Development of mouse and rat oocytes in chimeric reaggregated ovaries after interspecific exchange of somatic and germ cell components

In vitro production of viable eggs from undeveloped oocytes in mouse preantral follicles by reconstructing granulosa cell-oocyte complexes†

In vitro culture of ungrown oocytes in preantral follicles is one of the intriguing subjects being pursued to produce viable eggs in assisted reproductive technology. Previous studies have succeeded in obtaining mature eggs after in vitro culture of preantral follicles, while denuded undeveloped oocytes, which are obtained occasionally when collecting preantral follicles, seem to be almost useless. Moreover, methods to culture them efficiently to produce viable eggs have not been established yet. The present study was conducted to demonstrate in vitro culture of mouse denuded undeveloped oocytes by reconstructing granulosa cell-oocyte complexes, and to analyze cellular communication in reconstructed granulosa cell-oocyte complexes. Single denuded undeveloped oocytes were aggregated with 1 × 104 granulosa cells in wells with U-shaped bottoms in a low-binding cell culture plate for 8 days under either 20% or 5% O2, and then the reconstructed granulosa cell-oocyte complexes formed were cultured on a collagen-coated culture membrane insert for 4 days under 5% O2. At day 8 of culture, the rates of reconstructed granulosa cell-oocyte complexes formation were significantly higher in the culture group under 5% O2 (64.9%) than that under 20% O2 (42.3%; P < 0.001); furthermore, the formation of transzonal projections was observed. After maturation and fertilization, we produced matured eggs and blastocysts at higher rates (>90% and 61.9%, respectively) in the group cultured under 5% O2. After transferring 126 two- to four-cell stage embryos, six live pups were obtained. This is the first report that demonstrates production of viable eggs after in vitro culture of denuded undeveloped oocytes from preantral follicles by reconstruction of granulosa cell-oocyte complexes.

Innovations in poultry reproduction using cryopreserved avian germ cells

Meat and eggs from chicken are the major source of animal protein for the human population. The cryopreservation of poultry species is needed to guarantee sustainable production. Here, we describe the existing cryopreservation technologies for avian reproductive cells using embryonic germ cells, spermatozoa and ovarian tissues. We outline strategies to reconstitute chicken breeds from their cryopreserved embryonic germ cells using surrogate hosts and discuss the perspectives for genetic conservation and reconstitution of chicken and wild avian species using surrogate host animals.

Chronological and reproductive aging-associated changes in resistance to oxidative stress in post-reproductive female mice

Effort toward reproduction is often thought to negatively influence health and survival. Reproduction has been shown to influence metabolism, but the pathways and mechanisms have yet to be thoroughly elucidated. In the current experiments, our aim was to dissect the role of young and old ovarian tissues in the response to oxidative stress, through changes in liver oxidative stress response proteins. Liver proteins were analyzed in control mice at 4, 13, and 27 months of age and compared to 23-month-old mice which received young ovarian tissue transplants (intact or follicle-depleted) at 13 months of age. In control mice, of the 29 oxidative stress response proteins measured, 31% of the proteins decreased, 52% increased, and 17% were unchanged from 13 to 27 months. The greatest changes were seen during the period of reproductive failure, from 4 to 13 months of age. In transplanted mice, far more proteins were decreased from 13 to 23 months (93% in follicle-containing young ovary recipients; 62% in follicle-depleted young ovary recipients). Neither transplant group reflected changes seen in control mice between 13 and 27 months. Estradiol levels in transplant recipient mice were not increased compared with age-matched control mice. The current results suggest the presence of a germ cell- and estradiol-independent ovarian influence on aging-associated changes in the response to oxidative stress, which is manifest differently in reproductive-aged adults and post-reproductive-aged mice. The results presented here separate chronological and ovarian aging and the influence of estradiol in the response to aging-associated oxidative stress and support a novel, estradiol-independent role for the ovary in female health and survival.

Reconstruction of ovarian follicular-like structure by recellularization of a cell-free human ovarian scaffold with mouse fetal ovarian cells

The present study assessed the supportive roles of the decellularized human ovarian tissue in homing of mouse fetal ovarian cells into the scaffold as well as the formation of the follicular-like structure. The human ovarian cortical tissues were decellularized by three freeze-thaw cycles and then, treated with Triton X-100 for 15 h and 0.5% sodium dodecyl sulfate for 72 h. After isolation and preparation of mouse fetal ovarian cells (19 dpc) they were seeded into the decellularized scaffolds and cultured for 7 days, then using a light microscope, laser confocal scanning microscope, and scanning electron microscope these scaffolds were studied. Analysis of gene expression related to oocyte and follicular cells such as Ddx4, Nobox, Gdf9, and Connexin37 was assessed by real-time RT-PCR and the DDX4 and GDF9 proteins were detected by immunohistochemistry. The result showed that the human ovarian tissue was decellularized properly and the tissue elements and integrity were well preserved. After 7 days of in vitro culture, the fetal ovarian cells attached and penetrated into different sites and depths of the scaffold. The formed organoid within the scaffold showed large round, small polyhedral, and elongated spindle cells similar to the follicle structure. The molecular analysis and immunohistochemistry were confirmed an increase in the expression of genes and proteins related to oocyte and follicular cells in these reconstructed structures. In conclusion, the recellularization of human ovarian scaffolds by mouse fetal ovarian cells could support the follicular-like structure formation and it provides an in vitro model for follicle reconstitution and offers an alternative approach for clinical usage.

Analyzing the cellular and molecular atlas of ovarian mesenchymal cells provides a strategy against female reproductive aging

Ovarian mesenchymal cells (oMCs) constitute a distinct microenvironment that supports folliculogenesis under physiological conditions. Supplementation of exogenous non-ovarian mesenchymal-related cells has been reported to be an efficient approach to improve ovarian functions. However, the development and cellular and molecular characteristics of endogenous oMCs remain largely unexplored. In this study, we surveyed the single-cell transcriptomic landscape to dissect the cellular and molecular changes associated with the aging of oMCs in mice. Our results showed that the oMCs were composed of five ovarian differentiated MC (odMC) populations and one ovarian mesenchymal progenitor (oMP) cell population. These cells could differentiate into various odMCs via an oMP-derived route to construct the ovarian stroma structures. Comparative analysis revealed that ovarian aging was associated with decreased quantity of oMP cells and reduced quality of odMCs. Based on the findings of bioinformatics analysis, we designed different strategies involving supplementation with young oMCs to examine their effects on female fertility and health. Our functional investigations revealed that oMCs supplementation prior to ovarian senescence was the optimal method to improve female fertility and extend the reproductive lifespan of aged females in the long-term.

Rescue of follicle development after oocyte-induced ovary dysfunction and infertility in a model of POI

The mechanisms and aetiology underlying the development of premature ovarian insufficiency (POI) are poorly understood. However, the oocyte clearly has a role as demonstrated by the Double Mutant (DM) mouse model where ovarian dysfunction (6 weeks) is followed by POI (3 months) due to oocyte-specific deletion of complex and hybrid N- and O-glycans. The ovaries of DM mice contain more primary follicles (3a stage) accompanied by fewer developing follicles, indicating a block in follicle development. To investigate this block, we first analysed early follicle development in postnatal (8-day), pre-pubertal (3-week) and post-pubertal (6-week and 3-month) DM (C1galt1 F/F Mgat1 F/F:ZP3Cre) and Control (C1galt1 F/F Mgat1 F/F) mice. Second, we investigated if transplantation of DM ovaries into a "normal" endocrine environment would restore follicle development. Third, we determined if replacing DM ovarian somatic cells would rescue development of DM oocytes. At 3-week, DM primary 3a follicles contain large oocytes accompanied by early development of a second GC layer and increased GC proliferation. At 6-week, DM primary 3a follicles contain abnormally large oocytes, accompanied with decreased GC proliferation. Transplantation of DM ovaries into a 'normal' endocrine environment did not restore normal follicle development. However, replacing somatic cells by generating reaggregated ovaries (ROs) did enable follicle development to progress and thus highlighted intra-ovarian factors were responsible for the onset of POI in DM females. Thus, these studies demonstrate oocyte-initiated altered communication between GCs and oocytes results in abnormal primary follicles which fail to progress and leads to POI.

Thyroid hormone triiodothyronine does not protect ovarian reserve from DNA damage induced by X-ray and cisplatin

PURPOSE

Cancer therapy can induce premature ovarian insufficiency, necessitating methods for preserving fertility in female cancer patients. However, the only accepted clinical practice for doing so is cryopreservation of embryos, unfertilized ova, and ovarian tissue, despite potential options such as in vitro maturation of follicles. Therefore, considerable interest has arisen in fertoprotective agents, with research on rat ovarian granulosa cells suggesting that triiodothyronine (T3) regulates an anti-apoptosis mechanism that protects the ovarian reserve from paclitaxel-induced DNA damage. In this study, we used postnatal day 5 mouse ovary to confirm the existence of T3 thyroid hormone receptor (THR), as well as to investigate the potential protective effects of T3 against cisplatin- and X-ray-induced apoptosis. We also tested the potential anti-apoptotic effect of T3 in the breast cancer cell line MDA-MB-231.

METHODS

We treated cultured mouse ovaries with varying concentration of T3 and 4 μM cisplatin and 0.2 Gy X-ray. Real-time PCR, histological analysis, immunoblot analysis, and immunofluorescence were performed to assess the potential anti-apoptotic effects of T3.

RESULTS

We confirmed that THR alpha and beta are expressed in the mouse ovary. T3 (0.1, 1, 10, 100 nM, and 1 µM) does not protect ovarian reserve from cisplatin- or X-ray-induced apoptosis or DNA damage. Similarly, it does not protect mouse granulosa cells and MDA-MB-231 cells from cisplatin- or X-ray-induced apoptosis.

CONCLUSION

Our findings suggest that T3 is ineffective as a fertoprotective agent, and its candidacy as a potential agent to preserve fertility should be reconsidered.

Zinc dynamics regulate early ovarian follicle development

Zinc fluctuations regulate key steps in late oocyte and preimplantation embryo development; however, roles for zinc in preceding stages in early ovarian follicle development, when cooperative interactions exist between the oocyte and somatic cells, are unknown. To understand the roles of zinc during early follicle development, we applied single cell X-ray fluorescence microscopy, a radioactive zinc tracer, and a labile zinc probe to measure zinc in individual mouse oocytes and associated somatic cells within early follicles. Here, we report a significant stage-specific increase and compartmental redistribution in oocyte zinc content upon the initiation of early follicle growth. The increase in zinc correlates with the increased expression of specific zinc transporters, including two that are essential in oocyte maturation. While oocytes in follicles exhibit high tolerance to pronounced changes in zinc availability, somatic survival and proliferation are significantly more sensitive to zinc chelation or supplementation. Finally, transcriptomic, proteomic, and zinc loading analyses reveal enrichment of zinc targets in the ubiquitination pathway. Overall, these results demonstrate that distinct cell type-specific zinc regulations are required for follicle growth and indicate that physiological fluctuation in the localization and availability of this inorganic cofactor has fundamental functions in early gamete development.

Aging-associated changes in motor function are ovarian somatic tissue-dependent, but germ cell and estradiol independent in post-reproductive female mice exposed to young ovarian tissue

A critical mediator of evolution is natural selection, which operates by the divergent reproductive success of individuals and results in conformity of an organism with its environment. Reproductive function has evolved to support germline transmission. In mammalian ovaries, this requires healthy, active gonad function, and follicle development. However, healthy follicles do not contribute to germline transmission in a dead animal. Therefore, support of the health and survival of the organism, in addition to fertility, must be considered as an integral part of reproductive function. Reproductive and chronological aging both impose a burden on health and increase disease rates. Tremors are a common movement disorder and are often correlated with increasing age. Muscle quality is diminished with age and these declines are gender-specific and are influenced by menopause. In the current experiments, we evaluated aging-associated and reproduction-influenced changes in motor function, utilizing changes in tremor amplitude and grip strength. Tremor amplitude was increased with aging in normal female mice. This increase in tremor amplitude was prevented in aged female mice that received ovarian tissue transplants, both in mice that received germ cell-containing or germ cell-depleted ovarian tissue. Grip strength was decreased with aging in normal female mice. This decrease in grip strength was prevented in aged female mice that received either germ cell-containing or germ cell-depleted tissue transplants. As expected, estradiol levels decreased with aging in normal female mice. Estradiol levels did not change with exposure to young ovarian tissues/cells. Surprisingly, estradiol levels were not increased in aged females that received ovaries from actively cycling, young donors. Overall, tremor amplitude and grip strength were negatively influenced by aging and positively influenced by exposure to young ovarian tissues/cells in aged female mice, and this positive influence was independent of ovarian germ cells and estradiol levels. These findings provide a strong incentive for further investigation of the influence of ovarian somatic tissue on health. In addition, changes in tremor amplitude may serve as an additional marker of biological age.

Comparative analysis of vertebrates reveals that mouse primordial oocytes do not contain a Balbiani body

Oocytes spend the majority of their lifetime in a primordial state. The cellular and molecular biology of primordial oocytes is largely unexplored; yet, studying these is necessary to understand the mechanisms through which oocytes maintain cellular fitness for decades, and why they eventually fail with age. Here, we develop enabling methods for live-imaging based comparative characterization of Xenopus, mouse and human primordial oocytes. We show that primordial oocytes in all three vertebrate species contain active mitochondria, Golgi apparatus and lysosomes. We further demonstrate that human and Xenopus oocytes have a Balbiani body characterized by a dense accumulation of mitochondria in their cytoplasm. However, despite previous reports, we did not find a Balbiani body in mouse oocytes. Instead, we demonstrate what was previously used as a marker for the Balbiani body in mouse primordial oocytes is in fact a ring-shaped Golgi apparatus that is not functionally associated with oocyte dormancy. Our work provides the first insights into the organisation of the cytoplasm in mammalian primordial oocytes, and clarifies relative advantages and limitations of choosing different model organisms for studying oocyte dormancy.

Reversing complete mechanical transzonal projections disruption during mouse in vitro follicle culture with unaltered oocyte competence†

In vitro oocyte growth is widely studied as an alternative fertility preservation approach. Several animal models are used to generate extensive information on this complex process regulated by the constant and dynamic interaction between the oocyte and its somatic compartment throughout follicle growth and maturation. A two-dimensional attachment mouse secondary follicle culture system was used to assess the oocyte's capacity to overcome disconnection from its somatic companions at different developmental stages for final competence acquisition. To test this, complete mechanical denudation of oocytes from preantral (PA) and early antral (EA) follicles was performed. Established endpoints were the oocyte's potential to reconnect with somatic cells and the impact of connectivity disruption on mature oocyte quality. This study proves that oocytes from PA and EA cultured mouse follicles can overcome complete denudation, restoring likely functional transzonal projections with no significant differences in meiotic and developmental competence compared with those from intact cultured follicles. These novel findings constitute good premises for developing successful strategies to rescue human oocyte competence in the context of in vitro culture approaches such as nonhuman chorionic gonadotropin triggered in vitro maturation.

Strategies to Identify Genetic Variants Causing Infertility

Genetic causes are thought to underlie about half of infertility cases, but understanding the genetic bases has been a major challenge. Modern genomics tools allow more sophisticated exploration of genetic causes of infertility through population, family-based, and individual studies. Nevertheless, potential therapies based on genetic diagnostics will be limited until there is certainty regarding the causality of genetic variants identified in an individual. Genome modulation and editing technologies have revolutionized our ability to functionally test such variants, and also provide a potential means for clinical correction of infertility variants. This review addresses strategies being used to identify causative variants of infertility.

The effect of different concentrations of cerium oxide during pregnancy on ovarian follicle development in neonatal mice

OBJECTIVES

Cerium is a member of the rare metals group and widely used in drug delivery, gene therapy, molecular imaging and medicine. In this study, we investigated the effect of different doses of Cerium (IV) oxide (CeO₂ ) during pregnancy on neonatal mice ovaries, as well as its effect on blood biochemical parameters.

METHODS

Thirty pregnant NMRI mice were divided into five groups: Control and 4 groups treated with CeO₂ (10, 25, 80, 250 mg/kg.bw i.p) at the GD7 and GD14. The ovarian histological of neonatal (2 and 6 day-olds), as well as blood serum of neonates at 15-dpp were analyzed.

RESULTS

Count of ovarian primordial follicles in neonates at 2 dpp showed a significant decrease in the groups treated with 80 and 250 mg/kg.bw doses of CeO₂ . There was also a significant decrease in ovarian primordial and primary follicles in neonates at 6-dpp at 250 mg/kg.bw doses of CeO₂ in the control (P < 0.05). There was no significant difference in serum levels of malondialdehyde and total antioxidant capacity between the experimental and control groups.

CONCLUSIONS

Our results suggest that the effects of CeO₂ on the ovarian tissue of neonatal mice during pregnancy may be dose-dependent.

Interaction between oocytes, cortical germ cells and granulosa cells of the mouse and bat, following the dissociation-re-aggregation of adult ovaries

It is widely accepted that the oocyte plays a very active role in promoting the growth of the follicle by directing the differentiation of granulosa cells and secreting paracrine growth factors. In turn, granulosa cells regulate the development of the oocytes, establishing close bidirectional communication between germ and somatic cells. The presence of cortical cells with morphological characteristics, similar to primordial germ cells that express specific germline markers, stem cells and cell proliferation, known as adult cortical germ cells (ACGC) have been reported in phyllostomid bats. Using magnetic cell separation techniques, dissociation-cellular re-aggregation and organ culture, the behaviour of oocytes and ACGC was analyzed by interacting in vitro with mouse ovarian cells. Bat ACGC was mixed with disaggregated ovaries from a transgenic mouse that expressed green fluorescent protein. The in vitro reconstruction of the re-aggregates was evaluated. We examined the viability, integration, cellular interaction and ovarian morphogenesis by detecting the expression of Vasa, pH3, Cx43 and Laminin. Our results showed that the interaction between ovarian cells is carried out in the adult ovary of two species, without them losing their capacity to form follicular structures, even after having been enzymatically dissociated.

Meiosis: the chromosomal foundation of reproduction

Meiosis is the chromosomal foundation of reproduction, with errors in this important process leading to aneuploidy and/or infertility. In this review celebrating the 50th anniversary of the founding of the Society for the Study of Reproduction, the important chromosomal structures and dynamics contributing to genomic integrity across generations are highlighted. Critical unsolved biological problems are identified, and the advances that will lead to their ultimate resolution are predicted.

In vitro and in vivo mouse follicle development in ovaries and reaggregated ovaries

Follicle development requires complex and coordinated interactions between both the oocyte and its associated somatic cells. In ovarian dysfunction, follicle development may be abnormal due to defective somatic cell function; for example, premature ovarian insufficiency or malignancies. Replacing defective somatic cells, using the reaggregated ovary (RO) technique, may 'rescue' follicle development. ROs containing mature follicles have been generated when transplanted to a host mouse to develop. We have developed a RO culture technique and the aims were to determine how follicle development differed between transplanted and cultured ROs, and the influence of ovarian age (P2 vs P6). Mouse ROs were cultured for 14 days; P2 and P6 ovaries cultured as Controls. Follicle development was compared to ROs transplanted for 14 days and ovaries from P16 and P20 mice. ROs generated from either P2 or P6 exhibited similar follicle development in culture whereas in vivo follicle development was more advanced in P6 ROs. Follicles were more developed in cultured ROs than transplanted ROs. However, follicles in cultured ROs and ovaries had smaller oocytes with fewer theca and granulosa cells than in vivo counterparts. Our results demonstrate the fluidity of follicle development despite ovary dissociation and that environment is more important to basal lamina formation and theca cell development. Furthermore, follicle development within cultured ROs appears to be independent of oocyte nest breakdown and primordial follicle formation in source ovaries. Our results highlight the need for understanding follicle development in vitro, particularly in the development of the RO technique as a potential fertility treatment.

Mammalian Oocytes Locally Remodel Follicular Architecture to Provide the Foundation for Germline-Soma Communication

Germ cells develop in a microenvironment created by the somatic cells of the gonad [1-3]. Although in males, the germ and somatic support cells lie in direct contact, in females, a thick extracellular coat surrounds the oocyte, physically separating it from the somatic follicle cells [4]. To bypass this barrier to communication, narrow cytoplasmic extensions of the follicle cells traverse the extracellular coat to reach the oocyte plasma membrane [5-9]. These delicate structures provide the sole platform for the contact-mediated communication between the oocyte and its follicular environment that is indispensable for production of a fertilizable egg [8, 10-15]. Identifying the mechanisms underlying their formation should uncover conserved regulators of fertility. We show here in mice that these structures, termed transzonal projections (TZPs), are specialized filopodia whose number amplifies enormously as oocytes grow, enabling increased germ-soma communication. By creating chimeric complexes of genetically tagged oocytes and follicle cells, we demonstrate that follicle cells elaborate new TZPs that push through the extracellular coat to reach the oocyte surface. We further show that growth-differentiation factor 9, produced by the oocyte, drives the formation of new TZPs, uncovering a key yet unanticipated role for the germ cell in building these essential bridges of communication. Moreover, TZP number and germline-soma communication are strikingly reduced in reproductively aged females. Thus, the growing oocyte locally remodels follicular architecture to ensure that its developmental needs are met, and an inability of somatic follicle cells to respond appropriately to oocyte-derived cues may contribute to human infertility.

The miR-125 family is an important regulator of the expression and maintenance of maternal effect genes during preimplantational embryo development

Previously, we reported that Sebox is a new maternal effect gene (MEG) that is required for early embryo development beyond the two-cell (2C) stage because this gene orchestrates the expression of important genes for zygotic genome activation (ZGA). However, regulators of Sebox expression remain unknown. Therefore, the objectives of the present study were to use bioinformatics tools to identify such regulatory microRNAs (miRNAs) and to determine the effects of the identified miRNAs on Sebox expression. Using computational algorithms, we identified a motif within the 3′UTR of Sebox mRNA that is specific to the seed region of the miR-125 family, which includes miR-125a-5p, miR-125b-5p and miR-351-5p. During our search for miRNAs, we found that the Lin28a 3′UTR also contains the same binding motif for the seed region of the miR-125 family. In addition, we confirmed that Lin28a also plays a role as a MEG and affects ZGA at the 2C stage, without affecting oocyte maturation or fertilization. Thus, we provide the first report indicating that the miR-125 family plays a crucial role in regulating MEGs related to the 2C block and in regulating ZGA through methods such as affecting Sebox and Lin28a in oocytes and embryos.

Oocyte stem cells: fact or fantasy?

For many decades, the dogma prevailed that female mammals had a finite pool of oocytes at birth and this was gradually exhausted during a lifetime of reproductive function. However, in 2004, a new era began in the field of female oogenesis. A study was published that appeared to detect oocyte-stem cells capable of generating new eggs within mouse ovaries. This study was highly controversial and the years since this initial finding have produced extensive research and even more extensive debate into their possibility. Unequivocal evidence testifying to the existence of oocyte-stem cells (OSCs) has yet to be produced, meanwhile the spectrum of views from both sides of the debate are wide-ranging and surprisingly passionate. Although recent studies have presented some convincing results that germ cells exist and are capable of creating new oocytes, many questions remain. Are these cells present in humans? Do they exist in physiological conditions in a dormant state? This comprehensive review first examines where and how the dogma of a finite pool was established, how this has been challenged over the years and addresses the most pertinent questions as to the current status of their existence, their role in female fertility, and perhaps most importantly, if they do exist, how can we harness these cells to improve a woman's oocyte reserve and treat conditions such as premature ovarian insufficiency (POI: also known as premature ovarian failure, POF).

Gametic synapses, nanotubes and sperm RNAs - Redefining the origin of maternal determinants

The female germline cells, i.e., the oocytes/eggs, contain a subpopulation of unique organelles and molecules (RNA and proteins) collectively called "the maternal determinants" that are indispensable for the determination of cell fate in the developing embryo. Although it has been known for some time that somatic cells deliver low-molecular-weight molecules to the oocyte/egg, the paradigm has been that the larger molecules and organelles are synthesized by the female germline cells without input from the surrounding somatic cells. However, recent discoveries of novel types of intercellular connections such as gametic synapses and tunneling nanotubes, allowing the transfer of large, externally derived molecules to the oocyte/egg, may dismantle the paradigm of the transcriptional/translational self-containment of the female gamete and add novel and unexpected aspects to the origin and identity of maternal determinants. In addition, the discovery that sperm delivers various RNAs to the egg suggests that sperm may not only epigenetically modify the egg genome but also influence or modify information contained in the maternal determinants.

Ovarian stem cells: From basic to clinical applications

The field of reproductive biology has undergone significant developments in the last decade. The notion that there is a fixed reserve pool of oocytes before birth was established by Zuckerman in 1951. However, in 2004, an article published in nature challenged this central dogma of mammalian reproductive biology. Tilly’s group reported the existence of ovarian germline stem cells (GSCs) in postnatal ovaries of mice and suggested that the bone marrow could be an extragonadal source of ovarian GSCs. These findings were strongly criticized; however, several independent groups have since successfully isolated and characterized ovarian GSCs in postnatal mice. The ovarian GSCs are located in the ovarian surface epithelium and express markers of undifferentiated GSCs. When transplanted into mouse ovaries, mouse ovarian GSCs could differentiate and produce embryos and offspring. Similarly, in a recent study, ovarian GSCs were found to be present in the ovaries of women of reproductive age. Conversely, there is increasing evidence that stem cells responsible for maintaining a healthy state in normal tissue may be a source of some cancers, including ovarian cancer. Cancer stem cells (CSCs) have been found in many tissues, including ovaries. Some researchers have suggested that ovarian cancer may be a result of the transformation and dysfunction of ovarian GSCs with self-renewal properties. Drug resistant and metastasis-generating CSCs are responsible for many important problems affecting ovarian cancer patients. Therefore, the identification of CSCs will provide opportunities for the development of new therapeutic strategies for treatments for infertility and ovarian cancer. In this article, we summarize the current understanding of ovarian GSCs in adult mammals, and we also discuss whether there is a relationship between GSCs and CSCs.

Maternal age effect on mouse oocytes: new biological insight from proteomic analysis

The long-standing view of ‘immortal germline vs mortal soma’ poses a fundamental question in biology concerning how oocytes age in molecular terms. A mainstream hypothesis is that maternal ageing of oocytes has its roots in gene transcription. Investigating the proteins resulting from mRNA translation would reveal how far the levels of functionally available proteins correlate with mRNAs and would offer novel insights into the changes oocytes undergo during maternal ageing. Gene ontology (GO) semantic analysis revealed a high similarity of the detected proteome (2324 proteins) to the transcriptome (22 334 mRNAs), although not all proteins had a cognate mRNA. Concerning their dynamics, fourfold changes of abundance were more frequent in the proteome (3%) than the transcriptome (0.05%), with no correlation. Whereas proteins associated with the nucleus (e.g. structural maintenance of chromosomes and spindle-assembly checkpoints) were largely represented among those that change in oocytes during maternal ageing; proteins associated with oxidative stress/damage (e.g. superoxide dismutase) were infrequent. These quantitative alterations are either impoverishing or enriching. Using GO analysis, these alterations do not relate in any simple way to the classic signature of ageing known from somatic tissues. Given the lack of correlation, we conclude that proteome analysis of mouse oocytes may not be surrogated with transcriptome analysis. Furthermore, we conclude that the classic features of ageing may not be transposed from somatic tissues to oocytes in a one-to-one fashion. Overall, there is more to the maternal ageing of oocytes than mere cellular deterioration exemplified by the notorious increase of meiotic aneuploidy.

Recreating the female reproductive tract in vitro using iPSC technology in a linked microfluidics environment

The female reproductive tract produces hormones for reproductive function and cardiovascular, bone and sexual health; the tract supplies a finite number of gametes, and it supports fetal development. Diseases that affect each of the female reproductive tract organs, along with treatments that have direct, deleterious effects on the reproductive tract (for example, chemotherapeutics), are understudied due to the lack of model systems that phenocopy in vivo function. This review describes a path toward developing female reproductive tract mimics. The models use isolated primary support cells cultured onto a biological scaffold and within a microfluidic system to create a niche and support the desired differentiation of epithelia, germ and somatic cells from patient-derived induced pluripotent stem cells. Improving our fund of knowledge about reproductive tract biology and creating reproductive organs for patients who have lost gonadal, uterine or vaginal/ cervical function is a major step forward in women's health and an important advancement in personalized medicine.

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

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

Recovery of functional oocytes from cultured premeiotic germ cells after kidney capsule transplantation

The efficiency of in vitro culture systems for a premeiotic female germ cell is still low, mostly because of our incomplete understanding of the mechanisms controlling oogenesis and the obvious difficulties in reproducing the complex in vivo environment of such a process under in vitro conditions. Here we explored the possibility of recovering the developmental potential of mouse oocytes generated in vitro from premeiotic germ cells by transplantation under a kidney capsule of adult animals. To this aim, mouse embryonic ovaries of 12.5 days postcoitum cultured in vitro in a serum-free medium for 7 or 14 days, were transplanted beneath the kidney capsule of immunodeficient mice and analyzed after 21 (7+21 group) or 14 days (14+14 group). Cultured ovaries before transplantation showed delayed oocyte meiotic progression and follicle development. Interestingly, grafted ovaries of both groups, especially those of the 7+21 group, seemed able to restore the reproductive cycle of recipients. While the almost complete absence of primordial follicles was observed in grafted ovaries, oocytes from these ovaries showed transcript levels of genes associated to oocyte maturation similar to control. Moreover, the developmental stage of follicles and oocytes of the 7+21 group ovaries were comparable to that of 21 days post partum in vivo ovaries, whereas significant developmental delay were found in the 14+14 group ovaries. Nevertheless, oocytes retrieved from transplanted ovaries of both groups matured (around 80%) and were fertilized in vitro (around 20%-45%). Two-cell embryos from the fertilized oocytes developed to hatching blastocysts (about 50%) or gave rise to healthy live offspring (from 6% to 10%) when transplanted in a host mother. In conclusion, our results indicate that premeiotic female germ cells cultured in vitro up to primordial/primary follicle stages preserve their capability to complete oogenesis and can be fertilized and generate live pups after transplantation into a suitable in vivo environment.

Experimental evidence showing that no mitotically active female germline progenitors exist in postnatal mouse ovaries

It has been generally accepted for more than half a century that, in most mammalian species, oocytes cannot renew themselves in postnatal or adult life, and that the number of oocytes is already fixed in fetal or neonatal ovaries. This assumption, however, has been challenged over the past decade. In this study, we have taken an endogenous genetic approach to this question and generated a multiple fluorescent Rosa26(rbw/+);Ddx4-Cre germline reporter mouse model for in vivo and in vitro tracing of the development of female germline cell lineage. Through live cell imaging and de novo folliculogenesis experiments, we show that the Ddx4-expressing cells from postnatal mouse ovaries did not enter mitosis, nor did they contribute to oocytes during de novo folliculogenesis. Our results provide evidence that supports the traditional view that no postnatal follicular renewal occurs in mammals, and no mitotically active Ddx4-expressing female germline progenitors exist in postnatal mouse ovaries.

Phosphorylation of serine residues in the C-terminal cytoplasmic tail of connexin43 regulates proliferation of ovarian granulosa cells

Connexin43 (Cx43) forms gap junctions that couple the granulosa cells of ovarian follicles. In Cx43 knockout mice, follicle growth is restricted as a result of impaired granulosa cell proliferation. We have used these mice to examine the importance of specific Cx43 phosphorylation sites in follicle growth. Serines at residues 255, 262, 279, and 282 are MAP kinase substrates that, when phosphorylated, reduce junctional conductance. Mutant forms of Cx43 were constructed with these serines replaced with amino acids that cannot be phosphorylated. These mutants were transduced into Cx43 knockout ovarian somatic cells that were combined with wild-type oocytes and grafted into immunocompromised female mice permitting follicle growth in vivo. Despite residues 255 or 262 being mutated to prevent their being phosphorylated, recombinant ovaries constructed with these mutants were able to rescue the null phenotype, restoring complete folliculogenesis. In contrast, Cx43 with serine to alanine mutations at both residues 279 and 282 or at all four residues failed to rescue folliculogenesis; the mutant molecules were largely confined to intracellular sites, with few gap junctions. Using an in vitro proliferation assay, we confirmed a decrease in proliferation of granulosa cells expressing the double mutant construct. These results indicate that Cx43 phosphorylation by MAP kinase at serines 279 and 282 occurs in granulosa cells of early follicles and that this is involved in regulating follicle development.

MARF1 regulates essential oogenic processes in mice

Development of fertilization-competent oocytes depends on integrated processes controlling meiosis, cytoplasmic development, and maintenance of genomic integrity. We show that meiosis arrest female 1 (MARF1) is required for these processes in mammalian oocytes. Mutations of Marf1 cause female infertility characterized by up-regulation of a cohort of transcripts, increased retrotransposon expression, defective cytoplasmic maturation, and meiotic arrest. Up-regulation of protein phosphatase 2 catalytic subunit (PPP2CB) is key to the meiotic arrest phenotype. Moreover, Iap and Line1 retrotransposon messenger RNAs are also up-regulated, and, concomitantly, DNA double-strand breaks are elevated in mutant oocytes. Therefore MARF1, by suppressing levels of specific transcripts, is an essential regulator of important oogenic processes leading to female fertility and the development of healthy offspring.

Gas6 downregulation impaired cytoplasmic maturation and pronuclear formation independent to the MPF activity

Previously, we found that the growth arrest-specific gene 6 (Gas6) is more highly expressed in germinal vesicle (GV) oocytes than in metaphase II (MII) oocytes using annealing control primer (ACP)-PCR technology. The current study was undertaken to investigate the role of Gas6 in oocyte maturation and fertilization using RNA interference (RNAi). Interestingly, despite the specific and marked decrease in Gas6 mRNA and protein expression in GVs after Gas6 RNAi, nuclear maturation including spindle structures and chromosome segregation was not affected. The only discernible effect induced by Gas6 RNAi was a change in maturation promoting factor (MPF) activity. After parthenogenetic activation, Gas6 RNAi-treated oocytes at the MII stage had not developed further and arrested at MII (90.0%). After stimulation with Sr(2+), Gas6-silenced MII oocytes had markedly reduced Ca(2+) oscillation and exhibited no exocytosis of cortical granules. In these oocytes, sperm penetration occurred during fertilization but not pronucleus (PN) formation. By roscovitine and colcemid treatment, we found that the Gas6 knockdown affected cytoplasmic maturation directly, independent to the changed MPF activity. These results strongly suggest that 1) the Gas6 signaling itself is important to the cytoplasmic maturation, but not nuclear maturation, and 2) the decreased Gas6 expression and decreased MPF activity separately or mutually influence sperm head decondensation and PN formation.

Bidirectional communication between oocytes and follicle cells: ensuring oocyte developmental competence

Female fertility is determined to a large extent by the quality (developmental competence) of the oocyte as reflected in its ability to undergo meiosis, be fertilized, and give rise to a healthy embryo. Growth of the mammalian oocyte is coordinated with that of the follicle that encloses it by the actions of signals that pass in both directions between the germline and somatic components. This review summarizes what is known about the roles played by 2 different modes of intrafollicular signalling in oogenesis: paracrine factors activating receptors on the opposite cell type, and direct sharing of small molecules throughout the follicle via gap junction channels. Recent evidence indicates that these 2 modes of signalling interact to regulate oocyte growth and granulosa cell proliferation and that defects in either can contribute to female infertility.

The regulatory role of Dicer in folliculogenesis in mice

Dicer is the ribonuclease III for synthesis of mature functional microRNAs (miRNAs), which play an important role in regulating cell development. In the mouse ovary, the Dicer1 protein was expressed in both oocyte and granulosa cells of the follicle. In the present study, the role of miRNAs in mouse ovarian development was explored by using Dicer1 conditional knockout (cKO) mouse ovarian tissue (Amhr2 Cre/-; Dicer flox/flox), in which Dicer1 is deleted specifically in follicular granulosa cells. The morphology and gene expression profile of cKO and wild type (WT) mouse ovaries at various stages of development (day 4, day 8, 8 weeks and 8 months) were examined. Comparative analysis of the follicle number indicated that conditional inactivation of Dicer1 in the follicular granulosa cells led to an increased primordial follicle pool endowment, accelerated early follicle recruitment and more degenerate follicles in the cKO ovaries. In addition, significant differences were noted in the expression of some follicle development-related genes between cKO and WT mouse ovaries, such as Amh, Inhba, Cyp17a1, Cyp19a1, Zps, Gdf9 and Bmp15, suggesting the function of miRNAs in regulating gene expression is time- and gene-dependent. With the Dicer1 inactivation, mmu-mir-503, a miRNA that is more abundant in mouse ovary than in other tissues, was down-regulated significantly. Meanwhile, the expression of mmu-mir-503 decreased notably with follicle development in the gonadotropin-primed mouse ovary. Up-regulation of mmu-mir-503 in primary cultured granulosa cells resulted in the decreased expression of both the target gene and non-target gene at the transcriptional level, which involve genes related to granulosa cell proliferation and luteinization. In conclusion, Dicer1 plays important roles in follicular cell development through the differential regulation of gene expression.

Intact fetal ovarian cord formation promotes mouse oocyte survival and development

BACKGROUND

Female reproductive potential, or the ability to propagate life, is limited in mammals with the majority of oocytes lost before birth. In mice, surviving perinatal oocytes are enclosed in ovarian follicles for subsequent oocyte development and function in the adult. Before birth, fetal germ cells of both sexes develop in clusters, or germline cysts, in the undifferentiated gonad. Upon sex determination of the fetal gonad, germ cell cysts become organized into testicular or ovarian cord-like structures and begin to interact with gonadal somatic cells. Although germline cysts and testicular cords are required for spermatogenesis, the role of cyst and ovarian cord formation in mammalian oocyte development and female fertility has not been determined.

RESULTS

Here, we examine whether intact fetal ovarian germ and somatic cell cord structures are required for oocyte development using mouse gonad re-aggregation and transplantation to disrupt gonadal organization. We observed that germ cells from disrupted female gonad prior to embryonic day e13.5 completed prophase I of meiosis but did not survive following transplantation. Furthermore, re-aggregated ovaries from e13.5 to e15.5 developed with a reduced number of oocytes. Oocyte loss occurred before follicle formation and was associated with an absence of ovarian cord structure and ovary disorganization. However, disrupted ovaries from e16.5 or later were resistant to the re-aggregation impairment and supported robust oocyte survival and development in follicles.

CONCLUSIONS

Thus, we demonstrate a critical window of oocyte development from e13.5 to e16.5 in the intact fetal mouse ovary, corresponding to the establishment of ovarian cord structure, which promotes oocyte interaction with neighboring ovarian somatic granulosa cells before birth and imparts oocytes with competence to survive and develop in follicles. Because germline cyst and ovarian cord structures are conserved in the human fetal ovary, the identification of genetic components and molecular mechanisms of pre-follicle stage germ and somatic cell structures may be important for understanding human female infertility. In addition, this work provides a foundation for development of a robust fetal ovarian niche and transplantation based system to direct stem cell-derived oocyte differentiation as a potential therapeutic strategy for the treatment of infertility.

Transplantation directs oocyte maturation from embryonic stem cells and provides a therapeutic strategy for female infertility

Ten to 15% of couples are infertile, with the most common causes being linked to the production of few or no oocytes or sperm. Yet, our understanding of human germ cell development is poor, at least in part due to the inaccessibility of early stages to genetic and developmental studies. Embryonic stem cells (ESCs) provide an in vitro system to study oocyte development and potentially treat female infertility. However, most studies of ESC differentiation to oocytes have not documented fundamental properties of endogenous development, making it difficult to determine the physiologic relevance of differentiated germ cells. Here, we sought to establish fundamental parameters of oocyte development during ESC differentiation to explore suitability for basic developmental genetic applications using the mouse as a model prior to translating to the human system. We demonstrate a timeline of definitive germ cell differentiation from ESCs in vitro that initially parallels endogenous oocyte development in vivo by single-cell expression profiling and analysis of functional milestones including responsiveness to defined maturation media, shared genetic requirement of Dazl, and entry into meiosis. However, ESC-derived oocyte maturation ultimately fails in vitro. To overcome this obstacle, we transplant ESC-derived oocytes into an ovarian niche to direct their functional maturation and, thereby, present rigorous evidence of oocyte physiologic relevance and a potential therapeutic strategy for infertility.

Gene expression profile of rat ovarian tissue following xenotransplantation into immune-deficient mice

Immune-compromised mice have been used as gonadal tissue recipients to develop gametes of various mammalian species. The aim of this research was to determine gene expression differences between fresh and frozen–thawed rat xenotransplanted (XT) ovaries as well the gene expression differences between XT and sexually mature rat ovaries that were non-transplanted (NT). Ovaries from sexually immature female rats were transplanted under the kidney capsule of ovariectomized athymic nude mice either fresh or after freezing. The XT ovaries were collected ∼10–12 weeks after xenografting for microarray analysis. The NT ovaries were collected from sexually mature rats. Gene expression was very similar between fresh and cryopreserved XT ovaries: 125 genes were twofold up- or downregulated, but level of regulation was not statistically significant. Overall patterns of gene expression between XT and NT ovaries were very different indicated by the absence of diagonal relationship between XT and NT ovary gene expression. More than 3000 genes were significantly (P<0.01) up- or downregulated between XT and NT ovaries. Genes involved in metabolic processes, lipid metabolism, and growth were downregulated in XT ovaries, whereas genes involved in immune and inflammatory response were upregulated in XT ovaries. The results showed that ovarian tissue xenografting significantly alters genes responsible for ovarian metabolism and function and leads to an upregulation of genes responsible for graft rejection.

Granulosa cells regulate oocyte intracellular pH against acidosis in preantral follicles by multiple mechanisms

Mammalian oocytes grow within ovarian follicles in which the oocyte is coupled to surrounding granulosa cells by gap junctions. We report here that growing oocytes isolated from mouse preantral follicles are incapable of recovering from an experimentally induced acidosis, and that oocytes acquire the ability to manage acid loads by activating Na(+)/H(+) exchange during growth. By contrast, granulosa cells from similar preantral follicles possess substantial Na(+)/H(+) exchange capacity, which is attributable to the simultaneous action of two Na(+)/H(+) exchanger isoforms: NHE1 and NHE3. Granulosa cells were also found to possess a V-type H(+)-ATPase that drives partial acidosis recovery when Na(+)/H(+) exchange is inactivated. By monitoring intracellular pH (pH(i)) in small follicle-enclosed oocytes, we found that the oocyte has access to each of these acidosis-correcting activities, such that small follicle-enclosed oocytes readily recover from acidosis in a manner resembling granulosa cells. However, follicle-enclosed oocytes are unable to access these activities if gap-junction communication within the follicle is inhibited. Together, these experiments identify the NHE isoforms involved in regulating oocyte pH(i), indicate that gap junctions allow granulosa cells to exogenously regulate oocyte pH(i) against acidosis until the oocyte has acquired endogenous pH(i) regulation, and reveal that granulosa cells possess multiple mechanisms for carrying out this function.

Establishment of ovarian reconstruction system in culture for functional genomic analysis

In a previous study, we reported a list of genes expressed differentially in primordial and primary follicles [Park et al., Fertil. Steril., 83, 410-418 (2005)]. An innovative experimental system is required to evaluate the functions of these genes in folliculogenesis, particularly primordial-primary follicle transition. In this study, ovarian tissues were dissociated, and isolated cells were transfected using small interfering RNAs (siRNAs) for disrupting a specific gene, followed by ovarian reconstruction via calcium alginate encapsulation. The effects of RNA interference (RNAi) on follicular development were evaluated by a histological observation of the reconstructed ovarian tissue. Interestingly, follicular formation and development showed differences between control and experimental groups. Thus, even though this system includes some problems that need to be solved, ovarian reconstruction following the modification of gene expression of individual ovarian component cells could lead the way to methods of studying molecular mechanisms of primordial-primary follicle transition.

Stage-specific germ-somatic cell interaction directs the primordial folliculogenesis in mouse fetal ovaries

The mechanism regulating primordial follicle formation remains largely unexplored because of the developmental particularity of female germ cells and their ultimate functional structure as follicles. Using an in vitro follicle reconstitution culture model, we explored, in the present study, the possibility of producing transgenetic follicles in vitro. We found that mouse fetal ovarian germ cells progressively lose the flexibility for gene manipulation with their oogonia-oocyte transformation upon entering meiosis, the borderline of which was at around embryonic age of 13.5 days post coitus (dpc). Interestingly, we further observed that fetal ovarian cells, only at this age or beyond achieve the capacity to reform the follicles in culture. Screening of well-known marker gene (Zp1-3, Figalpha, and Cx43) expression in cultured fetal ovarian cells of various developmental ages revealed that Figalpha is one of the determining factors for normal primordial follicle formation. By conducting reciprocal follicle reconstitution experiments, we provided further evidence that a synchronized germ-somatic cell interaction determines the normal duration of primordial folliculogenesis. Besides uncovering a potentially important regulatory mechanism for normal oocyte differentiation and follicle formation, this observation offers an alternative approach to produce transgenic oocytes/follicles, and thus animal models.

Eggs forever?

A group of scientists from Harvard Medical School (Johnson et al., 2004) claims to have "established the existence of proliferative germ cells that sustain oocyte and follicle production in the postnatal mammalian ovary," expressing no doubts about their methods, results and conclusion. Johnson et al. based their conclusions of oocyte and follicular renewal from existing germline stem cells (GSC) in the postnatal mouse ovary on three types of observations: (1) A claimed discordance in follicle loss versus follicle atresia in the neonatal period and in the following pubertal and adult period; (2) immunohistochemical detection of proliferating GSC with meiotic capacity using combined markers for meiosis, germline, and mitosis; and (3) neo-folliculogenesis in ovarian chimeric grafting experiments with adult mice. Oogenesis is the process that transforms the proliferative oogonium into an oocyte through meiosis, followed by folliculogenesis and follicular and oocyte maturation. The most crucial part in producing a functional oocyte is firstly, initiation and completion of the first meiotic prophase, and secondly, enclosure of the resulting diplotene oocyte in a follicle. Neither of these two events has been shown to take place in Johnson et al.'s study of the postnatal mouse ovary. We hereby address the observations underpinning their hypothesis and conclude that it is premature to replace the paradigm that adult mammalian neo-oogenesis/folliculogenesis does not take place.

Granulosa cells regulate intracellular pH of the murine growing oocyte via gap junctions: development of independent homeostasis during oocyte growth

Oocytes grow within ovarian follicles in which the oocyte is coupled to the surrounding granulosa cells by gap junctions. It was previously found that small growing oocytes isolated from juvenile mice and freed of their surrounding granulosa cells (denuded) lacked the ability to regulate their intracellular pH (pH(i)), did not exhibit the pH(i)-regulatory HCO(3)(-)/Cl(-) and Na(+)/H(+) exchange activities found in fully-grown oocytes, and had low pH(i). However, both exchangers became active as oocytes grew near to full size, and, simultaneously, oocyte pH(i) increased by approximately 0.25 pH units. Here, we show that, in the more physiological setting of the intact follicle, oocyte pH(i) is instead maintained at approximately 7.2 throughout oocyte development, and the growing oocyte exhibits HCO(3)(-)/Cl(-) exchange, which it lacks when denuded. This activity in the oocyte requires functional gap junctions, as gap junction inhibitors eliminated HCO(3)(-)/Cl(-) exchange activity from follicle-enclosed growing oocytes and substantially impeded the recovery of the oocyte from an induced alkalosis, implying that oocyte pH(i) may be regulated by pH-regulatory exchangers in granulosa cells via gap junctions. This would require robust HCO(3)(-)/Cl(-) exchange activity in the granulosa cells, which was confirmed using oocytectomized (OOX) cumulus-oocyte complexes. Moreover, in cumulus-oocyte complexes with granulosa cells coupled to fully-grown oocytes, HCO(3)(-)/Cl(-) exchange activity was identical in both compartments and faster than in denuded oocytes. Taken together, these results indicate that growing oocyte pH(i) is controlled by pH-regulatory mechanisms residing in the granulosa cells until the oocyte reaches a developmental stage where it becomes capable of carrying out its own homeostasis.

Differential contributions of connexin37 and connexin43 to oogenesis revealed in chimeric reaggregated mouse ovaries

The gap junction proteins connexin37 and connexin43 are required for ovarian folliculogenesis in the mouse. To define their respective roles in oogenesis, chimeric ovaries containing either null mutant oocytes and wild-type granulosa cells or the reverse combination were grafted to the renal capsules of immunodeficient female mice. After three weeks, the oocytes were tested for meiotic competence and fertilizability in vitro. Ovaries composed of connexin43-deficient oocytes and wild-type granulosa cells produced antral follicles enclosing oocytes that could develop to at least the two-cell stage, demonstrating that oocytes need not express connexin43 to reach maturity. Conversely, both follicle development and oocyte maturation were impaired in ovaries containing either wild-type oocytes and connexin43-deficient granulosa cells or connexin37-deficient oocytes and wild-type granulosa cells. Thus absence of connexin43 from granulosa cells or connexin37 from oocytes is sufficient to compromise both oocyte and follicle development. Wild-type oocytes paired with connexin37-deficient granulosa cells generated antral follicles containing oocytes that developed to at least the two-cell stage. Therefore, connexin37 absence from granulosa cells need not impair fertility in mice. Dye transfer experiments revealed persistent oocyte-granulosa cell coupling in those follicles, indicating functional compensation by another connexin. The results indicate that mouse oocytes do not need to express connexin43 in order to develop into meiotically competent, fertilizable gametes, but must express connexin37 for communication with granulosa cells, a requirement for oogenesis.

Interplay between paracrine signaling and gap junctional communication in ovarian follicles

Intercellular communication is required for ovarian folliculogenesis. This is apparent in mice lacking connexin43 (Cx43, a gap junction protein strongly expressed in granulosa cells), or growth/differentiation factor-9 (GDF9, an oocyte-specific growth factor that stimulates granulosa cell proliferation and differentiation), or in mice expressing a mutant form of Kit ligand (KITL, a paracrine factor that, in the ovary, is secreted by granulosa cells to stimulate oocyte growth). In all of these mutant lines, follicle growth is impaired suggesting a possible interaction between paracrine signaling and gap junctional communication. To assess this possibility, we analyzed gene expression in mutant ovaries. Despite the lack of gap junctional coupling between granulosa cells of Cx43 null mutant ovaries, expression of the genes encoding KITL and its receptor, KIT, is maintained. Furthermore, GDF9 expression is maintained. In GDF9 null mutant ovaries, there is no apparent change in Cx43 expression and, correspondingly, the granulosa cells remain coupled. There is also no increase in granulosa cell apoptosis in ovaries lacking Cx43 or GDF9. Staining for proliferating cell nuclear antigen (PCNA) revealed that the granulosa cells of Cx43 null mutant ovaries have a reduced frequency of DNA synthesis. Using both radiolabeled thymidine incorporation and PCNA staining in vitro, we showed that recombinant GDF9 could restore the proliferation of coupling-deficient granulosa cells to the level of control cells. These results indicate that impaired folliculogenesis in mice lacking Cx43 is due at least in part to reduced responsiveness of granulosa cells to oocyte-derived GDF9, indicating an interaction between these two modes of intercellular communication.

Gonadotrophin regimens and oocyte quality in women with polycystic ovaries

The systemic endocrine environment during the later stages of follicle development has a crucial role in co-ordinating follicular and oocyte maturation before ovulation. Polycystic ovary syndrome (PCOS) is associated with abnormal circulating hormones, abnormal peri-follicular vascularity and significant abnormalities of granulosa cell function. After induction of ovulation, fertilization rates in vivo in women with PCOS are normal, but there is an increased risk of early pregnancy loss, particularly in obese patients. After in-vitro maturation of oocytes or following ovulation induction for IVF, oocyte and embryo quality in vitro are not obviously impaired in PCOS. In some reports however, specific endocrine abnormalities, such as hyperinsulinaemia/insulin resistance, have been noted to be associated with reduced fertilization rates and abnormal early embryonic development.

The mouse meiotic mutation mei1 disrupts chromosome synapsis with sexually dimorphic consequences for meiotic progression

mei1 (meiosis defective 1) is the first meiotic mutation in mice derived by phenotype-driven mutagenesis. It was isolated by using a novel technology in which embryonic stem (ES) cells were chemically mutagenized and used to generate families of mice that were screened for infertility. We report here that mei1/mei1 spermatocytes arrest at the zygotene stage of meiosis I, exhibiting failure of homologous chromosomes to properly synapse. Notably, RAD51 failed to associate with meiotic chromosomes in mutant spermatocytes, despite evidence for the presence of chromosomal breaks. Transcription of genes that are markers for the leptotene and zygotene stages, but not genes that are markers for the pachytene stage, was observed. mei1/mei1 females are sterile, and their oocytes also show severe synapsis defects. Nevertheless, unlike arrested spermatocytes, a small number of mutant oocytes proved capable of progressing to metaphase I and attempting the first meiotic division. However, their chromosomes were unpaired and were not organized properly at the metaphase plate or along the spindle fibers during segregation. mei1 was genetically mapped to chromosome (Chr) 15 in an interval that is syntenic to human Chr 22q13. This region, which has been completely sequenced, contains no known homologs of genes specifically required for meiosis in model organisms. Thus, mei1 may be a novel meiotic gene.

Failure of spermatogenesis in mice lacking connexin43

Connexin43 (Cx43), a gap junction protein encoded by the Gja1 gene, is expressed in several cell types of the testis. Cx43 gap junctions couple Sertoli cells with each other, Leydig cells with each other, and spermatogonia/spermatocytes with Sertoli cells. To investigate the role of this communication pathway in spermatogenesis, we studied postnatal testis development in mice lacking Cx43. Because such mice die shortly after birth, it was necessary to graft testes from null mutant fetuses under the kidney capsules of adult males for up to 3 wk. Grafted wild-type testes were used as controls. In our initial experiments with wild-type testes, histological examination indicated that the development of grafted testes kept pace with that of nongrafted testes in terms of the onset of meiosis, but this development required the presence of the host gonads. When excised grafts were stimulated in vitro with cAMP or LH, there was no significant difference in androgen production between null mutant and wild-type testes, indicating that the absence of Cx43 had not compromised steroidogenesis. Previous research has shown that Cx43 null mutant neonates have a germ cell deficiency that arises during fetal life, and our analysis of grafted testes demonstrated that this deficiency persists postnatally, giving rise to a "Sertoli cell only" phenotype. These results indicate that intercellular communication via Cx43 channels is required for postnatal expansion of the male germ line.

Intercellular communication via connexin43 gap junctions is required for ovarian folliculogenesis in the mouse

The ovarian follicle in mammals is a functional syncytium, with the oocyte being coupled with the surrounding cumulus granulosa cells, and the cumulus cells being coupled with each other and with the mural granulosa cells, via gap junctions. The gap junctions coupling granulosa cells in mature follicles contain several different connexins (gap junction channel proteins), including connexins 32, 43, and 45. Connexin43 immunoreactivity can be detected from the onset of folliculogenesis just after birth and persists through ovulation. In order to assess the importance of connexin43 gap junctions for postnatal folliculogenesis, we grafted ovaries from late gestation mouse fetuses or newborn pups lacking connexin43 (Gja1(-)/Gja1(-)) into the kidney capsules of adult females and allowed them to develop for up to 3 weeks (this was necessitated by the neonatal lethality caused by the mutation). By the end of the graft period, tertiary (antral) follicles had developed in grafted normal (wild-type or heterozygote) ovaries. Most follicles in Gja1(-)/Gja1(-) ovaries, however, failed to become multilaminar, with the severity of the effect depending on strain background. Dye transfer experiments indicated that intercellular coupling between granulosa cells is reduced, but not abolished, in the absence of connexin43, consistent with the presence of additional connexins. These results suggest that coupling between granulosa cells mediated specifically by connexin43 channels is required for continued follicular growth. Measurements of oocyte diameters revealed that oocyte growth in mutant follicles is retarded, but not arrested, despite the arrest of folliculogenesis. The mutant follicles are morphologically abnormal: the zona pellucida is poorly developed, the cytoplasm of both granulosa cells and oocytes is vacuolated, and cortical granules are absent from the oocytes. Correspondingly, the mutant oocytes obtained from 3-week grafts failed to undergo meiotic maturation and could not be fertilized, although half of the wild-type oocytes from 3-week grafted ovaries could be fertilized. We conclude that connexin43-containing gap junction channels are required for expansion of the granulosa cell population during the early stages of follicular development and that failure of the granulosa cell layers to develop properly has severe consequences for the oocyte.

Human infertility, reproductive cloning and nuclear transfer: a confusion of meanings

The Chief Medical Officer of Health of the United Kingdom has recommended that the 1990 Human Fertilisation and Embryology Act should be amended to allow cloning in humans for research purposes only. He also recommended that: "The transfer of an embryo created by cell nuclear replacement into the uterus of a woman (so called 'reproductive cloning') should remain a criminal offence" (recommendation 7, Ref. 1). This recommendation implies that nuclear replacement and cloning are the same. They are not. Nuclear transfer constitutes reproductive cloning only when the individual created is genetically identical to the nuclear donor. In this paper, we describe a possible future use of nuclear transfer for the treatment of infertile individuals. The treatment yields an individual that receives approximately equal genetic contributions from each parent. We use this example to illustrate how semantic confusion might lead to plausibly moral and justifiable treatments being legally banned. In doing so, we hope to encourage a more accurate and informed use of language in science, law and politics, so that legislation is properly informed by science and achieves what it intends. BioEssays 23:359-364, 2001.

Reproduction in the Noughties: will the scientists have all the fun?

The past 20-30 years have seen major advances in our understanding of human reproduction and in our ability to manipulate it, as well as major social changes in human reproductive and sexual attitudes. Many of these advances and changes developed out of the first successful in vitro fertilisation (IVF) of the human oocyte. It is also the case that 30 years ago few foresaw what was to come, and many were at best doubtful and often were very critical of the scientific work which led to human IVF and to many of the subsequent developments. This lack of foresight provides us with a lesson about the dangers that we face in looking forward and attempting to predict the future. This review will try to convey, not comprehensively but through examples, the flavour of current activities in Assisted Reproduction clinics and research laboratories around the world and what is being talked about for the future in respect of emergent patient demands and anticipated clinical needs. This clinically driven approach will form the basis for consideration of some underlying scientific aspects of reproductive research, some of the ethicolegal issues that may arise, and the implications of this anticipated future for our current approaches to medical education. The future will be considered not simply in terms of the New Reproduction itself but also in its interaction with the opportunities and challenges presented by the New Genetics. It is perhaps in the interaction between these two fields of endeavour that some of the most difficult challenges ahead lie.