Renewed debate over postnatal oogenesis in the mammalian ovary

Oogonial stem cells as a model to study age-associated infertility in women

Fertility is the first biological process to break down during aging, thereby making it a useful tool to understand fundamental processes of aging. Reproductive aging in females is associated with a loss of ovarian function characterised by a reduction in the number and quality of oocytes. The central dogma, namely that females are born with a fixed pool of oocytes that progressively decline with increasing maternal age, has been challenged by evidence supporting postnatal oogenesis in mammals. Reports demonstrating formation of new oocytes from newly discovered germline stem cells, referred to as oogonial stem cells (OSCs), has opened new avenues for treatment of female infertility. In this review we discuss why the OSCs possibly lose their regenerative potential over time, and focus specifically on the aging process in germline stem cells as a possible mechanism for understanding female age-related infertility and how we can slow or delay ovarian aging.

Review Article: Stem Cells in Human Reproduction

The derivation of human embryonic stem (hES) cells heralds a new era in stem cell research, generating excitement for their therapeutic potential in regenerative medicine. Pioneering work of embryologists, developmental biologists, and reproductive medicine practitioners in in vitro fertilization clinics has facilitated hES cell research. This review summarizes current research focused on optimizing hES cell culture conditions for good manufacturing practice, directing hES cell differentiation toward trophectoderm and germ cells, and approaches used to reprogram cells for pluripotent cell derivation. The identification of germ stem cells in the testis and the recent controversy over their existence in the ovary raise the possibility of harnessing them for treating young cancer survivors. There is also the potential to harvest fetal stem cells with pluripotent cell-like properties from discarded placental tissues. The recent identification of adult stem/progenitor cell activity in the human endometrium offers a new understanding of common gynecological diseases. Discoveries resulting from research into embryonic, germ, fetal, and adult stem cells are highly relevant to human reproduction.

Ovarian stem cells: From basic to clinical applications

The field of reproductive biology has undergone significant developments in the last decade. The notion that there is a fixed reserve pool of oocytes before birth was established by Zuckerman in 1951. However, in 2004, an article published in nature challenged this central dogma of mammalian reproductive biology. Tilly’s group reported the existence of ovarian germline stem cells (GSCs) in postnatal ovaries of mice and suggested that the bone marrow could be an extragonadal source of ovarian GSCs. These findings were strongly criticized; however, several independent groups have since successfully isolated and characterized ovarian GSCs in postnatal mice. The ovarian GSCs are located in the ovarian surface epithelium and express markers of undifferentiated GSCs. When transplanted into mouse ovaries, mouse ovarian GSCs could differentiate and produce embryos and offspring. Similarly, in a recent study, ovarian GSCs were found to be present in the ovaries of women of reproductive age. Conversely, there is increasing evidence that stem cells responsible for maintaining a healthy state in normal tissue may be a source of some cancers, including ovarian cancer. Cancer stem cells (CSCs) have been found in many tissues, including ovaries. Some researchers have suggested that ovarian cancer may be a result of the transformation and dysfunction of ovarian GSCs with self-renewal properties. Drug resistant and metastasis-generating CSCs are responsible for many important problems affecting ovarian cancer patients. Therefore, the identification of CSCs will provide opportunities for the development of new therapeutic strategies for treatments for infertility and ovarian cancer. In this article, we summarize the current understanding of ovarian GSCs in adult mammals, and we also discuss whether there is a relationship between GSCs and CSCs.

The intergenerational effects of war on the health of children

BACKGROUND

The short- and medium-term effects of conflict on population health are reasonably well documented. Less considered are its consequences across generations and potential harms to the health of children yet to be born.

DISCUSSION

Looking first at the nature and effects of exposures during conflict, and then at the potential routes through which harm may propagate within families, we consider the intergenerational effects of four features of conflict: violence, challenges to mental health, infection and malnutrition. Conflict-driven harms are transmitted through a complex permissive environment that includes biological, cultural and economic factors, and feedback loops between sources of harm and weaknesses in individual and societal resilience to them. We discuss the multiplicative effects of ongoing conflict when hostilities are prolonged.

SUMMARY

We summarize many instances in which the effects of war can propagate across generations. We hope that the evidence laid out in the article will stimulate research and--more importantly--contribute to the discussion of the costs of war; particularly in the longer-term in post-conflict situations in which interventions need to be sustained and adapted over many years.

Ovarian germline stem cells: an unlimited source of oocytes?

While there has been progress in directing the development of embryonic stem cells and induced pluripotent stem cells toward a germ cell state, their ability to serve as a source of functional oocytes in a clinically relevant model or situation has yet to be established. Recent studies suggest that the adult mammalian ovary is not endowed with a finite number of oocytes, but instead possesses stem cells that contribute to their renewal. The ability to isolate and promote the growth and development of such ovarian germline stem cells (GSCs) would provide a novel means to treat infertility in women. Although such ovarian GSCs are well characterized in nonmammalian model organisms, the findings that support the existence of adult ovarian GSCs in mammals have been met with considerable evidence that disputes their existence. This review details the lessons provided by model organisms that successfully utilize ovarian GSCs to allow for a continual and high level of female germ cell production throughout their life, with a specific focus on the cellular mechanisms involved in GSC self-renewal and oocyte development. Such an overview of the role that oogonial stem cells play in maintaining fertility in nonmammalian species serves as a backdrop for the data generated to date that supports or disputes the existence of GSCs in mammals as well as the future of this area of research in terms of its potential for any application in reproductive medicine.

An evolutionary perspective on adult female germline stem cell function from flies to humans

The concept that oogenesis continues into reproductive life has been well established in nonmammalian species. Recent studies of mice and women indicate that oocyte formation is also not, as traditionally believed, restricted to the fetal or perinatal periods. Analogous to de novo oocyte formation in flies and fish, newly formed oocytes in adult mammalian ovaries arise from germline stem cells (GSCs) or, more specifically, oogonial stem cells (OSCs). Studies of mice have confirmed that isolated OSCs, once delivered back into adult ovaries, are capable of generating fully functional eggs that fertilize to produce healthy embryos and offspring. Parallel studies of OSCs recently purified from ovaries of reproductive-age women indicate that these cells closely resemble their mouse ovary-derived counterparts, although the fertilization competency of oocytes generated by human OSCs awaits clarification. Despite the ability of OSCs to produce new oocytes during adulthood, oogenesis will still ultimately cease with age, contributing to ovarian failure. The causal mechanisms behind these events in mammals are unknown, but studies of flies have revealed that GSC niche dysfunction plays a critical role in age-related oogenic failure. Such insights derived from evaluation of nonmammalian species, in which postnatal oogenesis has been studied in depth, may aid in development of new strategies to alleviate ovarian failure and infertility in mammals.

The next (re)generation of ovarian biology and fertility in women: is current science tomorrow's practice?

Stem cell-based strategies for ovarian regeneration and oocyte production have been proposed as future clinical therapies for treating infertility in women. However, utilization of embryonic stem cells or induced pluripotent stem cells to produce oocytes has had limited success in vitro. A recent report of the isolation and characterization of endogenous oocyte-producing or oogonial stem cells (OSCs) from ovaries of reproductive age women describes the first stable and pure human female germ cell culture model in which a subset of cells appear to initiate and complete meiosis. In addition, purified human OSCs introduced into adult human ovarian cortical tissue generate oocytes that arrest at the diplotene stage of meiosis and successfully recruit granulosa cells to form new primordial follicles. This overview examines the current landscape of in vitro and in vivo gametogenesis from stem cells, with emphasis on generation of human oocytes. Future research objectives for this area of work, as well as potential clinical applications involving the use of human OSCs, are discussed.

Ovarian stem cells and aging

OBJECTIVE

To review successes to date in the field of ovarian stem cell research and discuss the evidence supporting their potential to rejuvenate the follicular pool during adult life; to present factors that may contribute to their competence; and to address the question of why menopause is an inevitable outcome of advanced age if ovarian stem cells exist.

METHOD

In a review of the literature, relevant articles were identified through a PubMed literature search from inception to July 2010.

RESULTS

The current concept that mammalian ovaries possess a static ovarian reserve is at odds with the experimental results discussed in this review. Ovarian stem cells are likely to be the source of germline stem cells during fetal and adult life, due to their potential to differentiate into competent oocytes given a suitable environment.

CONCLUSIONS

Stem cells in different compartments share properties such as pluripotency, self-renewal, and diminished regenerative potential in old age. Our model of ovarian stem cell aging suggests that menopause is driven by an age-related decline in ovarian stem cell function rather than depletion of a non-renewable follicular reserve. Understanding how ovarian stem cells interact with their surrounding environment moves us a step closer to controlling the female biological clock when it might be clinically desirable.

No evidence for the presence of oogonia in the human ovary after their final clearance during the first two years of life

BACKGROUND

Conflicting results of studies on mouse and human have either verified or refuted the presence of oogonia/primordial germ cells in the post-natal ovary. The aim of this study was to trace whether oogonia recognized by immunohistochemical methods in the first trimester human ovary were present also in peri- and post-natal ovaries.

METHODS

For this study, 82 human ovaries were collected: 25 from embryos from 5 to 10 weeks post conception (wpc), 2 at 18 wpc, 32 from 32 wpc to 2 years and 23 from 2 to 32 years. Of these, 80 ovaries were fixed and paraffin-embedded and 2 (8 year-old) ovaries were processed for plastic sections. Serial sections were prepared for immunohistochemical detection of markers for oogonia: tyrosine kinase receptor for stem cell factor (SCF)(C-KIT), stage-specific embryonic antigen-4 (SSEA4), homeobox gene transcription factor (NANOG), octamer binding transcription factor 4 (OCT4) and melanoma antigen-4 (Mage-A4), while noting that C-KIT also stains diplotene oocytes.

RESULTS

Almost all oogonia exclusively stained for SSEA4, NANOG, OCT4 and C-KIT, whereas MAGE-A4 only stained a small fraction. At birth only a few oogonia were stained. These disappeared before 2 years, leaving only diplotene oocytes stained for C-KIT. From 18 wpc to 2 years, the medulla contained conglomerates of healthy and degenerating oogonia and small follicles, waste baskets (WBs) and oogonia enclosed in growing follicles (FWB). Medulla of older ovaries contained groups of primordial, healthy follicles.

CONCLUSIONS

We found no evidence for the presence of oogonia in the human ovary after their final clearing during the first 2 years. We suggest that perinatal medullary WB and FWB give rise to the groups of small, healthy follicles in the medulla.

PAR6, a potential marker for the germ cells selected to form primordial follicles in mouse ovary

Partitioning-defective proteins (PAR) are detected to express mainly in the cytoplast, and play an important role in cell polarity. However, we showed here that PAR6, one kind of PAR protein, was localized in the nuclei of mouse oocytes that formed primordial follicles during the perinatal period, suggesting a new role of PAR protein. It is the first time we found that, in mouse fetal ovaries, PAR6 appeared in somatic cell cytoplasm and fell weak when somatic cells invaded germ cell cysts at 17.5 days post coitus (dpc). Meanwhile, the expression of PAR6 was observed in cysts, and became strong in the nuclei of some germ cells at 19.5 dpc and all primordial follicular oocytes at 3 day post parturition (dpp), and then obviously declined when the primordial follicles entered the folliculogenic growth phase. During the primordial follicle pool foundation, the number of PAR6 positive germ cells remained steady and was consistent with that of formed follicles at 3 dpp. There were no TUNEL (apoptosis examination) positive germ cells stained with PAR6 at any time studied. The number of follicles significantly declined when 15.5 dpc ovaries were treated with the anti-PAR6 antibody and PAR6 RNA interference. Carbenoxolone (CBX, a known blocker of gap junctions) inhibited the expression of PAR6 in germ cells and the formation of follicles. Our results suggest that PAR6 could be used as a potential marker of germ cells for the primordial follicle formation, and the expression of PAR6 by a gap junction-dependent process may contribute to the formation of primordial follicles and the maintenance of oocytes at the diplotene stage.

Collaborative work on evaluation of ovarian toxicity. 5) Two- or four-week repeated-dose studies and fertility study of busulfan in female rats

Busulfan, an antineoplastic agent that targets small follicles (primordial and primary follicles), was given orally to female Sprague-Dawley rats (0, 0.1, 0.5, or 1.5 mg/kg/day; n = 10 in each group) for 2 or 4 weeks to assess the optimal administration period for detection of the toxic effects on ovarian morphology. Isolated ovaries were used for histopathological analysis and follicle counts. In addition, a female fertility study was conducted by giving the same dose levels of busulfan from 2 weeks before mating to day 7 of pregnancy to determine the non-observed-adverse-effect-level (NOAEL) for female reproduction. In the 2-week study, all rats treated with busulfan showed normal estrous cyclicity and no toxicological changes in weight or histopathology of the ovaries. In the 4-week study, a decrease in small follicles was found histopathologically in 1 rat, even at 0.5 mg/kg, and in 4 rats at 1.5 mg/kg. Proliferating cell nuclear antigen immunohistochemistry of the follicles confirmed the above decrease in number of small follicles at 1.5 mg/kg. In the female fertility study, increases in dead embryos and post-implantation loss were found in rats at 1.5 mg/kg. Taken together, the NOAELs were 1.5 mg/kg for reproductive performance and 0.5 mg/kg for early embryonic development. In conclusion, the present study indicates that a 4-week administration period and appropriate assessment, including careful histopathological analysis of stage-based follicles are needed to detect small follicle depletion in a general toxicity study used as a first-titer screen.

Stem cell potential of the mammalian gonad

Stem cells have enormous potential for therapeutic application because of their ability to self-renew and differentiate into different cell types. Gonads, which consist of somatic cells and germ cells, are the only organs capable of transmitting genetic materials to the offspring. Germ-line stem cells and somatic stem cells have been found in the testis; however, the presence of stem cells in the ovary remains controversial. In this review, we discuss studies focusing on whether stem cell properties are present in the different cell types of male and female gonads and their implications on stem cell research.

The current status of evidence for and against postnatal oogenesis in mammals: a case of ovarian optimism versus pessimism?

Whether or not oogenesis continues in the ovaries of mammalian females during postnatal life was heavily debated from the late 1800s through the mid-1900s. However, in 1951 Lord Solomon Zuckerman published what many consider to be a landmark paper summarizing his personal views of data existing at the time for and against the possibility of postnatal oogenesis. In Zuckerman's opinion, none of the evidence he considered was inconsistent with Waldeyer's initial proposal in 1870 that female mammals cease production of oocytes at or shortly after birth. This conclusion rapidly became dogma, and remained essentially unchallenged until just recently, despite the fact that Zuckerman did not offer a single experiment proving that adult female mammals are incapable of oogenesis. Instead, 20 years later he reemphasized that his conclusion was based solely on an absence of data he felt would be inconsistent with the idea of a nonrenewable oocyte pool provided at birth. However, in the immortal words of Carl Sagan, an "absence of evidence is not evidence of absence." Indeed, building on the efforts of a few scientists who continued to question this dogma after Zuckerman's treatise in 1951, we reported several data sets in 2004 that were very much inconsistent with the widely held belief that germ cell production in female mammals ceases at birth. Perhaps not surprisingly, given the magnitude of the paradigm shift being proposed, this work reignited a vigorous debate that first began more than a century ago. Our purpose here is to review the experimental evidence offered in recent studies arguing support for and against the possibility that adult mammalian females replenish their oocyte reserve. "Never discourage anyone who continually makes progress, no matter how slow."-Plato (427-347 BC).

Stem cells: a perspective on oocytes

Stem cells, with their unlimited self-renewal feature and their ability to differentiate into almost every mature cell type in the body, have enormous potential for research and therapeutic application. In this article, we review the formation of primordial germ cells, the precursors of adult gametocytes, from their specification to their migration to prospective gonads. We discuss recent studies that obtained germ cells from stem cells in vitro. We place special emphasis on studies that challenge the current dogma in reproductive biology that female mammals are born with a set number of nonrenewable germ cells in the ovary by showing germ cell renewal in the adult ovary.

Expression of stem and germ cell markers within nonfollicle structures in adult mouse ovary

Recent studies have suggested that germline stem cells may generate new follicles in the adult murine ovary. In this study, the authors use a pou5f1-enhanced green fluorescent protein (EGFP) transgenic mouse model to study the expression of stem and germ cell markers in adult murine ovaries. Immunohistochemical analyses and reverse transcription polymerase chain reaction were performed to detect the expression of mouse vasa homologue, stem cells factor receptor, stage-specific embryonic antigen 1, synaptonemal complex proteins, disrupted meiotic, and growth differentiation factor-9 in GFP+ ovarian tissues. GFP+ cell aggregates of nonfollicle structures were identified and isolated from adult B6.CBA-Tg(pou5f1-EGFP)2Mnn/J transgenic mouse ovaries. This study shows the presence of cell aggregates that are distinct from ovarian follicles and are coexpressing germline and stem cell surface markers in adult murine ovaries. These cell aggregates may represent a mixed population of germ cells and germline stem cells. Further research is necessary to evaluate the plasticity of the potential stem cell population in these cell aggregates.

Ovarian germ cells

Surface cells in adult ovaries represent germ line-competent embryonic stem cells. They are a novel type of totipotent progenitors for distinct cell types including female germ cells/oocytes, with the potential for use in the autologous treatment of ovarian infertility and stem cell therapy. Ovarian infertility and stem cell therapy are complex scientific, therapeutic, and socioeconomic issues, which are accompanied by legal restrictions in many developed countries. We have described the differentiation of distinct cell types and the production of new eggs in cultures derived from adult human ovaries. The possibility of producing new eggs from ovarian surface epithelium representing totipotent stem cells supports new opportunities for the treatment of premature ovarian failure, whether idiopathic or after cytostatic chemotherapy treatment, as well as infertility associated with aged primary follicles, and infertility after natural menopause. The stem cells derived from adult human ovaries can also be used for stem cell research and to direct autologous stem cell therapy. This chapter describes general considerations regarding the egg origin from somatic progenitor cells, oogenesis and follicle formation in fetal and adult human ovaries (follicular renewal), including the promotional role of the immune system-related cells in vivo, and possible causes of ovarian infertility. It then provides detailed protocols for the separation and cultivation of adult ovarian stem cells.

BAX regulates follicular endowment in mice

It is believed that the endowment of primordial follicles in mammalian ovaries is finite. Once follicles are depleted, infertility ensues. Thus, the size of the initial endowment has consequences for fertility and reproductive longevity. Follicular endowment is comprised of various processes that culminate with the incorporation of meiosis-arrested oocytes into primordial follicles. Apoptosis is prominent during follicular endowment, and apoptosis regulatory genes are involved in its regulation. Conflicting data exist with regard to the role of the proapoptotic Bcl-2 associated X protein (BAX) in follicular endowment. Therefore, we investigated the role of BAX during follicular endowment in embryonic and neonatal ovaries. We found that BAX is involved in regulating follicular endowment in mice. Deletion ofBaxyields increased oocyte numbers in embryonic ovaries and increased follicle numbers in neonatal ovaries when compared with wild-type ovaries. Increased follicular endowment inBax−/−ovaries is not due to enhanced germ cell viability. Further, it is not due to an increased primordial germ cell (PGC) allotment, a delay in the onset of meiosis, or altered proliferative activity of oogonia. Instead, our data suggest that the regulatory activity of BAX in follicular endowment likely occurs during PGC migration, prior to PGC colonization of the gonad.

Establishment of ovarian reserve: a quantitative morphometric study of the developing human ovary

OBJECTIVE

To assess directly the dynamics of the formation of the ovarian reserve in the normal human ovary by evaluating the total number of follicles in developing ovaries when folliculogenesis occurs.

DESIGN

Histomorphometry-based follicle counts in complete serial tissue sections.

SETTING

Functional Anatomy Research Center, University of Milano.

PATIENT(S)

Thirteen fetuses, neonates, and one 8-month-old infant.

INTERVENTION(S)

Fifteen ovaries were completely cut, obtaining serial sections. Ovarian volume, volume fractions, density and total number of follicles per ovary were calculated using quantitative morphometric methods.

MAIN OUTCOME MEASURE(S)

Age-related dynamics of the establishment of ovarian reserve in human developing ovary at the end of the organogenesis.

RESULT(S)

The ovarian reserve (100,000 follicles at 15 weeks of postconceptional age) increased progressively to 680,000 follicles at 34 weeks. At 8 months of postnatal age the pool was still about 680,000 primordial follicles.

CONCLUSION(S)

The consistence of the primordial follicle pool during organogenesis shows an exponential increase until month 8 of prenatal life and it is subsequently maintained without modifications at least until month 8 of postnatal life.

Germline stem cells and neo-oogenesis in the adult human ovary

It remains unclear whether neo-oogenesis occurs in postnatal ovaries of mammals, based on studies in mice. We thought to test whether adult human ovaries contain germline stem cells (GSCs) and undergo neo-oogenesis. Rather than using genetic manipulation which is unethical in humans, we took the approach of analyzing the expression of meiotic marker genes and genes for germ cell proliferation, which are required for neo-oogenesis, in adult human ovaries covering an age range from 28 to 53 years old, compared to testis and fetal ovaries served as positive controls. We show that active meiosis, neo-oogenesis and GSCs are unlikely to exist in normal, adult, human ovaries. No early meiotic-specific or oogenesis-associated mRNAs for SPO11, PRDM9, SCP1, TERT and NOBOX were detectable in adult human ovaries using RT-PCR, compared to fetal ovary and adult testis controls. These findings are further corroborated by the absence of early meiocytes and proliferating germ cells in adult human ovarian cortex probed with markers for meiosis (SCP3), oogonium (OCT3/4, c-KIT), and cell cycle progression (Ki-67, PCNA), in contrast to fetal ovary controls. If postnatal oogenesis is confirmed in mice, then this species would represent an exception to the rule that neo-oogenesis does not occur in adults.

BAX is involved in regulating follicular growth, but is dispensable for follicle atresia in adult mouse ovaries

Mammalian females are endowed with a finite number of primordial follicles at birth or shortly thereafter. Immediately following the formation of the primordial follicle pool, cohorts of these follicles are recruited to begin growth, and this recruitment continues until the primordial follicle population is depleted. Once recruited, a follicle will either grow and ovulate or undergo atresia. Follicle atresia results from the apoptotic death of follicular cells. Members of the BCL-2 family of proteins are important regulators of apoptosis in most cells including in the ovary. Here, we tested the hypothesis that the proapoptotic BAX is an important regulator of follicle survival. We used a variety of histological and biochemical techniques to investigate the impact ofBaxdeletion on follicle growth and death. We observed that theBaxdeletion results in delayed vaginal opening and altered follicular growth. Young adultBax-deficient ovaries contained increased numbers of primordial follicles and a trend towards reduced numbers of growing follicles.Baxdeficiency led to a reduction in average litter size, and also a reduction in the number of oocytes ovulated in response to exogenous gonadotropins. In contrast,Baxdeficiency did not alter follicle atresia. In conclusion, BAX appears to be an important regulator of follicle growth, but is dispensable for follicle atresia in mice.

Quantification of healthy follicles in the neonatal and adult mouse ovary: evidence for maintenance of primordial follicle supply

Proliferation and partial meiotic maturation of germ cells in fetal ovaries is believed to establish a finite, non-renewable pool of primordial follicles at birth. The supply of primordial follicles in postnatal life should be depleted during folliculogenesis, either undergoing atresia or surviving to ovulation. Recent studies of mouse ovaries propose that intra- and extraovarian germline stem cells replenish oocytes and form new primordial follicles. We quantified all healthy follicles in C57BL/6 mouse ovaries from day 1 to 200 using unbiased stereological methods, immunolabelling of oocyte meiosis (germ cell nuclear antigen (GCNA)) and ovarian cell proliferation (proliferating cell nuclear antigen (PCNA)) and electronmicroscopy. Day 1 ovaries contained 7924+/-1564 (s.e.m.) oocytes or primordial follicles, declining on day 7 to 1987+/-203, with 200-800 oocytes ejected from individual ovaries on that day and day 12. Discarded oocytes and those subjacent to the surface epithelium were GCNA-positive indicating their incomplete meiotic maturation. From day 7 to 100 mean numbers of primordial follicles per ovary were not significantly depleted but declined at 200 days to 254+/-71. Mean numbers of all healthy follicles per ovary were not significantly different from day 7 to 100 (range 2332+/-349-3007+/-322). Primordial follicle oocytes were PCNA-negative. Occasional unidentified cells were PCNA-positive with mitotic figures observed in the cortex of day 1 and 12 ovaries. Although we found no evidence for ovarian germline stem cells, our data support the hypothesis of postnatal follicle renewal in postnatal and adult ovaries of C57BL/6 mice.

Tumor necrosis factor (TNF) receptor type 2 is an important mediator of TNF alpha function in the mouse ovary

It is believed that a finite pool of primordial follicles is established during embryonic and neonatal life. At birth, the mouse ovary consists of clusters of interconnected oocytes surrounded by pregranulosa cells. Shortly after birth these structures, termed germ cell cysts or nests (GCN), break down to facilitate primordial follicle formation. Tumor necrosis factor alpha (TNF) is a widely expressed protein with myriad functions. TNF is expressed in the ovary and may regulate GCN breakdown in rats. We investigated whether it participates in GCN breakdown and follicle formation in mice by using an in vitro ovary culture system as well as mutant animal models. We found that TNF and both receptors (TNFRSF1A and TNFRSF1B) are expressed in neonatal mouse ovaries and that TNF promotes oocyte death in neonatal ovaries in vitro. However, deletion of either receptor did not affect follicle endowment, suggesting that TNF does not regulate GCN breakdown in vivo. Tnfrsf1b deletion led to an apparent acceleration of follicular growth and a concomitant expansion of the primordial follicle population. This expansion of the primordial follicle population does not appear to be due to decreased primordial follicle atresia, although this cannot be ruled out completely. This study demonstrates that mouse oocytes express both TNF receptors and are sensitive to TNF-induced death. Additionally, TNFRSF1B is demonstrated to be an important mediator of TNF function in the mouse ovary and an important regulator of folliculogenesis.

Stem cells and fertility: what does the future hold?

PURPOSE OF REVIEW

The long-held belief that 'the total number of oocytes present in the mammalian ovary is generated during fetal ovarian development with no additional oocyte formation during reproductive life' has recently been challenged. This review aims to summarize the scientific evidence and discuss the criticism put forth by other investigators in the field. In addition, we will entertain possible future directions, underlining clinical implications of de-novo oocyte formation during adulthood.

RECENT FINDINGS

An initial report of oocyte generation from mouse stem cells in vitro was followed by a publication by Johnson et al., suggesting that new oocyte formation occurs in adult mice. Their more recent findings point toward bone marrow as a source of germline stem cells that give rise to new oocytes.

SUMMARY

The findings of Johnson et al. strongly suggest the generation of oocytes from stem cells in the adult mouse. Criticism by prominent investigators in the field has been voiced but not yet adequately supported by experimental evidence. Crucially, independent confirmation of the findings of Johnson et al. is also lacking. If proven to occur in human, de-novo oocyte formation from stem cells would have significant implications for fertility preservation.

Potential new strategies for the treatment of ovarian infertility and degenerative diseases with autologous ovarian stem cells

The 50-year-old and currently prevailing view that all oocytes in adult human ovaries persist from the fetal period of life is controversial as it clashes with Darwinian evolutionary theory. Studies of oogenesis and follicular renewal in adult human ovaries, and of the role of hormonal signals and third-party cells (tissue macrophages and T cells), could all be helpful in providing better understanding of the causes of ovarian infertility, its prevention and potential therapy. In addition, the authors recently reported differentiation of distinct cell types and the production of new eggs in cultures derived from premenopausal and postmenopausal human ovaries. It is possible that fertilisation of such eggs will open up new opportunities for providing genetically related children to infertile women for whom conventional in vitro fertilisation has failed. As ovarian stem cells appear to represent a new type of totipotent adult stem cell, they could also be utilised for autologous stem cell therapy of degenerative diseases, without any involvement of allogeneic embryonic stem cells and somatic cell nuclear transfer.

Eggs forever?

A group of scientists from Harvard Medical School (Johnson et al., 2004) claims to have "established the existence of proliferative germ cells that sustain oocyte and follicle production in the postnatal mammalian ovary," expressing no doubts about their methods, results and conclusion. Johnson et al. based their conclusions of oocyte and follicular renewal from existing germline stem cells (GSC) in the postnatal mouse ovary on three types of observations: (1) A claimed discordance in follicle loss versus follicle atresia in the neonatal period and in the following pubertal and adult period; (2) immunohistochemical detection of proliferating GSC with meiotic capacity using combined markers for meiosis, germline, and mitosis; and (3) neo-folliculogenesis in ovarian chimeric grafting experiments with adult mice. Oogenesis is the process that transforms the proliferative oogonium into an oocyte through meiosis, followed by folliculogenesis and follicular and oocyte maturation. The most crucial part in producing a functional oocyte is firstly, initiation and completion of the first meiotic prophase, and secondly, enclosure of the resulting diplotene oocyte in a follicle. Neither of these two events has been shown to take place in Johnson et al.'s study of the postnatal mouse ovary. We hereby address the observations underpinning their hypothesis and conclude that it is premature to replace the paradigm that adult mammalian neo-oogenesis/folliculogenesis does not take place.

Régénération des ovocytes : fantasme ou réalité ?

The central dogma of female reproductive biology has long held that oogenesis ceases around birth in almost all mammals. Consequently, the ovarian reserve is definitely fixed and will continuously deplete up to the menopause in humans. New papers by Johnson et al. (2004) and by Bukovsky et al. (2004, 2005) overturn this dogma since they claim that female mice and women, respectively, produce new oocytes during adult life. The following review is mainly devoted to an analysis of the study by Johnson et al. (2004) as well as unpublished data submitted for publication from the same authors. Two bodies of evidence were presented to support their hypothesis: morphologic and experimental. Whereas morphologic data can be easily refuted, experimental data remain disturbing. In conclusion, these recent observations need to be confirmed by others, and extended by additional supporting evidence before they could be considered to question the dogma of a fixed stock of oocytes at birth.

Foxl2 is required for commitment to ovary differentiation

Genetic control of female sex differentiation from a bipotential gonad in mammals is poorly understood. We find that mouse XX gonads lacking the forkhead transcription factor Foxl2 form meiotic prophase oocytes, but then activate the genetic program for somatic testis determination. Pivotal Foxl2 action thus represses the male gene pathway at several stages of female gonadal differentiation. This suggests the possible continued involvement of sex-determining genes in maintaining ovarian function throughout female reproductive life.