Mouse Ovarian Very Small Embryonic-Like Stem Cells Resist Chemotherapy and Retain Ability to Initiate Oocyte-Specific Differentiation

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.

Which side of the coin are you on regarding possible postnatal oogenesis?

It is well known that oocytes are produced during fetal development and that the total number of primary follicles is determined at birth. In humans, there is a constant loss of follicles after birth until about two years of age. The number of follicles is preserved until the resumption of meiosis at puberty and there is no renewal of the oocytes; this dogma was maintained in the last century because there were no suitable techniques to detect and obtain stem cells. However, following stem cell markers, several scientists have detected them in developing and adult human ovarian tissues, especially in the ovarian surface epithelial cells. Furthermore, many authors using different methodological strategies have indicated this possibility. This evidence has led many scientists to explore this hypothesis; there is no definitive consensus to accept this idea. Interestingly, oocyte retrieval from mature ovaries and other tissue sources of stem cells has contributed to the development of strategies for the retrieval of mature oocytes, useful for assisted reproductive technology. Here, we review the evidence and controversies on oocyte neooogenesis in adult women; in addition, we agree with the idea that this process may occur in adulthood and that its alteration may be related to various pathologies in women, such as polycystic ovary syndrome, premature ovarian insufficiency, diminished ovarian reserve and several infertility and genetic disorders.

Initiation of Cancer: The Journey From Mutations in Somatic Cells to Epigenetic Changes in Tissue-resident VSELs

Multiple theories exist to explain cancer initiation, although a consensus on this is crucial for developing effective therapies. 'Somatic mutation theory' suggests that mutations in somatic cells during DNA repair initiates cancer but this concept has several attached paradoxes. Research efforts to identify quiescent cancer stem cells (CSCs) that survive therapy and result in metastasis and recurrence have remained futile. In solid cancers, CSCs are suggested to appear during epithelial-mesenchymal transition by the dedifferentiation and reprogramming of epithelial cells. Pluripotent and quiescent very small embryonic-like stem cells (VSELs) exist in multiple tissues but remain elusive owing to their small size and scarce nature. VSELs are developmentally connected to primordial germ cells, undergo rare, asymmetrical cell divisions and are responsible for the regular turnover of cells to maintain tissue homeostasis throughout life. VSELs are directly vulnerable to extrinsic endocrine insults because they express gonadal and gonadotropin hormone receptors. VSELs undergo epigenetic changes due to endocrine insults and transform into CSCs. CSCs exhibit genomic instability and develop mutations due to errors during DNA replication while undergoing excessive proliferation and clonal expansion to form spheroids. Thus tissue-resident VSELs offer a connection between extrinsic insults and variations in cancer incidence reported in various body tissues. To conclude, cancer is indeed a stem cell disease with mutations occurring as a consequence. In addition to immunotherapy, targeting mutations, and Lgr5 + organoids for developing new therapeutics, targeting CSCs (epigenetically altered VSELs) by improving their niche and epigenetic status could serve as a promising strategy to treat cancer.

Resveratrol-βcd inhibited premature ovarian insufficiency progression by regulating granulosa cell autophagy

BACKGROUND

The ovarian environment of premature ovarian insufficiency (POI) patients exhibits immune dysregulation, which leads to excessive secretion of numerous proinflammatory cytokines that affect ovarian function. An abnormal level of macrophage polarization directly or indirectly inhibits the differentiation of ovarian granulosa cells and steroid hormone production, ultimately leading to POI. Resveratrol, as a health supplement, has been widely recognized for its safety. There is a substantial amount of evidence indicating that resveratrol and its analogs possess significant immune-regulatory functions. It has also been reported that resveratrol can effectively inhibit the progression of POI. However, the underlying immunological and molecular mechanisms through which resveratrol inhibits the progression of POI are still unclear.

RESULTS

Our preliminary reports have shown that resveratrol-βcd, the beta-cyclodextrin complex of resveratrol, significantly enhances the stability of resveratrol. Resveratrol-βcd could regulate the dysfunctional immune status of macrophages and T cells in the tumor microenvironment. In this study, we treated busulfan and cyclophosphamide (B/C)-treated mice, which were used as a POI model, with resveratrol-βcd. After resveratrol-βcd treatment, the levels of IL-6 in the ovaries were significantly increased, and the progression of POI was suppressed. IL-6 activated granulosa cells (GCs) through soluble IL-6R (sIL-6R), promoting autophagy in GCs. Resveratrol-βcd and IL-6 had a synergistic effect on enhancing autophagy in GCs and promoting E2 secretion.

CONCLUSIONS

We partially elucidated the immune mechanism by which resveratrol inhibits the progression of POI and the autophagy-regulating function of GCs. This provides a theoretical basis for using resveratrol to prevent POI in future studies and clinical guidance.

Chito-oligosaccharides and macrophages have synergistic effects on improving ovarian stem cells function by regulating inflammatory factors

BACKGROUND

Chronic low-grade inflammation and ovarian germline stem cells (OGSCs) aging are important reasons for the decline of ovarian reserve function, resulting in ovarian aging and infertility. Regulation of chronic inflammation is expected to promote the proliferation and differentiation of OGSCs, which will become a key means for maintaining and remodeling ovarian function. Our previous study demonstrated that Chitosan Oligosaccharides (Cos) promoted the OGSCs proliferation and remodelled the ovarian function through improving the secretion of immune related factors,but the mechanism remains unclear, and the role of macrophages, the important source of various inflammatory mediators in the ovary needs to be further studied. In this study, we used the method of macrophages and OGSCs co-culture to observe the effect and mechanism of Cos on OGSCs, and explore what contribution macrophages give during this process. Our finding provides new drug treatment options and methods for the prevention and treatment of premature ovarian failure and infertility.

METHODS

We used the method of macrophages and OGSCs co-culture to observe the effect and mechanism of Cos on OGSCs, and explore the important contribution of macrophages in it. The immunohistochemical staining was used to locate the OGSCs in the mouse ovary. Immunofluorescent staining, RT-qPCR and ALP staining were used to identify the OGSCs. CCK-8 and western blot were used to evaluate the OGSCs proliferation. β-galactosidase(SA-β-Gal) staining and western blot were used to detect the changing of cyclin-dependent kinase inhibitor 1A(P21), P53, Recombinant Sirtuin 1(SIRT1) and Recombinant Sirtuin 3(SIRT3). The levels of immune factors IL-2, IL-10, TNF-α and TGF-β were explored by using Western blot and ELISA.

RESULTS

We found that Cos promoted OGSCs proliferation in a dose-and time-dependent manner, accompanied by IL-2, TNF-α increase and IL-10, TGF-β decrease. Mouse monocyte-macrophages Leukemia cells(RAW) can also produce the same effect as Cos. When combined with Cos, it can enhance the proliferative effect of Cos in OGSCs, and further increase IL-2, TNF-α and further decrease IL-10, TGF-β. The macrophages can enhance the proliferative effect of Cos in OGSCs is also associated with the further increase in IL-2, TNF-α and the further decrease in IL-10, TGF-β. In this study, we determined that the anti-aging genes SIRT-1 and SIRT-3 protein levels were increased by Cos and RAW respectively, whereas the senescence-associated SA-β-Gal and aging genes P21 and P53 were decreased. Cos and RAW had a protective effect on OGSCs delaying aging. Furthermore, RAW can further decrease the SA-β-Gal and aging genes P21 and P53 by Cos, and further increase SIRT1 and SIRT3 protein levels in OGSCs by Cos.

CONCLUSION

In conclusion, Cos and macrophages have synergistic effects on improving OGSCs function and delaying ovarian aging by regulating inflammatory factors.

Sertoli cell-only syndrome: advances, challenges, and perspectives in genetics and mechanisms

Male infertility can be caused by quantitative and/or qualitative abnormalities in spermatogenesis, which affects men's physical and mental health. Sertoli cell-only syndrome (SCOS) is the most severe histological phenotype of male infertility characterized by the depletion of germ cells with only Sertoli cells remaining in the seminiferous tubules. Most SCOS cases cannot be explained by the already known genetic causes including karyotype abnormalities and microdeletions of the Y chromosome. With the development of sequencing technology, studies on screening new genetic causes for SCOS are growing in recent years. Directly sequencing of target genes in sporadic cases and whole-exome sequencing applied in familial cases have identified several genes associated with SCOS. Analyses of the testicular transcriptome, proteome, and epigenetics in SCOS patients provide explanations regarding the molecular mechanisms of SCOS. In this review, we discuss the possible relationship between defective germline development and SCOS based on mouse models with SCO phenotype. We also summarize the advances and challenges in the exploration of genetic causes and mechanisms of SCOS. Knowing the genetic factors of SCOS offers a better understanding of SCO and human spermatogenesis, and it also has practical significance for improving diagnosis, making appropriate medical decisions, and genetic counseling. For therapeutic implications, SCOS research, along with the achievements in stem cell technologies and gene therapy, build the foundation to develop novel therapies for SCOS patients to produce functional spermatozoa, giving them hope to father children.

Autologous Human Mesenchymal Stem Cell-Based Therapy in Infertility: New Strategies and Future Perspectives

Infertility could be associated with a few factors including problems with physical and mental health, hormonal imbalances, lifestyles, and genetic factors. Given that there is a concern about the rise of infertility globally, increased focus has been given to its treatment for the last several decades. Traditional assisted reproductive technology (ART) has been the prime option for many years in solving various cases of infertility; however, it contains significant risks and does not solve the fundamental problem of infertility such as genetic disorders. Attention toward the utilization of MSCs has been widely regarded as a promising option in the development of stem-cell-based infertility treatments. This narrative review briefly presents the challenges in the current ART treatment of infertility and the various potential applications of autologous MSCs in the treatment of these reproductive diseases.

Identification of oogonial stem cells in chicken ovary

OBJECTIVES

Oogonial stem cells (OSCs) are germ cells that can sustain neo-oogenesis to replenish the pool of primary follicles in adult ovaries. In lower vertebrates, fresh oocytes are produced by numerous OSCs through mitosis and meiosis during each reproduction cycle, but the OSCs in adult mammals are rare. The birds have retained many conserved features and developed unique features of ovarian physiology during evolution, and the presence of OSCs within avian species remain unknown.

MATERIALS AND METHODS

In this study, we investigated the existence and function of OSCs in adult chickens. The chicken OSCs were isolated and expanded in culture. We then used cell transplantation system to evaluate their potential for migration and differentiation in vivo.

RESULTS

DDX4/SSEA1-positive OSCs were identified in both the cortex and medulla of the adult chicken ovary. These putative OSCs undergo meiosis in the reproductively active ovary. Furthermore, the isolated OSCs were expanded in vitro for months and found to express germline markers similar to those of primordial germ cells. When transplanted into the bloodstream of recipient embryos, these OSCs efficiently migrated into developing gonads, initiated meiosis, and then derived oocytes in postnatal ovaries.

CONCLUSIONS

This study has confirmed the presence of functional OSCs in birds for the first time. The identification of chicken OSCs has great potential for improving egg laying and preserving endangered species.

Extremely Active Nano-formulation of Resveratrol (XAR™) attenuates and reverses chemotherapy-induced damage in mice ovaries and testes

BACKGROUND

Fertility preservation and restoration in cancer patients/survivors is the need of present times when increased numbers of patients get cured of cancer but face infertility as a serious side effect. Resveratrol has beneficial effects on chemoablated ovaries and testes in mice but has failed to enter the clinics because of extremely poor bioavailability. The present study was undertaken to evaluate the protective and curative effects of Extremely active Resveratrol (XAR™)- a nano-formulation of resveratrol with significantly improved bioavailability- on mouse ovary and testis after chemotherapy. Effects of XAR™ and FSH were compared on stimulation of follicle growth in adult mice ovaries. XAR™ (25 mg/kg) was administered for two days prior to chemotherapy to study the protective effects on the mouse gonads. XAR™ was also administered for 14 days post chemoablation to study the regenerative effects. Besides effect on numbers of primordial and growing follicles and spermatogenesis, the effect of XAR™ was also evaluated on the transcripts specific for ovarian/testicular stem/progenitor/germ cells, their proliferation, differentiation, meiosis, and the antioxidant indices.

RESULTS

Similar to FSH, XAR™ increased the numbers of primordial follicles (PF) as well as growing follicles. It protected the gonads from the adverse effects of chemotherapy and showed the ability to regenerate non-functional, chemoablated gonads. Besides stimulating follicle growth in adult ovaries similar to FSH, XAR™ also protected the testes from the adverse effects of chemotherapy and improved spermatogenesis. This was accompanied by improved anti-oxidant indices.

CONCLUSIONS

The results of the present study potentiate the use of XAR™ in pilot clinical studies to protect gonadal function during oncotherapy and also regenerate non-functional gonads in cancer survivors by improving antioxidant indices and stem cell-based tissue regeneration.

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.

GFP Tagged VSELs Help Delineate Novel Stem Cells Biology in Multiple Adult Tissues

Various types of stem cells are being researched upon to exploit their potential for regenerative medicine including pluripotent human embryonic stem (hES) cells derived from spare human embryos, induced pluripotent stem (iPS) cells by reprogramming somatic cells to a pluripotent state and multipotent mesenchymal stem/stromal cells (MSCs) obtained in vitro from multiple tissues. More than 50 independent groups have reported another novel population of pluripotent stem cells in adult tissues termed very small embryonic-like stem cells (VSELs). VSELs are developmentally linked to primordial germ cells, which rather than giving rise to the germ cells and later ceasing to exist, survive throughout life in multiple organs along with tissue-specific adult stem cells better described as lineage-restricted, tissue-committed progenitors with limited plasticity. VSELs survive total body irradiation in bone marrow, oncotherapy in the gonads, bilateral ovariectomy in the uterus and partial pancreatectomy in the pancreas of mice and participate in the regeneration of multiple organs under normal physiological conditions. VSELs and tissue-specific progenitor cells work together in a subtle manner, maintain life-long tissue homeostasis and their dysfunction leads to various pathologies including cancer. However, due to their quiescent state, VSELs have invariably eluded lineage-tracing studies reported so far. Present article reviews novel insights into VSELs biology and how VSELs enriched from GFP (green fluorescent protein) mice have enabled to delineate their role in various biological processes in vivo. VSELs biology needs to be understood in-depth as this alone will help evolve the field of regenerative medicine and win the war against cancer.

Artificial Oocyte: Development and Potential Application

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

Research Advances in Gametogenesis and Embryogenesis Using Pluripotent Stem Cells

The previous studies of human gametogenesis and embryogenesis have left many unanswered questions, which hinders the understanding of the physiology of these two vital processes and the development of diagnosis and treatment strategies for related diseases. Although many results have been obtained from animal studies, particularly mouse research, the results cannot be fully applied to humans due to species differences in physiology and pathology. However, due to ethical and material limitations, the direct study of human gametes and embryos is very difficult. The emergence and rapid development of organoids allow the construction of organoid systems that simulate gametogenesis and embryogenesis in vitro, and many studies have successfully established organoid systems for some parts of or even the entire processes of gametogenesis and embryogenesis. These studies typically start with the establishment of mouse models and then modify these models to obtain human organoid models. These organoid models can be used to obtain a better understanding of the signaling pathways, molecular mechanisms, genetics, and epigenetic changes involved in gametogenesis and embryogenesis and could also be applied to clinical applications, such as drug screening. Here, we discuss the formation of primordial stem cell-like cells (PGCLCs), and in vitro-induced gametes and embryoids using pluripotent stem cells (PSCs). We also analyze their applications and limitations.

Oocyte quality and aging

It is well known that female reproduction ability decreases during the forth decade of life due to age-related changes in oocyte quality and quantity; although the number of women trying to conceive has today increased remarkably between the ages of 36 to 44. The causes of reproductive aging and physiological aspects of this phenomenon are still elusive. With increase in the women's age, during Assisted Reproductive Technologies (ART) we have perceived a significant decline in the number and quality of retrieved oocytes, as well as in ovarian follicle reserves. This is because of increased aneuploidy due to factors such as spindle apparatus disruption; oxidative stress and mitochondrial damage. The aim of this review paper is to study data on the potential role of the aging process impacting oocyte quality and female reproductive ability. We present the current evidence that show the decreased oocyte quality with age, related to reductions in female reproductive outcome. The aging process is complicated and it is caused by many factors that control cellular and organism life span. Although the factors responsible for reduced oocyte quality remain unknown, the present review focuses on the potential role of ovarian follicle environment, oocyte structure and its organelles. To find a way to optimize oocyte quality and ameliorate clinical outcomes for women with aging-related causes of infertility.

Overview of follicle stimulating hormone and its receptors in reproduction and in stem cells and cancer stem cells

Follicle stimulating hormone (FSH) and its receptor (FSHR) have been reported to be responsible for several physiological functions and cancers. The responsiveness of stem cells and cancer stem cells towards the FSH-FSHR system make the function of FSH and its receptors more interesting in the context of cancer biology. This review is comprised of comprehensive information on FSH-FSHR signaling in normal physiology, gonadal stem cells, cancer cells, and potential options of utilizing FSH-FSHR system as an anti-cancer therapeutic target.

Endogenous, tissue-resident stem/progenitor cells in gonads and bone marrow express FSHR and respond to FSH via FSHR-3

Follicle stimulating hormone (FSH) is secreted by the anterior pituitary and acts on the germ cells indirectly through Granulosa cells in ovaries and Sertoli cells in the testes. Extragonadal action of FSH has been reported but is still debated. Adult tissues harbor two populations of stem cells including a reserve population of primitive, small-sized, pluripotent very small embryonic-like stem cells (VSELs) and slightly bigger, tissue-specific progenitors which include ovarian stem cells (OSCs) in ovaries, spermatogonial stem cells (SSCs) in testes, endometrial stem cells (EnSCs) in uterus and hematopoietic stem cells (HSCs) in the bone marrow. Data has accumulated in animal models showing FSHR expression on both VSELs and progenitors in ovaries, testes, uterus and bone marrow and eventually gets lost as the cells differentiate further. FSH exerts a direct action on the stem/progenitor cells via alternatively spliced FSHR-3 rather than the canonical FSHR-1. FSH stimulates VSELs to undergo asymmetrical cell divisions to self-renew and give rise to the progenitors that in turn undergo symmetrical cell divisions and clonal expansions followed by differentiation into specific cell types. Excessive self-renewal of VSELs results in cancer and this explains ubiquitous expression of embryonic markers including nuclear OCT-4 along with FSHR in cancerous tissues. Focus of this review is to compile published data to support this concept. FSHR expression in stem/progenitor cells was confirmed by immuno-fluorescence, Western blotting, in situ hybridization and by quantitative RT-PCR. Two different commercially available antibodies (Abcam, Santacruz) were used to confirm specificity of FSHR expression along with omission of primary antibody and pre-incubation of antibody with immunizing peptide as negative controls. Western blotting allowed detection of alternatively spliced FSHR isoforms. Oligoprobes and primers specific for Fshr-1 and Fshr-3 were used to study these alternately-sliced isoforms by in situ hybridization and their differential expression upon FSH treatment by qRT-PCR. To conclude, stem/progenitor cells in adult tissues express FSHR and directly respond to FSH via FSHR-3. These findings change the field of FSH-FSHR biology, call for paradigm shift, explain FSHR expression on cancer cells in multiple organs and provide straightforward explanations for various existing conundrums including extragonadal expression of FSHR.

Bone marrow-derived mesenchymal stem cells combined with gonadotropin therapy restore postnatal oogenesis of chemo-ablated ovaries in rats via enhancing very small embryonic-like stem cells

BACKGROUND

Very small embryonic-like stem cells (VSELs) are a rare population within the ovarian epithelial surface. They contribute to postnatal oogenesis as they have the ability to generate immature oocytes and resist the chemotherapy. These cells express markers of pluripotent embryonic and primordial germ cells.

OBJECTIVE

We aimed to explore the capability of VSELs in restoring the postnatal oogenesis of chemo-ablated rat ovaries treated with bone marrow-derived mesenchymal stem cells (BM-MSCs) combined with pregnant mare serum gonadotropin (PMSG).

METHODS

Female albino rats were randomly assigned across five groups: I (control), II (chemo-ablation), III (chemo-ablation + PMSG), IV (chemo-ablation + MSCs), and V (chemo-ablation + PMSG + MSCs). Postnatal oogenesis was assessed through measurement of OCT4, OCT4A, Scp3, Mvh, Nobox, Dazl4, Nanog, Sca-1, FSHr, STRA8, Bax, miR143, and miR376a transcript levels using qRT-PCR. Expression of selected key proteins were established as further confirmation of transcript expression changes. Histopathological examination and ovarian hormonal assessment were determined.

RESULTS

Group V displayed significant upregulation of all measured genes when compared with group II, III or IV. Protein expression confirmed the changes in transcript levels as group V displayed the highest average density in all targeted proteins. These results were confirmed histologically by the presence of cuboidal germinal epithelium, numerous primordial, unilaminar, and mature Graafian follicles in group V.

CONCLUSION

VSELs can restore the postnatal oogenesis in chemo-ablated ovaries treated by BM-MSCs combined with PMSG.

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.

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.

Experimental study for the establishment of a chemotherapy-induced ovarian insufficiency model in rats by using cyclophosphamide combined with busulfan

With an improvement in the survival rate of cancer patients, chemotherapy-induced premature ovarian insufficiency (POI) is increasingly affecting the quality of life of female patients. Currently, there are many relevant studies using mice as an animal model. However, a large coefficient of variation for weight in mice is not appropriate for endocrine-related studies, compared with rats; therefore, it is necessary to identify an appropriate experimental model in rats. In this study, cyclophosphamide combined with busulfan was used to establish an animal model. We compared several common modeling methods using chemotherapeutic drugs, cisplatin, cyclophosphamide, and 4-vinylcyclohexene diepoxide (VCD), and we found that the combination of cyclophosphamide and busulfan was more effective in establishing a POI model in rats with few side effects by analyzing general physical conditions, pathological tissue sections of heart, liver, lung, spleen, kidney, uterus, and ovary, serum hormone levels, and follicle counts; thus, providing a more reliable model basis for subsequent studies.

Two Stem Cell Populations Including VSELs and CSCs Detected in the Pericardium of Adult Mouse Heart

Adult mammalian heart is considered to be one of the least regenerative organs as it is not able to initiate endogenous regeneration in response to injury unlike in lower vertebrates and neonatal mammals. Evidence is now accumulating to suggest normal renewal and replacement of cardiomyocytes occurs even in middle-aged and old individuals. But underlying mechanisms leading to this are not yet clear. Do tissue-resident stem cells exist or somatic cells dedifferentiate leading to regeneration? Lot of attention is currently being focused on epicardium as it is involved in cardiac development, lodges multipotent progenitors and is a source of growth factors. Present study was undertaken to study the presence of stem cells in the pericardium. Intact adult mouse heart was subjected to partial enzymatic digestion to collect the pericardial cells dislodged from the surface. Pericardial cells suspension was processed to enrich the stem cells using our recently published protocol. Two populations of stem cells were successfully enriched from the pericardium of adult mouse heart along with distinct 'cardiospheres' with cytoplasmic continuity (formed by rapid proliferation and incomplete cytokinesis). These included very small embryonic-like stem cells (VSELs) and slightly bigger 'progenitors' cardiac stem cells (CSCs). Expression of pluripotent (Oct-4A, Sox-2, Nanog), primordial germ cells (Stella, Fragilis) and CSCs (Oct-4, Sca-1) specific transcripts was studied by RT-PCR. Stem cells expressed OCT-4, NANOG, SSEA-1, SCA-1 and c-KIT. c-KIT was expressed by cells of different sizes but only smaller CD45^-c-KIT⁺ VSELs possess regenerative potential. Inadvertent loss of stem cells while processing for different experiments has led to misperceptions & controversies existing in the field of cardiac stem cells and requires urgent rectification. VSELs/CSCs have the potential to regenerate damaged cardiac tissue in the presence of paracrine support provided by the mesenchymal stromal cells.

Adult tissue-resident stem cells-fact or fiction?

Life-long tissue homeostasis of adult tissues is supposedly maintained by the resident stem cells. These stem cells are quiescent in nature and rarely divide to self-renew and give rise to tissue-specific “progenitors” (lineage-restricted and tissue-committed) which divide rapidly and differentiate into tissue-specific cell types. However, it has proved difficult to isolate these quiescent stem cells as a physical entity. Recent single-cell RNAseq studies on several adult tissues including ovary, prostate, and cardiac tissues have not been able to detect stem cells. Thus, it has been postulated that adult cells dedifferentiate to stem-like state to ensure regeneration and can be defined as cells capable to replace lost cells through mitosis. This idea challenges basic paradigm of development biology regarding plasticity that a cell enters point of no return once it initiates differentiation. The underlying reason for this dilemma is that we are putting stem cells and somatic cells together while processing for various studies. Stem cells and adult mature cell types are distinct entities; stem cells are quiescent, small in size, and with minimal organelles whereas the mature cells are metabolically active and have multiple organelles lying in abundant cytoplasm. As a result, they do not pellet down together when centrifuged at 100–350g. At this speed, mature cells get collected but stem cells remain buoyant and can be pelleted by centrifuging at 1000g. Thus, inability to detect stem cells in recently published single-cell RNAseq studies is because the stem cells were unknowingly discarded while processing and were never subjected to RNAseq. This needs to be kept in mind before proposing to redefine adult stem cells.

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.

Kallikrein-kinin system and oxidative stress in cisplatin-induced ovarian toxicity

Kallikrein-kinin system (KKS) is involved in vascular reactivity and inflammatory response to cytotoxic drugs. Since cisplatin is a widely used chemotherapy and its cytotoxic mechanism can trigger inflammation and oxidative damage, in this work we evaluated the role of KKS in an animal model of cisplatin-induced ovarian toxicity. Biomarkers of ovarian stem cells, activity of KKS, inflammation and oxidative damage were measured in ovarian tissue of C57BL/6 female mice treated with vehicle or cisplatin (2.5 mg/kg). Cisplatin group presented greater number of atretic follicles, and lower numbers of antral and total viable follicles. Ki67, DDX4 and OCT-4 markers were similar between groups. Cisplatin triggered plasma and ovarian tissue kallikrein generation; and increased expression of bradykinin receptors B1 and B2. Neutrophil and macrophage infiltration markers increased. Superoxide anion generation also increased, while reduced glutathione levels decreased. These results suggest that KKS is activated and contributes to ovarian injury during cisplatin treatment.

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.

Stem cells survive oncotherapy & can regenerate non-functional gonads: A paradigm shift for oncofertility

A large proportion of patients who survive cancer are rendered infertile as an unwanted side effect of oncotherapy. Currently accepted approaches for fertility preservation involve banking eggs/sperm/embryos or ovarian/testicular tissue before oncotherapy for future use. Such approaches are invasive, expensive, technically challenging and depend on assisted reproductive technologies (ART). Establishing a gonadal tissue bank (for cancer patients) is also fraught with ethical, legal and safety issues. Most importantly, patients who find it difficult to meet expenses towards cancer treatment will find it difficult to meet expenses towards gonadal tissue banking and ART to achieve parenthood later on. In this review an alternative strategy to regenerate non-functional gonads in cancer survivors by targeting endogenous stem cells that survive oncotherapy is discussed. A novel population of pluripotent stem cells termed very small embryonic-like stem cells (VSELs), developmentally equivalent to late migratory primordial germ cells, exists in adult gonads and survives oncotherapy due to their quiescent nature. However, the stem-cell niche gets compromised by oncotherapy. Transplanting niche cells (Sertoli or mesenchymal cells) can regenerate the non-functional gonads. This approach is safe, has resulted in the birth of fertile offspring in mice and could restore gonadal function early in life to support proper growth and later serve as a source of gametes. This newly emerging understanding on stem cells biology can obviate the need to bank gonadal tissue and fertility may also be restored in existing cancer survivors who were earlier deprived of gonadal tissue banking before oncotherapy.

Heterogeneity of Stem Cells in the Ovary

Every organ in the body is thought to harbor two populations of stem cells, including the quiescent and the actively dividing, that leads to heterogeneity among them. It is generally believed that the ovary harbors a fixed number of follicles at birth that differentiate during fetal development from the primordial germ cells. The numbers of follicles decrease by age, leading to menopause. However, in 2004, it was suggested that ovary may harbor stem cells that are possibly involved in the formation of new follicles throughout reproductive life. Research over little more than a decade shows that ovarian stem cells include a quiescent population of very small embryonic-like stem cells (VSELs) and slightly bigger, actively dividing ovarian stem cells (OSCs). This heterogeneity among ovarian stem cells is similar to the presence of VSELs along with spermatogonial stem cells (SSCs) in the testis or hematopoietic stem cells (HSCs) in the hematopoietic system. VSELs express embryonic markers, including nuclear OCT-4, and are lodged in the ovary surface epithelium (OSE). Ovarian VSELs undergo asymmetric cell division to self-renew and give rise to OSCs that in turn undergo symmetric cell divisions and clonal expansion (germ cell nest) followed by meiosis to form an oocyte that gets assembled as a primordial follicle. Both VSELs and OSCs also express receptors for follicle-stimulating hormone (FSHR) and are directly activated by FSH to undergo neo-oogenesis and primordial follicle assembly. Whether stimulation of ovaries by FSH in Infertility Clinics activates the stem cells leading to the formation of multiple follicles needs further investigation. Epithelial cells lining the surface of ovary provide a niche to the stem cells under normal circumstances and undergo epithelial-mesenchymal transition (EMT) to form granulosa cells for primordial follicle assembly. Compromised function of the epithelial cells with age possibly leads to inability of stem cells to form follicles, leading to menopause. More than 90% of ovarian cancers arise in the OSE, possibly due to excessive self-renewal of VSELs. Altered biology of the OSE cells results in the formation of myofibroblasts by EMT and may provide a cancerous niche that supports excessive expansion of the stem cells lodged in the OSE, leading to ovarian cancer. Ovarian cancer cells express markers like OCT-4 and FSHR, which are also expressed by the VSELs lodged in the OSE, whereas the epithelial cells are distinctly negative for the same. Lot more research is required in the field to gain further understanding of ovarian stem cell biology.

Mesenchymal Stem Cells (MSCs) Therapy for Recovery of Fertility: a Systematic Review

In recent years, the mesenchymal stem cells (MSCs) have provided the new opportunities to treat different disorders including infertility. Different studies have suggested that the MSCs have ability to differentiate into germ-like cells under specific induction conditions as well as transplantation to gonadal tissues. The aim of this systematic review was to evaluate the results obtained from different studies on MSCs therapy for promoting fertility. This search was done in PubMed and Science Direct databases using key words MSCs, infertility, therapy, germ cell, azoospermia, ovarian failure and mesenchymal stem cell. Among the more than 11,400 papers, 53 studies were considered eligible for more evaluations. The obtained results indicated that the most studies were performed on MSCs derived from bone marrow and umbilical cord as compared with the other types of MSCs. Different evaluations on animal models as well as in vitro studies supported from their role in the recovery of spermatogenesis and folliculogenesis. Although the data obtained from this systematic review are promising, but the further studies need to assess the efficiency and safety of transplantation of these cells in fertility recovery.

Isolation and in vitro culture of ovarian stem cells in Chinese soft-shell turtle (Pelodiscus sinensis)

Gonadal cell lines provide valuable tools for studying gametogenesis, sex differentiation, and manipulating germ cells in reproductive biology. Female germline stem cells have been characterized and isolated from ovaries of mammalian species, including mice and human, but there has been very few studies on female germline stem cells in reptiles. Here, we described an ovarian stem cell-like line isolated and cultured from the Chinese soft-shell turtle (Pelodiscus sinensis), designated as PSO1. The cells showed high alkaline phosphatase activity with a normal diploid karyotype. As shown by reverse transcription-polymerase chain reaction, the cells were positive for the expression of germ cell-specific genes, vasa and dazl, as well as a stem cell marker, nanog, but negative for the expression of the folliculogenesis-specific gene, figla. Likewise, through fluorescent immunostaining analyses, both the Dazl and Vasa proteins were detected abundantly in the cytoplasm of perinuclear region, whereas Nanog and PCNA were dominantly observed in the nuclei in PSO1 cells. Moreover, PSO1 cells transfected with pCS2:h2b-egfp could properly express the fusion protein in the nuclei. Taken together, the findings suggested that the germline stem cells exist in the ovary of juvenile Chinese soft-shell turtle and these cells can be isolated for a long-term in vitro culture under experimental conditions. This study has provided a valuable basis for further investigations on the molecular mechanisms whereby the germline stem cells develop and differentiate into gametes in turtles. Also, it has paved the way for studies on oogenesis in turtles, even in the other reptiles.

Further characterization of adult sheep ovarian stem cells and their involvement in neo-oogenesis and follicle assembly

BACKGROUND

Stem cells in the ovary comprise of two distinct populations including very small embryonic-like stem cells (VSELs) and slightly bigger progenitors termed ovarian stem cells (OSCs). They are lodged in ovary surface epithelium (OSE) and are expected to undergo neo-oogenesis and primordial follicle (PF) assembly in adult ovaries. The ovarian stem cells express follicle stimulating hormone (FSH) receptors and are directly activated by FSH resulting in formation of germ cell nests (GCN) in vitro. Present study was undertaken to further characterize adult sheep OSCs and to understand their role during neo-oogenesis and PF assembly.

METHODS

Stem cells were collected by gently scraping the OSE cells and were characterized by H&E staining, immuno-localization, immuno-phenotyping and RT-PCR studies. Expression of FSH receptors and markers specific for stem cells (OCT-4, SSEA-4) and proliferation (PCNA) were studied on stem/progenitor cells in OSE culture and on adult sheep ovarian cortical tissue sections. Effect of FSH on stem cells was also studied in vitro. Asymmetric cell division (ACD) was monitored by studying expression of OCT-4 and NUMB.

RESULTS

Additional evidence was generated on the presence of two populations of stem cells in the OSE including VSELs and OSCs. FSHR expression was observed on both VSELs and OSCs by immuno-localization and immuno-phenotyping studies. FSH treatment in vitro stimulated VSELs that underwent ACD to self-renew and give rise to OSCs which divided rapidly by symmetric cell divisions (SCD) and clonal expansion with incomplete cytokinesis to form GCN. ACD was further confirmed by differential expression of OCT-4 in VSELs and NUMB in the OSCs. Immuno-histochemical expression of OCT-4, PCNA and FSHR was noted on stem cells located in the OSE in sheep ovarian sections. GCN and cohort of PF were observed in the ovarian cortex and provided evidence in support of neo-oogenesis from the stem cells.

CONCLUSION

Results of present study provide further evidence in support of two stem cells populations in adult sheep ovary. Both VSELs, OSCs and GCN express FSH receptors and FSH possibly regulates their function to undergo neo-oogenesis and primordial follicle assembly.

Improved Isolation, Proliferation, and Differentiation Capacity of Mouse Ovarian Putative Stem Cells

The recent discovery of ovarian stem cells in postnatal mammalian ovaries, also referred to as putative stem cells (PSCs), and their roles in mammalian fertility has challenged the long-existing theory that women are endowed with a certain number of germ cells. The rare amount of PSCs is the major limitation for utilizing them through different applications. Therefore, this study was conducted in six phases to find a way to increase the number of Fragilis- and mouse vasa homolog (MVH)-positive sorted cells from 14-day-old NMRI strain mice. Results showed that there is a population of Fragilis- and MVH-positive cells with pluripotent stem cell characteristics, which can be isolated and expanded for months in vitro. PSCs increase their proliferation capacity under the influence of some mitogenic agents, and our results showed that different doses of stem cell factor (SCF) induce PSC proliferation with the maximum increase observed at 50 ng/mL. SCF was also able to increase the number of Fragilis- and MVH-positive cells after sorting by magnetic-activated cell sorting and enhance colony formation efficiency in sorted cells. Differentiation capacity assay indicated that there is a basic level of spontaneous differentiation toward oocyte-like cells during 3 days of culture. However, relative gene expression was significantly higher in the follicle-stimulating hormone-treated groups, especially in the Fragilis- sorted PSCs. We suggest that higher number of PSCs provides us either a greater source of energy that can be injected into energy-impaired oocytes in women with a history of repeat IVF failure or a good source for research.

Making gametes from alternate sources of stem cells: past, present and future

Infertile couples including cancer survivors stand to benefit from gametes differentiated from embryonic or induced pluripotent stem (ES/iPS) cells. It remains challenging to convert human ES/iPS cells into primordial germ-like cells (PGCLCs) en route to obtaining gametes. Considerable success was achieved in 2016 to obtain fertile offspring starting with mouse ES/iPS cells, however the specification of human ES/iPS cells into PGCLCs in vitro is still not achieved. Human ES cells will not yield patient-specific gametes unless and until hES cells are derived by somatic cell nuclear transfer (therapeutic cloning) whereas iPS cells retain the residual epigenetic memory of the somatic cells from which they are derived and also harbor genomic and mitochondrial DNA mutations. Thus, they may not be ideal starting material to produce autologus gametes, especially for aged couples. Pluripotent, very small embryonic-like stem cells (VSELs) have been reported in adult tissues including gonads, are relatively quiescent in nature, survive oncotherapy and can be detected in aged, non-functional gonads. Being developmentally equivalent to PGCs (natural precursors to gametes), VSELs spontaneously differentiate into gametes in vitro. It is also being understood that gonadal stem cells niche is compromised by oncotherapy and with age. Improving the gonadal somatic niche could regenerate non-functional gonads from endogenous VSELs to restore fertility. Niche cells (Sertoli/mesenchymal cells) can be directly transplanted and restore gonadal function by providing paracrine support to endogenous VSELs. This strategy has been successful in several mice studies already and resulted in live birth in a woman with pre-mature ovarian failure.

Ovarian stem cells-resolving controversies

A recent review on ovarian stem cells by Horan and Williams entitled "Oocyte Stem Cells: Fact or Fantasy?" suggests that the debate on ovarian stem cells (OSCs) is still not over. They did not even discuss the presence of two distinct populations of stem cells in the ovary in their review. OSCs are located in the ovary surface epithelium and Tilly's group reported them in the size range of 5-8 μm whereas Virant-Klun's group has reported pluripotent, 2-4 μm OSCs. Our group reported OSCs of two distinct sizes including pluripotent very small embryonic-like stem cells (VSELs) which are smaller in size than RBCs (similar to those reported by Virant-Klun's group) and slightly bigger (similar to those reported by Tilly's group) tissue committed progenitors (OSCs) that presumably differentiate from VSELs. These stem/progenitor cells express receptors for follicle stimulating hormone (FSH) and are activated by FSH. Our opinion article provides explanation to several open-ended questions raised in the review on OSCs by Horan and Williams. VSELs survive chemotherapy; maintain life-long homeostasis; loss of their function due to a compromised niche results in age-related senescence and presence of overlapping pluripotent markers suggest that they may also be implicated in epithelial ovarian cancers.

Mouse Bone Marrow VSELs Exhibit Differentiation into Three Embryonic Germ Lineages and Germ & Hematopoietic Cells in Culture

Very small embryonic-like stem cells (VSELs) have been reported in various adult tissues, express pluripotent and primordial germ cells (PGCs) specific markers, are mobilized under stress/disease conditions, give rise to tissue committed progenitors and thus help regenerate and maintain homeostasis. The aim of the present study was to evaluate in vitro differentiation potential of VSELs using a quantitative approach. VSELs were collected from mouse bone marrow after 4 days of 5-fluorouracil (5-FU, 150 mg/Kg) treatment, further enriched by size based filtration and cultured on a feeder support in the presence of specific differentiation media. Cultured VSELs were found to differentiate into all three embryonic germ cell lineages, germ and hematopoietic cells after 14 days in culture. This was confirmed by studying Nestin, PDX-1, NKX2.5, DAZL, CD45 and other markers expression by various approaches. Very small, CD45 negative cells collected and enriched from GFP positive 5-FU treated mice bone marrow transitioned into CD45 positive cells in vitro thus demonstrating that VSELs can give rise to hematopoietic stem cells (HSCs). We envision that VSELs may be responsible for plasticity and ability of bone marrow cells to give rise to non-hematopoietic tissue progenitors of all 3 germ layers. Moreover the ability of VSELs to differentiate into germ cells as well as all the three lineages provides further evidence to support their pluripotent state and confirms developmental link between bone marrow VSELs and PGCs. The property of quiescence, no risk of teratoma formation and autologus source, make pluripotent VSELs a potential candidate to facilitate endogenous regeneration compared to cell replacement strategy envisioned using embryonic and induced pluripotent stem cells.

Overexpression of miR-21 in stem cells improves ovarian structure and function in rats with chemotherapy-induced ovarian damage by targeting PDCD4 and PTEN to inhibit granulosa cell apoptosis

BACKGROUND

Chemotherapy-induced premature ovarian failure (POF) is a severe complication affecting tumor patients at a childbearing age. Mesenchymal stem cells (MSCs) can partially restore the ovarian structure and function damaged by chemotherapy. miR-21 is a microRNA that can regulate cell apoptosis. This study discusses the repair effect and mechanism of MSCs overexpressing miR-21 on chemotherapy-induced POF.

METHODS

Rat MSCs and granulosa cells (GCs) were isolated in vitro. MSCs were transfected with miR-21 lentiviral vector (LV-miR-21) to obtain MSCs stably expressing miR-21 (miR-21-MSCs). The microenvironment of an ovary receiving chemotherapy was mimicked by adding phosphamide mustard (PM) into the cellular culture medium. The apoptosis rate and the mRNA and protein expression of target genes PTEN and PDCD4 were detected in MSCs. Apoptosis was induced by adding PM into the culture medium for GCs, which were cocultured with miR-21-MSCs. The apoptosis rate and the mRNA and protein expression of PTEN and PDCD4 were detected. The chemotherapy-induced POF model was built into rats by intraperitoneal cyclophosphamide injection. miR-21-MSCs were transplanted into the bilateral ovary. The rats were sacrificed at 15, 30, 45, and 60 days after the last injection. The ovarian weights, follicle count, estrous cycle, and sex hormone levels (estradiol (E2) and follicle-stimulating hormone (FSH)) were detected. Apoptosis of GCs was determined by TUNEL assay. The miR-21 and mRNA and protein expression of PTEN and PDCD4 were determined.

RESULTS

The apoptosis decreased in MSCs transfected with miR-21. The mRNA and protein expression of target genes PTEN and PDCD4 was downregulated. GCs cocultured with miR-21-MSCs showed a decreased apoptosis, an upregulation of miR-21, and a downregulation of PTEN and PDCD4. Following the injection of miR-21-MSCs, the ovarian weight and follicle counts increased; E₂ levels increased while FSH levels decreased, with less severe apoptosis of GCs. The miR-21 expression in the ovaries was upregulated, while the mRNA expression and protein expression of PTEN and PDCD4 were downregulated.

CONCLUSIONS

Overexpression of miR-21 in MSCs promoted efficacy against chemotherapy-induced POF and its improvement of the repair effect was related to the inhibition of GC apoptosis by targeting PTEN and PDCD4.

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.

Putative germline and pluripotent stem cells in adult mouse ovary and their in vitro differentiation potential into oocyte-like and somatic cells

According to classical knowledge of reproductive biology, in the ovary of female mammals there is a limited number of oocytes and there is no possibility of renewal if the oocytes are lost due to disease or injury. However, in recent years, the results of some studies on renewal and formation of oocytes and follicles in the adult mammalian ovary have led to the questioning of this opinion. The aim of our study is to demonstrate the presence of putative germline and pluripotent stem cells in the adult mouse ovary and their differentiation potential into germ and somatic cells. In ovary tissues and cells harvested from pre-differentiation step, the expression of pluripotent and germline stem cell markers was analysed by reverse transcription-polymerase chain reaction (RT-PCR), immunofluorescence staining and western blotting. Embryoid bodies that formed in this step were analysed using immunofluorescence staining and transmission electron microscopy. Ovarian stem cells were induced to differentiate into oocyte, osteoblast, chondrocyte and neural cells. Besides morphological observation, differentiated cells were analysed by RT-PCR, histochemical and immunofluorescence staining. Expression of germline and pluripotent stem cell markers both in mRNA and at the protein level were detected in the pre-differentiated cells and ovary tissues. As a result of the differentiation process, the formation of oocyte-like cells, osteoblasts, chondrocytes and neural cells was observed and characteristics of differentiated cells were confirmed using the methods mentioned above. Our study results revealed that the adult mouse ovary contains germline and pluripotent stem cells with the capacity to differentiate into oocyte-like cells, osteoblasts, chondrocytes and neural cells.

A Novel View of the Adult Stem Cell Compartment From the Perspective of a Quiescent Population of Very Small Embryonic-Like Stem Cells

Evidence has accumulated that adult hematopoietic tissues and other organs contain a population of dormant stem cells (SCs) that are more primitive than other, already restricted, monopotent tissue-committed SCs (TCSCs). These observations raise several questions, such as the developmental origin of these cells, their true pluripotent or multipotent nature, which surface markers they express, how they can be efficiently isolated from adult tissues, and what role they play in the adult organism. The phenotype of these cells and expression of some genes characteristic of embryonic SCs, epiblast SCs, and primordial germ cells suggests their early-embryonic deposition in developing tissues as precursors of adult SCs. In this review, we will critically discuss all these questions and the concept that small dormant SCs related to migratory primordial germ cells, described as very small embryonic-like SCs, are deposited during embryogenesis in bone marrow and other organs as a backup population for adult tissue-committed SCs and are involved in several processes related to tissue or organ rejuvenation, aging, and cancerogenesis. The most recent results on successful ex vivo expansion of human very small embryonic-like SC in chemically defined media free from feeder-layer cells open up new and exciting possibilities for their application in regenerative medicine.

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.

Used protocols for isolation and propagation of ovarian stem cells, different cells with different traits

Although existence of ovarian stem cells (OSCs) in mammalian postnatal ovary is still under controversy, however, it has been almost accepted that OSCs are contributing actively to folliculogenesis and neo-oogenesis. Recently, various methods with different efficacies have been employed for OSCs isolation from ovarian tissue, which these methods could be chosen depends on aim of isolation and accessible equipments and materials in lab. Although isolated OSCs from different methods have various traits and characterizations, which might become from their different nature and origin, however these stem cells are promising source for woman infertility treatment or source of energy for women with a history of repeat IVF failure in near future. This review has brought together and summarized currently used protocols for isolation and propagation of OSCs in vitro.

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.

Testicular Stem Cells Express Follicle-Stimulating Hormone Receptors and Are Directly Modulated by FSH

Testicular spermatogonial stem cells (SSCs) are a heterogeneous population of stem cells, and definitive marker for the most primitive subset that undergoes asymmetric cell division remains to be identified. A novel subpopulation of pluripotent, very small embryonic-like stem cells (VSELs) has been reported in both human and mouse testes. Follicle-stimulating hormone (FSH) receptors (FSHRs) are expressed on Sertoli cells in testis and on granulosa cells in ovary, but recently FSHRs are reported on VSELs in ovaries, bone marrow, and cord blood. The present study was aimed to investigate whether FSHRs are also expressed on testicular stem cells (VSELs and SSCs) and their possible modulation by FSH using intact and chemoablated (25 mg/kg busulfan) mice. Chemoablated testis was a better model to study stem cell biology since quiescent stem cells survive along with the Sertoli cells in the tubules. Proliferating cell nuclear antigen-positive, small-sized cells presumed to be VSELs were clearly visualized, and flow cytometry analysis revealed an increase in LIN^-/CD45^-/SCA-1⁺ VSELs from 0.045±0.008% to 0.1±0.03% of total cells in chemoablated testis after FSH treatment. Very small embryonic-like stem cells expressing nuclear octamer-binding transcription factor 4 (OCT-4) and SSCs with cytoplasmic OCT-4 were detected. Very small embryonic-like stem cells (Oct-4A, Sca-1, Nanog), SSCs (Oct-4), and proliferation (Pcna) specific transcripts were upregulated on FSH treatment. Stem cells expressed FSHR and were stimulated by FSH, and Fshr3 was the predominant transcript maximally modulated by FSH. Nuclear OCT-4 and SCA-1 (stem cell antigen 1) positive VSELs are the most primitive stem cells in testis, and FSH stimulates them to undergo asymmetric cell division including self-renewal and give rise to SSCs, which in turn proliferate rapidly and undergo clonal expansion and further differentiation.