Differentiation and development of human female germ cells during prenatal gonadogenesis: an immunohistochemical study

The dynamic expression of SOX17 in germ cells from human female foetus and adult ovaries after specification

Background

SOX17 has been identified as a critical factor in specification of human primordial germ cells, but whether SOX17 regulates development of germ cells after sex differentiation is poorly understood.

Methods

We collected specimens of gonadal ridge from an embryo (n=1), and ovaries of foetuses (n=23) and adults (n=3). Germ cells were labelled with SOX17, VASA (classic germ cells marker), phosphohistone H3 (PHH3, mitosis marker) and synaptonemal complex protein 3 (SCP3, meiosis marker).

Results

SOX17 was detected in both cytoplasm and nucleus of oogonia and oocytes of primordial and primary follicles from 15 to 28 gestational weeks (GW). However, it was exclusively expressed in cytoplasm of oogonia at 7 GW, and in nucleus of oocytes in secondary follicles. Co-expression rates of SOX17 in VASA+ germ cells ranged from 81.29% to 97.81% in foetuses. Co-staining rates of SOX17 and PHH3 or SCP3 were 0%-34% and 0%-57%, respectively. Interestingly, we distinguished a subpopulation of SOX17+VASA- germ cells in fetal ovaries. These cells clustered in the cortex and could be co-stained with the mitosis marker PHH3 but not the meiosis marker SCP3.

Conclusions

The dynamic expression of SOX17 was detected in human female germ cells. We discovered a population of SOX17+ VASA- germ cells clustering at the cortex of ovaries. We could not find a relationship between mitosis or meiosis and SOX17 or VASA staining in germ cells. Our findings provide insight into the potential role of SOX17 involving germ cells maturation after specification, although the mechanism is unclear and needs further investigation.

RNA Helicase Vasa as a Multifunctional Conservative Regulator of Gametogenesis in Eukaryotes

Being a conservative marker of germ cells across metazoan species, DEAD box RNA helicase Vasa (DDX4) remains the subject of worldwide investigations thanks to its multiple functional manifestations. Vasa takes part in the preformation of primordial germ cells in a group of organisms and contributes to the maintenance of germline stem cells. Vasa is an essential player in the piRNA-mediated silencing of harmful genomic elements and in the translational regulation of selected mRNAs. Vasa is the top hierarchical protein of germ granules, liquid droplet organelles that compartmentalize RNA processing factors. Here, we survey current advances and problems in the understanding of the multifaceted functions of Vasa proteins in the gametogenesis of different eukaryotic organisms, from nematodes to humans.

Development of the human ovary: Fetal through pubertal ovarian morphology, folliculogenesis and expression of cellular differentiation markers

A definition of normal human fetal and early postnatal ovarian development is critical to the ability to accurately diagnose the presence or absence of functional ovarian tissue in clinical specimens. Through assembling an extensive histologic and immunohistochemical developmental ontogeny of human ovarian specimens from 8 weeks of gestation through 16 years of postnatal, we present a comprehensive immunohistochemical mapping of normal protein expression patterns in the early fetal through post-pubertal human ovary and detail a specific expression-based definition of the early stages of follicular development. Normal fetal and postnatal ovarian tissue is defined by the presence of follicular structures and characteristic immunohistochemical staining patterns, including granulosa cells expressing Forkhead Box Protein L2 (FOXL2). However, the current standard array of immunohistochemical markers poorly defines ovarian stromal tissue, and additional work is needed to identify new markers to advance our ability to accurately identify ovarian stromal components in gonadal specimens from patients with disorders of sexual differentiation.

Genomic instability and the link to infertility: A focus on microsatellites and genomic instability syndromes

Infertility is associated to multiple types of different genomic instabilities and is a genetic feature of genomic instability syndromes. While the mismatch repair machinery contributes to the maintenance of genome integrity, surprisingly its potential role in infertility is overlooked. Defects in mismatch repair mechanisms contribute to microsatellite instability and genomic instability syndromes, due to the inability to repair newly replicated DNA. This article reviews the literature to date to elucidate the contribution of microsatellite instability to genomic instability syndromes and infertility. The key findings presented reveal microsatellite instability is poorly researched in genomic instability syndromes and infertility.

Deciphering Sex-Specific Differentiation of Human Fetal Gonads: Insight From Experimental Models

Sex-specific gonadal differentiation is initiated by the expression of SRY in male foetuses. This promotes a signalling pathway directing testicular development, while in female foetuses the absence of SRY and expression of pro-ovarian factors promote ovarian development. Importantly, in addition to the initiation of a sex-specific signalling cascade the opposite pathway is simultaneously inhibited. The somatic cell populations within the gonads dictates this differentiation as well as the development of secondary sex characteristics via secretion of endocrine factors and steroid hormones. Opposing pathways SOX9/FGF9 (testis) and WNT4/RSPO1 (ovary) controls the development and differentiation of the bipotential mouse gonad and even though sex-specific gonadal differentiation is largely considered to be conserved between mice and humans, recent studies have identified several differences. Hence, the signalling pathways promoting early mouse gonad differentiation cannot be directly transferred to human development thus highlighting the importance of also examining this signalling in human fetal gonads. This review focus on the current understanding of regulatory mechanisms governing human gonadal sex differentiation by combining knowledge of these processes from studies in mice, information from patients with differences of sex development and insight from manipulation of selected signalling pathways in ex vivo culture models of human fetal gonads.

Transcriptional control of human gametogenesis

The pathways of gametogenesis encompass elaborate cellular specialization accompanied by precise partitioning of the genome content in order to produce fully matured spermatozoa and oocytes. Transcription factors are an important class of molecules that function in gametogenesis to regulate intrinsic gene expression programs, play essential roles in specifying (or determining) germ cell fate and assist in guiding full maturation of germ cells and maintenance of their populations. Moreover, in order to reinforce or redirect cell fate in vitro, it is transcription factors that are most frequently induced, over-expressed or activated. Many reviews have focused on the molecular development and genetics of gametogenesis, in vivo and in vitro, in model organisms and in humans, including several recent comprehensive reviews: here, we focus specifically on the role of transcription factors. Recent advances in stem cell biology and multi-omic studies have enabled deeper investigation into the unique transcriptional mechanisms of human reproductive development. Moreover, as methods continually improve, in vitro differentiation of germ cells can provide the platform for robust gain- and loss-of-function genetic analyses. These analyses are delineating unique and shared human germ cell transcriptional network components that, together with somatic lineage specifiers and pluripotency transcription factors, function in transitions from pluripotent stem cells to gametes. This grand theme review offers additional insight into human infertility and reproductive disorders that are linked predominantly to defects in the transcription factor networks and thus may potentially contribute to the development of novel treatments for infertility.

A new perspective on the function of Tissue Non-Specific Alkaline Phosphatase: from bone mineralization to intra-cellular lipid accumulation

Tissue-nonspecific alkaline phosphatase (TNAP) is one of four isozymes, which include germ cell, placental and intestinal alkaline phosphatases. The TNAP isozyme has 3 isoforms (liver, bone and kidney) which differ by tissue expression and glycosylation pattern. Despite a long history of investigation, the exact function of TNAP in many tissues is largely unknown. Only the bone isoform has been well characterised during mineralization where the enzyme hydrolyses pyrophosphate to inorganic phosphate, which combines with calcium to form hydroxyapatite crystals deposited as new bone. The inorganic phosphate also increases gene expression of proteins that support tissue mineralization. Recent studies have shown that TNAP is expressed in preadipocytes from several species, and that inhibition of TNAP activity causes attenuation of intracellular lipid accumulation in these and other lipid-storing cells. The mechanism by which TNAP stimulates lipid accumulation is not known; however, proteins that are important for controlling phosphate levels in bone are also expressed in adipocytes. This review examines the evidence that inorganic phosphate generated by TNAP promotes transcription that enhances the expression of the regulators of lipid storage and consequently, that TNAP has a major function of lipid metabolism.

Accelerated loss of oogonia and impaired folliculogenesis in females with Turner syndrome start during early fetal development

STUDY QUESTION

How are germ cell numbers and initiation of folliculogenesis affected in fetal Turner syndrome (TS) ovaries?

SUMMARY ANSWER

Germ cell development was severely affected already in early second trimester pregnancies, including accelerated oogonia loss and impaired initiation of primordial follicle formation in TS ovaries, while the phenotype in TS mosaic ovaries was less severe.

WHAT IS KNOWN ALREADY

Females with TS are characterized by premature ovarian insufficiency (POI). This phenotype is proposed to be a consequence of germ cell loss during development, but the timing and mechanisms behind this are not characterized in detail. Only few studies have evaluated germ cell development in fetal TS and TS mosaic ovaries, and with a sparse number of specimens included per study.

STUDY DESIGN, SIZE, DURATION

This study included a total of 102 formalin-fixed and paraffin-embedded fetal ovarian tissue specimens. Specimens included were from fetuses with 45,X (N = 42 aged gestational week (GW) 12-20, except one GW 40 sample), 45,X/46,XX (N = 7, aged GW 12-20), and from controls (N = 53, aged GW 12-42) from a biobank (ethics approval # H-2-2014-103).

PARTICIPANTS/MATERIALS, SETTING, METHODS

The number of OCT4 positive germ cells/mm2, follicles (primordial and primary)/mm2 and cPARP positive cells/mm2 were quantified in fetal ovarian tissue from TS, TS mosaic and controls following morphological and immunohistochemical analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

After adjusting for gestational age, the number of OCT4+ oogonia was significantly higher in control ovaries (N = 53) versus 45,X ovaries (N = 40, P < 0.001), as well as in control ovaries versus 45,X/46,XX mosaic ovaries (N = 7, P < 0.043). Accordingly, the numbers of follicles were significantly higher in control ovaries versus 45,X and 45,X/46,XX ovaries from GW 16-20 with a median range of 154 (N = 11) versus 0 (N = 24) versus 3 (N = 5) (P < 0.001 and P < 0.015, respectively). The number of follicles was also significantly higher in 45,X/46,XX mosaic ovaries from GW 16-20 compared with 45,X ovaries (P < 0.005). Additionally, the numbers of apoptotic cells determined as cPARP+ cells/mm2 were significantly higher in ovaries 45,X (n = 39) versus controls (n = 15, P = 0.001) from GW 12-20 after adjusting for GW.

LIMITATIONS, REASONS FOR CAUTION

The analysis of OCT4+ cells/mm2, cPARP+ cells/mm2 and follicles (primordial and primary)/mm2 should be considered semi-quantitative as it was not possible to use quantification by stereology. The heterogeneous distribution of follicles in the ovarian cortex warrants a cautious interpretation of the exact quantitative numbers reported. Moreover, only one 45,X specimen and no 45,X/46,XX specimens aged above GW 20 were available for this study, which unfortunately made it impossible to assess whether the ovarian folliculogenesis was delayed or absent in the TS and TS mosaic specimens.

WIDER IMPLICATIONS OF THE FINDINGS

This human study provides insights about the timing of accelerated fetal germ cell loss in TS. Knowledge about the biological mechanism of POI in girls with TS is clinically useful when counseling patients about expected ovarian function and fertility preservation strategies.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the International Center for Research and Research Training in Endocrine Disruption of Male Reproduction and Child Health (EDMaRC).

TRIAL REGISTRATION NUMBER

N/A.

The safety of VASApos presumptive adult ovarian stem cells

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

Sex-Specific Isolation and Propagation of Human Premeiotic Fetal Germ Cells and Germ Cell-Like Cells

The second trimester of human development is marked by asynchronous gonadal development hampering the isolation of homogenous populations of early and late fetal germ cells (FGCs). We evaluated the feasibility of using surface markers TNAP, PDPN, EPCAM and ITGA6 to isolate FGCs as well as human primordial germ cell-like cells (hPGCLCs) derived from embryonic stem cells (hESCs) from both sexes by fluorescence-activated cell sorting (FACS). Our results suggest that a combination of TNAP and PDPN was sufficient to separate populations of premeiotic FGCs and hPGCLCs in both sexes. This combination of antibodies also proved efficient in separating 'mitotic' from 'retinoic-acid responsive' female FGCs. Furthermore, we report that the differentiation efficiency of TNAP+PDPN+ hPGCLCs from hESCs was sex-independent, but the ability to propagate differed considerably between the sexes. In contrast to male, female hPGCLCs retained their characteristics and exhibited robust colony-forming ability when cultured for five days in medium containing LIF, forskolin and FGF2. We conclude that marked sex differences exist in the isolation and propagation of human FGCs and hPGCLCs. Our study provides novel insights relevant for the optimization of in vitro gametogenesis in humans.

The use of autologous platelet-rich plasma (PRP) versus no intervention in women with low ovarian reserve undergoing fertility treatment: a non-randomized interventional study

PURPOSE

To investigate the impact of a 3-month course of intracortical injections of autologous platelet-rich plasma (PRP) upon ovarian reserve markers versus no intervention in women with low ovarian reserve prior to undergoing assisted reproductive technology (ART).

METHODS

Prospective controlled, non-randomized comparative study conducted in a private fertility clinic, in Venezuela. Women with abnormal ovarian reserve markers (FSH, AMH and AFC) who declined oocyte donation were allocated to one of the following groups according to patient choice: monthly intracortical ovarian PRP injections for three cycles, or no intervention. Primary outcomes were the change in FSH, AMH and AFC pre- and post-treatment. Secondary outcomes included the number of oocytes collected and fertilized, biochemical/clinical pregnancy rates and miscarriage and live birth rates.

RESULTS

Eighty-three women were included, of which 46 received PRP treatment and 37 underwent no intervention. Overall median age was 41 years (IQR 39-44). There were no demographic differences between the study groups. At the 3-month follow-up, women treated with PRP experienced a significant improvement in FSH, AMH and AFC, whereas there was no change in the control group. Furthermore, overall rates of biochemical (26.1% versus 5.4%, P = 0.02) and clinical pregnancy (23.9% versus 5.4%, P = 0.03) were higher in the PRP group, while there was no difference in the rates of first trimester miscarriage and live birth between groups.

CONCLUSION

PRP injections are effective and safe to improve markers of low ovarian reserve prior to ART, although further evidence is required to evaluate the impact of PRP on pregnancy outcomes.

Human germ cell tumours from a developmental perspective

Human germ cell tumours (GCTs) are derived from stem cells of the early embryo and the germ line. They occur in the gonads (ovaries and testes) and also in extragonadal sites, where migrating primordial germ cells are located during embryogenesis. This group of heterogeneous neoplasms is unique in that their developmental potential is in effect determined by the latent potency state of their cells of origin, which are reprogrammed to omnipotent, totipotent or pluripotent stem cells. Seven GCT types, defined according to their developmental potential, have been identified, each with distinct epidemiological and (epi)genomic features. Heritable predisposition factors affecting the cells of origin and their niches likely explain bilateral, multiple and familial occurrences of the different types of GCTs. Unlike most other tumour types, GCTs are rarely caused by somatic driver mutations, but arise through failure to control the latent developmental potential of their cells of origin, resulting in their reprogramming. Consistent with their non-mutational origin, even the malignant tumours of the group are characterized by wild-type TP53 and high sensitivity for DNA damage. However, tumour progression and the rare occurrence of treatment resistance are driven by embryonic epigenetic state, specific (sub)chromosomal imbalances and somatic mutations. Thus, recent progress in understanding GCT biology supports a comprehensive developmental pathogenetic model for the origin of all GCTs, and provides new biomarkers, as well as potential targets for treatment of resistant disease.

Gene Expression in the Supporting Cells at the Onset of Meiosis in Rabbit Gonads

Subsequent to somatic gonadal sexual differentiation, germ cells enter meiosis or mitotic arrest in the ovary or testis, respectively. Among mice, these processes occur almost synchronically in fetal gonads and depend, among other factors, on the levels of retinoic acid (RA). In contrast to those in mice, rabbit germ cells enter meiosis or mitotic arrest after birth and coexist with proliferating germ cells. Here, we studied the somatic cell context in which germ cells enter meiosis or mitotic arrest in the rabbit. Using confocal immunofluorescence and real-time PCR, we studied the expression profiles of ALDH1A1 and ALDH1A2 and, comprising 2 genes required for RA synthesis, 2 meiosis markers STRA8 and SYCP3 as well as 2 genes involved in meiosis inhibition, CYP26B1 and NANOS2. We found that although both meiosis and mitotic arrest initiate after birth, these 2 processes are regulated in a way similar to the human fetal gonad. Current results reinforce the value of the neonatal rabbit gonad as an alternative experimental model for analyzing the direct effect of environmental factors during critical stages of germ cell establishment.

The timeline development of female canine germ cells

During the sex differentiation, the primordial germ cells (PGCs) pass through a differentiation, becoming spermatogonial cells in males and oocytes in females. In this phase, there is difference in gene expression and differentiation potency between males and females. Specific cell markers have been essential in the PGC meiosis beginning and become oocyte cells. However, there are few studies about germline in domestic animals. The domestic dog (Canis lupus familiaris) is an interesting animal model to be used in the investigation about the mammal development because it has several biochemical and physiological similarities to humans. In addition, some additional investigations about dogs may contribute to a better understanding of the biology and genetic components, improving clinical veterinary and zoological sciences. Here, we elucidated by immunofluorescence and quantitative reverse transcriptase-polymerase chain reaction (RT-qPCR), the dynamics of the expression of pluripotent (POU5F1 and NANOG) and germline (DDX4, DAZL and DPPA3) markers that are very important in the development of female canine germ cells during 35-50 days post-fertilization (dpf). The female canine germ cells were positive for pluripotent markers during middle developmental period. The number of DDX4, DAZL and DPPA3 cells increased along the germ cell maturation from 45 to 50 dpf. We provided an expression analysis of the pluripotent and germline markers in paraffin sections using the middle and later periods in female canine germ cells. The results can contribute the understanding about the timeline of each marker along the maturation of female canine germ cells. These results have a great significance to demonstrate the germ cell profile changes because it may allow the development of protocols about in vitro germ cell derivation.

Serum and follicular fluid Stem Cell Factor assay in IVF poor responder and normal responder patients: a predictive biomarker of oocyte retrieval

OBJECTIVE

The aim of the study is to investigate serum stem cell factor (SCF) concentrations as potential biomarker for oocyte retrieval efficiency in IVF patients with poor prognosis.

METHODS

A pilot case-control study was performed on 30 poor and 30 normal responders that were stimulated with antagonist protocol. SCF concentrations were evaluated in samples of serum and follicular fluid obtained by all patients on the day of oocyte retrieval. At the time of oocyte retrieval, follicular fluid from at least two follicles ≥ 14 mm and two follicles < 14 mm was collected for SCF determination.

RESULTS

We did not find any statistical difference when comparing serum and follicular fluid levels of SCF in both poor- and normal-responder patients, the same results were achieved when poor-responder patients were stratified according to the number of MII oocytes retrieved. Moreover, levels of SCF (OR 1.000, 0.994-1.006) or in follicular fluid from ovarian follicles ≥ 14 mm (OR 0.995, CI 0.989-1.001) or from ovarian follicles < 14 mm (OR 1.003, CI 0.999-1.0069), were not significantly associated with the chances of ongoing pregnancies in poor-responder patients.

CONCLUSION

SCF was unable to predict oocyte retrieval efficiency or the chances of reaching embryo transfer.

The ovarian germinal reserve and apoptosis-related proteins in the infant and adolescent human ovary

Background

Normal pubertal ovary displays all stages of follicular development and a biased BAX/BCL2 protein ratio in favor of pro-apoptotic BAX protein comparable to the adult ovary. However, adolescents suffering malignant extra-gonadal disease show a limited follicle development after cytotoxic drug treatment and a reduced capacity of in vitro follicle growth. We evaluated the expression of pro- and anti-apoptotic members of the BCL2 gene family, the FAS/FAS-L proteins from the extrinsic apoptosis pathway, the germ-cell-specific marker VASA, the pluripotency marker OCT3/4, and markers of early and late apoptosis in the ovary of pubertal patients with malignant extra-gonadal disease, which received or not pre-surgery chemotherapy, entering a cryopreservation program.

Results

Ovarian biopsies from 12 adolescent girls were screened for follicle count and expression of VASA, OCT3/4, BAX, BCL2, MCL1L and S, cleaved-BID, FAS/FAS-L and CASPASE 3 through immunohistochemistry, western blot and RT-PCR. All stages of folliculogenesis, from primordial to antral follicle, were present in all 12 patients analyzed. VASA and most of the screened apoptosis-related genes showed a pattern of immune-expression comparable to that previously reported. OCT3/4 showed a cytoplasmic localization in the great majority of the primordial follicles; however, in some cases the localization was nuclear. In addition, OCT3/4B showed a significant reduction compared to OCT3/4A. Unexpectedly, BCL2 was detected at all stages of folliculogenesis, associated to the Balbiani’s body in the primordial follicles, regardless of whether patients had or had not received chemotherapy, ruling out the possibility that its expression is a protective response to chemotherapy.

Conclusions

These findings reveal new information on the morphological status of the follicular reserve and the expression of apoptosis-related genes in histologically normal adolescent ovary from patients undergoing extragonadal cancer. The unexpected expression of apoptosis-inhibiting BCL2 protein, both in patients that had or had not received chemotherapy, opens a new avenue for thorough investigations. Moreover, the nuclear localization of OCT3/4 protein in primordial follicle-enclosed oocytes suggests a possible increased activity of ovarian stem cells in response to chemotherapy and/or extragonadal cancer. This new information can be essential for a better managing of in vitro culture of follicles that can be removed by filtration from preserved ovarian tissue, especially in girls that entered a cryopreservation program.

Quantitative Proteomic Profiling of the Human Ovary from Early to Mid-Gestation Reveals Protein Expression Dynamics of Oogenesis and Folliculogenesis

The in vivo gene networks involved in coordinating human fetal ovarian development remain obscure. In this study, quantitative mass spectrometry was conducted on ovarian tissue collected at key stages during the first two trimesters of human gestational development, confirming the expression profiling data using immunofluorescence, as well as in vitro modeling with human oogonial stem cells (OSCs) and human embryonic stem cells (ESCs). A total of 3,837 proteins were identified in samples spanning developmental days 47-137. Bioinformatics clustering and Ingenuity Pathway Analysis identified DNA mismatch repair and base excision repair as major pathways upregulated during this time. In addition, MAEL and TEX11, two key meiosis-related proteins, were identified as highly expressed during the developmental window associated with fetal oogenesis. These findings were confirmed and extended using in vitro differentiation of OSCs into in vitro derived oocytes and of ESCs into primordial germ cell-like cells and oocyte-like cells, as models. In conclusion, the global protein expression profiling data generated by this study have provided novel insights into human fetal ovarian development in vivo and will serve as a valuable new resource for future studies of the signaling pathways used to orchestrate human oogenesis and folliculogenesis.

Human foetal ovary shares meiotic preventing factors with the developing testis

STUDY QUESTION

How can pre-meiotic germ cells persist in the human foetal ovary?

SUMMARY ANSWER

Numerous oogonia escaping meiotic entry were retrieved throughout human ovarian development simultaneously with the expression of signalling pathways preventing meiosis, typically described in the rodent embryonic testis.

WHAT IS KNOWN ALREADY

The transition from mitosis to meiosis is a key event in female germ cells that remains poorly documented in research on the human ovary. Previous reports described a strikingly asynchronous differentiation in the human female germ line during development, with the persistence of oogonia among oocytes and follicles during the second and third trimesters. The possible mechanisms allowing some cells to escape meiosis remain elusive.

STUDY DESIGN SIZE, DURATION

In order to document the extent of this phenomenon, we detailed the expression profile of germ cell differentiation markers using 73 ovaries ranging from 6.4 to 35 weeks post-fertilization.

PARTICIPANTS/MATERIALS SETTING, METHODS

Pre-meiotic markers were detected by immunohistochemistry or qRT-PCR. The expression of the main meiosis-preventing factors identified in mice was analysed, and their functionality assessed using organ cultures.

MAIN RESULTS AND THE ROLE OF CHANCE

Oogonia stained for AP2γ could be traced from the first trimester until the end of the third trimester. Female germ cell differentiation is organized both in time and space in a centripetal manner in the foetal human ovary. Unexpectedly, some features usually ascribed to rodent pre-spermatogonia could be observed in human foetal ovaries, such as NANOS2 expression and quiescence in some germ cells. The two main somatic signals known to inhibit meiosis in the mouse embryonic testis, CYP26B1 and FGF9, were detected in the human ovary and act simultaneously to repress STRA8 and meiosis in human foetal female germ cells.

LARGE SCALE DATA

N/A.

LIMITATIONS REASON FOR CAUTION

Our conclusions relied partly on in vitro experiments. Germ cells were not systematically identified with immunostaining and some may have thus escaped analysis.

WIDER IMPLICATIONS OF THE FINDINGS

We found evidence that a robust repression of meiotic entry is taking place in the human foetal ovary, possibly explaining the exceptional long-lasting presence of pre-meiotic germ cells until late gestational age. This result calls for a redefinition of the markers known as classical male markers, which may in fact characterize mammalian developing gonads irrespectively of their sex.

STUDY FUNDING/COMPETING INTEREST(S)

This research was supported by the Université Paris Diderot-Paris 7 and Université Paris-Sud, CEA, INSERM, and Agence de la Biomédecine. The authors declare no conflict of interest.

A pilgrim's progress: Seeking meaning in primordial germ cell migration

Comparative studies of primordial germ cell (PGC) development across organisms in many phyla reveal surprising diversity in the route of migration, timing and underlying molecular mechanisms, suggesting that the process of migration itself is conserved. However, beyond the perfunctory transport of cellular precursors to their later arising home of the gonads, does PGC migration serve a function? Here we propose that the process of migration plays an additional role in quality control, by eliminating PGCs incapable of completing migration as well as through mechanisms that favor PGCs capable of responding appropriately to migration cues. Focusing on PGCs in mice, we explore evidence for a selective capacity of migration, considering the tandem regulation of proliferation and migration, cell-intrinsic and extrinsic control, the potential for tumors derived from failed PGC migrants, the potential mechanisms by which migratory PGCs vary in their cellular behaviors, and corresponding effects on development. We discuss the implications of a selective role of PGC migration for in vitro gametogenesis.

Disorganization of the germ cell pool leads to primary ovarian insufficiency

The mammalian ovary is an organ that controls female germ cell development, storing them and releasing mature oocytes for transporting to the oviduct. During the fetal stage, female germ cells change from a proliferative state to meiosis before forming follicles with the potential for the growth of surrounding somatic cells. Understanding of molecular and physiological bases of germ cell development in the fetal ovary contributed not only to the elucidation of genetic disorders in primary ovarian insufficiency (POI), but also to the advancement of novel treatments for patients with POI. Accumulating evidence indicates that mutations in NOBOX, DAZL and FIGLAgenes are associated with POI. In addition, cell biology studies revealed the important roles of these genes as essential translational factors for germ cell development. Recent insights into the role of the PI3K (phosphatidylinositol 3-kinase)-Akt signaling pathway in primordial follicle activation allowed the development of a new infertility treatment, IVA (in vitro activation), leading to successful pregnancy/delivery in POI patients. Furthermore, elucidation of genetic dynamics underlying female germ cell development could allow regeneration of oocytes from ES (embryonic stem)/iPS (induced pluripotent stem) cells in mammals. The purpose of this review is to summarize basic findings related to female germ cell development and potential clinical implications, especially focusing on POI etiologies. We also summarize evolving new POI therapies based on IVA as well as oocyte regeneration.

Insights into female germ cell biology: from in vivo development to in vitro derivations

Understanding the mechanisms of human germ cell biology is important for developing infertility treatments. However, little is known about the mechanisms that regulate human gametogenesis due to the difficulties in collecting samples, especially germ cells during fetal development. In contrast to the mitotic arrest of spermatogonia stem cells in the fetal testis, female germ cells proceed into meiosis and began folliculogenesis in fetal ovaries. Regulations of these developmental events, including the initiation of meiosis and the endowment of primordial follicles, remain an enigma. Studying the molecular mechanisms of female germ cell biology in the human ovary has been mostly limited to spatiotemporal characterizations of genes or proteins. Recent efforts in utilizing in vitro differentiation system of stem cells to derive germ cells have allowed researchers to begin studying molecular mechanisms during human germ cell development. Meanwhile, the possibility of isolating female germline stem cells in adult ovaries also excites researchers and generates many debates. This review will mainly focus on presenting and discussing recent in vivo and in vitro studies on female germ cell biology in human. The topics will highlight the progress made in understanding the three main stages of germ cell developments: namely, primordial germ cell formation, meiotic initiation, and folliculogenesis.

Long-Term Oocyte-Like Cell Development in Cultures Derived from Neonatal Marmoset Monkey Ovary

We use the common marmoset monkey (Callithrix jacchus) as a preclinical nonhuman primate model to study reproductive and stem cell biology. The neonatal marmoset monkey ovary contains numerous primitive premeiotic germ cells (oogonia) expressing pluripotent stem cell markers including OCT4A (POU5F1). This is a peculiarity compared to neonatal human and rodent ovaries. Here, we aimed at culturing marmoset oogonia from neonatal ovaries. We established a culture system being stable for more than 20 passages and 5 months. Importantly, comparative transcriptome analysis of the cultured cells with neonatal ovary, embryonic stem cells, and fibroblasts revealed a lack of germ cell and pluripotency genes indicating the complete loss of oogonia upon initiation of the culture. From passage 4 onwards, however, the cultured cells produced large spherical, free-floating cells resembling oocyte-like cells (OLCs). OLCs strongly expressed several germ cell genes and may derive from the ovarian surface epithelium. In summary, our novel primate ovarian cell culture initially lacked detectable germ cells but then produced OLCs over a long period of time. This culture system may allow a deeper analysis of early phases of female primate germ cell development and-after significant refinement-possibly also the production of monkey oocytes.

Analysis of POU5F1, c-Kit, PLAP, AP2γ and SALL4 in gonocytes of patients with cryptorchidism

Cryptorchidism is a risk factor for the development of testicular germ cell tumors (TGCTs). The most common type of TGCT in cryptorchidism is seminoma. The intratubular germ cell neoplasia unclassified (ITGCNU) is a histological pattern preceding the development of seminomas and non-seminomas. It was suggested that in patients with cryptorchidism, the gonocytes remained undifferentiated with pluripotent abilities expressing proteins like POU domain class 5 transcription factor 1 (POU5F1), tyrosine kinase receptor c-Kit, placental-like alkaline phosphatase (PLAP), the transcription factor AP2γ and sal-like protein 4 (SALL4) that confer to the gonocytes this ability and therefore make them susceptible to develop ITGCNU. The aim of the present study was to determine if the gonocytes of patients with cryptorchidism express POU5F1, c-Kit, PLAP, AP2γ and SALL4 proteins after their differentiation period. Based on this, we evaluated samples of testicular tissue from newborns to 16-year old subjects with or without cryptorchidism in search of POU5F1, c-Kit, PLAP, AP2γ and SALL4 using immunocytochemical method, the results of which were validated by RT-PCR. The results showed that control subjects witnessed a down-regulation in the expression of these five proteins in the first year of life, which eventually disappeared. On the other hand, it was determined that 21.6% (8/37) of the patients with cryptorchidism continued to express, at least, one of the proteins analyzed in this study after the second year of life. And only 5.4% (2/37) of the patients were positive to the five markers. These data sustain the proposed hypothesis that in cryptorchid patients, ITGCNU arises from gonocytes that fail in their differentiation process to spermatogonia with conservation of the proteins (POU5F1, c-Kit, PLAP, AP2γ and SALL4) that maintain pluripotency and undifferentiated characteristics and which are responsible for making the gonocytes susceptible to malignancy. However, we cannot guarantee that these patients present neoplastic transformation.

Pathogenesis of germ cell neoplasia in testicular dysgenesis and disorders of sex development

Development of human gonads is a sex-dimorphic process which evolved to produce sex-specific types of germ cells. The process of gonadal sex differentiation is directed by the action of the somatic cells and ultimately results in germ cells differentiating to become functional gametes through spermatogenesis or oogenesis. This tightly controlled process depends on the proper sequential expression of many genes and signalling pathways. Disturbances of this process can be manifested as a large spectrum of disorders, ranging from severe disorders of sex development (DSD) to - in the genetic male - mild reproductive problems within the testicular dysgenesis syndrome (TDS), with large overlap between the syndromes. These disorders carry an increased but variable risk of germ cell neoplasia. In this review, we discuss the pathogenesis of germ cell neoplasia associated with gonadal dysgenesis, especially in individuals with 46,XY DSD. We summarise knowledge concerning development and sex differentiation of human gonads, with focus on sex-dimorphic steps of germ cell maturation, including meiosis. We also briefly outline the histopathology of germ cell neoplasia in situ (GCNIS) and gonadoblastoma (GDB), which are essentially the same precursor lesion but with different morphological structure dependent upon the masculinisation of the somatic niche. To assess the risk of germ cell neoplasia in different types of DSD, we have performed a PubMed search and provide here a synthesis of the evidence from studies published since 2006. We present a model for pathogenesis of GCNIS/GDB in TDS/DSD, with the risk of malignancy determined by the presence of the testis-inducing Y chromosome and the degree of masculinisation. The associations between phenotype and the risk of neoplasia are likely further modulated in each individual by the constellation of the gene polymorphisms and environmental factors.

Increased DNA damage and repair deficiency in granulosa cells are associated with ovarian aging in rhesus monkey

PURPOSE

Ovarian aging is closely tied to the decline in ovarian follicular reserve and oocyte quality. During the prolonged reproductive lifespan of the female, granulosa cells connected with oocytes play critical roles in maintaining follicle reservoir, oocyte growth and follicular development. We tested whether double-strand breaks (DSBs) and repair in granulosa cells within the follicular reservoir are associated with ovarian aging.

METHODS

Ovaries were sectioned and processed for epi-fluorescence microscopy, confocal microscopy, and immunohistochemistry. DNA damage was revealed by immunstaining of γH2AX foci and telomere damage by γH2AX foci co-localized with telomere associated protein TRF2. DNA repair was indicated by BRCA1 immunofluorescence.

RESULTS

DSBs in granulosa cells increase and DSB repair ability, characterized by BRCA1 foci, decreases with advancing age. γH2AX foci increase in primordial, primary and secondary follicles with advancing age. Likewise, telomere damage increases with advancing age. In contrast, BRCA1 foci in granulosa cells of primordial, primary and secondary follicles decrease with monkey age. BRCA1 positive foci in the oocyte nuclei also decline with maternal age.

CONCLUSIONS

Increased DSBs and reduced DNA repair in granulosa cells may contribute to ovarian aging. Discovery of therapeutics that targets these pathways might help maintain follicle reserve and postpone ovarian dysfunction with age.

Alkaline phosphatase in stem cells

Alkaline phosphatase is an enzyme commonly expressed in almost all living organisms. In humans and other mammals, determinations of the expression and activity of alkaline phosphatase have frequently been used for cell determination in developmental studies and/or within clinical trials. Alkaline phosphatase also seems to be one of the key markers in the identification of pluripotent embryonic stem as well as related cells. However, alkaline phosphatases exist in some isoenzymes and isoforms, which have tissue specific expressions and functions. Here, the role of alkaline phosphatase as a stem cell marker is discussed in detail. First, we briefly summarize contemporary knowledge of mammalian alkaline phosphatases in general. Second, we focus on the known facts of its role in and potential significance for the identification of stem cells.

No evidence for neo-oogenesis may link to ovarian senescence in adult monkey

Female germline or oogonial stem cells transiently residing in fetal ovaries are analogous to the spermatogonial stem cells or germline stem cells (GSCs) in adult testes where GSCs and meiosis continuously renew. Oocytes can be generated in vitro from embryonic stem cells and induced pluripotent stem cells, but the existence of GSCs and neo-oogenesis in adult mammalian ovaries is less clear. Preliminary findings of GSCs and neo-oogenesis in mice and humans have not been consistently reproducible. Monkeys provide the most relevant model of human ovarian biology. We searched for GSCs and neo-meiosis in ovaries of adult monkeys at various ages, and compared them with GSCs from adult monkey testis, which are characterized by cytoplasmic staining for the germ cell marker DAZL and nuclear expression of the proliferative markers PCNA and KI67, and pluripotency-associated genes LIN28 and SOX2, and lack of nuclear LAMIN A, a marker for cell differentiation. Early meiocytes undergo homologous pairing at prophase I distinguished by synaptonemal complex lateral filaments with telomere perinuclear distribution. By exhaustive searching using comprehensive experimental approaches, we show that proliferative GSCs and neo-meiocytes by these specific criteria were undetectable in adult mouse and monkey ovaries. However, we found proliferative nongermline somatic stem cells that do not express LAMIN A and germ cell markers in the adult ovaries, notably in the cortex and granulosa cells of growing follicles. These data support the paradigm that adult ovaries do not undergo germ cell renewal, which may contribute significantly to ovarian senescence that occurs with age.

The neonatal marmoset monkey ovary is very primitive exhibiting many oogonia

Oogonia are characterized by diploidy and mitotic proliferation. Human and mouse oogonia express several factors such as OCT4, which are characteristic of pluripotent cells. In human, almost all oogonia enter meiosis between weeks 9 and 22 of prenatal development or undergo mitotic arrest and subsequent elimination from the ovary. As a consequence, neonatal human ovaries generally lack oogonia. The same was found in neonatal ovaries of the rhesus monkey, a representative of the old world monkeys (Catarrhini). By contrast, proliferating oogonia were found in adult prosimians (now called Strepsirrhini), which is a group of 'lower' primates. The common marmoset monkey (Callithrix jacchus) belongs to the new world monkeys (Platyrrhini) and is increasingly used in reproductive biology and stem cell research. However, ovarian development in the marmoset monkey has not been widely investigated. Herein, we show that the neonatal marmoset ovary has an extremely immature histological appearance compared with the human ovary. It contains numerous oogonia expressing the pluripotency factors OCT4A, SALL4, and LIN28A (LIN28). The pluripotency factor-positive germ cells also express the proliferation marker MKI67 (Ki-67), which has previously been shown in the human ovary to be restricted to premeiotic germ cells. Together, the data demonstrate the primitiveness of the neonatal marmoset ovary compared with human. This study may introduce the marmoset monkey as a non-human primate model to experimentally study the aspects of primate primitive gonad development, follicle assembly, and germ cell biology in vivo.

Involvement of doublesex and mab-3-related transcription factors in human female germ cell development demonstrated by xenograft and interference RNA strategies

We identified three doublesex and mab-3-related transcription factors (DMRT) that were sexually differentially expressed in human fetal gonads and present in the ovaries at the time of meiotic initiation. These were also identified in murine embryonic female germ cells. Among these, we focused on DMRTA2 (DMRT5), whose function is unknown in the developing gonads, and clarified its role in human female fetal germ cells, using an original xenograft model. Early human fetal ovaries (8-11 weeks post-fertilization) were grafted into nude mice. Grafted ovaries developed normally, with no apparent overt changes, when compared with ungrafted ovaries at equivalent developmental stages. Appropriate germ cell density, mitotic/meiotic transition, markers of meiotic progression and follicle formation were evident. Four weeks after grafting, mice were treated with siRNA, specifically targeting human DMRTA2 mRNA. DMRTA2 inhibition triggered an increase in undifferentiated FUT4-positive germ cells and a decrease in the percentage of meiotic γH2AX-positive germ cells, when compared with mice that were injected with control siRNA. Interestingly, the expression of markers associated with pre-meiotic germ cell differentiation was also impaired, as was the expression of DMRTB1 (DMRT6) and DMRTC2 (DMRT7). This study reveals, for the first time, the requirement of DMRTA2 for normal human female embryonic germ cell development. DMRTA2 appears to be necessary for proper differentiation of oogonia, prior to entry into meiosis, in the human species. Additionally, we developed a new model of organ xenografting, coupled with RNA interference, which provides a useful tool for genetic investigations of human germline development.

Primordial germ-cell development and epigenetic reprogramming in mammals

Primordial germ cells (PGCs) are the embryonic precursors of the gametes and represent the founder cells of the germline. Specification of PGCs is a critical divergent point during embryogenesis. Whereas the somatic lineages will ultimately perish, cells of the germline have the potential to form a new individual and hence progress to the next generation. It is therefore critical that the genome emerges intact and carrying the appropriate epigenetic information during its passage through the germline. To ensure this fidelity of transmission, PGC development encompasses extensive epigenetic reprogramming. The low cell numbers and relative inaccessibility of PGCs present a challenge to those seeking mechanistic understanding of the crucial developmental and epigenetic processes in this most fascinating of lineages. Here, we present an overview of PGC development in the mouse and compare this with the limited information available for other mammalian species. We believe that a comparative approach will be increasingly important to uncover the extent to which mechanisms are conserved and reveal the critical steps during PGC development in humans.

The dynamics of the primordial follicle reserve

The female germline comprises a reserve population of primordial (non-growing) follicles containing diplotene oocytes arrested in the first meiotic prophase. By convention, the reserve is established when all individual oocytes are enclosed by granulosa cells. This commonly occurs prior to or around birth, according to species. Histologically, the ‘reserve’ is the number of primordial follicles in the ovary at any given age and is ultimately depleted by degeneration and progression through folliculogenesis until exhausted. How and when the reserve reaches its peak number of follicles is determined by ovarian morphogenesis and germ cell dynamics involving i) oogonial proliferation and entry into meiosis producing an oversupply of oocytes and ii) large-scale germ cell death resulting in markedly reduced numbers surviving as the primordial follicle reserve. Our understanding of the processes maintaining the reserve comes primarily from genetically engineered mouse models, experimental activation or destruction of oocytes, and quantitative histological analysis. As the source of ovulated oocytes in postnatal life, the primordial follicle reserve requires regulation of i) its survival or maintenance, ii) suppression of development (dormancy), and iii) activation for growth and entry into folliculogenesis. The mechanisms influencing these alternate and complex inter-related phenomena remain to be fully elucidated. Drawing upon direct and indirect evidence, we discuss the controversial concept of postnatal oogenesis. This posits a rare population of oogonial stem cells that contribute new oocytes to partially compensate for the age-related decline in the primordial follicle reserve.

A developmental stage-specific switch from DAZL to BOLL occurs during fetal oogenesis in humans, but not mice

The Deleted in Azoospermia gene family encodes three germ cell-specific RNA-binding proteins (DAZ, DAZL and BOLL) that are essential for gametogenesis in diverse species. Targeted disruption of Boll in mice causes male-specific spermiogenic defects, but females are apparently fertile. Overexpression of human BOLL promotes the derivation of germ cell-like cells from genetically female (XX), but not male (XY) human ES cells however, suggesting a functional role for BOLL in regulating female gametogenesis in humans. Whether BOLL is expressed during oogenesis in mammals also remains unclear. We have therefore investigated the expression of BOLL during fetal oogenesis in humans and mice. We demonstrate that BOLL protein is expressed in the germ cells of the human fetal ovary, at a later developmental stage than, and almost mutually-exclusive to, the expression of DAZL. Strikingly, BOLL is downregulated, and DAZL re-expressed, as primordial follicles form, revealing BOLL expression to be restricted to a narrow window during fetal oogenesis. By quantifying the extent of co-expression of DAZL and BOLL with markers of meiosis, we show that this window likely corresponds to the later stages of meiotic prophase I. Finally, we demonstrate that Boll is also transiently expressed during oogenesis in the fetal mouse ovary, but is simultaneously co-expressed within the same germ cells as Dazl. These data reveal significant similarities and differences between the expression of BOLL homologues during oogenesis in humans and mice, and raise questions as to the validity of the Boll(-/-) mouse as a model for understanding BOLL function during human oogenesis.

Activation of the aryl hydrocarbon receptor by a component of cigarette smoke reduces germ cell proliferation in the human fetal ovary

Fetal life is a critical time for female fertility, when germ cells complete proliferation, initiate meiosis and ultimately form the lifetime stock of primordial follicles. Female fertility may be reduced by in utero exposure to cigarette smoke, which contains ligands for the aryl hydrocarbon receptor (AhR). The AhR is a critical regulator of ovarian germ cell survival in mice; thus activation of this receptor in the ovaries of fetuses exposed to maternal cigarette smoke in utero may provide a mechanism by which female fertility is reduced in later life. We have therefore investigated AhR expression in the human fetal ovary, and examined the effects of an AhR ligand present in cigarette smoke, on germ cells in human fetal ovaries cultured in vitro. The results showed that AHR mRNA expression increased 2-fold between first and late second trimester (P = 0.008). AhR protein was confined to germ cells at all gestations, but varied from expression in most germ cells during the first trimester, to only patchy expression by clusters of germ cells at later gestations. Culture of human fetal ovaries with the AhR ligand 9,10-dimethyl-1,2-benzanthracene-3,4-dihydrodiol (DMBA-DHD; a component of cigarette smoke) did not affect germ cell number in vitro, but significantly reduced the proportion of proliferating germ cells by 29% (as assessed by phospho-histone H3 staining (P = 0.04)). Germ cell apoptosis was not significantly affected. These results reveal that germ cells in the human fetal ovary express AhR from the proliferative stage of development through entry into meiosis and beyond, and demonstrate that AhR ligands found in cigarette smoke have the capacity to impair human fetal ovarian germ cell proliferation.

Exposure to chemical cocktails before or after conception--- the effect of timing on ovarian development

Exposure of female fetuses to environmental chemicals (ECs) during pregnancy results in a disturbed ovarian adult phenotype. We investigated the influence of pre- and/or post-conception exposure to low-level mixtures of ECs on the structure and function of the fetal ovine ovary. We examined ovarian morphology, expression of oocyte and granulosa cell-specific genes and proteome. Female fetuses were collected at day 110 of gestation, from dams exposed continuously until, and after mating, by grazing in pastures treated with sewage sludge as a fertiliser (TT) or in control fields treated with inorganic fertiliser (CC). In addition, in a cross-over design, fetal ovaries were collected from dams maintained on sludge pastures up to the time of mating but then transferred to control pastures (TC) and, reciprocally, those transferred from control to treated pastures at mating (CT). On examination, the proportion of type 1a follicles (activating primordial follicles) was significantly lower in animals from the CT groups compared with CC and TT groups (P<0.05). Of the 23 ovarian gene transcripts studied, 14 were altered in the ovaries of exposed fetuses (CT, TC, and TT) relative to controls, with the largest number of changes observed in cross-exposure pattern groups (CT or TC). Continuous EC exposure (TT) produced fewer transcript alterations and only two genes (INHBA and GSN) presented differential profiles between CC and TT. Fetal ovarian proteome analysis (2-DE gels) showed, across all exposure groups, 86 differentially expressed protein spots compared to controls. Animals in the CT group exhibited the highest number (53) while TC and TT presented the same number of affected protein spots (42). Fetal ovarian proteins with altered expression included MVP (major vault protein) and several members of the heat-shock family (HSPA4L, HSP90AA1 and HSF1). The present findings indicate that continuous maternal EC exposure before and during gestation, are less deleterious for fetal ovarian development than a change in maternal EC exposure between pre and post-conception. The pathways by which the ovary responds to this chemical stress were common in TT, CT, TC exposed foetuses. In addition to the period of pregnancy, the pre-conception period appears also as crucial for conditioning long-term effects of EC exposure on ovarian development and primordial follicle reserve and hence future fertility.

Antiandrogen flutamide affects folliculogenesis during fetal development in pigs

Androgen deficiency during prenatal development may affect the expression of genes involved in the folliculogenesis regulation. In order to study the effect of antiandrogen on fetal ovarian development, pregnant gilts were injected with flutamide (for 7 days, 50 mg/kg bodyweight per day) or corn oil (control groups) starting on gestation days 43 (GD50), 83 (GD90), or 101 (GD108). The obtained fetal ovaries were fixed for histology and immunohistochemistry or frozen for real-time PCR. Morphological evaluation, TUNEL assay, and expression of selected factors (Ki-67, GATA binding transcription factor 4 (GATA4), E-Cadherin and tumor necrosis factor a (TNFa)) were performed. On GD90 and GD108, ovaries following flutamide administration showed a higher number of egg nests and lower number off ollicles than those in respective control groups. An increased mRNA and protein expression of Ki-67 was observed in flutamide-treated groups compared with controls on GD50 and GD108 but decreased expression was found on GD90. In comparison to control groups a higher percentage of TUNEL-positive cells was shown after flutamide exposure on GD50 and GD90 and a lower percentage of apoptotic cells was observed on GD108. These data were consistent with changes in TNF (TNFa) mRNA expression, which increased on GD90 and decreased on GD108. E-cadherin mRNA and protein expression was upregulated on GD50 and downregulated on GD90 and GD108. In conclusion diminished androgen action in porcine fetal ovaries during mid- and late gestation leads to changes in the expression of genes crucial for follicle formation. Consequently, delayed folliculogenesis was observed on GD90 and GD108. It seems however that androgens exhibit diverse biological effects depending on the gestational period.

Molecular characteristics of malignant ovarian germ cell tumors and comparison with testicular counterparts: implications for pathogenesis

This review focuses on the molecular characteristics and development of rare malignant ovarian germ cell tumors (mOGCTs). We provide an overview of the genomic aberrations assessed by ploidy, cytogenetic banding, and comparative genomic hybridization. We summarize and discuss the transcriptome profiles of mRNA and microRNA (miRNA), and biomarkers (DNA methylation, gene mutation, individual protein expression) for each mOGCT histological subtype. Parallels between the origin of mOGCT and their male counterpart testicular GCT (TGCT) are discussed from the perspective of germ cell development, endocrinological influences, and pathogenesis, as is the GCT origin in patients with disorders of sex development. Integrated molecular profiles of the 3 main histological subtypes, dysgerminoma (DG), yolk sac tumor (YST), and immature teratoma (IT), are presented. DGs show genomic aberrations comparable to TGCT. In contrast, the genome profiles of YST and IT are different both from each other and from DG/TGCT. Differences between DG and YST are underlined by their miRNA/mRNA expression patterns, suggesting preferential involvement of the WNT/β-catenin and TGF-β/bone morphogenetic protein signaling pathways among YSTs. Characteristic protein expression patterns are observed in DG, YST and IT. We propose that mOGCT develop through different developmental pathways, including one that is likely shared with TGCT and involves insufficient sexual differentiation of the germ cell niche. The molecular features of the mOGCTs underline their similarity to pluripotent precursor cells (primordial germ cells, PGCs) and other stem cells. This similarity combined with the process of ovary development, explain why mOGCTs present so early in life, and with greater histological complexity, than most somatic solid tumors.

The role of ovarian surface epithelium in folliculogenesis during fetal development of the bovine ovary: a histological and immunohistochemical study

Although many aspects of ovarian differentiation have been established, comparatively little is known about prenatal follicle formation and differentiation of bovine ovaries. The objective of this investigation was to study the role of the surface epithelium during the development of germ cell nests, germ cell cords and follicle formation in the fetal bovine ovary. Associated important proliferation and apoptotic features were further investigated. Additionally, the expression pattern of the S100 protein was detected. A strong increase of mitotic figures was detected in the surface epithelium, germ cell nests and germ cell cords of ovaries with a crown-rump length (CRL) of 13.0-58.0 cm. Oocytes were positively stained with S100 in bovine ovaries from fetuses with a CRL of 21.0 cm. The staining intensity enhanced parallel to increasing oocyte and follicle sizes during the ovary development. In later stages, a strong staining for S100 was observed in healthy oocytes in contradistinction to atretic oocytes where no expression of the S100 protein could be found. In conclusion, increasing mitosis index of surface epithelium cells, as well as oogonia directly beneath the surface epithelium, in combination with open surface connection during stages from a CRL of 11.0-94.0 cm of bovine fetal ovaries could play an important role in the period of time of ongoing folliculogenesis and derivation of granulosa cells. Additionally, S100-positive oocytes in primordial and later follicle stages joined by a high rate of Ki67-positive index in surrounding granulosa cells indicate that in the oocytes the S100 protein can perhaps be a useful marker for intact oocytes in bovine ovaries.

Effect of supernatant from fibroblasts from the progeny of female rats with chronic liver disease of different origin on the morphology and function of cultured peritoneal macrophages

We studied the effects of supernatant from ovarian fibroblasts derived from the progeny of female rats with experimental chronic liver disease of different genesis on the function of cultured allogenic peritoneal macrophages. Addition of fibroblast supernatant derived from animals with liver disease to the culture medium suppressed the function of peritoneal macrophages from intact newborn rats, which manifested in reduction of the adhesion characteristics and decrease of their phagocytic parameter and phagocytic index.

Humans put their eggs in more than one basket

Primordial germ cell (PGC) development in the human fetus remains relatively uncharted. A new study suggests that epigenetic reprogramming and sex differentiation in human PGCs occur asynchronously over an extended time period. This finding raises questions and implications for in vitro PGC differentiation.

A new model of development of the mammalian ovary and follicles

Ovarian follicular granulosa cells surround and nurture oocytes, and produce sex steroid hormones. It is believed that during development the ovarian surface epithelial cells penetrate into the ovary and develop into granulosa cells when associating with oogonia to form follicles. Using bovine fetal ovaries (n = 80) we identified a novel cell type, termed GREL for Gonadal Ridge Epithelial-Like. Using 26 markers for GREL and other cells and extracellular matrix we conducted immunohistochemistry and electron microscopy and chronologically tracked all somatic cell types during development. Before 70 days of gestation the gonadal ridge/ovarian primordium is formed by proliferation of GREL cells at the surface epithelium of the mesonephros. Primordial germ cells (PGCs) migrate into the ovarian primordium. After 70 days, stroma from the underlying mesonephros begins to penetrate the primordium, partitioning the developing ovary into irregularly-shaped ovigerous cords composed of GREL cells and PGCs/oogonia. Importantly we identified that the cords are always separated from the stroma by a basal lamina. Around 130 days of gestation the stroma expands laterally below the outermost layers of GREL cells forming a sub-epithelial basal lamina and establishing an epithelial-stromal interface. It is at this stage that a mature surface epithelium develops from the GREL cells on the surface of the ovary primordium. Expansion of the stroma continues to partition the ovigerous cords into smaller groups of cells eventually forming follicles containing an oogonium/oocyte surrounded by GREL cells, which become granulosa cells, all enclosed by a basal lamina. Thus in contrast to the prevailing theory, the ovarian surface epithelial cells do not penetrate into the ovary to form the granulosa cells of follicles, instead ovarian surface epithelial cells and granulosa cells have a common precursor, the GREL cell.

The infant and pubertal human ovary: Balbiani's body-associated VASA expression, immunohistochemical detection of apoptosis-related BCL2 and BAX proteins, and DNA fragmentation

STUDY QUESTION

How do apoptosis-related BCL2 and BAX genes, known to regulate death or survival of germ cells in fetal and adult life, and germ-cell-specific VASA protein behave from birth to puberty in the human ovary?

SUMMARY ANSWER

In resting primordial follicles in both infant and pubertal ovaries, BCL2 family members and germ-cell-specific VASA behave as in fetal life. After birth, once follicles leave the resting reserve to enter the growing follicular pool, detection of apoptosis-related genes moves from the germ cell to granulosa cells and VASA expression is lost.

WHAT IS KNOWN ALREADY

In the human ovary, around 85% of the 7 × 10⁶ potential oocytes produced at mid-gestation are lost before birth, an extra 10% before puberty, and loss continues throughout reproductive life until germinal exhaustion of the ovary. Oocyte loss is mainly driven through a balanced expression of BCL2 gene family members. Apoptosis-inducing BAX gene shows a sustained expression throughout fetal and adult life, whereas apoptosis-inhibiting BCL2 is detectable during the proliferative stage of primordial germ cells and oogonia in the fetal ovary and proliferation of granulosa cells in growing follicles in the adult ovary. The germ-cell marker VASA is detectable in the fetal ovary from early oogenesis and is conspicuously expressed in primordial follicles, where in late pregnancy it is associated with the Balbiani's vitelline space. VASA expression is not detectable in the adult ovary.

STUDY DESIGN, SIZE, DURATION

This is a qualitative analysis involving infant/pubertal paraffin-embedded human ovaries screened for apoptosis-related proteins, DNA fragmentation and germ-cell identity.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Ovaries from 13 patients ranging in age from 4 to 16 years, undergoing gynaecological surgical procedures due to benign pathology, were studied. Tissues were fixed in 10% formalin, paraffin-embedded and processed for immunohistochemistry to screen the temporal and cellular localization of germ-cell-specific VASA protein and BCL2 and BAX apoptosis-related proteins. In addition, a terminal deoxynucleotidyl transferase-mediated deoxiuridinetriphosphate nick-end labelling (TUNEL) assay was performed to detect DNA fragmentation. General histology and tissue integrity were assessed by haematoxylin-eosin staining.

MAIN RESULTS AND THE ROLE OF CHANCE

VASA showed a differential pattern of expression; in the resting primordial follicle reserve in infant and pubertal ovaries, it was associated with the Balbiani's body space in the germ cell. VASA remained detectable in primary follicles leaving the resting reserve, but once follicles entered the growing pool it became undetectable. This pattern of VASA expression is the same as in the fetal ovary. BAX was expressed both in the resting primordial reserve and in the pool of growing follicles, whereas BCL2 was detected only in granulosa cells in antral follicles in the growing pool. Apoptosis-related protein expression moved from the germ cell to the somatic stratum when primordial follicles left the resting reserve to enter the pool of growing follicles, irrespective of female age. Most TUNEL-positive cells were detected in the granulosa cells of antral follicles. No TUNEL-positive cells were found in resting primordial follicles.

LIMITATIONS, REASONS FOR CAUTION

The study was limited by the qualitative nature of the immuno-histochemical analysis and the TUNEL assay. The results neither quantify the levels of germ-cell death nor exclude other concurrent cell death mechanisms that could act in the regulation of female germ-cell number.

WIDER IMPLICATIONS OF THE FINDING

This study provides missing knowledge about apoptosis and germ-cell-specific VASA expression in the human ovary between birth and puberty and the participation of BCL2 and BAX genes in the balance between death and survival throughout female germ-line development. Intracellular localization of VASA in resting follicles emerges as a possible marker with prognostic value that needs further investigation, especially in infant patients entering ovarian cryo-preservation programmes. This knowledge will be valuable in optimizing the rescue and clinical use of germ cells to restore fertility in women.

STUDY FUNDING/COMPETING INTEREST(S)

No external funding was obtained for this study. The authors have no conflicts of interest to declare.

Immunohistochemical approaches to the study of human fetal ovarian development

The development of primordial germ cells into oocytes within primordial follicles involves a complex sequence of proliferation, developmental commitment, entry and arrest in meiosis, and association with surrounding somatic cells. These processes occur over the first few months of development in the human, with multiple stages of development present at any one time point. Immunohistochemistry has been hugely instructive in identifying the various key stages in ovarian development, by allowing simultaneous visualization of different stages of germ cell development, and their spatial arrangement. These studies allow comparison with other species and have identified key differences between human and murine ovarian development as well as giving a basis for functional studies. In this chapter we describe the main methodologies used in immunohistochemistry, using both chromogen and fluorescence approaches, and both single and double antigen detection.

Prevalence of c-KIT mutations in gonadoblastoma and dysgerminomas of patients with disorders of sex development (DSD) and ovarian dysgerminomas

Activating c-KIT mutations (exons 11 and 17) are found in 10–40% of testicular seminomas, the majority being missense point mutations (codon 816). Malignant ovarian dysgerminomas represent ∼3% of all ovarian cancers in Western countries, resembling testicular seminomas, regarding chromosomal aberrations and c-KIT mutations. DSD patients with specific Y-sequences have an increased risk for Type II Germ Cell Tumor/Cancer, with gonadoblastoma as precursor progressing to dysgerminoma. Here we present analysis of c-KIT exon 8, 9, 11, 13 and 17, and PDGFRA exon 12, 14 and 18 by conventional sequencing together with mutational analysis of c-KIT codon 816 by a sensitive and specific LightCycler melting curve analysis, confirmed by sequencing. The results are combined with data on TSPY and OCT3/4 expression in a series of 16 DSD patients presenting with gonadoblastoma and dysgerminoma and 15 patients presenting pure ovarian dysgerminomas without DSD. c-KIT codon 816 mutations were detected in five out of the total of 31 cases (all found in pure ovarian dysgerminomas). A synonymous SNP (rs 5578615) was detected in two patients, one DSD patient (with bilateral disease) and one patient with dysgerminoma. Next to these, three codon N822K mutations were detected in the group of 15 pure ovarian dysgerminomas. In total activating c-KIT mutations were found in 53% of ovarian dysgerminomas without DSD. In the group of 16 DSD cases a N505I and D820E mutation was found in a single tumor of a patient with gonadoblastoma and dysgerminoma. No PDGFRA mutations were found. Positive OCT3/4 staining was present in all gonadoblastomas and dysgerminomas investigated, TSPY expression was only seen in the gonadoblastoma/dysgerminoma lesions of the 16 DSD patients. This data supports the existence of two distinct but parallel pathways in the development of dysgerminoma, in which mutational status of c-KIT might parallel the presence of TSPY.

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

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

Delayed Recognition of Disorders of Sex Development (DSD): A Missed Opportunity for Early Diagnosis of Malignant Germ Cell Tumors

Disorders of sex development (DSD) are defined as a congenital condition in which development of chromosomal, gonadal or anatomical sex is atypical. DSD patients with gonadal dysgenesis or hypovirilization, containing part of the Y chromosome (GBY), have an increased risk for malignant type II germ cell tumors (GCTs: seminomas and nonseminomas). DSD may be diagnosed in newborns (e.g., ambiguous genitalia), or later in life, even at or after puberty. Here we describe three independent male patients with a GCT; two were retrospectively recognized as DSD, based on the histological identification of both carcinoma in situ and gonadoblastoma in a single gonad as the cancer precursor. Hypospadias and cryptorchidism in their history are consistent with this conclusion. The power of recognition of these parameters is demonstrated by the third patient, in which the precursor lesion was diagnosed before progression to invasiveness. Early recognition based on these clinical parameters could have prevented development of (metastatic) cancer, to be treated by systemic therapy. All three patients showed a normal male 46,XY karyotype, without obvious genetic rearrangements by high-resolution whole-genome copy number analysis. These cases demonstrate overlap between DSD and the so-called testicular dysgenesis syndrome (TDS), of significant relevance for identification of individuals at increased risk for development of a malignant GCT.