Germline stem cells in the postnatal mammalian ovary: a phenomenon of prosimian primates and mice?

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

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

Reproductive aging and menopause-like transition in the menstruating spiny mouse (Acomys cahirinus)

STUDY QUESTION

Does the naturally menstruating spiny mouse go through menopause?

SUMMARY ANSWER

Our study is the first to show a natural and gradual menopausal transition in a rodent.

WHAT IS KNOWN ALREADY

Age-related depletion of the human ovarian reserve (OvR) leads to menopause, the permanent cessation of menstruation and reproduction. Current rodent models of menopause are inappropriate for inferences of the human condition, as reproductive senescence is abrupt or induced through ovariectomy. The spiny mouse is the only confirmed rodent with a naturally occurring menstrual cycle.

STUDY DESIGN, SIZE, DURATION

Histological assessment of virgin spiny mice occurred in females aged 6 months (n = 14), 1 year (n = 7), 2 years (n = 13), 3 years (n = 9) and 4 years (n = 9). Endocrinology was assessed in a further 9 females per age group. Five animals per group were used for ovarian stereology with additional ovaries collected at prenatal Day 35 (n = 3), day of birth (n = 5), postnatal Days 35 (n = 5) and 100 (n = 5) and 15 months (n = 5).

PARTICIPANTS/MATERIALS, SETTING, METHODS

Morphological changes in the reproductive system were examined using hematoxylin and eosin stains. Proliferating cell nuclear antigen immunohistochemistry assessed endometrial proliferation and sex steroids estradiol and testosterone were assayed using commercial ELISA kits.

MAIN RESULTS AND THE ROLE OF CHANCE

The proportion of females actively cycling was 86% at 6 months, 71% at 1 year, 69% at 2 years, 56% at 3 years and 44% at 4 years. Uterine and ovarian weights declined steadily from 1 year in all groups and corresponded with loss of uterine proliferation (P < 0.01). Estradiol was significantly decreased at 1 and 2 years compared to 6-month-old females, before becoming erratic at 3 and 4 years, with no changes in testosterone across any age. Fully formed primordial follicles were observed in prenatal ovaries. Aging impacted on both OvR and growing follicle numbers (P < 0.001-0.0001). After the age of 3 years, the follicle decline rate increased more than 5-fold.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This is a descriptive study in a novel research rodent whereby reagents validated for use in the spiny mouse were limited.

WIDER IMPLICATIONS OF THE FINDINGS

The gradual, rather than sudden, menopausal transition suggests that the spiny mouse is a more appropriate perimenopausal model than the current rodent models in which to examine the neuroendocrine pathways that encompass all hormonal interactions in the hypothalamic-pituitary-gonadal axis. The logistic, ethical and economic advantages of such a model may reduce our reliance on primates in menopause research and enable more thorough and invasive investigation than is possible in humans.

STUDY FUNDING/COMPETING INTEREST(S)

Hudson Institute is supported by the Victorian State Government Operational Infrastructure Scheme. The authors declare no competing interests.

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

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

In Vitro Generation of Oocytes from Ovarian Stem Cells (OSCs): In Search of Major Evidence

The existence of ovarian stem cells (OSCs) in women as well as their physiological role in post-menopausal age are disputed. However, accumulating evidence demonstrated that, besides the animal models including primarily mice, even in adult women putative OSCs obtained from ovarian cortex are capable to differentiate in vitro into oocyte-like cells (OLCs) expressing molecular markers typical of terminal stage of oogonial cell lineage. Recent studies describe that, similarly to mature oocytes, the OSC-derived OLCs also contain haploid karyotype. As proof of concept of their stem commitment, OSCs from mice differentiated to oocytes in vitro are suitable to be fertilized and implanted in sterilized animals resulting in embryo development. Despite enthusiasm for these data, which definitely require extended confirmation before considering potential application in humans for treatment of ovarian insufficiency, OSCs appear suitable for other clinical uses, restoring the endocrine derangements in premature ovarian failure or for fertility preservation in oncologic patients after anti-cancer treatments. In this context, the selection of viable oocytes generated from OSCs before chemotherapy protocols would overcome the potential adjunct oncogenic risk in women bearing hormone-dependent tumors who are repeatedly stimulated with high dose estrogens to induce oocyte maturation for their egg recruitment and cryopreservation.

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

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

Cell therapy for chemically induced ovarian failure in mice

Cell therapy has been linked to an unexplained return of ovarian function and fertility in some cancer survivors. Studies modeling this in mice have shown that cells transplantation generates donor-derived oocytes in chemotherapy-treated recipients. This study was conducted to further clarify the impact of cell transplantation from different sources on female reproductive function after chemotherapy using a preclinical mouse model. Methods. Female mice were administered 7.5 mg/kg cisplatin followed by cell transplantation (one week later) using GFP+ female cell donors. For cell tracking, adipose derived stem cell GFP+ (ADSC), female germline stem cell GFP+/MVH+ (FGSC), or ovary cell suspension GFP+ mice were transplanted into cisplatin-treated wild-type recipients. After 7 or 14 days animals were killed and histological analysis, IHQ for GFP cells, and ELISA for estradiol were performed. Results. Histological examinations showed that ADSC, ovary cell suspension, and FGSC transplant increase the number of follicles with apparent normal structure in the cells recipient group euthanized on day 7. Cell tracking showed GFP+ samples 7 days after transplant. Conclusion. These data suggest that intraovarian injection of ADSCs and FGSC into mice with chemotherapy-induced ovarian failure diminished the damage caused by cisplatin.

Ovarian germline stem cells

It has long been established that germline stem cells (GSCs) are responsible for lifelong gametogenesis in males, and some female invertebrates (for example, Drosophila) and lower vertebrates (for example, teleost fish and some prosimians) also appear to rely on GSCs to replenish their oocyte reserve in adulthood. However, the presence of such cells in the majority of female mammals is controversial, and the idea of a fixed ovarian reserve determined at birth is the prevailing belief among reproductive biologists. However, accumulating evidence demonstrates the isolation and culture of putative GSCs from the ovaries of adult mice and humans. Live offspring have been reportedly produced from the culture of adult mouse GSCs, and human GSCs formed primordial follicles using a mouse xenograft model. If GSCs were present in adult female ovaries, it could be postulated that the occurrence of menopause is not due to the exhaustion of a fixed supply of oocytes but instead is a result of GSC and somatic cell aging. Alternatively, they may be benign under normal physiological conditions. If their existence were confirmed, female GSCs could have many potential applications in both basic science and clinical therapies. GSCs not only may provide a valuable model for germ cell development and maturation but may have a role in the field of fertility preservation, with women potentially being able to store GSCs or GSC-derived oocytes from their own ovaries prior to infertility-inducing treatments. Essential future work in this field will include further independent corroboration of the existence of GSCs in female mammals and the demonstration of the production of mature competent oocytes from GSCs cultured entirely in vitro.

Combating ovarian aging depends on the use of existing ovarian follicles, not on putative oogonial stem cells

Ovarian aging is characterized by both a reduction in egg quality and a drastic reduction in the number of ovarian follicles. It has been generally accepted for 60 years that a fixed population of primordial follicles is established in the ovaries during early life, and in most mammalian species, oocytes cannot renew themselves in postnatal or adult life. This dogma, however, has been challenged over the past decade. In this review, we summarize the recent studies on primordial follicles and putative oogonial stem cells and discuss what resources in the ovary might be more reliable and promising source tools for combating ovarian aging.

Stem cells and the reproductive system: historical perspective and future directions

Recent findings in stem cell biology have presented new perspectives and opportunities for the treatment of reproductive disease. In a departure from the long held dogma of embryologically fixed numbers of oocytes, current literature suggests that human ovaries contain stem cells which form new oocytes even in adulthood and that these stem cells can be cultured in vitro to develop into mature oocytes. These findings have provided new hope and broader options for fertility preservation. Evidence of endometrial regeneration by bone marrow stem cells in endometrial tissue of women who received bone marrow transplant highlight potential for the novel treatments of uterine disorders and supports new theories for the etiology of endometriosis - ectopic transdifferentiation of stem cells. Further, endometrial derived stem cells have been demonstrated to be useful in the treatment of several chronic and often debilitating diseases, including Parkinson's Disease and Diabetes. Other cells that may present future therapeutic benefits for a myriad of disease states include placental and fetal cells which enter maternal circulation during pregnancy and can later promote parenchymal regeneration in maternal tissue. These findings highlight novel functions of the uterus and ovaries. They demonstrate that the uterus is a dynamic organ permeable to fetal stem cells capable of transdifferentiation as well as a renewable source of multipotent stem cells. While we still have much to understand about stem cells, their potential applications in reproductive biology and medicine are countless.

Seeking the origin of female germline stem cells in the mammalian ovary

The function of female germline stem cells (FGSCs, also called oogonial stem cells) in the adult mammalian ovary is currently debated in the scientific community. As the evidence to support or discard the possible crucial role of this new class of germ cells in mammals has been extensively discussed, in this review, we wonder which could be their origin. We will assume that FGSCs are present in the post-natal ovaries and speculate as to what origin and characteristics such cells could have. We believe that the definition of these features might shed light on future experimental approaches that could clarify the ongoing debate.

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.

[Current progress and future direction in the biology of ovarian germ stem cells in mammals]

Whether or not oogenesis continues after birth in mammalian ovaries remains controversial. Since the 1950's, it has been generally accepted that oogenesis takes place during embryogenesis in mammals and ceases at birth. At birth, germ cells in mammalian ovaries have progressed to the diplotene stage of meiotic prophase and have formed primordial follicles with surrounding somatic cells. These primordial follicles represent follicle reserves of the reproductive life. However, this view has been recently challenged by a growing body of evidence showing the isolation and propagation of germ stem cells from mouse and human ovaries. These ovarian germ stem cells are capable of regenerating functional oocytes when transplanted back into recipient ovaries. Despite the discovery of the potential germ stem cells in mammalian ovaries, it remains uncertain whether these cells exist and function in ovaries under physiological conditions. Herein we review the current progress and future direction in this infant area.

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

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

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.

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.

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.

Ovarian follicular dynamics: a review with emphasis on the bovine species. Part I: Folliculogenesis and pre-antral follicle development

Recent scientific research into pre-antral follicular dynamics has resulted in the discovery of a wide range of hormones and local factors that influence primordial follicle activation and contribute to follicular development. The putative role of several of these mediators in the follicle growth process has been elucidated by genetic and molecular investigations. Crucial questions, such as the mechanism for primordial follicle initiation and the interplay between oocyte and granulosa cells in this process, remain however unresolved. This review article commences with a description of the embryogenesis of the ovary and follicles. Next, the different stages in the development from primordial to pre-antral follicle are discussed. Thereafter, a short overview of the various in vitro models for the study of follicular dynamics is presented. Finally, an in-depth discussion of pre-antral follicle development engages in the current hypotheses regarding primordial follicle activation, and the role of gonadotrophins and angiogenesis.

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).

Putative stem cells with an embryonic character isolated from the ovarian surface epithelium of women with no naturally present follicles and oocytes

There have been some proposals that stem cells exist in the ovarian surface epithelium (OSE) of the adult human ovary; however, no direct evidence of such cells has been given until now. The aim of this study was to isolate the putative ovarian stem cells (OSCs) from the OSE layer in women with no naturally present oocytes and follicles--20 postmenopausal women and five women with premature ovarian failure. Small round cells with a bubble-like structure and diameters from 2 to 4 microm were isolated from the material obtained by OSE scraping. They expressed early embryonic developmental markers such as stage-specific embryonic antigen-4 and Oct-4, Nanog, Sox-2, and c-kit transcription markers, and they displayed prominent c-kit immunohistochemical staining. These cells were separated by density gradient centrifugation and grown in vitro, where they proliferated. Some of them grew intensively and reached a diameter of approximately 20 microm after 5-7 days. In the OSE cell culture, oocyte-like cells developed, which reached a diameter of up to 95 microm and expressed Oct-4A, Oct-4B, c-kit, VASA, and ZP2 transcription markers, corresponding to early oocytes. They did not express SCP3 meiotic marker. In conclusion, the discovered cells are proposed to represent the adult OSCs with the expression of embryonic stem cell markers. The expression of germ lineage marker c-kit points toward their primordial germ cell ancestry. A new term "embryonic-like stem cells of the adult" is proposed for embryonic-like stem cells that might persist in various tissues and organs of adults. These findings could be used for further studies aimed at the autologous treatment of ovarian infertility and degenerative diseases.

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.

Serious doubts over "Eggs forever?"

A recent commentary in this journal by Byskov et al. (2005) claims that, despite published results from numerous independent lines of investigation from our laboratory and others, there does not "exist any evidence for neo-folliculogenesis in the adult mammalian ovary." While we agree with Byskov et al. that our work represents a radical departure from the age-old dogma that mammalian females permanently lose the capacity for oocyte and follicle production during the perinatal period, careful examination of all of the available data leaves no doubt that adult female mammals retain the capacity for oogenesis and folliculogenesis. These findings do not change the fact that exhaustion of the oocyte pool occurs with advancing chronological age--a process responsible for driving the menopause in women--but rather question the basic mechanism underlying age-related ovarian failure. In this regard, studies of aging male mice have demonstrated that testicular atrophy is associated with a dramatic decline in the number, activity and quality of germline stem cells that maintain spermatogenesis during adulthood (Zhang et al., 2006). Therefore, to the contrary of the opinion of Byskov et al. that such a process would be "considered exceptional among stem cells," it is certainly reasonable to hypothesize that a similar deterioration of female germline stem cell function underlies the decline in oocyte quality and the onset of ovarian failure in aging females. Further, while we accept that a departure from conventional thinking can take years to gain widespread acceptance, we feel this resistance to change should not be construed as the sole means to voice opinions about the validity of our data or the maturity of our principal conclusion.

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.

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.

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.

Clinical applications and limitations of current ovarian stem cell research: a review

The publication of a report in Nature in 2004 by the Tilly group suggesting that mouse ovaries are capable of generating oocytes de novo post-natally, has sparked interest in a problem long thought to have been resolved from classical studies in a variety of mammalian species. Within a nearly two year time period, laboratories around the world have taken up the challenge to dogma raised by this initial report, either to test this concept in an experimental basic science setting or give direction to clinical applications that could result, were the original premises of this work in the mouse valid for extrapolation to humans. This review provides a status report for this promising area of research, (1) to summarize recent findings in the literature with respect to the validity of the original hypothesis proffered by the Tilly group, and, (2) to gauge the potential utility of ovarian stem cells as a treatment for certain forms of human infertility.

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.

Origin of germ cells and formation of new primary follicles in adult human ovaries

Recent reports indicate that functional mouse oocytes and sperm can be derived in vitro from somatic cell lines. We hypothesize that in adult human ovaries, mesenchymal cells in the tunica albuginea (TA) are bipotent progenitors with a commitment for both primitive granulosa and germ cells. We investigated ovaries of twelve adult women (mean age 32.8 +/- 4.1 SD, range 27-38 years) by single, double, and triple color immunohistochemistry. We show that cytokeratin (CK)+ mesenchymal cells in ovarian TA differentiate into surface epithelium (SE) cells by a mesenchymal-epithelial transition. Segments of SE directly associated with ovarian cortex are overgrown by TA, forming solid epithelial cords, which fragment into small (20 micron) epithelial nests descending into the lower ovarian cortex, before assembling with zona pellucida (ZP)+ oocytes. Germ cells can originate from SE cells which cover the TA. Small (10 micron) germ-like cells showing PS1 meiotically expressed oocyte carbohydrate protein are derived from SE cells via asymmetric division. They show nuclear MAPK immunoexpression, subsequently divide symmetrically, and enter adjacent cortical vessels. During vascular transport, the putative germ cells increase to oocyte size, and are picked-up by epithelial nests associated with the vessels. During follicle formation, extensions of granulosa cells enter the oocyte cytoplasm, forming a single paranuclear CK+ Balbiani body supplying all the mitochondria of the oocyte. In the ovarian medulla, occasional vessels show an accumulation of ZP+ oocytes (25-30 microns) or their remnants, suggesting that some oocytes degenerate. In contrast to males, adult human female gonads do not preserve germline type stem cells. This study expands our previous observations on the formation of germ cells in adult human ovaries. Differentiation of primitive granulosa and germ cells from the bipotent mesenchymal cell precursors of TA in adult human ovaries represents a most sophisticated adaptive mechanism created during the evolution of female reproduction. Our data indicate that the pool of primary follicles in adult human ovaries does not represent a static but a dynamic population of differentiating and regressing structures. An essential mission of such follicular turnover might be elimination of spontaneous or environmentally induced genetic alterations of oocytes in resting primary follicles.