were developed using technologies intended to target the ovarian surface epithelium (OSE), the cell type long believed to be the origin of EOC. More recent evidence has highlighted what is likely a more prevalent role of the secretory cell of the fallopian tube in the ontogeny of EOC, however none of the GEM of EOC have demonstrated successful targeting of this important cell type.

The precise technologies exploited to develop the existing GEM of EOC are varied and carry with them advantages and disadvantages. The use of tissue specific promoters to model disease has been very successful, but the lack of any truly specific OSE or oviductal secretory cell promoters makes the outcomes of these models quite unpredictable. Effecting genetic change by the administration of adenoviral vectors expressing Cre recombinase may alleviate the perceived need for tissue specific promoters, however the efficiencies of infection of different cell types is subject to numerous biological parameters that may lead to preferential targeting of certain cell populations.

One important future avenue of GEM of EOC is the evaluation of the role of genetic modifiers. We have found that genetic background can lead to contrasting phenotypes in one model of ovarian cancer, and data from other laboratories have also hinted that the exact genetic background of the model may influence the resulting phenotype. The different genetic backgrounds may modify the biology of the tumors in a manner that will be relevant to human disease, but they may also be modifying parameters which impact the response of the host to the technologies employed to develop the model.

Immunoregulation of follicular renewal, selection, POF, and menopause in vivo, vs. neo-oogenesis in vitro, POF and ovarian infertility treatment, and a clinical trial

The immune system plays an important role in the regulation of tissue homeostasis ("tissue immune physiology"). Function of distinct tissues during adulthood, including the ovary, requires (1) Renewal from stem cells, (2) Preservation of tissue-specific cells in a proper differentiated state, which differs among distinct tissues, and (3) Regulation of tissue quantity. Such morphostasis can be executed by the tissue control system, consisting of immune system-related components, vascular pericytes, and autonomic innervation. Morphostasis is established epigenetically, during morphogenetic (developmental) immune adaptation, i.e., during the critical developmental period. Subsequently, the tissues are maintained in a state of differentiation reached during the adaptation by a "stop effect" of resident and self renewing monocyte-derived cells. The later normal tissue is programmed to emerge (e.g., late emergence of ovarian granulosa cells), the earlier its function ceases. Alteration of certain tissue differentiation during the critical developmental period causes persistent alteration of that tissue function, including premature ovarian failure (POF) and primary amenorrhea. In fetal and adult human ovaries the ovarian surface epithelium cells called ovarian stem cells (OSC) are bipotent stem cells for the formation of ovarian germ and granulosa cells. Recently termed oogonial stem cells are, in reality, not stem but already germ cells which have the ability to divide. Immune system-related cells and molecules accompany asymmetric division of OSC resulting in the emergence of secondary germ cells, symmetric division, and migration of secondary germ cells, formation of new granulosa cells and fetal and adult primordial follicles (follicular renewal), and selection and growth of primary/preantral, and dominant follicles. The number of selected follicles during each ovarian cycle is determined by autonomic innervation. Morphostasis is altered with advancing age, due to degenerative changes of the immune system. This causes cessation of oocyte and follicular renewal at 38 +/-2 years of age due to the lack of formation of new granulosa cells. Oocytes in primordial follicles persisting after the end of the prime reproductive period accumulate genetic alterations resulting in an exponentially growing incidence of fetal trisomies and other genetic abnormalities with advanced maternal age. The secondary germ cells also develop in the OSC cultures derived from POF and aging ovaries. In vitro conditions are free of immune mechanisms, which prevent neo-oogenesis in vivo. Such germ cells are capable of differentiating in vitro into functional oocytes. This may provide fresh oocytes and genetically related children to women lacking the ability to produce their own follicular oocytes. Further study of "immune physiology" may help us to better understand ovarian physiology and pathology, including ovarian infertility caused by POF or by a lack of ovarian follicles with functional oocytes in aging ovaries. The observations indicating involvement of immunoregulation in physiological neo-oogenesis and follicular renewal from OSC during the fetal and prime reproductive periods are reviewed as well as immune system and age-independent neo-oogenesis and oocyte maturation in OSC cultures, perimenopausal alteration of homeostasis causing disorders of many tissues, and the first OSC culture clinical trial.

Attributes of Oct4 in stem cell biology: perspectives on cancer stem cells of the ovary

Epithelial ovarian cancer (EOC) remains the most lethal of all the gynaecological malignancies with drug resistance and recurrence remaining the major therapeutic barrier in the management of the disease. Although several studies have been undertaken to understand the mechanisms responsible for chemoresistance and subsequent recurrence in EOC, the exact mechanisms associated with chemoresistance/recurrence continue to remain elusive. Recent studies have shown that the parallel characteristics commonly seen between embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSC) are also shared by a relatively rare population of cells within tumors that display stem cell-like features. These cells, termed 'cancer initiating cells' or 'cancer stem cells (CSCs)' have been shown not only to display increased self renewal and pluripotent abilities as seen in ESCs and iPSCs, but are also highly tumorigenic in in vivo mouse models. Additionally, these CSCs have been implicated in tumor recurrence and chemoresistance, and when isolated have consistently shown to express the master pluripotency and embryonic stem cell regulating gene Oct4. This article reviews the involvement of Oct4 in cancer progression and chemoresistance, with emphasis on ovarian cancer. Overall, we highlight why ovarian cancer patients, who initially respond to conventional chemotherapy subsequently relapse with recurrent chemoresistant disease that is essentially incurable.

β-catenin/Tcf-signaling appears to establish the murine ovarian surface epithelium (OSE) and remains active in selected postnatal OSE cells

Background

Wnts are a family of secreted signaling molecules involved in a number of developmental processes including the establishment of cell fate, polarity and proliferation. Recent studies also implicate wnts in epithelial adult stem cell maintenance, renewal and differentiation. Wnts transduce their signal through one of three signaling pathways. The best studied, the wnt/β-catenin pathway, leads to an increase in intracellular β-catenin which acts as a co-transcription factor with members of the Tcf/Lef family. A number of wnts are expressed in the ovary, specifically in the membrana granulosa and ovarian surface epithelium (OSE). We investigated the spatio-temporal pattern of β-catenin/Tcf expression in the OSE using responsive transgenic (TopGal) mice.

Results

The generated β-galactosidase response (lacZ⁺) identified the cell population that overlies the medio-lateral surface of the indifferent gonad at embryonic day (E) 11.5. From E12.5 onwards, lacZ expression disappeared in cells covering the testis but remained with ovary development. LacZ⁺ OSE cells were present throughout embryonic and postnatal ovarian development but demonstrated an age-dependent decrease to a small proportion when animals were weaned and remained at this proportion with aging. Flow cytometric (FACS) and ovarian section analyses showed lacZ⁺ cells constitute approximately 20% of OSE in postnatal (day 1) mice which fell to 8% in 5 day-old animals while in prepubertal and adult mice this accounted for only 0.2% of OSE. Apoptosis was undetected in OSE of neonates and β-catenin/Tcf-signaling cells were proliferative in neonatal mice indicating that neither cell death nor proliferation failure was responsible for the proportion alteration. It appeared that lacZ⁺ cells give rise to lacZ^- cells and this was confirmed in cell cultures. The DNA-binding dye DyeCycle Violet was used to set up the side population (SP) assay aimed at identifying subpopulations of OSE cells with chemoresistance phenotype associated with ABCG2 transporter activity. FACS analysis revealed lacZ⁺ cells exhibit cytoprotective mechanisms as indicated by enrichment within the SP.

Conclusions

The study raises the possibility that wnt/β-catenin-signaling cells constitute a progenitor cell population and could underlie the pronounced histopathology observed for human ovarian cancer.

LIN28 is selectively expressed by primordial and pre-meiotic germ cells in the human fetal ovary

Germ cell development requires timely transition from primordial germ cell (PGC) self-renewal to meiotic differentiation. This is associated with widespread changes in gene expression, including downregulation of stem cell-associated genes, such as OCT4 and KIT, and upregulation of markers of germ cell differentiation and meiosis, such as VASA, STRA8, and SYCP3. The stem cell-expressed RNA-binding protein Lin28 has recently been demonstrated to be essential for PGC specification in mice, and LIN28 is expressed in human germ cell tumors with phenotypic similarities to human fetal germ cells. We have therefore examined the expression of LIN28 during normal germ cell development in the human fetal ovary, from the PGC stage, through meiosis to the initiation of follicle formation. LIN28 transcript levels were highest when the gonad contained only PGCs, and decreased significantly with increasing gestation, coincident with the onset of germ cell differentiation. Immunohistochemistry revealed LIN28 protein expression to be germ cell-specific at all stages examined. All PGCs expressed LIN28, but at later gestations expression was restricted to a subpopulation of germ cells, which we demonstrate to be primordial and premeiotic germ cells based on immunofluorescent colocalization of LIN28 and OCT4, and absence of overlap with the meiosis marker SYCP3. We also demonstrate the expression of the LIN28 target precursor pri-microRNA transcripts pri-LET7a/f/d and pri-LET-7g in the human fetal ovary, and that expression of these is highest at the PGC stage, mirroring that of LIN28. The spatial and temporal restriction of LIN28 expression and coincident peaks of expression of LIN28 and target pri-microRNAs suggest important roles for this protein in the maintenance of the germline stem cell state and the regulation of microRNA activity in the developing human ovary.