Inferring biological mechanisms from spatial analysis: prediction of a local inhibitor in the ovary

Overactivation or Apoptosis: Which Mechanisms Affect Chemotherapy-Induced Ovarian Reserve Depletion?

Dormant primordial follicles (PMF), which constitute the ovarian reserve, are recruited continuously into the cohort of growing follicles in the ovary throughout female reproductive life. Gonadotoxic chemotherapy was shown to diminish the ovarian reserve pool, to destroy growing follicle population, and to cause premature ovarian insufficiency (POI). Three primary mechanisms have been proposed to account for this chemotherapy-induced PMF depletion: either indirectly via over-recruitment of PMF, by stromal damage, or through direct toxicity effects on PMF. Preventative pharmacological agents intervening in these ovotoxic mechanisms may be ideal candidates for fertility preservation (FP). This manuscript reviews the mechanisms that disrupt follicle dormancy causing depletion of the ovarian reserve. It describes the most widely studied experimental inhibitors that have been deployed in attempts to counteract these affects and prevent follicle depletion.

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.

Evidence for a fragile X messenger ribonucleoprotein 1 (FMR1) mRNA gain-of-function toxicity mechanism contributing to the pathogenesis of fragile X-associated premature ovarian insufficiency

Fragile X-associated premature ovarian insufficiency (FXPOI) is among a family of disorders caused by expansion of a CGG trinucleotide repeat sequence located in the 5' untranslated region (UTR) of the fragile X messenger ribonucleoprotein 1 (FMR1) gene on the X chromosome. Women with FXPOI have a depleted ovarian follicle population, resulting in amenorrhea, hypoestrogenism, and loss of fertility before the age of 40. FXPOI is caused by expansions of the CGG sequence to lengths between 55 and 200 repeats, known as a FMRI premutation, however the mechanism by which the premutation drives disease pathogenesis remains unclear. Two main hypotheses exist, which describe an mRNA toxic gain-of-function mechanism or a protein-based mechanism, where repeat-associated non-AUG (RAN) translation results in the production of an abnormal protein, called FMRpolyG. Here, we have developed an in vitro granulosa cell model of the FMR1 premutation by ectopically expressing CGG-repeat RNA and FMRpolyG protein. We show that expanded CGG-repeat RNA accumulated in intranuclear RNA structures, and these aggregates were able to cause significant granulosa cell death independent of FMRpolyG expression. Using an innovative RNA pulldown, mass spectrometry-based approach we have identified proteins that are specifically sequestered by CGG RNA aggregates in granulosa cells in vitro, and thus may be deregulated as consequence of this interaction. Furthermore, we have demonstrated reduced expression of three proteins identified via our RNA pulldown (FUS, PA2G4 and TRA2β) in ovarian follicles in a FMR1 premutation mouse model. Collectively, these data provide evidence for the contribution of an mRNA gain-of-function mechanism to FXPOI disease biology.

A New Understanding, Guided by Single-Cell Sequencing, of the Establishment and Maintenance of the Ovarian Reserve in Mammals

BACKGROUND

Oocytes are a finite and non-renewable resource that are maintained in primordial follicle structures. The ovarian reserve is the totality of primordial follicles, present from birth, within the ovary and its establishment, size, and maintenance dictates the duration of the female reproductive lifespan. Understanding the cellular and molecular dynamics relevant to the establishment and maintenance of the reserve provides the first steps necessary for modulating both individual human and animal reproductive health as well as population dynamics.

SUMMARY

This review details the key stages of establishment and maintenance of the ovarian reserve, encompassing germ cell nest formation, germ cell nest breakdown, and primordial follicle formation and activation. Furthermore, we spotlight several formative single-cell sequencing studies that have significantly advanced our knowledge of novel molecular regulators of the ovarian reserve, which may improve our ability to modulate female reproductive lifespans.

KEY MESSAGES

The application of single-cell sequencing to studies of ovarian development in mammals, especially when leveraging genetic and environmental models, offers significant insights into fertility and its regulation. Moreover, comparative studies looking at key stages in the development of the ovarian reserve across species has the potential to impact not just human fertility, but also conservation biology, invasive species management, and agriculture.

Spatial Statistical Models: An Overview under the Bayesian Approach

Spatial documentation is exponentially increasing given the availability of Big Data in the Internet of Things, enabled by device miniaturization and data storage capacity. Bayesian spatial statistics is a useful statistical tool to determine the dependence structure and hidden patterns in space through prior knowledge and data likelihood. However, this class of modeling is not yet well explored when compared to adopting classification and regression in machine-learning models, in which the assumption of the spatiotemporal independence of the data is often made, that is an inexistent or very weak dependence. Thus, this systematic review aims to address the main models presented in the literature over the past 20 years, identifying the gaps and research opportunities. Elements such as random fields, spatial domains, prior specification, the covariance function, and numerical approximations are discussed. This work explores the two subclasses of spatial smoothing: global and local.

Ovary Development: Insights From a Three-Dimensional Imaging Revolution

The ovary is an indispensable unit of female reproduction and health. However, the study of ovarian function in mammals is hindered by unique challenges, which include the desynchronized development of oocytes, irregular distribution and vast size discrepancy of follicles, and dynamic tissue remodeling during each hormonal cycle. Overcoming the limitations of traditional histology, recent advances in optical tissue clearing and three-dimensional (3D) visualization offer an advanced platform to explore the architecture of intact organs at a single cell level and reveal new relationships and levels of organization. Here we summarize the development and function of ovarian compartments that have been delineated by conventional two-dimensional (2D) methods and the limits of what can be learned by these approaches. We compare types of optical tissue clearing, 3D analysis technologies, and their application to the mammalian ovary. We discuss how 3D modeling of the ovary has extended our knowledge and propose future directions to unravel ovarian structure toward therapeutic applications for ovarian disease and extending female reproductive lifespan.

Altered germline cyst formation and oogenesis in Tex14 mutant mice

During oocyte differentiation in mouse fetal ovaries, sister germ cells are connected by intercellular bridges, forming germline cysts. Within the cyst, primary oocytes form via gaining cytoplasm and organelles from sister germ cells through germ cell connectivity. To uncover the role of intercellular bridges in oocyte differentiation, we analyzed mutant female mice lacking testis-expressed 14 (TEX14), a protein involved in intercellular bridge formation and stabilization. In Tex14 homozygous mutant fetal ovaries, germ cells divide to form a reduced number of cysts in which germ cells remained connected via syncytia or fragmented cell membranes, rather than normal intercellular bridges. Compared with wild-type cysts, homozygous mutant cysts fragmented at a higher frequency and produced a greatly reduced number of primary oocytes with precocious cytoplasmic enrichment and enlarged volume. By contrast, Tex14 heterozygous mutant germline cysts were less fragmented and generate primary oocytes at a reduced size. Moreover, enlarged primary oocytes in homozygous mutants were used more efficiently to sustain folliculogenesis than undersized heterozygous mutant primary oocytes. Our observations directly link the nature of fetal germline cysts to oocyte differentiation and development.

Summary: Altered germline cyst formation and fragmentation due to defective germ cell connectivity leads to changes in oocyte differentiation and development in Tex14 mutant mice.

Spatial and temporal changes in follicle distribution in the human ovarian cortex

RESEARCH QUESTION

How does follicle distribution evolve in the human ovarian cortex between the ages of 20 and 35 years?

DESIGN

Fragments of ovarian cortex from women undergoing unilateral oophorectomy for fertility preservation were obtained for quantitative histological assessment, including recording the two-dimensional coordinates of the follicles. Data were analysed using spatial statistical methods.

RESULTS

A total of 53 ovarian cortex tissue samples, containing 1-803 follicles each, were obtained from 14 women aged 20-35 years. Primordial and transitory follicles lay in a clustered manner in the human ovarian cortex, with an average cluster radius of around 270 µm (95% confidence interval 154-377 µm; n = 49). Follicle density declined with age (P = 0.006, n = 13), and the distance from the nearest neighbouring follicle increased (P = 0.004, n = 13). Cluster radius decreased with age (P = 0.02, n = 13), but the degree of clustering tended to increase (P = 0.11, n = 13). In the majority of the samples, follicles at different stages lay in different clusters (P < 0.05, n = 13).

CONCLUSIONS

This study shows that primordial and transitory follicles lie in different clusters in the human ovarian cortex. Spatio-temporal computer simulation suggests that interfollicular signals may hinder follicle loss and may therefore drive clustered follicle distribution. In clinical practice, the woman's age should be taken into account when assessing follicle density, as follicle distribution is increasingly clustered with advancing age.

Biopsying, fragmentation and autotransplantation of fresh ovarian cortical tissue in infertile women with diminished ovarian reserve

STUDY QUESTION

Can ovarian biopsying per se and/or autotransplantation of fragmented ovarian cortical tissue activate dormant follicles and increase the number of recruitable follicles for IVF/ICSI in women with diminished ovarian reserve (DOR)?

SUMMARY ANSWER

Ovarian biopsying followed by immediate autotransplantation of fragmented cortical tissue failed to increase the number of recruitable follicles for IVF/ICSI 10 weeks after the procedure either at the graft site or in the biopsied ovary, but 12 of the 20 women subsequently had a clinical pregnancy during the 1-year follow-up.

WHAT IS KNOWN ALREADY

Infertile women with DOR constitute a group of patients with poor reproductive outcome mainly due to the low number of mature oocytes available for IVF/ICSI. Recent studies have shown that in vitro activation of residual dormant follicles by both chemical treatment and tissue fragmentation has resulted in return of menstrual cycles and pregnancies in a fraction of amenorrhoeic women with premature ovarian insufficiency.

STUDY DESIGN, SIZE, DURATION

This is a prospective clinical cohort study including 20 women with DOR treated at the fertility clinic, Rigshospitalet, Denmark, during April 2016-December 2017. Non-pregnant patients were on average followed for 280 days (range 118-408), while women who conceived were followed until delivery. Study follow-up of non-pregnant patients ended in September 2018.

PARTICIPANTS, MATERIALS, SETTING, METHODS

The study included infertile women aged 30-39 years with preserved menstrual cycles, indication for IVF/ICSI and repeated serum measurements of anti-Müllerian hormone (AMH) ≤ 5 pmol/L. Patients were randomized to have four biopsies taken from either the left or the right ovary by laparoscopy followed by fragmentation of the cortical tissue to an approximate size of 1 mm3 and autotransplanted to a peritoneal pocket. The other ovary served as a control. Patients were followed weekly for 10 weeks with recording of hormone profile, antral follicle count (AFC), ovarian volume and assessment for ectopic follicle growth. After 10 weeks, an IVF/ICSI-cycle with maximal ovarian stimulation was initiated.

MAIN RESULTS AND THE ROLE OF CHANCE

No difference in the number of mature follicles after ovarian stimulation 10 weeks after the procedure in the biopsied versus the control ovaries was observed (1.0 vs. 0.7 follicles, P = 0.35). In only three patients, growth of four follicles was detected at the graft site 24-268 days after the procedure. From one of these follicles, a metaphase II (MII) oocyte was retrieved and fertilized, but embryonic development failed. Overall AMH levels did not change significantly after the procedure (P = 0.2). The AFC increased by 0.14 (95% CI: 0.06;0.21) per week (P < 0.005), and the biopsied ovary had on average 0.6 (95% CI: 0.3;-0.88) follicles fewer than the control ovary (P = 0.01). Serum levels of androstenedione and testosterone increased significantly by 0.63 nmol/L (95% CI: 0.21;1.04) and 0.11 nmol/L (95% CI: 0.01;0.21) 1 week after the procedure, respectively, and testosterone increased consecutively over the 10 weeks by 0.0095 nmol/L (95% CI: 0.0002;0.0188) per week (P = 0.045). In 7 of the 20 patients, there was a serum AMH elevation 5 to 8 weeks after the procedure. In this group, mean AMH increased from 2.08 pmol/L (range 1.74-2.34) to 3.94 pmol/L (range 3.66-4.29) from Weeks 1-4 to Weeks 5-8. A clinical pregnancy was obtained in 12 of the 20 (60%) patients with and without medically assisted reproduction (MAR) treatments. We report a cumulated live birth rate per started IVF/ICSI cycle of 18.4%.

LIMITATIONS, REASON FOR CAUTION

Limitations of the study were the number of patients included and the lack of a non-operated control group. Moreover, 9 of the 20 women had no male partner at inclusion and were treated with donor sperm, but each of these women had an average of 6.8 (range 4-9) unsuccessful MAR treatments with donor sperm prior to inclusion.

WIDER IMPLICATIONS OF THE FINDINGS

Although 12 out of 20 patients became pregnant during the follow-up period, the current study does not indicate that biopsying, fragmenting and autotransplanting of ovarian cortical tissue increase the number of recruitable follicles for IVF/ICSI after 10 weeks. However, a proportion of the patients may have a follicular response in Weeks 5-8 after the procedure. It could therefore be relevant to perform a future study on the possible effects of biopsying per se that includes stimulation for IVF/ICSI earlier than week 10.

STUDY FUNDING/COMPETING INTEREST(S)

This study is part of the ReproUnion collaborative study, co-financed by the European Union, Interreg V ÖKS. The funders had no role in the study design, data collection and interpretation, or decision to submit the work for publication. None of the authors have a conflict of interest.

TRIAL REGISTRATION NUMBER

NCT02792569.

Spatial Analysis of Growing Follicles in the Human Ovary to Inform Tissue Engineering Strategies

Cancer survivorship has increased considerably, but common cancer treatments may threaten female reproductive health and fertility. In females, standard fertility preservation techniques include egg and embryo banking and ovarian tissue cryopreservation, but these methods are not suitable for all individuals. Emerging fertility preservation technologies include in vitro follicle growth and ovarian bioprosthetics. Although these platforms hold tremendous promise, they remain in the preclinical phase likely because of our inability to adequately phenocopy the complexity of the in vivo ovarian environment. The goal of this study was to use an established research archive of fixed human ovarian tissue established through the Oncofertility Consortium to better understand the dynamics and milieu of growing follicles within the human ovary. We performed a histological analysis of the immediate surroundings of primary and secondary stage follicles. We evaluated oocyte and follicle diameters of these growing follicles, analyzed their growth trajectories, and mapped their precise relationships to other stage follicles within a defined area. We also stratified our findings according to participant age and previous treatment history. Our results serve as in vivo benchmarks for follicles grown in vitro and provide insight into how follicles should be seeded spatially within bioprosthetic ovaries, potentially improving the efficacy and clinical translation of these emerging technologies. Impact statement Life-preserving cancer treatments have greatly increased survivorship. However, treatments often have off-target health consequences that threaten female reproductive health and fertility. Although several standard fertility preservation options exist, there is a constant need to explore and expand options for all populations. In vitro follicle growth and ovarian bioprosthetics are new experimental procedures, which are currently limited to proof of concept. In this study, we analyzed human ovarian tissue from a deidentified biospecimen repository to characterize the growing follicle landscape with the ultimate goal of informing bioengineering practices. This spatial analysis pinpoints the geometry of growing follicles within the human ovary and provides a framework for paralleling this environment in ex vivo platforms.

Ovarian damage from chemotherapy and current approaches to its protection

BACKGROUND

Anti-cancer therapy is often a cause of premature ovarian insufficiency and infertility since the ovarian follicle reserve is extremely sensitive to the effects of chemotherapy and radiotherapy. While oocyte, embryo and ovarian cortex cryopreservation can help some women with cancer-induced infertility achieve pregnancy, the development of effective methods to protect ovarian function during chemotherapy would be a significant advantage.

OBJECTIVE AND RATIONALE

This paper critically discusses the different damaging effects of the most common chemotherapeutic compounds on the ovary, in particular, the ovarian follicles and the molecular pathways that lead to that damage. The mechanisms through which fertility-protective agents might prevent chemotherapy drug-induced follicle loss are then reviewed.

SEARCH METHODS

Articles published in English were searched on PubMed up to March 2019 using the following terms: ovary, fertility preservation, chemotherapy, follicle death, adjuvant therapy, cyclophosphamide, cisplatin, doxorubicin. Inclusion and exclusion criteria were applied to the analysis of the protective agents.

OUTCOMES

Recent studies reveal how chemotherapeutic drugs can affect the different cellular components of the ovary, causing rapid depletion of the ovarian follicular reserve. The three most commonly used drugs, cyclophosphamide, cisplatin and doxorubicin, cause premature ovarian insufficiency by inducing death and/or accelerated activation of primordial follicles and increased atresia of growing follicles. They also cause an increase in damage to blood vessels and the stromal compartment and increment inflammation. In the past 20 years, many compounds have been investigated as potential protective agents to counteract these adverse effects. The interactions of recently described fertility-protective agents with these damage pathways are discussed.

WIDER IMPLICATIONS

Understanding the mechanisms underlying the action of chemotherapy compounds on the various components of the ovary is essential for the development of efficient and targeted pharmacological therapies that could protect and prolong female fertility. While there are increasing preclinical investigations of potential fertility preserving adjuvants, there remains a lack of approaches that are being developed and tested clinically.

A Gatekeeping Role of ESR2 to Maintain the Primordial Follicle Reserve

Over the entire reproductive lifespan in mammals, a fixed number of primordial follicles serve as the source of mature oocytes. Uncontrolled and excessive activation of primordial follicles can lead to depletion of the ovarian reserve. We observed that disruption of estrogen receptor β (ESR2) signaling results in increased activation of primordial follicles in Esr2-null (Esr2-/-) rats. However, follicle assembly was unaffected, and the total number of follicles remained comparable between neonatal wild-type and Esr2-/- ovaries. While the activated follicle counts were increased in Esr2-/- ovary, the number of primordial follicles were markedly decreased. Excessive recruitment of primordial follicles led to premature ovarian senescence in Esr2-/- rats and was associated with reduced levels of serum AMH and estradiol. Disruption of ESR2 signaling through administration of a selective antagonist (PHTPP) increased the number of activated follicles in wildtype rats, whereas a selective agonist (DPN) decreased follicle activation. In contrast, primordial follicle activation was not increased in the absence of ESR1, indicating that the regulation of primordial follicle activation is ESR2 specific. Follicle activation was also increased in Esr2 mutants lacking the DNA binding domain, suggesting a role for the canonical transcriptional activation function. Both primordial and activated follicles express ESR2, suggesting a direct regulatory role for ESR2 within these follicles. We also detected that loss of ESR2 augmented the activation of AKT, ERK, and mTOR pathways. Our results indicate that the lack of ESR2 upregulated both granulosa and oocyte factors, which can facilitate AKT and mTOR activation in Esr2-/- ovaries leading to increased activation of primordial follicles.

Nuclear exclusion of SMAD2/3 in granulosa cells is associated with primordial follicle activation in the mouse ovary

Maintenance and activation of the limited supply of primordial follicles in the ovary are important determinants of reproductive lifespan. Currently, the molecular programme that maintains the primordial phenotype and the early events associated with follicle activation are not well defined. Here, we have systematically analysed these events using microscopy and detailed image analysis. Using the immature mouse ovary as a model, we demonstrate that the onset of granulosa cell (GC) proliferation results in increased packing density on the oocyte surface and consequent GC cuboidalization. These events precede oocyte growth and nuclear translocation of FOXO3a, a transcription factor important in follicle activation. Immunolabelling of the TGFβ signalling mediators and transcription factors SMAD2/3 revealed a striking expression pattern specific to GCs of small follicles. SMAD2/3 were expressed in the nuclei of primordial GCs but were mostly excluded in early growing follicles. In activated follicles, GC nuclei lacking SMAD2/3 generally expressed Ki67. These findings suggest that the first phenotypic changes during follicle activation are observed in GCs, and that TGFβ signalling is fundamental for regulating GC arrest and the onset of proliferation.

Being a good egg in the 21st century

INTRODUCTION

Women are increasingly having children at a later age, but this can conflict with declining fertility in the later 30's and thereafter.

AREAS OF AGREEMENT

Declining egg quality and quantity with age are well-established, although egg quality can only be surmised from reproductive success or failure.

AREAS OF CONTROVERSY

Whether increasing the number of eggs that can be obtained from ovarian stimulation is of value, and whether there are precursor cells within the adult ovary that could become mature eggs.

GROWING POINTS

There is increasing use of donated eggs by older women to enhance their chances of conception. The storage of frozen eggs for potential use later in life is also becoming more common.

AREAS TIMELY FOR DEVELOPING RESEARCH

Understanding of growth initiation of follicles and development of an artificial ovary may lead to the ability to affect fertility and reproductive lifespan.

Low-dose metronomic delivery of cyclophosphamide is less detrimental to granulosa cell viability, ovarian function, and fertility than maximum tolerated dose delivery in the mouse

Chemotherapy can cause early menopause or infertility in women and have a profound negative impact on the quality of life of young female cancer survivors. Various factors are known to influence the risk of chemotherapy-induced ovarian failure, including the drug dose and treatment duration; however, the scheduling of dose administration has not yet been evaluated as an independent risk factor. We hypothesized that low-dose metronomic (LDM) chemotherapy scheduling would be less detrimental to ovarian function than the traditional maximum tolerated dose (MTD) strategy. In vitro, MTD cyclophosphamide exposure resulted in decreased proliferation and increased granulosa cell apoptosis, while cells treated with LDM cyclophosphamide were not different from untreated controls. Treatments of MTD cyclophosphamide induced high levels of follicle atresia and enhanced follicle recruitment in mice. In contrast, LDM delivery of an equivalent dose of cyclophosphamide reduced growing follicle numbers, but was not associated with higher levels of follicle atresia or recruitment. MTD cyclophosphamide induced significant vascular disruption and DNA damage in vivo, while LDM chemotherapy with equal cumulative amounts of cyclophosphamide was not different from controls. MTD chemotherapy also had a negative effect on mouse-fertility outcomes. Our findings suggest that LDM scheduling could potentially minimize the long-term effects of cyclophosphamide on female fertility by preventing follicle depletion from enhanced activation.

Effects of vitrification on the viability of alginate encapsulated isolated bovine pre-antral follicles

PURPOSE

Individual follicle cryopreservation techniques, without hydrogel support, are labor-intensive and a substantial proportion of isolated follicles are lost during handling and after warming. Therefore, the viability and morphology of isolated bovine (as a model for human) pre-antral follicles after vitrification and warming, when encapsulated in alginate beads, were investigated.

METHODS

Bovine pre-antral follicles were mechanically isolated and divided into four different groups: (1) culture in 2% alginate beads (3D system) and vitrification in beads using mesh cups (3DVIT), (2) culture in 2% alginate beads (3DCUL), (3) culture in 96-well plates (2D system) and vitrification using High Security Vitrification straws® (2DVIT), (4) culture in a 2D system (2DCUL). The same vitrification and warming protocols were used for embedded (3DVIT) and non-embedded follicles (2DVIT).

RESULTS

No differences were observed in follicle viability between group 2DCUL and 3DCUL. Group 3DVIT showed the lowest viability (45.9%) according to calcein and neutral red staining among all groups. Group 2DVIT displayed the highest viability (87.5%) and largest percentage of follicles with a well-preserved morphology.

CONCLUSIONS

Our results show that, using a vitification protocol optimized for non-embedded follicles, 2D culture is more effective in vitrifying isolated follicles. However, embedding in alginate allow to handle follicles more efficiently, i.e., without excessive manipulation and thus less labor-intensive in combination with a reduced loss of follicles during the procedure. Based on the increased work efficiency, but lower viability and higher proportion of follicles showing impaired morphology, we consider it advantageous to optimize the protocol for the vitrification of embedded follicles to increase survival and maintain morphology after vitrification.

Further insights into the impact of mouse follicle stage on graft outcome in an artificial ovary environment

STUDY QUESTION

Are mouse preantral follicles differently affected by isolation, encapsulation and/or grafting procedures according to stage?

SUMMARY ANSWER

Isolated secondary follicles showed superior ability to survive and grow after transplantation, which was not related to a particular effect of the isolation and/or grafting procedure, but rather to their own ability to induce neoangiogenesis.

WHAT IS KNOWN ALREADY

Isolated and encapsulated mouse preantral follicles can survive (6-27%) and grow (80-100%) in a fibrin matrix with a low concentration of fibrinogen and thrombin (F12.5/T1) after short-term transplantation.

STUDY DESIGN, SIZE, DURATION

An in vivo experimental model using 20 donor Naval Medical Research Institute (NMRI) mice (6-25 weeks of age) and 14 recipient severe combined immunodeficient (SCID) mice (11-39 weeks of age) was applied. Each NMRI mouse underwent mechanical disruption of both ovaries and isolation of primordial-primary and secondary follicles with ovarian stromal cells, in order to encapsulate them in an F12.5/T1 matrix. Twelve out of 40 fibrin clots were immediately fixed as controls (D0) (10 for histology and 2 for scanning electron microscopy [SEM]) and the others (n = 28) were grafted to the inner part of the peritoneum for 2 (16 fibrin clots) or 7 (12 fibrin clots) days (D2 and D7).

PARTICIPANTS/MATERIALS, SETTING, METHODS

This study involved the participation of the Gynecology Research Unit (Universitè Catholique de Louvain) and the Physiological Sciences Department (University of Brasília). Specific techniques were used to analyze the follicle recovery rate (hematoxylin-eosin staining), vascularization (CD34) and follicle ultrastructure (transmission electron microscopy [TEM] and SEM).

MAIN RESULTS AND THE ROLE OF CHANCE

After follicle isolation and encapsulation, a statistically higher percentage of normal follicles was observed in the secondary group (62%) than in the primordial-primary group (47%). Follicle recovery rates were 34% and 62% for primordial-primary and secondary follicles on D2, respectively, and 12% and 42% on D7, confirming that secondary follicles survive better than primordial-primary follicles after grafting. Concerning vascularization, both follicle stages exhibited similar vascularization to that seen in control mouse ovary on D7, but a significantly higher number of vessels and greater vessel surface area were detected in the secondary follicle group. Despite structural differences in fiber density between fibrin clots and ovarian tissue observed by SEM and TEM, preantral follicles appeared to be well encapsulated in the matrix, also showing a normal ultrastructure after grafting.

LARGE SCALE DATA

Not applicable.

LIMITATIONS, REASONS FOR CAUTION

As demonstrated by our results during the isolation procedure, we encapsulated a significantly higher number of round structures in the primordial-primary group than in the secondary group, which could partially explain the lower recovery rate of early-stage follicles in our previous study. However, it is not excluded that the physical and mechanical properties of the fibrin matrix may also play a role in follicle survival and growth, so further investigations are needed.

WIDER IMPLICATIONS OF THE FINDINGS

This research represents one more key step in the creation of the artificial ovary.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by grants from the Fonds National de la Recherche Scientifique de Belgique (FNRS) to C.A. Amorim as a research associate at FRS-FNRS and (grant 5/4/150/5 awarded to M.M. Dolmans), Fonds Spéciaux de Recherche, Fondation St Luc, Foundation Against Cancer, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-Brazil) (grant #013/14 CAPES/WBI awarded to C.M. Lucci, with F. Paulini receiving a post-doctoral fellowship), and Wallonie-Bruxelles International, and donations from the Ferrero family. None of the authors have any competing interests to declare in relation to the topic.

Imaging the ovary

During each reproductive cycle, the ovary exhibits tissue remodelling and cyclic vasculature changes associated with hormonally regulated folliculogenesis, follicle rupture, luteal formation and regression. However, the relationships among different types of follicles and corpora lutea are unclear, and the role of ovarian vasculature in folliculogenesis and luteal dynamics has not been extensively investigated. Understanding of ovarian physiology and pathophysiology relies upon elucidation of ovarian morphology and architecture. This paper summarizes the literature on traditional approaches to the imaging of ovarian structures and discusses recent advances in ovarian imaging. Traditional in-vivo ultrasound, together with histological and electron microscopic approaches provide detailed views of the ovary at organ, tissue and molecular levels. However, in-vivo imaging is limited to antral and larger follicles whereas histological imaging is mainly two-dimensional in nature. Also discussed are emerging approaches in the use of near-infrared fluorophores to image follicles in live animals to detect preantral follicles as well as visualizing ovarian structures using CLARITY in fixed whole ovaries to elucidate three-dimensional interrelationships among follicles, corpora lutea and ovarian vasculature. Advances in ovarian imaging techniques provide new understanding of ovarian physiology and allow for the development of better tools to diagnose ovarian pathophysiology.

Laparoscopic ovarian biopsy pick-up method for goats

Biopsy pick-up (BPU) has been considered a safe method to harvest ovarian fragments from live animals. However, no studies have been reported on the use of BPU to collect in vivo ovarian tissue in goats. The goals of this study were: (i) to test different biopsy needle sizes to collect ovarian tissue in situ using the BPU method (Experiment 1), and (ii) to study ovarian tissue features such as preantral follicle density, morphology, class distribution, and stromal cell density in ovarian fragments obtained in vivo through a laparoscopic BPU method (Experiment 2). In Experiment 1, goat ovaries (n = 20) were collected in a slaughterhouse and subjected to in situ BPU. Three needles (16, 18, and 20G) were tested. In Experiment 2, the most efficient biopsy needle from Experiment 1 was used to perform laparoscopic BPU in goats (n = 8). In Experiment 1, the recovery rate was greater (P < 0.05; range 50-62%) with 16G and 18G needles than the 20G (17%) needle. The mean weight of ovarian fragments collected by the 16G needle was greater (P < 0.05) than the 18G and the 20G needle. In Experiment 2, 62 biopsy attempts were performed and 52 ovarian fragments were collected (90% success rate). Overall, 2054 preantral follicles were recorded in 5882 histological sections analyzed. Mean preantral follicular density was 28.4 ± 1.3 follicles per cm². The follicular density differed (P < 0.05) among animals and ovarian fragments within the same animal. The mean stromal cell density in the ovarian fragments was 37.1 ± 0.5 cells per 2500 μm², and differed (P < 0.05) among animals. Moreover, preantral follicle density and stromal cell density were associated (P < 0.001). The percentage of morphologically normal follicles was 70.1 ± 1.2, and differed (P < 0.05) among animals. The majority (79%) of the morphologically normal follicles was classified as primordial follicles, and differed (P < 0.05) among animals and between ovaries. In summary, a laparoscopic BPU method has been developed to harvest ovarian tissue in vivo with a satisfactory success rate in goats. Furthermore, this study described for the first time that goat ovarian biopsy fragments have a high heterogeneity in follicular density, morphology, class distribution, and stromal cell density.

CLARITY reveals dynamics of ovarian follicular architecture and vasculature in three-dimensions

Optimal distribution of heterogeneous organelles and cell types within an organ is essential for physiological processes. Unique for the ovary, hormonally regulated folliculogenesis, ovulation, luteal formation/regression and associated vasculature changes lead to tissue remodeling during each reproductive cycle. Using the CLARITY approach and marker immunostaining, we identified individual follicles and corpora lutea in intact ovaries. Monitoring lifetime changes in follicle populations showed age-dependent decreases in total follicles and percentages of advanced follicles. Follicle development from primordial to preovulatory stage was characterized by 3 × 10⁵-fold increases in volume, decreases in roundness, and decreased clustering of same stage follicles. Construction of follicle-vasculature relationship maps indicated age- and gonadotropin-dependent increases in vasculature and branching surrounding follicles. Heterozygous mutant mice with deletion of hypoxia-response element in the vascular endothelial growth factor A (VEGFA) promoter showed defective ovarian vasculature and decreased ovulatory responses. Unilateral intrabursal injection of axitinib, an inhibitor of VEGF receptors, retarded neo-angiogenesis that was associated with defective ovulation in treated ovaries. Our approach uncovers unique features of ovarian architecture and essential roles of vasculature in organizing follicles to allow future studies on normal and diseased human ovaries. Similar approaches could also reveal roles of neo-angiogenesis during embryonic development and tumorigenesis.

Proteomic Analysis of Fetal Ovaries Reveals That Primordial Follicle Formation and Transition Are Differentially Regulated

Primordial follicle formation represents a critical phase of the initiation of embryonic reproductive organ development, while the primordial follicle transition into primary follicle determines whether oestrus or ovulation will occur in female animals. To identify molecular mechanism of new proteins which are involved in ovarian development, we employed 2D-DIGE to compare the protein expression profiles of primordial follicles and primary follicles of fetal ovaries in pigs. Fetal ovaries were collected at distinct time-points of the gestation cycle (g55 and g90). The identified proteins at the g55 time-point are mainly involved in the development of anatomical structures [reticulocalbin-1 (RCN1), reticulocalbin-3 (RCN3)], cell differentiation (actin), and stress response [heterogeneous nuclear ribonucleoprotein K (HNRNPK)]. Meanwhile, at the g90 stage, the isolated proteins with altered expression levels were mainly associated with cell proliferation [major vault protein (MVP)] and stress response [heat shock-related 70 kDa protein 2 (HSPA2)]. In conclusion, our work revealed that primordial follicle formation is regulated by RCN1, RCN3, actin, and HNRNPK, while the primordial follicle transformation to primary follicle is regulated by MVP and HSPA2. Therefore, our results provide further information for the prospective understanding of the molecular mechanism(s) involved in the regulation of the ovarian follicle development.

Observation of the dynamics of follicular development in the ovary

The number of ovulated oocytes is different among mammals but does not vary much within the same species. In order to sustain periodic ovulation, follicular development must be coordinated at the tissue level. Elucidating the regulatory mechanisms of follicular development is difficult because the ovary has a complicated structure and it takes a long time for primordial follicles to develop into Graafian follicles. Therefore, it is not possible to observe follicular development by conventional experiments. The authors previously developed a new ovarian tissue culture method that enabled the observation of follicular development from the early follicle stage. These findings indicated that follicular interactions are important in regulating follicular development and ovulation. This review describes the current methods of observing follicular development in the ovary and the regulatory mechanisms of follicular development.

Non-growing follicle density is increased following adriamycin, bleomycin, vinblastine and dacarbazine (ABVD) chemotherapy in the adult human ovary

STUDY QUESTION

Do the chemotherapeutic regimens of ABVD (adriamycin, bleomycin, vinblastine and dacarbazine) or OEPA-COPDAC (combined vincristine, etoposide, prednisone, doxorubicin (OEPA) and cyclophosphamide, vincristine, prednisone, dacarbazine (COPDAC)) used to treat Hodgkin lymphoma (HL), affect the density, morphology and in vitro developmental potential of human ovarian follicles?

SUMMARY ANSWER

Ovarian tissue from women treated with ABVD contained a higher density of non-growing follicles (NGFs) per cubic millimetre and increased numbers of multiovular follicles but showed reduced in vitro growth compared with patients with lymphoma who had not received chemotherapy, patients treated with OEPA-COPDAC, age-matched healthy women and age-related model-predicted values.

WHAT IS KNOWN ALREADY

Chemotherapy regimens can cause a loss of follicles within the ovary, which depends on the drugs given. Early stage HL is commonly treated by ABVD, a non-alkylating regimen that apparently has ovarian sparing qualities; thus it is important to investigate the histological appearance and distribution of follicles within ABVD-treated ovarian tissue.

STUDY DESIGN, SIZE, DURATION

Thirteen ovarian biopsies were obtained from HL patients (six adolescents and seven adults) and one biopsy from a non-HL patient. Two HL patients and the non-HL patient had received no treatment prior to biopsy collection. The remaining 11 HL patients received one of two regimens: ABVD or OEPA-COPDAC. Tissue was analysed histologically and compared to biopsies from healthy women, and in a subgroup of patients, tissue was cultured for 6 days in vitro.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Ovarian biopsies were obtained from patients undergoing ovarian cryopreservation for fertility preservation and from healthy women at the time of Caesarian section ('obstetric tissue'). Follicle number and maturity were evaluated in sections of ovarian cortical tissue, and compared to an age-related model of mean follicle density and to age-matched contemporaneous biopsies. The developmental potential of follicles was investigated after 6 days of tissue culture.

MAIN RESULTS AND THE ROLE OF CHANCE

A total of 6877 follicles were analysed. ABVD-treated tissue contained a higher density of NGFs per cubic millimetre (230 ± 17) (mean ± SEM) than untreated (110 ± 54), OEPA-COPDAC-treated (50 ± 27) and obstetric (20 ± 4) tissue (P < 0.01), with follicle density 9-21 SD higher than predicted by an age-related model. Biovular and binucleated NGFs occurred frequently in ABVD-treated and in adolescent-untreated tissue but were not observed in OEPA-COPDAC-treated or obstetric tissue, although OEPA-COPDAC-treated tissue contained a high proportion of morphologically abnormal oocytes (52% versus 23% in untreated, 22% in ABVD-treated and 25% in obstetric tissue; P < 0.001). Activation of follicle growth in vitro occurred in all groups, but in ABVD-treated samples there was very limited development to the secondary stage, whereas in untreated samples from lymphoma patients growth was similar to that observed in obstetric tissue (untreated; P < 0.01 versus ABVD-treated, NS versus obstetric).

LARGE SCALE DATA

N/A LIMITATIONS, REASONS FOR CAUTION: Although a large number of follicles were analysed, these data were derived from a small number of biopsies. The mechanisms underpinning these observations have yet to be determined and it is unclear how they relate to future fertility.

WIDER IMPLICATIONS OF THE FINDINGS

This study confirms that the number of NGFs is not depleted following ABVD treatment, consistent with clinical data that female fertility is preserved. Our findings demonstrate that immature follicle density can increase as well as decrease following at least one chemotherapy treatment. This is the first report of morphological and follicle developmental similarities between ABVD-treated tissue and the immature human ovary. Further experiments will investigate the basis for the marked increase in follicle density in ABVD-treated tissue.

STUDY FUNDING/COMPETING INTERESTS

Funded by UK Medical Research Council Grants G0901839 and MR/L00299X/1. The authors have no competing interests.

Effect of first line cancer treatment on the ovarian reserve and follicular density in girls under the age of 18 years

OBJECTIVE

To study the impact of first-line antineoplastic treatment on ovarian reserve in young girls returning for ovarian tissue cryopreservation (OTC) in connection with a relapse.

DESIGN

Retrospective case-control study.

SETTING

University hospitals.

PATIENT(S)

Sixty-three girls under the age of 18 years who underwent OTC before (group 1: 31 patients) and after (group 2: 32 patients) their initial cancer treatment.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Follicular densities (follicles/mm³) measured from an ovarian cortical biopsy before OTC. The ovarian volume (mL) of entire ovaries excised for OTC was also monitored.

RESULT(S)

There was no statistically significant difference in the mean age or follicular density between groups 1 and 2 (334 ± 476/mm³ vs. 327 ± 756/mm³). In contrast, the ovarian volume and total number of ovarian cortex chips cryopreserved were statistically significantly lower in patients who received gonadotoxic treatment before OTC (mean ± standard deviation [SD]: ovarian volume, 5.3 ± 3.1 mL vs. 2.9 ± 2.1 mL, respectively; number of cortex chips: 21.3 ± 8.1 vs. 15.2 ± 7.1, respectively). The reduction in the estimated ovarian reserve ranged from 10% to 20% in children to around 30% in adolescent girls (>10 years).

CONCLUSION(S)

Girls under the age of 10 tolerate a gonadotoxic insult better than adolescents, who may experience up to a 30% reduction in the ovarian reserve via first-line gonadotoxic treatment, which at present is considered to have little effect on the follicle pool. This information will improve counseling of young female cancer patients in deciding whether to undergo fertility preservation treatment.

Fibrin-alginate hydrogel supports steroidogenesis, in vitro maturation of oocytes and parthenotes production from caprine preantral follicles cultured in group

This study aimed to establish a culture system that improves the in vitro development of caprine preantral follicles. In a first experiment, follicles were encapsulated as a single unit per bead and cultured singly or in groups or with five follicles in the same alginate (ALG) bead for 18 days. In a subsequent experiment, the "five follicles per bead" design was chosen to culture in ALG, fibrin-alginate (FA) or hyaluronate (HA) for 18 days. In a third experiment, we chose the five follicles per bead in FA to culture for 30 days. The culture set-up of five follicles per ALG bead increased antrum formation and follicle diameter compared to the other culture designs (p < .05). Moreover, under this condition, 44.44% of the oocytes from in vitro cultured preantral follicles reached meiotic resumption. A significant increase of follicle diameter occurred in attachment system and FA (p < .05), but the ALG condition reached the highest among all groups on day 18 (p < .05). Follicles encapsulated in matrix produced more estradiol and progesterone than attachment system (p < .05). The expression of MMP-9 mRNA was higher in FA than in other groups (p < .05) and similar to antral follicles from in vivo control (p > .05). Only FA group resulted in oocytes matured. After 30 days, oocytes from preantral follicles in vitro grown in FA developed to eight-cell parthenotes. In conclusion, a culture system using FA supported the development of caprine preantral follicles cultured in group and included in the same bead of hydrogel, improving the oocyte maturation and producing parthenotes.

Effect of High Fat Dietary Intake during Maternal Gestation on Offspring Ovarian Health in a Pig Model

Excessive fat intake is a global health concern as women of childbearing age increasingly ingest a high fat diet. We therefore determined the association of a maternal high fat diet in pregnancy with offspring ovarian health during the gestation and postnatal female offspring in pig a model. Thirty-two Yorkshire gilts with similar bodyweights mated at the third estrus were randomly assigned to two nutrition levels of either a control (CON, crude fat: 7.27%) or a high fat diet (HFD, crude fat: 11.78%). Ovary samples were collected during the fetal (Day 55 (g55) and Day 90 of gestation (g90)) and offspring (prepuberty Day 160 (d160) and age at puberty) period to detect ovary development, antioxidant status and apoptosis cells. Maternal HFD did not influence notch signaling gene expression, which regulates primordial follicle formation and transformation, and ovarian histological effect at g55 and g90. However, maternal HFD reduced the numbers of large follicles at d160 and small follicle numbers upon puberty compared to CON in offspring. The results also revealed that the antioxidant index of total antioxidative capability (T-AOC), cytoplasmic copper/zinc superoxide dismutase (CuZn-SOD), glutathione peroxidase (GPx) activities and mRNA expression were higher in the CON than the HFD at g90 and d160, whereas, malondialdehyde (MDA) concentration was decreased in the CON. Maternal HFD increased the inhibitor of the apoptosis-related gene of B-cell lymphoma-2 (bcl2) mRNA expression at g90 and d160, whereas, pro-apoptotic-related gene bcl-2 assaciated X protein (bax) was reduced. These data show that the maternal high fat diet does not delay fetal ovarian development, but it changes ovarian health by the induction of oxidative stress and accelerating cell apoptosis in offspring.

Control of Oocyte Reawakening by Kit

In mammals, females are born with finite numbers of oocytes stockpiled as primordial follicles. Oocytes are “reawakened” via an ovarian-intrinsic process that initiates their growth. The forkhead transcription factor Foxo3 controls reawakening downstream of PI3K-AKT signaling. However, the identity of the presumptive upstream cell surface receptor controlling the PI3K-AKT-Foxo3 axis has been questioned. Here we show that the receptor tyrosine kinase Kit controls reawakening. Oocyte-specific expression of a novel constitutively-active Kit^D818V allele resulted in female sterility and ovarian failure due to global oocyte reawakening. To confirm this result, we engineered a novel loss-of-function allele, Kit^L. Kit inactivation within oocytes also led to premature ovarian failure, albeit via a contrasting phenotype. Despite normal initial complements of primordial follicles, oocytes remained dormant with arrested oocyte maturation. Foxo3 protein localization in the nucleus versus cytoplasm explained both mutant phenotypes. These genetic studies provide formal genetic proof that Kit controls oocyte reawakening, focusing future investigations into the causes of primary ovarian insufficiency and ovarian aging.

In mammals, oocyte reawakening controls female fertility, the onset of the menopause, and thus, overall aging. We demonstrate here through complementary genetic experiments that Kit is the upstream receptor regulating oocyte reawakening. Although other cell surface receptors have been proposed as candidates, the data have remained contradictory, and definitive genetic evidence in support of any one receptor has been lacking. We engineered two novel Kit alleles in mice, one an activating (gain-of-function) mutation, the other an inactivating (loss-of-function) mutation. These alleles permitted us to conduct elegant genetic experiments whereby Kit was activated or inactivated in the oocytes of newborn mice. The results were complementary and striking. Oocyte-specific Kit activation resulted in female sterility due to reawakening of all oocytes, leading to premature ovarian failure. In contrast, Kit inactivation also led to female sterility and ovarian failure, but through a contrasting and opposite phenotype: a complete failure of primordial follicle reawakening. Additional studies demonstrated that Foxo3, a known regulator of reawakening, was the mediator of both phenotypes, linking our findings to prior discoveries. These complementary genetic experiments thus definitively incriminate Kit as the upstream receptor regulating reawakening.

Immunolocalization of the Anti-Müllerian Hormone (AMH) in Caprine Follicles and the Effects of AMH on In Vitro Culture of Caprine Pre-antral Follicles Enclosed in Ovarian Tissue

The aims of this study were to evaluate the localization, by immunohistochemistry, of the anti-Müllerian hormone (AMH) in goat ovaries and to investigate its effects on the in vitro survival and development of caprine pre-antral follicles enclosed in fragments of ovarian tissue. Pre-antral follicles were cultured in vitro for 1 or 7 days in α-MEM(+) in the absence or presence of kit ligand (KL; 50 ng/ml, positive control) or AMH (50 or 150 ng/ml). The results showed that AMH was localized in oocytes and granulosa cells from the primordial follicle to antral follicle stages. Addition of AMH maintained the percentage of developing follicles, similar to that in the uncultured control; however, the percentage of developing follicles was significantly lower than that in the cultured control and KL. Nonetheless, addition of AMH to the culture medium did not affect survival rates and follicular growth. In conclusion, this study demonstrated that the expression of AMH varies according to the compartment and stage of follicular development. Furthermore, AMH inhibits the activation of caprine primordial follicles.

Crowding and Follicular Fate: Spatial Determinants of Follicular Reserve and Activation of Follicular Growth in the Mammalian Ovary

Initiation of growth of resting ovarian follicles is a key phenomenon for providing an adequate number of mature oocytes in each ovulation, while preventing premature exhaustion of primordial follicle reserve during the reproductive lifespan. Resting follicle dynamics strongly suggest that primordial follicles are under constant inhibitory influences, by mechanisms and factors whose nature remains ill defined. In this work, we aimed to assess the influence of spatial determinants, with special attention to clustering patterns and crowding, on the fate of early follicles in the adult mouse and human ovary. To this end, detailed histological and morphometric analyses, targeting resting and early growing follicles, were conducted in ovaries from mice, either wild type (WT) or genetically modified to lack kisspeptin receptor expression (Kiss1r KO), and healthy adult women. Kiss1r KO mice were studied as model of persistent hypogonadotropism and anovulation. Different qualitative and quantitative indices of the patterns of spatial distribution of resting and early growing follicles in the mouse and human ovary, including the Morisita’s index of clustering, were obtained. Our results show that resting primordial follicles display a clear-cut clustered pattern of spatial distribution in adult mouse and human ovaries, and that resting follicle aggrupation is inversely correlated with the proportion of follicles initiating growth and entering into the growing pool. As a whole, our data suggest that resting follicle crowding, defined by changes in density and clustered pattern of distribution, is a major determinant of follicular activation and the fate of ovarian reserve. Uneven follicle crowding would constitute the structural counterpart of the major humoral regulators of early follicular growth, with potential implications in ovarian ageing and pathophysiology.

Exposure Duration-Dependent Ovarian Recovery in Methoxychlor-Treated Mice

The pesticide methoxychlor (MXC) is known to target ovarian antral follicles in the mouse. In previous in vivo studies, MXC administration for 20 days increased atresia, but did not affect female fertility immediately after dosing. Thus, we hypothesized that perhaps not enough time had elapsed between the onset of MXC-induced atresia and actual follicle loss to result in reduced fertility. The current study was undertaken to determine whether MXC treatment for 20 days results in reduced antral follicle numbers and fertility at 30 and 60 days after dosing. To test this hypothesis, adult CD-1 female mice were dosed with vehicle control or MXC (64 mg/kg/day) for 20 days. At 30 and 60 days postdosing, the mice were either subjected to fertility tests or their ovaries were collected and subjected to histological evaluation of follicle numbers and atresia. The results indicate that at 30 days after the completion of dosing, MXC significantly increased atresia and reduced primordial and total follicle numbers, but did not affect fertility compared to controls. At 60 days after completion of dosing, MXC did not significantly affect fertility, follicle numbers, or atresia compared to controls. Collectively, these data indicate that the ovary may be able to recover from MXC treatment for 20 days.

Identification of a β-galactosidase transgene that provides a live-cell marker of transcriptional activity in growing oocytes and embryos

Identifying the events and molecular mechanisms that regulate oocyte growth has emerged as a key objective of research in human fertility, fuelled by evidence from human and animal studies indicating that disease and environmental factors can act on oocytes to affect the health of the resulting individual and by efforts to grow oocytes in vitro to enable fertility preservation of cancer survivors. Techniques that monitor the development of growing oocytes would be valuable tools to assess the progression of growth under different conditions. Most methods used to assess oocytes grown in vitro are indirect, however, relying on characteristics of the somatic compartment of the follicle, or compromise the oocyte, preventing its subsequent culture or fertilization. We investigated the utility of T-cell factor/lymphoid enhancer-binding factor (TCF/Lef)-LacZ transgene expression as a predictor of global transcriptional activity in oocytes and early embryos. Using a fluorescent β-galactosidase substrate combined with live-cell imaging, we show that TCF/Lef-LacZ transgene expression is detectable in growing oocytes, lost in fully grown oocytes and resumes in late two-cell embryos. Transgene expression is likely regulated by a Wnt-independent mechanism. Using chromatin analysis, LacZ expression and methods to monitor and inhibit transcription, we show that TCF/Lef-LacZ expression mirrors transcriptional activity in oocytes and preimplantation embryos. Oocytes and preimplantation embryos that undergo live-cell imaging for TCF/Lef-LacZ expression are able to continue development in vitro. TCF/Lef-LacZ reporter expression in living oocytes and early embryos is thus a sensitive and faithful marker of transcriptional activity that can be used to monitor and optimize conditions for oocyte growth.

Predictive value of antral follicle count and anti-Müllerian hormone for follicle and oocyte developmental competence during the early prepubertal period in a sheep model

Circulating anti-Müllerian hormone (AMH) and antral follicle count (AFC) are addressed as suitable markers of oocyte quantity and quality during adulthood. To investigate whether AFC and circulating AMH could predict follicle development and oocyte quality during the prepubertal period we used 40-day-old ewe lambs with high, intermediate and low AFC (≥30, 16-29 and≤15 follicles respectively). The analysis of the response to the exogenous FSH ovarian reserve test showed a positive correlation between AFC, AMH plasma levels, total follicle number and the number of large follicles (≥3mm) grown after exogenous FSH administration. The incorporation of abattoir-derived oocytes collected from ovaries with different AFC in an in vitro embryo production system showed that a high AFC can predict oocyte quality in prepubertal ovaries, reflecting an ovarian status suitable for follicular development. The histological quantification of the ovarian reserve evidenced that AFC was not predictive of differences in either the number of healthy follicles or the size of the primordial follicle pool in prepubertal ovaries. Further studies are needed to investigate the implication on the reproductive performance of the significant inter-individual differences found in the present study in AFC and circulating AMH in the early prepubertal period.

Follicle dynamics and global organization in the intact mouse ovary

Quantitative analysis of tissues and organs can reveal large-scale patterning as well as the impact of perturbations and aging on biological architecture. Here we develop tools for imaging of single cells in intact organs and computational approaches to assess spatial relationships in 3D. In the mouse ovary, we use nuclear volume of the oocyte to read out quiescence or growth of oocyte-somatic cell units known as follicles. This in-ovary quantification of non-growing follicle dynamics from neonate to adult fits a mathematical function, which corroborates the model of fixed oocyte reserve. Mapping approaches show that radial organization of folliculogenesis established in the newborn ovary is preserved through adulthood. By contrast, inter-follicle clustering increases during aging with different dynamics depending on size. These broadly applicable tools can reveal high dimensional phenotypes and age-related architectural changes in other organs. In the adult mouse pancreas, we find stochastic radial organization of the islets of Langerhans but evidence for localized interactions among the smallest islets.

Computer-generated ovaries to assist follicle counting experiments

Precise estimation of the number of follicles in ovaries is of key importance in the field of reproductive biology, both from a developmental point of view, where follicle numbers are determined at specific time points, as well as from a therapeutic perspective, determining the adverse effects of environmental toxins and cancer chemotherapeutics on the reproductive system. The two main factors affecting follicle number estimates are the sampling method and the variation in follicle numbers within animals of the same strain, due to biological variability. This study aims at assessing the effect of these two factors, when estimating ovarian follicle numbers of neonatal mice. We developed computer algorithms, which generate models of neonatal mouse ovaries (simulated ovaries), with characteristics derived from experimental measurements already available in the published literature. The simulated ovaries are used to reproduce in-silico counting experiments based on unbiased stereological techniques; the proposed approach provides the necessary number of ovaries and sampling frequency to be used in the experiments given a specific biological variability and a desirable degree of accuracy. The simulated ovary is a novel, versatile tool which can be used in the planning phase of experiments to estimate the expected number of animals and workload, ensuring appropriate statistical power of the resulting measurements. Moreover, the idea of the simulated ovary can be applied to other organs made up of large numbers of individual functional units.

Fibrin-mediated delivery of an ovarian follicle pool in a mouse model of infertility

The cryopreservation and autotransplantation of ovarian tissue is emerging as a powerful approach for preserving fertility. However, for cancer patients, it may not be possible to transplant ovarian tissue due to the risk of re-seeding disease. We investigated strategies for transplantation of individually isolated follicles to minimize the risk of re-introducing cancer cells present within the vasculature of ovarian stroma. Procedures for large-scale isolation of early-stage follicles and their encapsulation into fibrin hydrogels were developed. For in vivo validation studies, mice were ovariectomized and transplanted with encapsulated follicles into the ovarian bursa. A substantial increase in the number of secondary follicles was observed in the graft at 9 days after transplantation, and antral follicles by day 21, demonstrating primordial follicle recruitment into the growing pool. Initially, elevated follicle-stimulating hormone levels declined substantially by day 21, indicating feedback from the graft; presence of corpora lutea showed the graft's capability of restoring hormone cyclicity. Taken together, the transplanted follicles were able to engraft, mature, and restore ovarian function in an infertile mouse. This biomaterial may, thus, provide a platform for follicle transplantation with a low risk of cancer contamination and for developing strategies that preserve fertility for women facing a cancer diagnosis.

The ovarian reserve of primordial follicles and the dynamic reserve of antral growing follicles: what is the link?

The growing follicles develop from a reserve of primordial follicles constituted early in life. From this pre-established reserve, a second ovarian reserve is formed, which consists of gonadotropin-responsive small antral growing follicles and is a dynamic reserve for ovulation. Its size, evaluated by direct antral follicular count or endocrine markers, determines the success of assisted reproductive technologies in humans and embryo production biotechnologies in animals. Strong evidence indicates that these two reserves are functionally related. The size of both reserves appears to be highly variable between individuals of similar age, but the equilibrium size of the dynamic reserve in adults seems to be specific to each individual. The dynamics of both follicular reserves appears to result from the fine tuning of regulations involving two main pathways, the phosphatase and tensin homolog (PTEN)/phosphatidylinositol-3 kinase (PI3K)/3-phosphoinositide-dependent protein kinase-1 (PDPK1)/v-akt murine thymoma viral oncogene homolog 1 (AKT1) and the bone morphogenetic protein (BMP)/anti-Müllerian hormone (AMH)/SMAD signaling pathways. Mutations in genes encoding the ligands, receptors, or signaling effectors of these pathways can accelerate or modulate the exhaustion rate of the ovarian reserves, causing premature ovarian insufficiency (POI) or increase in reproductive longevity, respectively. With female aging, the decline in primordial follicle numbers parallels the decrease in the size of the dynamic reserve of small antral follicles and the deterioration of oocyte quality. Recent progress in our knowledge of signaling pathways and their environmental and hormonal control during adult and fetal life opens new perspectives to improve the management of the ovarian reserves.

Fetal cyclophosphamide exposure induces testicular cancer and reduced spermatogenesis and ovarian follicle numbers in mice

Exposure to radiation during fetal development induces testicular germ cell tumors (TGCT) and reduces spermatogenesis in mice. However, whether DNA damaging chemotherapeutic agents elicit these effects in mice remains unclear. Among such agents, cyclophosphamide (CP) is currently used to treat breast cancer in pregnant women, and the effects of fetal exposure to this drug manifested in the offspring must be better understood to offer such patients suitable counseling. The present study was designed to determine whether fetal exposure to CP induces testicular cancer and/or gonadal toxicity in 129 and in 129.MOLF congenic (L1) mice. Exposure to CP on embryonic days 10.5 and 11.5 dramatically increased TGCT incidence to 28% in offspring of 129 mice (control value, 2%) and to 80% in the male offspring of L1 (control value 33%). These increases are similar to those observed in both lines of mice by radiation. In utero exposure to CP also significantly reduced testis weights at 4 weeks of age to ∼70% of control and induced atrophic seminiferous tubules in ∼30% of the testes. When the in utero CP-exposed 129 mice reached adulthood, there were significant reductions in testicular and epididymal sperm counts to 62% and 70%, respectively, of controls. In female offspring, CP caused the loss of 77% of primordial follicles and increased follicle growth activation. The results indicate that i) DNA damage is a common mechanism leading to induction of testicular cancer, ii) increased induction of testis cancer by external agents is proportional to the spontaneous incidence due to inherent genetic susceptibility, and iii) children exposed to radiation or DNA damaging chemotherapeutic agents in utero may have increased risks of developing testis cancer and having reduced spermatogenic potential or diminished reproductive lifespan.

The two classes of primordial follicles in the mouse ovary: their development, physiological functions and implications for future research

Ovarian follicles are the basic functional units in the mammalian ovary. This review summarizes early pioneering studies and focuses on recent progress that has shown that there are two distinct classes of primordial follicles in the ovary: the first wave of primordial follicles that are activated immediately after they are formed and the adult primordial follicles that are activated gradually in later life. These two separate classes have been proposed for two decades, but sufficient experimental evidence to support this hypothesis has only been obtained recently using newly developed follicular tracing techniques in genetically modified mouse models. These two follicle populations differ from each other primarily in terms of their developmental dynamics and their contributions to ovarian physiology. It is apparent now that these two follicle populations should be treated separately, and such knowledge will hopefully lead to a more in-depth understanding of how distinct types of primordial follicles contribute to physiologic and pathologic alterations of the mammalian ovary.

Follicle activation and 'burn-out' contribute to post-transplantation follicle loss in ovarian tissue grafts: the effect of graft thickness

STUDY QUESTION

What are the effects of thin ovarian grafts compared with grafts of the standard thickness on follicle loss post-transplantation?

SUMMARY ANSWER

Transplantation of reduced-thickness ovarian grafts led to intense activation and 'burn-out' a short time after transplantation resulting in significant folllicle loss.

WHAT IS KNOWN ALREADY

Transplantation of fresh and frozen-thawed ovarian tissue has been proved successful, but techniques vary and are not optimised, often resulting in significant follicular loss. Follicle loss is mostly related to the freezing-thawing process and to post-transplantation hypoxia.

STUDY DESIGN, SIZE, DURATION

Bovine ovarian tissue strips (n = 55) were prepared in two groups of conventional-thickness strips (1-2 mm) or thin strips (0.5-0.9 mm). Fresh or frozen-thawed samples were xenotransplanted into sterilized immune-deficient mice (n = 49). Non-transplanted conventional size fresh samples were used as controls (n = 6). Grafts from all study groups were recovered after 7 days for analysis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Morphometric differential counting of follicle classes was performed by two observers. Immunohistochemistry was conducted for proliferation (Ki67), cortical fibrosis (Masson tri-chrome) and blood-vessel density (CD31). Results were expressed as the mean number of dormant or growing follicle (GF) type per section or total follicle counts per graft. Blood-vessel density was calculated per mm(2). P-values <0.05 were considered statistically significant.

MAIN RESULTS AND THE ROLE OF CHANCE

The loss of all follicle types, and most noteably of primordial follicles (PMFs), was observed 7 days post-transplantation (P < 0.05). The relatively high number of GFs and the positive Ki67 staining in all recovered grafts indicated that follicle activation was depleting the resting follicle pool. The reduced graft thickness had an adverse effect on the number of recovered follicles, especially on the resting non-GFs in the fresh, and more so in the frozen-thawed, samples (P < 0.05). Extensive stromal fibrosis and high blood-vessel density were observed in all grafts with no advantage in the thin prepared grafts.

LIMITATIONS, REASONS FOR CAUTION

This study used only one species of ovaries (bovine) for xenotransplantation. The immediate post-transplantation events were not visualized directly nor were the molecules involved in follicle activation studied.

WIDER IMPLICATIONS OF THE FINDINGS

Follicle activation and 'burn-out' appear to be important in follicle loss after transplantation. Reducing graft thickness in an attempt to improve freezing conditions and reduce post-transplantation ischemia has adverse effects on the graft follicle pool due to increased activation and loss. Agents which prevent 'burn-out' will potentially improve follicle pool survival.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by research grants from the Israeli Science Foundation (No. 1675/10), the Israeli Jack Craps foundation and the Israel Cancer Research Fund (ICRF No. 12-3081). The authors have no competing interest to declare.

Statistical comparison of spatial point patterns in biological imaging

In biological systems, functions and spatial organizations are closely related. Spatial data in biology frequently consist of, or can be assimilated to, sets of points. An important goal in the quantitative analysis of such data is the evaluation and localization of differences in spatial distributions between groups. Because of experimental replications, achieving this goal requires comparing collections of point sets, a noticeably challenging issue for which no method has been proposed to date. We introduce a strategy to address this problem, based on the comparison of point intensities throughout space. Our method is based on a statistical test that determines whether local point intensities, estimated using replicated data, are significantly different or not. Repeating this test at different positions provides an intensity comparison map and reveals domains showing significant intensity differences. Simulated data were used to characterize and validate this approach. The method was then applied to two different neuroanatomical systems to evaluate its ability to reveal spatial differences in biological data sets. Applied to two distinct neuronal populations within the rat spinal cord, the method generated an objective representation of the spatial segregation established previously on a subjective visual basis. The method was also applied to analyze the spatial distribution of locus coeruleus neurons in control and mutant mice. The results objectively consolidated previous conclusions obtained from visual comparisons. Remarkably, they also provided new insights into the maturation of the locus coeruleus in mutant and control animals. Overall, the method introduced here is a new contribution to the quantitative analysis of biological organizations that provides meaningful spatial representations which are easy to understand and to interpret. Finally, because our approach is generic and punctual structures are widespread at the cellular and histological scales, it is potentially useful for a large spectrum of applications for the analysis of biological systems.

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.

The immature human ovary shows loss of abnormal follicles and increasing follicle developmental competence through childhood and adolescence

STUDY QUESTION

Do the ovarian follicles of children and adolescents differ in their morphology and in vitro growth potential from those of adults?

SUMMARY ANSWER

Pre-pubertal ovaries contained a high proportion of morphologically abnormal non-growing follicles, and follicles showed reduced capacity for in vitro growth.

WHAT IS KNOWN ALREADY

The pre-pubertal ovary is known to contain follicles at the early growing stages. How this changes over childhood and through puberty is unknown, and there are no previous data on the in vitro growth potential of follicles from pre-pubertal and pubertal girls.

STUDY DESIGN, SIZE, DURATION

Ovarian biopsies from five pre-pubertal and seven pubertal girls and 19 adult women were analysed histologically, cultured in vitro for 6 days, with growing follicles then isolated and cultured for a further 6 days.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Ovarian biopsies were obtained from girls undergoing ovarian tissue cryopreservation for fertility preservation, and compared with biopsies from adult women. Follicle stage and morphology were classified. After 6 days in culture, follicle growth initiation was assessed. The growth of isolated secondary follicles was assessed over a further 6 days, including analysis of oocyte growth.

MAIN RESULTS AND THE ROLE OF CHANCE

Pre-pubertal ovaries contained a high proportion of abnormal non-growing follicles (19.4 versus 4.85% in pubertal ovaries; 4004 follicles analysed; P = 0.02) characterized by indistinct germinal vesicle membrane and absent nucleolus. Follicles with this abnormal morphology were not seen in the adult ovary. During 6 days culture, follicle growth initiation was observed at all ages; in pre-pubertal samples there was very little development to secondary stages, while pubertal samples showed similar growth activation to that seen in adult tissue (pubertal group: P = 0.02 versus pre-pubertal, ns versus adult). Isolated secondary follicles were cultured for a further 6 days. Those from pre-pubertal ovary showed limited growth (P < 0.05 versus both pubertal and adult follicles) and no change in oocyte diameter over that period. Follicles from pubertal ovaries showed increased growth; this was still reduced compared with follicles from adult women (P < 0.05) but oocyte growth was proportionate to follicle size.

LIMITATIONS, REASONS FOR CAUTION

These data derive from only a small number of ovarian biopsies, although large numbers of follicles were analysed. It is unclear whether the differences between groups are related to puberty, or just age.

WIDER IMPLICATIONS OF THE FINDINGS

These findings show that follicles from girls of all ages can be induced to grow in vitro, which has important implications for some patients who are at high risk of malignant contamination of their ovarian tissue. The reduced growth of isolated follicles indicates that there are true intrafollicular differences in addition to potential differences in their local environment, and that there are maturational processes occurring in the ovary through childhood and adolescence, which involve the loss of abnormal follicles, and increasing follicle developmental competence.

Study funding/competing interest(s)

Funded by MRC grants G0901839 and G1100357. No competing interests.

Investigations of TGF-β signaling in preantral follicles of female mice reveal differential roles for bone morphogenetic protein 15

Bone morphogenetic protein 15 (BMP15) and growth differentiation factor 9 (GDF9) are 2 closely related TGF-β ligands implicated as key regulators of follicle development and fertility. Animals harboring mutations of these factors often exhibit a blockage in follicle development beyond the primary stage and therefore little is known about the role of these ligands during subsequent (preantral) stages. Preantral follicles isolated from immature mice were cultured with combinations of BMP15, GDF9, and activin receptor-like kinase (ALK) inhibitors. Individually, GDF9 and BMP15 promoted follicle growth during the first 24 hours, whereas BMP15 subsequently (48-72 h) caused follicle shrinkage and atresia with increased granulosa cell apoptosis. Inhibition of ALK6 prevented the BMP15-induced reduction in follicle size and under basal conditions promoted a rapid increase in granulosa cell proliferation, suggesting BMP15 signals through ALK6, which in turn acts to restrain follicle growth. In the presence of GDF9, BMP15 no longer promoted atresia and in fact follicle growth was increased significantly more than with either ligand alone. This cooperative effect was accompanied by differential expression of Id1-3, Smad6-7, and Has2 and was blocked by the same ALK5 inhibitor used to block GDF9 signaling. Immunostaining for SMAD2/3 and SMAD1/5/8, representing the 2 main branches of TGF-β signaling, supported the fact that both canonical pathways have the potential to be active in growing follicles, whereas primordial follicles only express SMAD2/3. Overall results highlight differential effects of the 2 main TGF-β signaling pathways during preantral follicle growth.

Multiple follicle culture supports primary follicle growth through paracrine-acting signals

In vitro follicle growth in alginate hydrogels is a unique and versatile method for studying ovarian and follicle biology that may also have implications for fertility preservation. Current culture systems support the development of isolated mouse follicles from the secondary stage onward. However, it has been a challenge to grow smaller follicles in vitro due to the dissociation of the oocyte from companion somatic cells. Recent work has demonstrated that coculturing primary follicles with mouse embryonic fibroblasts or ovarian stromal cells supports follicle survival and growth. In this study, we demonstrate that follicles themselves can exert a beneficial coculture effect. When primary follicles were cultured in groups of five or ten (multiple follicle culture), there was increased growth and survival. The multiple follicle culture approach maintained follicle integrity and resulted in the formation of antral stage follicles containing meiotically competent gametes. The growth and survival of primary follicles were highly number dependent, with the most significant enhancement observed when the largest number of follicles was grown together. Our data suggest that the follicle unit is necessary to produce the secreted factors responsible for the supportive effects of multiple follicle culture, as neither denuded oocytes, oocyte-secreted factors, nor granulosa cells alone were sufficient to support early follicle growth in vitro. Therefore, there may be signaling from both the oocyte and the follicle that enhances growth but requires both components in a feedback mechanism. This work is consistent with current in vivo models for follicle growth and thus advances the movement to recapitulate the ovarian environment in vitro.

Interpreting human follicular recruitment and antimüllerian hormone concentrations throughout life

The changes in the relationships between circulating antimüllerian hormone, the size of the primordial follicle pool, and follicular recruitment before and through the reproductive years have now been clarified, and show dynamic changes through sexual development. The constant relationship between the number of follicles and circulating antimüllerian hormone exists only after the age of 25 years, implying that the association between follicular recruitment and follicular survival to the later stages of development is not constant across the reproductive life course. This commentary assesses the factors that may underlie these relationships and their clinical implications for reproductive health.

Regulation of the ovarian reserve by members of the transforming growth factor beta family

Genetic or environmental factors that affect the endowment of oocytes, their assembly into primordial follicles, or their subsequent entry into the growing follicle pool can disrupt reproductive function and may underlie disorders such as primary ovarian insufficiency. Mouse models have been instrumental in identifying genes important in ovarian development, and a number of genes now associated with ovarian dysfunction in women were first identified as causing reproductive defects in knockout mice. The transforming growth factor beta (TGFB) family consists of developmentally important growth factors that include the TGFBs, anti-Müllerian hormone (AMH), activins, bone morphogenetic proteins (BMPs), and growth and differentiation factor 9 (GDF9). The ovarian primordial follicle pool is the source of oocytes in adults. Development of this pool can be grossly divided into three key processes: (1) establishment of oocytes during embryogenesis followed by (2) assembly and (3) activation of the primordial follicle. Disruptions in any of these processes may cause reproductive dysfunction. Most members of the TGFB family show pivotal roles in each of these areas. Understanding the phenotypes of various mouse models for this protein family will be directly relevant to understanding how disruptions in TGFB family signaling result in reproductive diseases in women and will present new areas for development of tailored diagnostics and interventions for infertility.