Mouse ovarian germ cell cysts undergo programmed breakdown to form primordial follicles

CKAP5 deficiency induces premature ovarian insufficiency

BACKGROUND

Premature ovarian insufficiency (POI) is characterized by ovarian dysfunction that develops from diminished ovarian reserve (DOR). The exact aetiology of POI remains poorly understood. This study aims to elucidate the role of CKAP5 in the regulation of ovarian function and fertility.

METHODS

Bulk RNA sequencing of granulosa cells was conducted in the control group and in the patients with DOR to screen for candidate genes, which were further validated by gene burden analysis in a next-generation sequencing cohort of POI and control individuals. Additionally, ovarian reserve was evaluated in heterozygous Ckap5 knockout mice, alongside the ovarian and oocyte single-cell transcriptome analysis. The regulatory mechanism of CKAP5 was studied through in vivo and in vitro experiments.

FINDINGS

CKAP5 was identified as a key hub gene associated with ovarian ageing. Heterozygous Ckap5 knockout mice exhibited a POI-like phenotype, characterized by a reduced primordial follicle pool and accelerated follicular atresia. CKAP5 promotes autophagy via ATG7 and simultaneously supports DNA damage repair through the ATM. Finally, a variant in CKAP5 (NM_0001008938.4, c.630 + 7_630 + 11delCAAAA) was identified in patients with POI, resulting in protein truncation and loss of function.

INTERPRETATION

CKAP5 deficiency induces premature ovarian insufficiency in both humans and mice.

FUNDING

The National Key R&D Program of China (2017YFC1001100), the National Natural Science Foundation of China (81501248, 81471453 and 81801295), the Health Research Project of Hunan Provincial Health Commission (W20243018), the Science and Technology Innovation Program of Hunan Province (2021RC3031), the National Natural Science Foundation of Hunan Province (2022JJ30066), the Scientific Research Program of Hunan Provincial Health Commission (202205033471 and 21B0058), the Open Research Fund of Hunan Provincial Key Laboratory of Regional Hereditary Birth Defects Prevention and Control (HPKL2023013).

Single-Cell Transcriptomic Analysis of the Potential Mechanisms of Follicular Development in Stra8-Deficient Mice

Follicle development is a critical process in mammalian reproduction, with significant implications for ovarian reserve and fertility. Stra8 is a known key factor regulating the initiation of meiosis; however, oocyte-like cells still appear in Stra8-deficient mice. Nevertheless, the underlying mechanism remains unclear and requires further investigation. Therefore, we used single-cell RNA sequencing to construct a comprehensive transcriptional atlas of ovarian cells from both wild-type and Stra8-deficient mice at embryonic stages E14.5 and E16.5. With stringent quality control, we obtained a total of 14,755 single cells of six major cell types. A further fine-scale analysis of the germ cell clusters revealed notable heterogeneity between wild-type and Stra8-deficient mice. Compared to the wild-type mice, the deficiency in Stra8 led to the downregulation of meiosis-related genes (e.g., Pigp, Tex12, and Sycp3), and the upregulation of apoptosis-related genes (e.g., Fos, Jun, and Actb), thereby hindering the meiotic process. Notably, we observed that, following Stra8 deficiency, the expression levels of Sub1 and Stk31 remained elevated at this stage. Furthermore, an RNA interference analysis confirmed the potential role of these genes as regulatory factors in the formation of primordial follicle-like cells. Additionally, Stra8 deficiency disrupted the signaling between germ cells and pregranulosa cells that is mediated by Mdk-Sdc1, leading to the abnormal expression of the PI3K/AKT signaling pathway. Together, these results shed light on the molecular processes governing germ cell differentiation and folliculogenesis, emphasizing the complex role of Stra8 in ovarian function.

Ovarian stimulation by promoting basal follicular growth

BACKGROUND

Most methods of ovarian stimulation rely on gonadotropin modulation. However, abnormal anti-Müllerian hormone concentrations are frequent in infertility, suggesting that defects in the gonadotropin-independent period of folliculogenesis preceding cyclic recruitment (i.e., basal follicular growth) may often occur. We need to better understand basal follicular growth and determine how to improve it.

METHODS

Section I summarizes a literature search concerning preantral and early antral folliculogenesis, cyclic recruitment, and selection. Section II presents current knowledge about interventions involving early antral folliculogenesis and cyclic recruitment.

RESULTS

While folliculogenesis following cyclic recruitment is gonadotropin-dependent, basal follicular growth is not. Basal follicular growth is regulated by follicle-stimulating hormone and local communication between the oocyte and its granulosa and thecal cells involving gap junctions and many autocrine/paracrine factors. This local communication sustains growth synergistically with follicle-stimulating hormone, but also suppresses this hormone to induce granulosa cell differentiation. As a follicle develops, its responsiveness to gonadotropin progressively increases. Section II describes 4 interventions affecting early antral folliculogenesis, including granulocyte colony-stimulating factor priming, bromocriptine rebound, carbohydrate metabolism intervention, and danazol priming, which have improved embryo development and live birth rate in patients with previous failures.

CONCLUSION

Basal follicular growth modulation can increase live birth rates.

Ovarian Mechanobiology: Understanding the Interplay Between Mechanics and Follicular Development

The ovary is a dynamic organ where mechanical forces profoundly regulate follicular development, oocyte maturation, and overall reproductive function. These forces, originating from the extracellular matrix (ECM), granulosa and theca cells, and ovarian stroma, influence cellular behavior through mechanotransduction, translating mechanical stimuli into biochemical responses. This review explores the intricate interplay between mechanical cues and ovarian biology, focusing on key mechanosensitive pathways such as Hippo signaling, the PI3K/AKT pathway, and cytoskeletal remodeling, which govern follicular dormancy, activation, and growth. Additionally, it examines how ovarian aging disrupts the mechanical microenvironment, with ECM stiffening and altered mechanotransduction contributing to a decline in ovarian reserve and reproductive potential. Emerging technologies, including 3D culture systems and organ-on-chip platforms, are highlighted for their ability to replicate the ovarian microenvironment and advance drug discovery and therapeutic interventions. By integrating mechanobiological principles, this review aims to enhance our understanding of ovarian function and provide new strategies for preserving fertility and combating infertility.

A simple immunohistochemical method for perinatal mammalian ovaries revealed different kinetics of oocyte apoptosis caused by DNA damage and asynapsis

Oocytes with meiotic defects are assumed to be eliminated by apoptosis in the perinatal period. However, oocyte apoptosis caused by meiotic defects has not been well analyzed, partly because of the great technical demands of tissue sectioning perinatal ovaries. In the present study, we applied a squash method for immunohistochemical analysis of perinatal mouse ovaries as a substitute for tissue sectioning. As a result, we could show different kinetics of apoptosis caused by DMC1- and SPO11-deficiencies, indicating that DNA damage-induced apoptosis precedes asynapsis-induced apoptosis in mouse oocytes. Double-mutant analysis revealed that only asynapsis-induced apoptosis was significantly dependent on HORMAD2. The present method is simple, easy, and able to analyze a sufficient number of oocytes to detect infrequent events in a single specimen, accelerating detailed immunohistochemical analyses of mammalian ovaries during the fetal and perinatal periods.

Autophagy in reproduction and pregnancy-associated diseases

As advantageous as sexual reproduction is during progeny generation, it is also an expensive and treacherous reproductive strategy. The viviparous eukaryote has evolved to survive stress before, during, and after pregnancy. An important and conserved intracellular pathway for the control of metabolic stress is autophagy. The autophagy process occurs in multiple stages through the coordinated action of autophagy-related genes. This review summarizes the evidence that autophagy is an integral component of reproduction. Additionally, we discuss emerging in vitro techniques that will enable cellular and molecular studies of autophagy and its associated pathways in reproduction. Finally, we discuss the role of autophagy in the pathogenesis and progression of several pregnancy-related disorders such as preterm birth, preeclampsia, and intra-uterine growth restriction, and its potential as a therapeutic target.

A tug-of-war between germ cell motility and intercellular bridges controls germline cyst formation in mice

Gametes in many species develop in cysts-clusters of germ cells formed by incomplete cytokinesis-that remain connected through intercellular bridges (ICBs). These connections enable sharing of cytoplasmic components between germ cells and, in the female germ line, enrich select cells in the cyst to become the oocyte(s). In mice, germline cysts of variable sizes are generated during embryonic development, thought to result from cyst fractures. Studies of fixed samples failed to capture fracture events, and thus, the mechanism remained elusive. Here, we use high-resolution live imaging of germ cells within their native tissue environment to visualize germline cyst dynamics. With this novel approach, we reveal a striking motile phenotype of gonad-resident germ cells and show that this randomly oriented cell-autonomous motile behavior during cyst formation underlies fracture events. Conversely, we show that stabilized ICBs help resist excessive fracturing. Additionally, we find that motility and thus fracture rates gradually decrease during development in a sex-dependent manner, completely ceasing by the end of cyst-forming divisions. These results lead to a model where the opposing activities of developmentally regulated cell motility and stable ICBs give rise to cysts of variable sizes. We corroborate these results by developing a model that uses experimentally measured fracture rates to simulate cyst formation and fracture and show that it can reproduce experimentally measured cyst sizes in both male and female. Understanding how variable cysts form will enable further studies of mammalian oocyte selection and establishment of the ovarian reserve.

Polycomb in female reproductive health: patterning the present and programming the future

Epigenetic modifications regulate chromatin accessibility, gene expression, cell differentiation and tissue development. As epigenetic modifications can be inherited via mitotic and meiotic cell divisions, they enable a heritable memory of cell identity and function and can alter inherited characteristics in the next generation. Tight regulation of epigenetic information is critical for normal cell function and is often disrupted in diseases including cancer, metabolic, neurological and inherited congenital conditions. The ovary performs critical functions in female reproductive health and fertility, including oocyte and sex-hormone production. Oocytes undergo extensive epigenetic programming including the establishment of maternal genomic imprints, which are critical for offspring health and development. Epigenetic modifiers also regulate ovarian somatic cells, such as granulosa and theca cells which support oocytes and produce hormones. While ovarian dysfunction contributes to serious ovarian conditions such as primary ovarian insufficiency (POI), polycystic ovary syndrome (PCOS) and ovarian cancers, the roles of epigenetic modifications in the ovary and their contribution to ovarian dysfunction are not properly understood. Here we review recent advancements in understanding Polycomb proteins, important epigenetic modifiers that have emerging roles in ovarian development and maternal epigenetic inheritance. Polycomb group proteins (PcGs) contribute to the faithful establishment of epigenetic information in oocytes, a process essential for normal offspring development in mice. Emerging evidence also indicates that PcGs regulate ovarian function and female fertility. Understanding these and similar mechanisms will provide greater insight into the epigenetic regulation of ovarian and oocyte function, and how its disruption can impact reproductive health and maternal inheritance.

Transcriptomic signatures of WNT-driven pathways and granulosa cell-oocyte interactions during primordial follicle activation

Primordial follicle activation (PFA) is a pivotal event in female reproductive biology, coordinating the transition from quiescent to growing follicles. This study employed comprehensive single-cell RNA sequencing to gain insights into the detailed regulatory mechanisms governing the synchronized dormancy and activation between granulosa cells (GCs) and oocytes with the progression of the PFA process. Wntless (Wls) conditional knockout (cKO) mice served as a unique model, suppressing the transition from pre-GCs to GCs, and disrupting somatic cell-derived WNT signaling in the ovary. Our data revealed immediate transcriptomic changes in GCs post-PFA in Wls cKO mice, leading to a divergent trajectory, while oocytes exhibited modest transcriptomic alterations. Subpopulation analysis identified the molecular pathways affected by WNT signaling on GC maturation, along with specific gene signatures linked to dormant and activated oocytes. Despite minimal evidence of continuous up-regulation of dormancy-related genes in oocytes, the loss of WNT signaling in (pre-)GCs impacted gene expression in oocytes even before PFA, subsequently influencing them globally. The infertility observed in Wls cKO mice was attributed to compromised GC-oocyte molecular crosstalk and the microenvironment for oocytes. Our study highlights the pivotal role of the WNT-signaling pathway and its molecular signature, emphasizing the importance of intercellular crosstalk between (pre-)GCs and oocytes in orchestrating folliculogenesis.

The mouse Balbiani body regulates primary oocyte quiescence via RNA storage

In mammalian females, the transition from dormancy in primordial follicles to follicular development is critical for maintaining ovarian function and reproductive longevity. In mice, the quiescent primary oocyte of the primordial follicle contains a Balbiani body (B-body), an organelle aggregate comprised of a spherical structure of Golgi complexes. Here we show that the structure of the B-body is maintained by microtubules and actin. The B-body stores mRNA-capping enzyme and 597 mRNAs associated with mRNA-decapping enzyme 1 A (DCP1A). Gene ontology analysis results indicate that proteins encoded by these mRNAs function in enzyme binding, cellular component organization and packing of telomere ends. Pharmacological depolymerization of microtubules or actin led to B-body disassociation and nascent protein synthesis around the dissociated B-bodies within three hours. An increased number of activated developing follicles were observed in ovaries with prolonged culture and the in vivo mouse model. Our results indicate that the mouse B-body is involved in the activation of dormant primordial follicles likely via translation of the B-body-associated RNAs in primary oocytes.

Single-cell sequencing reveals the transcriptional alternations of 17β-estradiol suppressing primordial follicle formation in neonatal mouse ovaries

Estrogen has been implicated in multiple biological processes, but the variation underlying estrogen-mediated primordial follicle (PF) formation remains unclear. Here, we show that 17β-estradiol (E2) treatment of neonatal mice led to the inhibition of PF formation and cell proliferation. Single-cell RNA sequencing (scRNA-seq) revealed that E2 treatment caused significant changes in the transcriptome of oocytes and somatic cells. E2 treatment disrupted the synchronised development of oocytes, pre-granulosa (PG) cells and stromal cells. Mechanistically, E2 treatment disrupted several signalling pathways critical to PF formation, especially down-regulating the Kitl and Smad1/3/4/5/7 expression, reducing the frequency and number of cell communication. In addition, E2 treatment influenced key gene expression, mitochondrial function of oocytes, the recruitment and maintenance of PG cells, the cell proliferation of somatic cells, as well as disordered the ovarian microenvironment. This study not only revealed insights into the regulatory role of estrogen during PF formation, but also filled in knowledge of dramatic changes in perinatal hormones, which are critical for the physiological significance of understanding hormone changes and reproductive protection.

Sustained fertility from first-wave follicle oocytes that pause their growth

Ovulation results from the cyclical recruitment of non-renewing, quiescent oocytes for growth. Therefore, the primordial follicles that are established during development from an oocyte encapsulated by granulosa cells are thought to comprise the lifelong ovarian reserve 1-4. However, using oocyte lineage tracing in mice, we observed that a subset of oocytes recruited for growth in the first juvenile wave remain paused for many months before continuing growth, ovulation, fertilization and development into healthy offspring. This small subset of genetically-labeled fetal oocytes, labeled with Sycp3-CreERT2, is distinguished by earlier entry and slower dynamics of meiotic prophase I. While labeled oocytes were initially found in both primordial follicles and growing follicles of the first wave, they disappeared from primordial follicles by puberty. Unexpectedly, these first-wave labeled growing oocytes persisted throughout reproductive lifespan and contributed to offspring at a steady rate beyond 12 months of age, suggesting that follicles can pause mid-growth for extended periods then successfully resume. These results challenge the conclusion from lineage tracing of granulosa cells that first-wave follicles make a limited contribution to fertility5 and furthermore suggest that growth-paused oocytes comprise a second and previously unrecognized ovarian reserve.

The effect of bisphenol A on the Notch (Notch2 and Jagged2) signaling pathway in the follicular development of the neonatal rat ovary

The formation of primordial follicles determines the pool size of follicles in the ovary, and is crucial for female reproductivity. Oocyte nest breakdown, and the formation of primordial follicles, largely depend upon the communication between oocytes and the surrounding pregranulosa cells. The neurogenic locus notch homolog protein (Notch) signaling pathway is the key player for this cell-to-cell communication, and is responsible for primordial folliculogenesis. However, different endocrine disruptors, including bisphenol A (BPA; a plasticizer and a constituent of reusable plastic containers) may affect the Notch signaling pathway, and might induce ovary dysfunction via Notch signaling. Consequently, we investigated the possible influence of BPA treatment on the proportional distribution of the follicular stages, follicle numbers, levels of apoptosis, and on Notch2 and Jagged2 expressions in the ovary. BPA was administered at doses of either 50 µg/kg/day or 50 mg/kg/day, at different time intervals, during neonatal and fetal periods in vivo. After collecting the ovaries from the various experimental groups, follicles were counted, and frequency of apoptosis was determined by TUNEL assay. In addition, Notch2 and Jagged2 expressions were assessed by immunohistochemical staining and qPCR. In summary, BPA treatment affected the follicle numbers and apoptosis level, and Notch2 and Jagged2 expressions varied with follicular stage. It was also observed that these parameters were dose and time dependent with respect to BPA exposure.

The maintenance of oocytes in the mammalian ovary involves extreme protein longevity

Women are born with all of their oocytes. The oocyte proteome must be maintained with minimal damage throughout the woman's reproductive life, and hence for decades. Here we report that oocyte and ovarian proteostasis involves extreme protein longevity. Mouse ovaries had more extremely long-lived proteins than other tissues, including brain. These long-lived proteins had diverse functions, including in mitochondria, the cytoskeleton, chromatin and proteostasis. The stable proteins resided not only in oocytes but also in long-lived ovarian somatic cells. Our data suggest that mammals increase protein longevity and enhance proteostasis by chaperones and cellular antioxidants to maintain the female germline for long periods. Indeed, protein aggregation in oocytes did not increase with age and proteasome activity did not decay. However, increasing protein longevity cannot fully block female germline senescence. Large-scale proteome profiling of ~8,890 proteins revealed a decline in many long-lived proteins of the proteostasis network in the aging ovary, accompanied by massive proteome remodeling, which eventually leads to female fertility decline.

Perinatal bisphenol S exposure exacerbates the oxidative burden and apoptosis in neonatal ovaries by suppressing the mTOR/autophagy axis

Bisphenol S (BPS) is an emerging environmental endocrine disruptor capable of crossing the placental barrier, resulting in widespread exposure to pregnant women due to its extensive usage. However, the impact of perinatal maternal exposure to BPS on reproductive health in offspring and the underlying molecular mechanism remain underexplored. In this study, gestational ICR mice were provided with drinking water containing 3.33 mg/L BPS to mimic possible human exposure in some countries. Results demonstrated that BPS accelerated the breakdown of germ-cell cysts and the assembly of primordial follicles in neonates, leading to oocyte over-loss. Furthermore, the expression levels of folliculogenesis-related genes (Kit, Nobox, Gdf9, Sohlh2, Kitl, Bmp15, Lhx8, Figla, and Tgfb1) decreased, thus compromising oocyte quality and disrupting early folliculogenesis dynamics. BPS also disrupted other aspects of offspring reproduction, including advancing puberty onset, disrupting the estrus cycle, and impairing fertility. Further investigation found that BPS exposure inhibited the activities and expression levels of antioxidant-related enzymes in neonatal ovaries, leading to the substantial accumulation of MDA and ROS. The increased oxidative burden exacerbated the intracellular apoptotic signaling, manifested by increased expression levels of pro-apoptotic markers (Bax, Caspase 3, and Caspase 9) and decreased expression levels of anti-apoptotic marker (Bcl2). Concurrently, BPS inhibited autophagy by increasing p-mTOR/mTOR and decreasing p-ULK1/ULK1, subsequently down-regulating autophagy flux-related biomarkers (LC3b/LC3a and Beclin-1) and impeding the degradation of autophagy substrate p62. However, the imbalanced crosstalk between autophagy, apoptosis and oxidative stress homeostasis was restored after rapamycin treatment. Collectively, the findings demonstrated that BPS exposure induced reproductive disorders in offspring by perturbing the mTOR/autophagy axis, and such autophagic dysfunction exacerbated redox imbalance and promoted excessive apoptosis. These results provide novel mechanistic insights into the role of autophagy in mitigating BPS-induced intergenerational reproductive dysfunction.

EPAS1 expression contributes to maintenance of the primordial follicle pool in the mouse ovary

Oxygen availability can have profound effects on cell fate decisions and survival, in part by regulating expression of hypoxia-inducible factors (HIFs). In the ovary, HIF expression has been characterised in granulosa cells, however, any requirement in oocytes remains relatively undefined. Here we developed a Hif2a/Epas1 germline-specific knockout mouse line in which females were fertile, however produced 40% fewer pups than controls. No defects in follicle development were detected, and quality of MII oocytes was normal, as per assessments of viability, intracellular reactive oxygen species, and spindle parameters. However, a significant diminishment of the primordial follicle pool was evident in cKO females that was attributed to accelerated follicle loss from postnatal day 6 onwards, potentially via disruption of the autophagy pathway. These data demonstrate the importance of HIF signalling in oocytes, particularly at the primordial follicle stage, and lend to the importance of controlling oxygen tension in the development of in vitro growth and maturation approaches for assisted reproduction.

A Human Homozygous HELQ Missense Variant Does Not Cause Premature Ovarian Insufficiency in a Mouse Model

Disruption of meiosis and DNA repair genes is associated with female fertility disorders like premature ovarian insufficiency (POI). In this study, we identified a homozygous missense variant in the HELQ gene (c.596 A>C; p.Gln199Pro) through whole exome sequencing in a POI patient, a condition associated with disrupted ovarian function and female infertility. HELQ, an enzyme involved in DNA repair, plays a crucial role in repairing DNA cross-links and has been linked to germ cell maintenance, fertility, and tumour suppression in mice. To explore the potential association of the HELQ variant with POI, we used CRISPR/Cas9 to create a knock-in mouse model harbouring the equivalent of the human HELQ variant identified in the POI patient. Surprisingly, Helq knock-in mice showed no discernible phenotype, with fertility levels, histological features, and follicle development similar to wild-type mice. Despite the lack of observable effects in mice, the potential role of HELQ in human fertility, especially in the context of POI, should not be dismissed. Larger studies encompassing diverse ethnic populations and alternative functional approaches will be necessary to further examine the role of HELQ in POI. Our results underscore the potential uncertainties associated with genomic variants and the limitations of in vivo animal modelling.

Making human eggs in a dish: are we close?

In the space of 50 years, we have seen incredible achievements in human reproductive medicine. With these leaps forward, it is no wonder that there is a major interest in women's reproductive health research, including extension of reproductive lifespan. Substantial effort is currently being made to address this challenge, including from the commercial sector. In vitro gametogenesis (IVG) in mice is a spectacular breakthrough and has the potential to offer hope to women with intractable infertility. However, with such lofty goals, some reflection may be called for: mastering all of the techniques required for complete and safe IVG in women is likely to be extraordinarily difficult.

Female Germline Cysts in Animals: Evolution and Function

Germline cysts are syncytia formed by incomplete cytokinesis of mitotic germline precursors (cystoblasts) in which the cystocytes are interconnected by cytoplasmic bridges, permitting the sharing of molecules and organelles. Among animals, such cysts are a nearly universal feature of spermatogenesis and are also often involved in oogenesis. Recent, elegant studies have demonstrated remarkable similarities in the oogenic cysts of mammals and insects, leading to proposals of widespread conservation of these features among animals. Unfortunately, such claims obscure the well-described diversity of female germline cysts in animals and ignore major taxa in which female germline cysts appear to be absent. In this review, I explore the phylogenetic patterns of oogenic cysts in the animal kingdom, with a focus on the hexapods as an informative example of a clade in which such cysts have been lost, regained, and modified in various ways. My aim is to build on the fascinating insights of recent comparative studies, by calling for a more nuanced view of evolutionary conservation. Female germline cysts in the Metazoa are an example of a phenomenon that-though essential for the continuance of many, diverse animal lineages-nevertheless exhibits intriguing patterns of evolutionary innovation, loss, and convergence.

Maternal vitamin B1 is a determinant for the fate of primordial follicle formation in offspring

The mediation of maternal-embryonic cross-talk via nutrition and metabolism impacts greatly on offspring health. However, the underlying key interfaces remain elusive. Here, we determined that maternal high-fat diet during pregnancy in mice impaired preservation of the ovarian primordial follicle pool in female offspring, which was concomitant with mitochondrial dysfunction of germ cells. Furthermore, this occurred through a reduction in maternal gut microbiota-related vitamin B1 while the defects were restored via vitamin B1 supplementation. Intriguingly, vitamin B1 promoted acetyl-CoA metabolism in offspring ovaries, contributing to histone acetylation and chromatin accessibility at the promoters of cell cycle-related genes, enhancement of mitochondrial function, and improvement of granulosa cell proliferation. In humans, vitamin B1 is downregulated in the serum of women with gestational diabetes mellitus. In this work, these findings uncover the role of the non-gamete transmission of maternal high-fat diet in influencing offspring oogenic fate. Vitamin B1 could be a promising therapeutic approach for protecting offspring health.

An oocyte meiotic midbody cap is required for developmental competence in mice

Embryo development depends upon maternally derived materials. Mammalian oocytes undergo extreme asymmetric cytokinesis events, producing one large egg and two small polar bodies. During cytokinesis in somatic cells, the midbody and subsequent assembly of the midbody remnant, a signaling organelle containing RNAs, transcription factors and translation machinery, is thought to influence cellular function or fate. The role of the midbody and midbody remnant in gametes, in particular, oocytes, remains unclear. Here, we examined the formation and function of meiotic midbodies (mMB) and mMB remnants using mouse oocytes and demonstrate that mMBs have a specialized cap structure that is orientated toward polar bodies. We show that that mMBs are translationally active, and that mMB caps are required to retain nascent proteins in eggs. We propose that this specialized mMB cap maintains genetic factors in eggs allowing for full developmental competency.

Reconstituted ovaries self-assemble without an ovarian surface epithelium

Three-dimensional (3D) stem cell models of the ovary have the potential to benefit women's reproductive health research. One such model, the reconstituted ovary (rOvary) self-assembles with pluripotent stem cell-derived germ cells creating a 3D ovarian mimic competent to support the differentiation of functional oocytes inside follicles. In this study, we evaluated the cellular composition of the rOvary revealing the capacity to generate multiple follicles surrounded by NR2F2+ stroma cells. However, the rOvary does not develop a surface epithelium, the source of second-wave pre-granulosa cells, or steroidogenic theca. Therefore, the rOvary models represent the self-assembly of activated follicles in a pre-pubertal ovary poised but not yet competent for hormone production.

Cell-cell interactions during the formation of primordial follicles in humans

Gametogenesis is a complex and sex-specific multistep process during which the gonadal somatic niche plays an essential regulatory role. One of the most crucial steps during human female gametogenesis is the formation of primordial follicles, the functional unit of the ovary that constitutes the pool of follicles available at birth during the entire reproductive life. However, the relation between human fetal germ cells (hFGCs) and gonadal somatic cells during the formation of the primordial follicles remains largely unexplored. We have discovered that hFGCs can form multinucleated syncytia, some connected via interconnecting intercellular bridges, and that not all nuclei in hFGC-syncytia were synchronous regarding meiotic stage. As hFGCs progressed in development, pre-granulosa cells formed protrusions that seemed to progressively constrict individual hFGCs, perhaps contributing to separate them from the multinucleated syncytia. Our findings highlighted the cell-cell interaction and molecular dynamics between hFGCs and (pre)granulosa cells during the formation of primordial follicles in humans. Knowledge on how the pool of primordial follicle is formed is important to understand human infertility.

Protective effect of nebivolol on rat ovary exposed to deltamethrin toxicity

PURPOSE

We aimed to investigate the antioxidant activity of nebivolol against possible damage to the ovarian tissue due to the application of deltamethrin as a toxic agent, by evaluating histopathological proliferating cell nuclear antigen (PCNA) and tumor necrosis factor-alpha (TNF-α) signal molecules immunohistochemically.

METHODS

The animals were divided into three groups as control, deltamethrin and deltamethrin + nebivolol groups. Vaginal smears were taken after the animals were mated and detected on the first day of pregnancy. After the sixth day, deltamethrin (0.5 mL of 30 mg/kg BW undiluted ULV), and 2 mL of sterile nebivolol solution were administered intraperitoneally every day for 6-21 periods. After routine histopathological follow-up, the ovarian tissue was stained with hematoxylin and eosin stain.

RESULTS

Control group showed normal histology of ovarium. In deltamethrin group, hyperplasic cells, degenerative follicles, pyknotic nuclei, inflammation and hemorrhagic areas were observed. Nebivolol treatment restored these pathologies. Deltamethrin treatment increased TNF-α and PCNA reaction. However, nebivolol decreased the expression.

CONCLUSIONS

It was thought that deltamethrin toxicity adversely affected follicle development by inducing degeneration and apoptotic process in preantral and antra follicle cells, and nebivolol administration might reduce inflammation and slow down the apoptotic signal in the nuclear phase and regulate reorganization.

An oocyte meiotic midbody cap is required for developmental competence in mice

Embryo development depends upon maternally derived materials. Mammalian oocytes undergo extreme asymmetric cytokinesis events, producing one large egg and two small polar bodies (PB). During cytokinesis in somatic cells, the midbody (MB) and subsequent assembly of the midbody remnant (MBR), a signaling organelle containing RNAs, transcription factors and translation machinery, is thought to influence cellular function or fate. The role of the MB and MBR in gametes, in particular, oocytes, remains unclear. Here, we examined the formation and function of meiotic MBs (mMB) and mMB remnants (mMBRs) using mouse oocytes and demonstrate that mMBs have a specialized meiotic mMB cap structure that is orientated toward PBs. We show that that mMBs are translationally active, and that mMB caps are required to retain nascent proteins in eggs. We propose that this specialized mMB cap maintains genetic factors in eggs allowing for full developmental competency.

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.

Prolonged fasting induces significant germ cell loss in chickens after hatching

Germ cell loss is a crucial biological event during germ cell development. The number of female germ cells determines the reproductive performance and egg production of hens. Various intrinsic and extrinsic factors affect germ cell loss, such as germ cell nest breakdown in early life and nutritional deficiencies during daily husbandry. Here, we examined the effect of fasting on the germ cell number of chicks. The results showed that 72 h fasting resulted in a higher germ cell loss than that by 24 h fasting in chicks. The RNA-seq analysis revealed that the genes of ribosome pathway were down-regulated and the biological processes of protein processing in endoplasmic reticulum were inhibited in starved chicks. Furthermore, in female chicks treated with 72 h fasting, the qPCR of ovaries showed down-regulation of ribosome-related genes, and transmission electron microscopy imaging of ovaries showed fewer ribosomes. The blood biochemical indices indicated that 72 h fasting reduced the liver functions and affected the glucose metabolism, lipid metabolites and ion metabolites. In summary, the present results concluded negative impacts on the germ cell pool by prolonged fasting in the early life of chicks and manifested that adequate management should be cared for fasted time for breeding.

Beyond apoptosis: evidence of other regulated cell death pathways in the ovary throughout development and life

BACKGROUND

Regulated cell death is a fundamental component of numerous physiological processes; spanning from organogenesis in utero, to normal cell turnover during adulthood, as well as the elimination of infected or damaged cells throughout life. Quality control through regulation of cell death pathways is particularly important in the germline, which is responsible for the generation of offspring. Women are born with their entire supply of germ cells, housed in functional units known as follicles. Follicles contain an oocyte, as well as specialized somatic granulosa cells essential for oocyte survival. Follicle loss-via regulated cell death-occurs throughout follicle development and life, and can be accelerated following exposure to various environmental and lifestyle factors. It is thought that the elimination of damaged follicles is necessary to ensure that only the best quality oocytes are available for reproduction.

OBJECTIVE AND RATIONALE

Understanding the precise factors involved in triggering and executing follicle death is crucial to uncovering how follicle endowment is initially determined, as well as how follicle number is maintained throughout puberty, reproductive life, and ovarian ageing in women. Apoptosis is established as essential for ovarian homeostasis at all stages of development and life. However, involvement of other cell death pathways in the ovary is less established. This review aims to summarize the most recent literature on cell death regulators in the ovary, with a particular focus on non-apoptotic pathways and their functions throughout the discrete stages of ovarian development and reproductive life.

SEARCH METHODS

Comprehensive literature searches were carried out using PubMed and Google Scholar for human, animal, and cellular studies published until August 2022 using the following search terms: oogenesis, follicle formation, follicle atresia, oocyte loss, oocyte apoptosis, regulated cell death in the ovary, non-apoptotic cell death in the ovary, premature ovarian insufficiency, primordial follicles, oocyte quality control, granulosa cell death, autophagy in the ovary, autophagy in oocytes, necroptosis in the ovary, necroptosis in oocytes, pyroptosis in the ovary, pyroptosis in oocytes, parthanatos in the ovary, and parthanatos in oocytes.

OUTCOMES

Numerous regulated cell death pathways operate in mammalian cells, including apoptosis, autophagic cell death, necroptosis, and pyroptosis. However, our understanding of the distinct cell death mediators in each ovarian cell type and follicle class across the different stages of life remains the source of ongoing investigation. Here, we highlight recent evidence for the contribution of non-apoptotic pathways to ovarian development and function. In particular, we discuss the involvement of autophagy during follicle formation and the role of autophagic cell death, necroptosis, pyroptosis, and parthanatos during follicle atresia, particularly in response to physiological stressors (e.g. oxidative stress).

WIDER IMPLICATIONS

Improved knowledge of the roles of each regulated cell death pathway in the ovary is vital for understanding ovarian development, as well as maintenance of ovarian function throughout the lifespan. This information is pertinent not only to our understanding of endocrine health, reproductive health, and fertility in women but also to enable identification of novel fertility preservation targets.

Replication Protein A1 is essential for DNA damage repair during mammalian oogenesis

Persistence of unrepaired DNA damage in oocytes is detrimental and may cause genetic aberrations, miscarriage, and infertility. RPA, an ssDNA-binding complex, is essential for various DNA-related processes. Here we report that RPA plays a novel role in DNA damage repair during postnatal oocyte development after meiotic recombination. To investigate the role of RPA during oogenesis, we inactivated RPA1 (replication protein A1), the largest subunit of the heterotrimeric RPA complex, specifically in oocytes using two germline-specific Cre drivers (Ddx4-Cre and Zp3-Cre). We find that depletion of RPA1 leads to the disassembly of the RPA complex, as evidenced by the absence of RPA2 and RPA3 in RPA1-deficient oocytes. Strikingly, severe DNA damage occurs in RPA1-deficient GV-stage oocytes. Loss of RPA in oocytes triggered the canonical DNA damage response mechanisms and pathways, such as activation of ATM, ATR, DNA-PK, and p53. In addition, the RPA deficiency causes chromosome misalignment at metaphase I and metaphase II stages of oocytes, which is consistent with altered transcript levels of genes involved in cytoskeleton organization in RPA1-deficient oocytes. Absence of the RPA complex in oocytes severely impairs folliculogenesis and leads to a significant reduction in oocyte number and female infertility. Our results demonstrate that RPA plays an unexpected role in DNA damage repair during mammalian folliculogenesis.

The effects of endogenous FSH and its receptor on oogenesis and folliculogenesis in female Alligator sinensis

BACKGROUND

The precise mechanisms of hormone action responsible for the full course of events modulating folliculogenesis in crocodilian have not been determined, although histological features have been identified.

RESULTS

The Alligator sinensis ovarian morphological characteristics observed at 1, 15, 30, 60, 90, and 300 days post hatching(dph) revealed that the dynamic changes in germ cells varied in different meiotic and developmental stages, confirming that the processes of folliculogenesis were protracted and asynchronous. The presence of endogenous follicle-stimulating hormone(FSH) mRNA and protein expression within the cerebrum at 1 dph, in parallel with the increase in germ cells within the germ cell nests(Nest) from 1 dph to 15 dph, suggested that endocrine regulation of the pituitary-gonad axis is an early event in oogonia division. Furthermore, the endogenous expression of FSH showed a trend of negative feedback augmentation accompanied by the exhaustion of maternal yolk E₂ observed at 15 dph. Such significant elevation of endogenous FSH levels was observed to be related to pivotal events in the transition from mitosis to meiosis, as reflected by the proportion of oogonia during premeiosis interphase, with endogenous FSH levels reaching a peak at the earliest time step of 1 dph. In addition, the simultaneous upregulation of premeiotic marker STRA8 mRNA expression and the increase in endogenous FSH further verified the above speculation. The strongly FSHr-positive label in the oocytes within Pre-previtellogenic follicles was synchronized with the significant elevation of ovarian cAMP detected at 300 dph, which suggested that diplotene arrest maintenance during early vitellogenesis might be FSH dependent. In addition, preferential selection in asynchronous meiotic initiation has been supposed to act on somatic supportive cells and not directly on germ cells via regulation of FSH that in turn affects downstream estrogen levels. This suggestion was verified by the reciprocal stimulating effect of FSH and E₂ on the accelerated meiotic marker SYCP3 and by the inhibited cell apoptosis demonstrated in ovarian cell culture in vitro.

CONCLUSION

The corresponding results contribute an expansion of the understanding of physiological processes and shed some light on the specific factors responsible for gonadotropin function in the early folliculogenesis of crocodilians.

Branched germline cysts and female-specific cyst fragmentation facilitate oocyte determination in mice

During mouse gametogenesis, germ cells derived from the same progenitor are connected via intercellular bridges forming germline cysts, within which asymmetrical or symmetrical cell fate occurs in female and male germ cells, respectively. Here, we have identified branched cyst structures in mice, and investigated their formation and function in oocyte determination. In fetal female cysts, 16.8% of the germ cells are connected by three or four bridges, namely branching germ cells. These germ cells are preferentially protected from cell death and cyst fragmentation and accumulate cytoplasm and organelles from sister germ cells to become primary oocytes. Changes in cyst structure and differential cell volumes among cyst germ cells suggest that cytoplasmic transport in germline cysts is conducted in a directional manner, in which cellular content is first transported locally between peripheral germ cells and further enriched in branching germ cells, a process causing selective germ cell loss in cysts. Cyst fragmentation occurs extensively in female cysts, but not in male cysts. Male cysts in fetal and adult testes have branched cyst structures, without differential cell fates between germ cells. During fetal cyst formation, E-cadherin (E-cad) junctions between germ cells position intercellular bridges to form branched cysts. Disrupted junction formation in E-cad-depleted cysts led to an altered ratio in branched cysts. Germ cell-specific E-cad knockout resulted in reductions in primary oocyte number and oocyte size. These findings shed light on how oocyte fate is determined within mouse germline cysts.

Oogenesis in Caenorhabditis elegans

BACKGROUND

The nematode, Caenorhabditis elegans has proven itself as a valuable model for investigating metazoan biology. C. elegans have a transparent body, an invariant cell lineage, and a high level of genetic conservation which makes it a desirable model organism. Although used to elucidate many aspects of somatic biology, a distinct advantage of C. elegans is its well annotated germline which allows all aspects of oogenesis to be observed in real time within a single animal. C. elegans hermaphrodites have two U-shaped gonad arms which produce their own sperm that is later stored to fertilise their own oocytes. These two germlines take up much of the internal space of each animal and germ cells are therefore the most abundant cell present within each animal. This feature and the genetic phenotypes observed for mutant worm gonads have allowed many novel findings that established our early understanding of germ cell dynamics. The mutant phenotypes also allowed key features of meiosis and germ cell maturation to be unveiled.

SUMMARY

This review will focus on the key aspects that make C. elegans an outstanding model for exploring each feature of oogenesis. This will include the fundamental steps associated with germline function and germ cell maturation and will be of use for those interested in exploring reproductive metazoan biology.

KEY MESSAGES

Since germ cell biology is highly conserved in animals, much can be gained from study of a simple metazoan like C. elegans. Past findings have enhanced understanding on topics that would be more laborious or challenging in more complex animal models.

Toxic effect window of ovarian development in female offspring mice induced by prenatal prednisone exposure with different doses and time

BACKGROUND

Prednisone is one of the most used synthetic glucocorticoids during pregnancy. Epidemiological investigations suggested that prenatal prednisone therapy could affect fetal development, but systematic studies on its effects on ovarian development and the "toxic effect window" remained scarce.

METHODS

In this study, by simulating clinical application characteristics, Kunming mice were given prednisone by oral gavage with different doses (0.25 or 1.0 mg/kg·d) or at different time gestational days (GD) (GD0-9, GD10-18, or GD0-18). Blood and ovaries of fetal mice were collected on GD18, and the serum estradiol level and the related function indexes of ovarian granulosa cells and oocytes were detected.

RESULTS

Compared with the control group, prenatal prednisone exposure (PPE) induced pathological injury and enhanced cell proliferation in fetal mice ovary. Furthermore, the expression of steroid synthesis functional genes in pre-granulosa cells, the oocyte function markers, and developmentally related genes was enhanced with different doses or at different time of PPE. The Hippo signaling was activated in the fetal ovary of PPE groups. The above changes were most significant in the low or high-dose and full-term PPE groups.

CONCLUSION

PPE caused various cell developmental toxicity in the fetal ovary, especially in the low or high-dose, full-term exposure groups. The potential mechanism might be related to the activation of the Hippo signaling pathway.

Go with the flow - bulk transport by molecular motors

Cells are the smallest building blocks of all living eukaryotic organisms, usually ranging from a couple of micrometers (for example, platelets) to hundreds of micrometers (for example, neurons and oocytes) in size. In eukaryotic cells that are more than 100 µm in diameter, very often a self-organized large-scale movement of cytoplasmic contents, known as cytoplasmic streaming, occurs to compensate for the physical constraints of large cells. In this Review, we discuss cytoplasmic streaming in multiple cell types and the mechanisms driving this event. We particularly focus on the molecular motors responsible for cytoplasmic movements and the biological roles of cytoplasmic streaming in cells. Finally, we describe bulk intercellular flow that transports cytoplasmic materials to the oocyte from its sister germline cells to drive rapid oocyte growth.

Postnatal oogenesis leads to an exceptionally large ovarian reserve in naked mole-rats

In the long-lived naked mole-rat (NMR), the entire process of oogenesis occurs postnatally. Germ cell numbers increase significantly in NMRs between postnatal days 5 (P5) and P8, and germs cells positive for proliferation markers (Ki-67, pHH3) are present at least until P90. Using pluripotency markers (SOX2 and OCT4) and the primordial germ cell (PGC) marker BLIMP1, we show that PGCs persist up to P90 alongside germ cells in all stages of female differentiation and undergo mitosis both in vivo and in vitro. We identified VASA+ SOX2+ cells at 6 months and at 3-years in subordinate and reproductively activated females. Reproductive activation was associated with proliferation of VASA+ SOX2+ cells. Collectively, our results suggest that highly desynchronized germ cell development and the maintenance of a small population of PGCs that can expand upon reproductive activation are unique strategies that could help to maintain the NMR's ovarian reserve for its 30-year reproductive lifespan.

Meiotic Development of Canine Oocytes from Poly-Ovular and Mono-Ovular Follicles after In Vitro Maturation

Poly-ovular follicles are defined as those with more than one oocyte present in single follicles. The occurrence frequency of this follicle type is higher in canines than that in other species. This study aimed to evaluate the in vitro meiotic maturation of dog oocytes from this follicle type in comparison to those from mono-ovular follicles of various sizes (small antral, medium antral, and large antral) considering different phases of the estrus cycle (anestrus, proestrus, estrus, and diestrus). Canine oocytes were obtained separately from the poly-ovular and mono-ovular antral follicles from the ovaries of adult females. In each experimental replicate, cumulus-oocyte complexes (COCs) from poly-ovular and mono-ovular follicles were incubated in supplemented TCM-199 at 38.5 °C and 5% CO₂ for 72 h. After culturing, the meiotic development of each oocyte was evaluated using epifluorescence microscopy. Meiotic stages were classified into germinal vesicle (GV), germinal vesicle breakdown (GVBD), first metaphase (MI), and second metaphase (MII). Data were evaluated using an analysis of variance. Oocytes from poly-ovular follicles at all phases exhibited a higher (p < 0.05) percentage of oocytes arrested at the GV stage than those from mono-ovular follicles, showing the highest rate of GV in small antral follicles during anestrus. In contrast, there were no differences in MII rates (p < 0.05) in oocytes from mono-ovular and poly-ovular follicles during the estrus and diestrus phases in all sizes evaluated, with the highest MII rate in estrus. These results suggest that oocytes from poly-ovular follicles can resume meiosis at a slower rate than those from mono-ovular follicles; however, the maturation in vitro of such oocytes is possible. Furthermore, the relationship between the maturation capacity of oocytes from both poly-ovular and mono-ovular follicles depends on the ovarian cycle and follicular development.

Factors within the Developing Embryo and Ovarian Microenvironment That Influence Primordial Germ Cell Fate

BACKGROUND

Primordial germ cell (PGC) fate is dictated by the designation, taxis, and influence of the surrounding embryonic somatic cells. Whereas gonadal sex determination results from a balance of factors within the tissue microenvironment.

SUMMARY

Our understanding of mammalian ovary development is formed in large part from developmental time courses established using murine models. Genomic tools where genes implicated in the PGC designation or gonadal sex determination have been modulated through complete or conditional knockouts in vivo, and studies in in situ models with inhibitors or cultures that alter the native gonadal environment have pieced together the interplay of pioneering transcription factors, co-regulators and chromosomes critical for the progression of PGCs to oocytes. Tools such as pluripotent stem cell derivation, genomic modifications, and aggregate differentiation cultures have yielded some insight into the human condition. Additional understanding of sex determination, both gonadal and anatomical, may be inferred from phenotypes that arise from de novo or inherited gene variants in humans who have differences in sex development.

KEY MESSAGES

This review highlights major factors critical for PGC specification and migration, and in ovarian gonad specification by reviewing seminal murine models. These pathways are compared to what is known about the human condition from expression profiles of fetal gonadal tissue, use of human pluripotent stem cells, or disorders resulting from disease variants. Many of these pathways are challenging to decipher in human tissues. However, the impact of new single-cell technologies and whole-genome sequencing to reveal disease variants of idiopathic reproductive tract phenotypes will help elucidate the mechanisms involved in human ovary development.

Regulation of Oocyte Apoptosis: A View from Gene Knockout Mice

Apoptosis is a form of programmed cell death that plays a critical role in cellular homeostasis and development, including in the ovarian reserve. In humans, hundreds of thousands of oocytes are produced in the fetal ovary. However, the majority die by apoptosis before birth. After puberty, primordial follicles develop into mature follicles. While only a large dominant follicle is selected to ovulate, smaller ones undergo apoptosis. Despite numerous studies, the mechanism of oocyte death at the molecular level remains elusive. Over the last two and a half decades, many knockout mouse models disrupting key genes in the apoptosis pathway have been generated. In this review, we highlight some of the phenotypes and discuss distinct and overlapping roles of the apoptosis regulators in oocyte death and survival. We also review how the transcription factor p63 and its family members may trigger oocyte apoptosis in response to DNA damage.

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

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

Reorganization, specialization, and degradation of oocyte maternal components for early development

Background

Oocyte components are maternally provided, solely determine oocyte quality, and coordinately determine embryo quality with zygotic gene expression. During oocyte maturation, maternal organelles are drastically reorganized and specialized to support oocyte characteristics. A large number of maternal components are actively degraded after fertilization and gradually replaced by zygotic gene products. The molecular basis and the significance of these processes on oocyte/embryo quality are not fully understood.

Methods

Firstly, recent findings in organelle characteristics of other cells or oocytes from model organisms are introduced for further understanding of oocyte organelle reorganization/specialization. Secondly, recent progress in studies on maternal components degradation and their molecular mechanisms are introduced. Finally, future applications of these advancements for predicting mammalian oocyte/embryo quality are discussed.

Main findings

The significance of cellular surface protein degradation via endocytosis for embryonic development, and involvement of biogenesis of lipid droplets in embryonic quality, were recently reported using mammalian model organisms.

Conclusion

Identifying key oocyte component characteristics and understanding their dynamics may lead to new applications in oocyte/embryo quality prediction and improvement. To implement these multidimensional concepts, development of new technical approaches that allow us to address the complexity and efficient studies using model organisms are required.

Metzincin metalloproteases in PGC migration and gonadal sex conversion

Development of a functional gonad includes migration of primordial germ cells (PGCs), differentiations of somatic and germ cells, formation of primary follicles or spermatogenic cysts with somatic gonadal cells, development and maturation of gametes, and subsequent releasing of mature germ cells. These processes require extensive cellular and tissue remodeling, as well as broad alterations of the surrounding extracellular matrix (ECM). Metalloproteases, including MMPs (matrix metalloproteases), ADAMs (a disintegrin and metalloproteinases), and ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs), are suggested to have critical roles in the remodeling of the ECM during gonad development. However, few research articles and reviews are available on the functions and mechanisms of metalloproteases in remodeling gonadal ECM, gonadal development, or gonadal differentiation. Moreover, most studies focused on the roles of transcription and growth factors in early gonad development and primary sex determination, leaving a significant knowledge gap on how differentially expressed metalloproteases exert effects on the ECM, cell migration, development, and survival of germ cells during the development and differentiation of ovaries or testes. We will review gonad development with focus on the evidence of metalloprotease involvements, and with an emphasis on zebrafish as a model for studying gonadal sex differentiation and metalloprotease functions.

Genetic control of meiosis surveillance mechanisms in mammals

Meiosis is a specialized cell division that generates haploid gametes and is critical for successful sexual reproduction. During the extended meiotic prophase I, homologous chromosomes progressively pair, synapse and desynapse. These chromosomal dynamics are tightly integrated with meiotic recombination (MR), during which programmed DNA double-strand breaks (DSBs) are formed and subsequently repaired. Consequently, parental chromosome arms reciprocally exchange, ultimately ensuring accurate homolog segregation and genetic diversity in the offspring. Surveillance mechanisms carefully monitor the MR and homologous chromosome synapsis during meiotic prophase I to avoid producing aberrant chromosomes and defective gametes. Errors in these critical processes would lead to aneuploidy and/or genetic instability. Studies of mutation in mouse models, coupled with advances in genomic technologies, lead us to more clearly understand how meiosis is controlled and how meiotic errors are linked to mammalian infertility. Here, we review the genetic regulations of these major meiotic events in mice and highlight our current understanding of their surveillance mechanisms. Furthermore, we summarize meiotic prophase genes, the mutations that activate the surveillance system leading to meiotic prophase arrest in mouse models, and their corresponding genetic variants identified in human infertile patients. Finally, we discuss their value for the diagnosis of causes of meiosis-based infertility in humans.

Postnatal developmental expression and localization of apelin and apelin receptor protein in the ovary and uterus of mice

Postnatal ovarian and uterine development is crucial to accomplished female fertility. Thus, the investigations of factors that present in pre-pubertal stages are important as it might be responsible for the regulation of ovarian and uterine function. Apelin, an adipokine and its receptor (APJ) are present in female reproductive organs. However, no study has reported its postnatal expression in uterus and ovary. Thus, we investigated the postnatal developmental changes in expression and localization of apelin and APJ in the ovary and uterus of mice. Postnatal ovary and uterus were collected from postnatal day (PND) 1, 7, 14, 21, 42, 65 and performed western blot analysis and immunohistochemistry. Uterine APJ is elevated in PND14 and PND65, whereas, ovarian APJ elevated in PND7, PND14, and PND65. Apelin expression in both ovary and uterus showed intense staining at PND65 and PND14. Our results showed that apelin and APJ abundance was lower at PND21 in uterus and ovary. In conclusion, apelin and APJ are developmentally regulated in the ovary and uterus, and its localization in the different compartments of ovary and uterus suggest its distribution specific physiological role in the uterus and ovary.

Tracking the dynamics of female germ cell development during peri-hatch periods using a gene-edited chicken model

In hens, egg production depends on the development of germ cells in the ovary. Germ cells are established before birth, and their number gradually decreases during their lifespan. Therefore, it is essential to determine the time points of massive germ cell loss and the underlying mechanism. In this study, a gene-edited chicken with mCherry fluorescence specifically expressed in the germline was generated by the integration of the mCherry gene into the 3'-end of the DAZL locus, which facilitated the isolation of germ cells from the gonads of DAZL-mCherry embryos or chicks and quantification using flow cytometry based on the observation of red fluorescence. The results demonstrated the dynamics of germ cell development from embryos at 17 d of hatching (dh) to chickens at 7 d post-hatch (dph) and revealed a substantial loss of germ cells in the late embryonic stage (18 -19 dh) and post-hatch period (2 -3 dph). Additionally, the number of germ cells in DAZL × Guangxi Ma chicken was significantly higher than that in DAZL × Lohmann Pink chicken at 19 dh and 3 dph (P < 0.05). Furthermore, the numbers of germ cells positively correlated with the body weight in DAZL × Lohmann Pink chicken. In conclusion, our results showed the dynamics of germ cell development in chicken ovaries during peri-hatch periods and indicated the time point of substantial germ cell loss. The results provide evidence for further exploration of the underlying mechanism and serve as a reference for chicken breeding and management.

Ovarian Reserve Disorders, Can We Prevent Them? A Review

The ovarian reserve is finite and begins declining from its peak at mid-gestation until only residual follicles remain as women approach menopause. Reduced ovarian reserve, or its extreme form, premature ovarian insufficiency, stems from multiple factors, including developmental, genetic, environmental exposures, autoimmune disease, or medical/surgical treatment. In many cases, the cause remains unknown and resulting infertility is not ultimately addressed by assisted reproductive technologies. Deciphering the mechanisms that underlie disorders of ovarian reserve could improve the outcomes for patients struggling with infertility, but these disorders are diverse and can be categorized in multiple ways. In this review, we will explore the topic from a perspective that emphasizes the prevention or mitigation of ovarian damage. The most desirable mode of fertoprotection is primary prevention (intervening before ablative influence occurs), as identifying toxic influences and deciphering the mechanisms by which they exert their effect can reduce or eliminate exposure and damage. Secondary prevention in the form of screening is not recommended broadly. Nevertheless, in some instances where a known genetic background exists in discrete families, screening is advised. As part of prenatal care, screening panels include some genetic diseases that can lead to infertility or subfertility. In these patients, early diagnosis could enable fertility preservation or changes in family-building plans. Finally, Tertiary Prevention (managing disease post-diagnosis) is critical. Reduced ovarian reserve has a major influence on physiology beyond fertility, including delayed/absent puberty or premature menopause. In these instances, proper diagnosis and medical therapy can reduce adverse effects. Here, we elaborate on these modes of prevention as well as proposed mechanisms that underlie ovarian reserve disorders.

Reconstitution of reproductive organ system that produces functional oocytes

Reproductive organs have unique developmental and functional properties that enable them to manage both germ cell development and the endocrine system in a sex-dependent manner. Proper reconstitution of the reproductive organs, therefore, will contribute to a deeper understanding of the mechanisms underlying germ cell development and sex-determination. However, reproductive organs have not yet been systematically reconstituted from pluripotent stem cells. This is largely due to technical problems in the reconstitution of the germ cell and somatic cell lineages, which have very different developmental trajectories. Accordingly, faithful construction of reproductive organoids requires that the reconstitution and evaluation of these two different cell lineages be performed separately. Here, we update the state-of-the-art in the reconstitution of reproductive organoids that produce functional oocytes.

Tamoxifen Activates Dormant Primordial Follicles in Mouse Ovaries

Our previous study found that 17β-estradiol (E2) suppresses primordial follicle activation and growth in cultured mouse ovaries. In this study, we administered tamoxifen, an estrogen receptor antagonist, into the abdominal cavity of mice to clarify the relationship between primordial follicle activation and the physiological concentration of E2 in mouse ovaries. The results showed that tamoxifen promoted primordial follicle activation. Administration of tamoxifen promoted degradation of the extracellular matrix surrounding primordial follicles in the ovaries. Furthermore, tamoxifen decreased the expression of stefin A, an inhibitor of cathepsins that digest some proteins and extracellular matrix, in the ovaries. Mechanical stress produced by the extracellular matrix reportedly suppresses the activation of primordial follicles. The collective results show that tamoxifen can promote primordial follicle activation through the degradation of the extracellular matrix surrounding primordial follicles. Our results indicate that E2 suppresses primordial follicle activation in vivo and that tamoxifen may be useful as a therapeutic agent against infertility.

Uncoupling cell division and cytokinesis during germline development in metazoans

The canonical eukaryotic cell cycle ends with cytokinesis, which physically divides the mother cell in two and allows the cycle to resume in the newly individualized daughter cells. However, during germline development in nearly all metazoans, dividing germ cells undergo incomplete cytokinesis and germ cells stay connected by intercellular bridges which allow the exchange of cytoplasm and organelles between cells. The near ubiquity of incomplete cytokinesis in animal germ lines suggests that this is an ancient feature that is fundamental for the development and function of this tissue. While cytokinesis has been studied for several decades, the mechanisms that enable regulated incomplete cytokinesis in germ cells are only beginning to emerge. Here we review the current knowledge on the regulation of germ cell intercellular bridge formation, focusing on findings made using mouse, Drosophila melanogaster and Caenorhabditis elegans as experimental systems.

Gonadal Sex Differentiation and Ovarian Organogenesis along the Cortical-Medullary Axis in Mammals

In most mammals, the sex of the gonads is based on the fate of the supporting cell lineages, which arises from the proliferation of coelomic epithelium (CE) that surfaces on the bipotential genital ridge in both XY and XX embryos. Recent genetic studies and single-cell transcriptome analyses in mice have revealed the cellular and molecular events in the two-wave proliferation of the CE that produce the supporting cells. This proliferation contributes to the formation of the primary sex cords in the medullary region of both the testis and the ovary at the early phase of gonadal sex differentiation, as well as to that of the secondary sex cords in the cortical region of the ovary at the perinatal stage. To support gametogenesis, the testis forms seminiferous tubules in the medullary region, whereas the ovary forms follicles mainly in the cortical region. The medullary region in the ovary exhibits morphological and functional diversity among mammalian species that ranges from ovary-like to testis-like characteristics. This review focuses on the mechanism of gonadal sex differentiation along the cortical-medullary axis and compares the features of the cortical and medullary regions of the ovary in mammalian species.

Perfluorooctanoic acid (PFOA) promotes follicular growth and alters expression of genes that regulate the cell cycle and the Hippo pathway in cultured neonatal mouse ovaries

Perfluorooctanoic acid (PFOA) is a synthetic chemical resistant to biodegradation and is environmentally persistent. PFOA is found in many consumer products and is a major source of water contamination. While PFOA has been identified as a contaminant of concern for reproductive health, little is known about the effects of PFOA on ovarian follicular development and growth. Recent evidence indicates that the Hippo pathway is an important regulator of ovarian physiology. Here, we investigated the effects of PFOA on ovarian folliculogenesis during the neonatal period of development and potential impacts on the Hippo signaling pathway. Post-natal day 4 (PND4) neonatal ovaries from CD-1 mice were cultured with control medium (DMSO <0.01% final concentration) or PFOA (50 μM or 100 μM). After 96 h, ovaries were collected for histological analysis of folliculogenesis, gene and protein expression, and immunostaining. Results revealed that PFOA (50 μM) increased the number of secondary follicles, which was accompanied by increases in mRNA transcripts and protein of marker of proliferation marker Ki67 with no impacts on apoptosis markers Bax, Bcl2, or cleaved caspase-3. PFOA treatment (50 μM and 100 μM) stimulated an upregulation of transcripts for cell cycle regulators Ccna2, Ccnb2, Ccne1, Ccnd1, Ccnd2, and Ccnd3. PFOA also increased abundance of transcripts of Hippo pathway components Mst1/2, Lats1, Mob1b, Yap1, and Taz, as well as downstream Hippo pathway targets Areg, Amotl2, and Cyr61, although it decreased transcripts for anti-apoptotic Birc5. Inhibition of the Hippo pathway effector YAP1 with Verteporfin resulted in the attenuation of PFOA-induced follicular growth and proliferation. Together, these findings suggest that occupationally relevant levels of PFOA (50 μM) can stimulate follicular activation in neonatal ovaries potentially through activation of the Hippo pathway.