Ovarian Reserve Of Primordial Follicles Research Articles

Mathematical recapitulation of the end stages of human ovarian aging.

Ovarian aging in women can be described as highly unpredictable within individuals but predictable across large populations. We showed previously that modeling an individual woman's ovarian reserve of primordial follicles using mathematical random walks replicates the natural pattern of growing follicles exiting the reserve. Compiling many simulations yields the observed population distribution of the age at natural menopause (ANM). Here, we have probed how stochastic control of primordial follicle loss might relate to the distribution of the preceding menopausal transition (MT), when women begin to experience menstrual cycle irregularity. We show that identical random walk model conditions produce both the reported MT distribution and the ANM distribution when thresholds are set for growing follicle availability. The MT and ANM are shown to correspond to gaps when primordial follicles fail to grow for 7 and 12 days, respectively. Modeling growing follicle supply is shown to precisely recapitulate epidemiological data and provides quantitative criteria for the MT and ANM in humans.

BPA disrupts meiosis I in oogonia by acting on pathways including cell cycle regulation, meiosis initiation and spindle assembly

The negative in utero effects of bisphenol A (BPA) on female reproduction are of concern since the ovarian reserve of primordial follicles is constituted during the fetal period. This time-window is difficult to access, particularly in humans. Animal models and explant culture systems are, therefore, vital tools for investigating EDC impacts on primordial germ cells (PGCs). Here, we investigated the effects of BPA on prophase I meiosis in the fetal sheep ovary. We established an in vitro model of early gametogenesis through retinoic acid (RA)-induced differentiation of sheep PGCs that progressed through meiosis. Using this system, we demonstrated that BPA (3 ×10-7 M & 3 ×10-5 M) exposure for 20 days disrupted meiotic initiation and completion in sheep oogonia and induced transcriptomic modifications of exposed explants. After exposure to the lowest concentrations of BPA (3 ×10-7 M), only 2 probes were significantly up-regulated corresponding to NR2F1 and TMEM167A transcripts. In contrast, after exposure to 3 × 10-5 M BPA, 446 probes were deregulated, 225 were down- and 221 were up-regulated following microarray analysis. Gene Ontology (GO) annotations of differentially expressed genes revealed that pathways mainly affected were involved in cell-cycle phase transition, meiosis and spindle assembly. Differences in key gene expression within each pathway were validated by qRT-PCR. This study provides a novel model for direct examination of the molecular pathways of environmental toxicants on early female gametogenesis and novel insights into the mechanisms by which BPA affects meiosis I. BPA exposure could thereby disrupt ovarian reserve formation by inhibiting meiotic progression of oocytes I and consequently by increasing atresia of primordial follicles containing defective oocytes.

Abstract P4-10-16: Increased chemotherapy-induced ovarian reserve loss and impaired oocyte quality in a mouse ATM knock-down model: An augury for women with ATM-pathway-pathogenic-variants-associated breast cancer?

Abstract Rationale: Breast cancer chemotherapy commonly causes the depletion of ovarian primordial follicle reserve, which results in premature ovarian insufficiency and infertility. It is critical to understand the mechanism of this ovarian damage to be able to develop pharmacological fertility preservation methods. We have previously shown that the main mechanism of ovarian follicle loss induced by cyclophosphamide (C) and doxorubicin (A) is via the induction of double-strand DNA breaks (DSBs) and the resulting apoptotic death of primordial follicles. We have also shown that some oocytes may repair this damage by activating the ATM-mediated DNA DSB repair pathway (ATM-Pathway). We have also demonstrated that the oocytes of women with BRCA1 pathogenic variants are more prone to DNA damage and such women are more likely to lose ovarian reserve after AC-based chemotherapy. BRCA1 is a key member of the ATM-Pathway. An increasing number of women are diagnosed with breast cancer associated with mutations of the other members of the ATM-Pathway. This includes ATM, which is the orchestrator of the ATM-Pathway. Therefore, we hypothesized that ATM-deficient oocytes may be more prone to chemotherapy-induced damage. In addition, given our recent findings of the role of declining BRCA1 function in oocyte aging, we hypothesized that ATM function is important in oocyte and embryo quality. Having both increased liability to chemotherapy-induced loss and accelerated ovarian aging can be a double-whammy for women with breast cancer that are associated with the pathogenic variants of the ATM-Pathway genes. Methods: Germinal Vesicle (GV) stage oocytes were collected from 6-wk old mice 48h after ovarian stimulation. Oocytes were then microinjected with 50uM ATM small interfering RNA (siRNA). The scrambled-siRNA-injected and un-injected oocytes served as negative controls. In separate experiments, effective ATM-knockdown (~70%) was confirmed by single-cell real-time-qPCR. Then, oocytes were subjected to subsequent in vitro maturation followed by parthenogenesis or 4h exposure to the active metabolite of cyclophosphamide, 4-hydroperoxycyclophosphamide (4-HC, 20µg/mL). Experiments were repeated ≥ thrice. One-way ANOVA and non-parametric Kruskal-Wallis posthoc tests were used for statistical analysis.Results: ATM-knockdown resulted in a significant reduction of oocyte maturation compared with scrambled or uninjected (29%, n=157 vs. 78%, n=172 vs. 84%, n=151) (p<0.001 siATM vs. the controls), suggesting altered oocyte quality. The siATM parthenotes had lower cleavage and blastocyst rates (15% and 0%, n=20,) compared with scrambled (63% and 18%, n=22), and uninjected controls (66% and 24%, n=21) (p<0.05 compared to either control), and demonstrated impaired morphology. Moreover, ATM-knock-down resulted in increased oocyte sensitivity to chemotherapy; with significantly lower survival rates (15%, n=37) compared to scrambled (55%, n=48) or un-injected controls (57% n=48; p<0.001) after the 4-HC exposure. Conclusions: This mouse model indicates that intact ATM function is critical in both oocyte resistance against chemotherapy and the maintenance of egg and embryo quality. Clinical studies are justified to determine if women with ATM pathogenic variants have impaired egg and embryo quality and are more prone to chemotherapy-induced loss of ovarian reserve and fertility. Citation Format: Kutluk H Oktay, Xiu Tan, Katarzyna Szymanska-Vandendriessche. Increased chemotherapy-induced ovarian reserve loss and impaired oocyte quality in a mouse ATM knock-down model: An augury for women with ATM-pathway-pathogenic-variants-associated breast cancer? [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-10-16.

PI3K/PTEN/AKT Signaling Pathways in Germ Cell Development and Their Involvement in Germ Cell Tumors and Ovarian Dysfunctions.

Several studies indicate that the PI3K/PTEN/AKT signaling pathways are critical regulators of ovarian function including the formation of the germ cell precursors, termed primordial germ cells, and the follicular pool maintenance. This article reviews the current state of knowledge of the functional role of the PI3K/PTEN/AKT pathways during primordial germ cell development and the dynamics of the ovarian primordial follicle reserve and how dysregulation of these signaling pathways may contribute to the development of some types of germ cell tumors and ovarian dysfunctions.

The Interconnections Between Somatic and Ovarian Aging in Murine Models.

The mammalian female is born with a limited ovarian reserve of primordial follicles. These primordial follicles are slowly activated throughout the reproductive lifecycle, thereby determining lifecycle length. Once primordial follicles are exhausted, women undergo menopause, which is associated with several metabolic perturbations and a higher mortality risk. Long before exhaustion of the reserve, females experience severe declines in fertility and health. As such, significant efforts have been made to unravel the mechanisms that promote ovarian aging and insufficiency. In this review, we explain how long-living murine models can provide insights in the regulation of ovarian aging. There is now overwhelming evidence that most life-span-extending strategies, and long-living mutant models simultaneously delay ovarian aging. Therefore, it appears that the same mechanisms that regulate somatic aging may also be modulating ovarian aging and germ cell exhaustion. We explore several potential contributing mechanisms including insulin resistance, inflammation, and DNA damage-all of which are hallmarks of cellular aging throughout the body including the ovary. These findings are in alignment with the disposable soma theory of aging, which dictates a trade-off between growth, reproduction, and DNA repair. Therefore, delaying ovarian aging will not only increase the fertility window of middle age females, but may also actively prevent menopausal-related decline in systemic health parameters, compressing the period of morbidity in mid-to-late life in females.

Cisplatin- and cyclophosphamide-induced primordial follicle depletion is caused by direct damage to oocytes.

It is well established that DNA-damaging chemotherapies can cause infertility and ovarian endocrine failure by depleting the ovarian reserve of primordial follicles. Currently, no effective pharmacological therapies exist for the preservation of long-term fertility and ovarian function in female cancer patients, due to a limited understanding of the mechanisms of chemotherapy-induced follicle depletion. This study investigated the cellular targets, molecular mechanisms, and temporal course of ovarian reserve depletion following treatment with commonly used chemotherapeutic drugs. Adult female C57BL/6 mice were injected i.p. with saline, cisplatin (5mg/kg), or cyclophosphamide (300mg/kg); ovaries were harvested after 8 or 24 hours. Follicle counts showed depletion of all follicular stages 24 hours after administration of cisplatin or cyclophosphamide. Eight hours post-treatment, H2A histone family member X (γH2AX) immunofluorescence showed DNA double-stranded breaks at all follicular stages, including within primordial follicle oocytes. This staining was resolving by 24 hours, indicating that primordial follicle oocytes begin to undergo either apoptosis or repair in this timeframe. γH2AX-positive follicles were further examined to identify the specific cell types damaged. In primordial, transitional, and primary follicles, only oocytes sustained DNA damage, whereas in secondary and antral follicles, only somatic cells were affected. TUNEL staining confirmed that apoptosis occurs in these targeted cell types. Whilst multi-drug and multi-dose regimens were not examined, this study conclusively shows that cyclophosphamide and cisplatin cause direct damage to primordial follicle oocytes, which then undergo apoptosis. Therefore, future pharmacological strategies to prevent chemotherapy-induced infertility in females must specifically prevent primordial follicle oocyte death.

Fibroblast growth factor 21 coordinates adiponectin to mediate the beneficial effects of low-protein diet on primordial follicle reserve.

BackgroundGlobal consumption of protein per capita is rising, while rates of infertility are increasing. However, a clear relationship between protein intake and reproductive health has not been demonstrated. The activation of the quiescent primordial follicles is the first step of folliculogenesis, and their activation must be tightly controlled to prevent premature exhaustion of the ovarian follicular reserve.MethodsThe primordial follicle reserve of wild-type or liver-specific ablation of fibroblast growth factor 21 (FGF21) in mice, subjected to limited or excessive protein diets or oral gavage test, were detected in vivo. Mouse ovary organ cultures were used to examine the direct role of metabolites or metabolic hormones on primordial follicle activation.FindingsMouse primordial follicle activation, was reduced by restricted protein intake and was accelerated by excessive protein intake, in an ovarian mTORC1 signaling-dependent manner. Furthermore, restricted or excessive protein intake resulted in an augmentation or decline of oocyte number and fertility at older age, respectively. Liver-specific ablation of FGF21, which resulted in a reduction of 87% in circulating FGF21, abrogated the preserving effect of low-protein intake on primordial follicle pool. Interestingly, FGF21 had no direct effect on the activation of primordial follicles, but instead required an adipokine adiponectin. Moreover, AdipoRon, an oral adiponectin receptor agonist, prevented the over-activation effect of excessive protein intake on primordial follicle activation.InterpretationDietary protein consumption controlled ovarian primordial follicle reserve and fertility, which required coordination between FGF21 and adiponectin.FundNatural Science Foundation of China (Grant 31772616).

FGF21 Coordinates Adiponectin to Mediate the Beneficial Effects of Low-Protein Diet on Primordial Follicle Reserve

Global per capita consumption of protein is rising, while infertility is becoming prevailing. Here we found that mouse primordial follicle activation, the first step of folliculogenesis, was reduced by restricted protein intake and was accelerated by excessive protein intake, in an ovarian mTORC1 signaling-dependent manner. Furthermore, restricted or excessive protein intake resulted in an augmentation or decline of oocyte number and fertility at older age, respectively. Liver-specific ablation of fibroblast growth factor 21 (FGF21), which resulted in a reduction of 87% in circulating FGF21, abrogated the preserving effect of low-protein intake on primordial follicle pool. Interestingly, FGF21 had no direct effect on the activation of primordial follicles, but instead required an adipokine adiponectin. Moreover, AdipoRon, an oral adiponectin receptor agonist, prevented the over-activation effect of excessive protein intake on primordial follicle activation. Collectively, dietary protein consumption controlled ovarian primordial follicle reserve and fertility, which required coordination between FGF21 and adiponectin. Funding Statement: This study was funded in part by National Natural Science Foundation of China (Grant 31772616). Declaration of Interests: None of the authors have a conflict of interests to declare. Ethics Approval Statement: All animal procedures in this study were handled in accordance with Guide for the Care and Use of Laboratory Animals (National Research Council, Bethesda, MD, USA), and the Institutional Animal Care and Research Committee of Sichuan Agricultural University (SICAU-2015-034).

Janus kinase JAK1 maintains the ovarian reserve of primordial follicles in the mouse ovary

Is the Janus kinase and signal transducer and activator of transcription (JAK-STAT) signalling pathway involved in ovarian follicle development and primordial follicle activation? JAK1 is a key factor involved in the regulation of primordial follicle activation and maintenance of the ovarian reserve. A series of integrated, intrinsic signalling pathways (including PI3K/AKT, mTOR and KITL) are responsible for regulating the ovarian reserve of non-growing primordial follicles and ultimately female fertility. The JAK-STAT signal transduction pathway is highly conserved with established roles in cell division and differentiation. Key pathway members (specifically JAK1, STAT3 and SOCS4) have been previously implicated in early follicle development. A laboratory animal study was undertaken using the C57Bl/6 inbred mouse strain as a model for human ovarian follicle development. To determine which Jak genes were most abundantly expressed during primordial follicle activation, mRNA expression was analysed across a developmental time-course, with ovaries collected from female mice at post-natal days 1 (PND1), 4 (PND4), 8 (PND8), as well as at 6 weeks (6WK) and 7 months (7MTH) (n ≥ 4). Functional analysis of JAK1 was performed on PND2 mouse ovaries subjected to in vitro explant culture treated with 12.5 μM Ruxolitinib (JAK inhibitor) or vehicle control (DMSO) for 48 h prior to histological assessment (n ≥ 4). The expression and localization of the JAK family during ovarian follicle development in the C57Bl/6 inbred mouse strain were evaluated using quantitative PCR, immunoblotting and immunolocalisation. Functional studies were undertaken using the JAK inhibitor Ruxolitinib to investigate the underpinning cellular mechanisms via biochemical in vitro inhibition and histological assessment of intact neonate ovaries. All experiments were replicated at least three times using tissue from different mice unless otherwise stated. Jak1 is the predominant Jak mRNA expressed in the C57Bl/6 mouse ovary across all developmental time-points assessed (P ≤ 0.05). Forty-eight hour inhibition of JAK1 with Ruxolitinib of PND2 ovaries in vitro demonstrated concomitant acceleration of primordial follicle activation and apoptosis (P ≤ 0.001) and upregulation of downstream JAK-STAT pathway members STAT3 and suppressors of cytokine signalling 4 (SOCS4). N/A. Results are shown in one species, the C57Bl/6 mouse strain as an established model of human ovary development. Ruxolitinib also inhibits JAK2, with decreased efficacy. However, Jak2 mRNA had limited expression in the mouse ovary, particularly at the neonatal stages of follicle development, thus any effect of Ruxolitinib on primordial follicle activation was unlikely to be mediated via this isoform. This study supports a key role for JAK1 in the maintenance and activation of primordial follicles, with potential for targeting the JAK-STAT pathway as a method of regulating the ovarian reserve and female fertility. This project has been funded by the Australian National Health and Medical Research Council (G1600095) and The Hunter Medical Research Institute Bob and Terry Kennedy Children's Research Project Grant in Pregnancy & Reproduction (G1501433). All authors declare no conflict of interests.

Loss of PUMA protects the ovarian reserve during DNA-damaging chemotherapy and preserves fertility

Female gametes are stored in the ovary in structures called primordial follicles, the supply of which is non-renewable. It is well established that DNA-damaging cancer treatments can deplete the ovarian reserve of primordial follicles, causing premature ovarian failure and infertility. The precise mechanisms underlying this chemotherapy-driven follicle loss are unclear, and this has limited the development of targeted ovarian-protective agents. To address this fundamental knowledge gap, we used gene deletion mouse models to examine the role of the DNA damage-induced pro-apoptotic protein, PUMA, and its transcriptional activator TAp63, in primordial follicle depletion caused by treatment with cyclophosphamide or cisplatin. Cyclophosphamide caused almost complete destruction of the primordial follicle pool in adult wild-type (WT) mice, and a significant destructive effect was also observed for cisplatin. In striking contrast, Puma−/− mice retained 100% of their primordial follicles following either genotoxic treatment. Furthermore, elimination of PUMA alone completely preserved fertility in cyclophosphamide-treated mice, indicating that oocytes rescued from DNA damage-induced death can repair themselves sufficiently to support reproductive function and offspring health. Primordial follicles were also protected in TAp63−/− mice following cisplatin treatment, but not cyclophosphamide, suggesting mechanistic differences in the induction of apoptosis and depletion of the ovarian reserve in response to these different chemotherapies. These studies identify PUMA as a crucial effector of apoptosis responsible for depletion of primordial follicles following exposure to cyclophosphamide or cisplatin, and this indicates that inhibition of PUMA may be an effective ovarian-protective strategy during cancer treatment in women.

Dacarbazine depletes the ovarian reserve in mice and depletion is enhanced with age

Dacarbazine is commonly administered for the treatment of cancers prevalent in reproductive age females. However, investigations of off-target effects of dacarbazine on the ovary are limited. We assessed the impact of dacarbazine on the ovarian reserve of primordial follicles, essential for fertility. Eight week and 6 month old C57BL/6 J mice were administered with dacarbazine or saline on day (d)0 and d7, then sacrificed after 12 hours (h), or 14d (n = 4–5/group). Follicle numbers, follicle density, serum AMH and corpora lutea were quantified and estrous cyclicity monitored. In reproductively young mice, dacarbazine did not affect primordial follicle numbers at 12 h, but resulted in a 36% reduction at 14d (p < 0.05). Dacarbazine-mediated primordial follicle depletion was accelerated with age, with a 24% (p < 0.05) and 36% (p < 0.01) reduction at 12 h and 14d. Follicle density remained unchanged between treatment groups at either age. Dacarbazine depleted antral follicles at 14d (p < 0.05), at both ages. Despite partial reduction of antral follicles, serum AMH, estrous cyclicity and corpora lutea (indicative of ovulation) remained unchanged between treatment groups, at both ages. Importantly, diminished ovarian reserve can result in premature ovarian insufficiency and infertility, thus, fertility preservation options should be considered for young female patients prior to dacarbazine treatment.

The effect of ovarian reserve and receptor signalling on granulosa cell apoptosis during human follicle development.

The poor oocyte quality in older women has previously been linked to the depletion of the ovarian reserve of primordial follicles and an increase in granulosal apoptosis. Granulosa cells were collected from 198 follicles and individually analysed by flow cytometry. In the young IVF patients, the level of apoptosis was inversely proportional to the expression of bone morphogenetic protein (BMPR1B) and follicle stimulating hormone (FSH) receptors. Conversely, in the older patients this relationship became dysregulated. In the older patients, at the time of preovulatory maturation, the reduced apoptosis reflects the poor mitogenic growth turnover rate of healthy follicles rather than the death rate in an atretic follicle. Restoring an optimum receptor density and down-regulation of receptors may improve oocyte quality and the pregnancy rate in older women.

Infertility and ovarian follicle reserve depletion are associated with dysregulation of the FSH and LH receptor density in human antral follicles

The low take-home baby rate in older women in Australia (5.8%) undergoing IVF (5.8%) is linked to the depletion of the ovarian reserve of primordial follicles. Oocyte depletion causes an irreversible change to ovarian function. We found that the young patient FSH receptor and LH receptor expression profile on the granulosa cells collected from different size follicles were similar to the expression profile reported in natural cycles in women and sheep. This was reversed in the older patients with poor ovarian reserve. The strong correlation of BMPR1B and FSH receptor density in the young was not present in the older women; whereas, the LH receptor and BMPR1B correlation was weak in the young but was strongly correlated in the older women. The reduced fertilisation and pregnancy rate was associated with a lower LH receptor density and a lack of essential down-regulation of the FSH and LH receptor. The mechanism regulating FSH and LH receptor expression appears to function independently, in vivo, from the dose of FSH gonadotrophin, rather than in response to it. Restoring an optimum receptor density may improve oocyte quality and the pregnancy rate in older women.

Ovarian aging and the activation of the primordial follicle reserve in the long-lived Ames dwarf and the short-lived bGH transgenic mice

The aim of this study was to evaluate the effect of growth hormone (GH) in the maintenance of the ovarian primordial follicle reserve. Ovaries from 16 mo old GH-deficient Ames Dwarf (df/df) and Normal (N/df) mice were used. A subgroup of df/df and N mice received GH or saline injections for six weeks starting at 14 mo of age. In addition, ovaries from 12 mo old mice overexpressing bovine GH (bGH) and controls were used. df/df mice had higher number of primordial and total follicles than N/df mice (p < 0.05), while GH treatment decreased follicle counts in both genotypes (p < 0.05). In addition, bGH mice had lower number of primordial and total follicles than the controls (p < 0.05). pFoxO3a levels were higher in mice treated with GH and in bGH mice (p < 0.05) when comparing with age match controls. These results indicate that increased circulating GH is associated with a reduced ovarian primordial follicle reserve and increased pFoxO3a content in oocytes.

The Ovarian Reserve of Primordial Follicles and the Dynamic Reserve of Antral Growing Follicles: What Is the Link?1

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.

Role of the Antiapoptotic Proteins BCL2 and MCL1 in the Neonatal Mouse Ovary1

The mammalian ovarian lifespan is determined at the time of birth through a delicate balance of oocyte survival and apoptosis as primordial follicles form, and the mechanism by which germ cells die is not understood. We hypothesized that two BCL2 family proteins, BCL2 and MCL1, may be responsible for regulating neonatal oocyte survival. Previous work has shown that BCL2 is important for germ cell survival in adult mouse ovaries, but no work has been done to examine its role at the time of birth. To elucidate the effects of BCL2 in the neonatal ovary, we examined ovaries of both Bcl2-overexpressing and knockout transgenic mice. When compared to wild-type mice, neither Bcl2 overexpression nor abrogation significantly altered ovarian histology. Another BCL2 family protein, MCL1, is expressed in human oocytes during ovarian development, suggesting a role for MCL1 in oocyte survival. To test this, we first examined the expression of MCL1 in the newborn mouse ovary. MCL1 was localized to both oocytes and somatic cells during primordial follicle formation. Subsequently, we used an in vitro organ culture system to identify a role for MCL1 in oocyte survival. We found that inhibition of MCL1 with an antibody to MCL1 in culture resulted in a reduced number of germ cells and an increase in cyst breakdown. Our data demonstrate that while BCL2 is not likely involved in perinatal oocyte survival, MCL1 may be an important regulator of the ovarian primordial follicle reserve.

Follicle-stimulating hormone increases primordial follicle reserve in mature female hypogonadal mice

Ovarian primordial follicle reserve is considered hormonally independent or subject to depletion by FSH-driven follicle recruitment. To explore specific in vivo effects of FSH on early follicle populations in the absence of luteinizing hormone (LH) activity, we examined mature hypogonadal (hpg), gonadotrophin-deficient mice expressing transgenic (tg) human FSH. Sustained expression of tg-FSH (5.3 +/- 0.3 IU/l) increased ovary weights fourfold and significantly elevated total primordial follicle numbers twofold in tg-FSH hpg (4209 +/- 457) relative to non-tg hpg (2079 +/- 391) and wild-type (2043 +/- 195) age-matched ovaries. Absolute primary follicle numbers in tg-FSH hpg ovaries were similar to non-tg hpg and wild-type ovaries. Furthermore, tg-FSH quantitatively increased secondary and antral follicles in hpg ovaries to numbers equivalent to wild-type, but did not induce ovulation, indicating a selective FSH response without LH. Circulating inhibin B and inhibin A levels were significantly increased in tg-FSH hpg females compared with hpg controls, and inhibin B correlated with antral number, consistent with FSH-driven antral follicle formation. These findings revealed that sustained pituitary-independent FSH activity, in the absence of endogenous gonadotrophins, promotes an increase in primordial follicle reserve despite also stimulating follicular growth in mature females. Therefore, the tg-FSH hpg ovary presents a novel paradigm to evaluate specific gonadotrophin effects on follicle reserve and recruitment.

Apoptose ovocytaire et évolution de la réserve ovarienne

In mammals, the ovarian reserve of primordial follicles is constituted early in fetus, then it becomes progressively exhausted by both follicular development and oocyte apoptosis. At least two third of the oocytes present in the reserve die by apoptosis before birth. Hypotheses of mechanism underlying this process are 1) a “quality control” leading to eliminate meiotic anomalies 2) a deficit in survival factors produced by somatic neighbouring cells 3) a “self sacrifice” or “altruistic death”, as previously described in Drosophila. After birth, growth factors and cytokines are main actors of the dialogue which exists between granulosa cells and the oocyte and determines oocyte survival. Mitochondrial factors belonging to bcl–2 family, metabolites of sphingolipids and the aromatic hydrocarbon receptor participate to oocyte apoptosis and can modulate numerical changes in the ovarian reserve. The reserve is quantitatively and qualitatively under both genetic and environmental control. Therapy to preserve ovarian reserve will benefit from better knowledge of molecular and cellular mechanisms regulating oocyte apoptosis. Moreover, recent identification of genes implicated in ovarian premature insufficiency will allow to propose new perspectives to prolong ovarian lifespan.