Activin A accelerates the progression of fetal oocytes throughout meiosis and early oogenesis in the mouse

Meiosis and retinoic acid in the mouse fetal gonads: An unforeseen twist

In mammals, differentiation of germ cells is crucial for sexual reproduction, involving complex signaling pathways and environmental cues defined by the somatic cells of the gonads. This review examines the long-standing model positing that all-trans retinoic acid (ATRA) acts as a meiosis-inducing substance (MIS) in the fetal ovary by inducing expression of STRA8 in female germ cells, while CYP26B1 serves as a meiosis-preventing substance (MPS) in the fetal testis by degrading ATRA and preventing STRA8 expression in the male germ cells until postnatal development. Recent genetic studies in the mouse challenge this paradigm, revealing that meiosis initiation in female germ cells can occur independently of ATRA signaling, with key roles played by other intrinsic factors like DAZL and DMRT1, and extrinsic signals such as BMPs and vitamin C. Thus, ATRA can no longer be considered as 'the' long-searched MIS. Furthermore, evidence indicates that CYP26B1 does not prevent meiosis by degrading ATRA in the fetal testis, but acts by degrading an unidentified MIS or synthesizing an equally unknown MPS. By emphasizing the necessity of genetic loss-of-function approaches to accurately delineate the roles of signaling molecules such ATRA in vivo, this chapter calls for a reevaluation of the mechanisms instructing and preventing meiosis initiation in the fetal ovary and testis, respectively. It highlights the need for further research into the molecular identities of the signals involved in these processes.

Immunolocalization of activin and inhibin at different stages of follicular development in the lizard Sceloporus torquatus

The activins and inhibins are glycoproteins with a role in the follicular development of vertebrates, that are found in follicular fluid and somatic follicular cells, with a different pattern among taxa. The principal function of activin (Act) is to modulate the follicle-stimulating hormone (FSH) synthesis and secretion, whereas inhibin (Inh) downregulates it. Both factors are modulators of intraovarian follicular recruitment, oocyte maturation, cell proliferation, and steroidogenic activity. Our aim was to characterize the immunolocalization of Act and Inh in the ovarian follicles during the reproductive cycle of the lizard Sceloporus torquatus. Act was detected in the granulosa cells and oocyte cortex in the different stages of follicular development. On the other hand, we identified Inh in the oocyte cortex and the cytoplasm of pyriform and small cells of previtellogenic follicles. Also, we found immunoreactivity in the oocyte cortex, theca, and small cells of vitellogenic and preovulatory follicles. Our data provide evidence that Act and Inh have changes related to the stage of follicular development. This dynamic appears to be conserved among vertebrates and is fundamental to ensure an adequate follicular development in this specie.

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

Background

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

Methods

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

Results

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

Conclusions

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

The Beginning of Meiosis in Mammalian Female Germ Cells: A Never-Ending Story of Intrinsic and Extrinsic Factors

Meiosis is the unique division of germ cells resulting in the recombination of the maternal and paternal genomes and the production of haploid gametes. In mammals, it begins during the fetal life in females and during puberty in males. In both cases, entering meiosis requires a timely switch from the mitotic to the meiotic cell cycle and the transition from a potential pluripotent status to meiotic differentiation. Revealing the molecular mechanisms underlying these interrelated processes represents the essence in understanding the beginning of meiosis. Meiosis facilitates diversity across individuals and acts as a fundamental driver of evolution. Major differences between sexes and among species complicate the understanding of how meiosis begins. Basic meiotic research is further hindered by a current lack of meiotic cell lines. This has been recently partly overcome with the use of primordial-germ-cell-like cells (PGCLCs) generated from pluripotent stem cells. Much of what we know about this process depends on data from model organisms, namely, the mouse; in mice, the process, however, appears to differ in many aspects from that in humans. Identifying the mechanisms and molecules controlling germ cells to enter meiosis has represented and still represents a major challenge for reproductive medicine. In fact, the proper execution of meiosis is essential for fertility, for maintaining the integrity of the genome, and for ensuring the normal development of the offspring. The main clinical consequences of meiotic defects are infertility and, probably, increased susceptibility to some types of germ-cell tumors. In the present work, we report and discuss data mainly concerning the beginning of meiosis in mammalian female germ cells, referring to such process in males only when pertinent. After a brief account of this process in mice and humans and an historical chronicle of the major hypotheses and progress in this topic, the most recent results are reviewed and discussed.

Single-cell transcriptomic profiling provides insights into the toxic effects of Zearalenone exposure on primordial follicle assembly

Rationale: Zearalenone (ZEN), a pollutant in our daily diet, seriously threatens the reproductive health of humans and animals. The primordial follicle (PF) assembly in the mouse occurs during the perinatal period, which determines the whole ovarian reserve in reproductive lifespan. In the current investigation, we administered ZEN orally to perinatal mice from 16.5 days post coitum (dpc) to postnatal day 3 (PD3), and single-cell RNA sequencing (scRNA-seq) was performed on PD0 and PD3 ovarian tissues in the offspring to check ZEN toxic to primordial follicle formation at the single cell level. Methods: Ovarian tissues (in vivo) were examined by single cell RNA sequencing analysis, Immunostaining, and Western blotting. Ovarian tissues (in vitro) were examined by qRT-PCR, Immunostaining, and Western blotting. Results: We found that ZEN exposure altered the developmental trajectory of both germ cells and granulosa cells. Furthermore, after establishing the cell-cell communication network between germ cells and granulosa cells, we found that this was perturbed by ZEN exposure, especially during the Hippo signaling pathway. Conclusions: This study showed that ZEN affected the status of germ cells and granulosa cells through the Hippo signaling pathway and blocked the assembly of PFs. This research contributes to our deeper understanding of the mechanisms of toxicity in different cell types and the disruption of normal intercellular signaling by ZEN exposure.

Maternal Bisphenol S exposure affects the reproductive capacity of F1 and F2 offspring in mice

Bisphenol S (BPS) is an endocrine disruptor which is widely used in commercial plastic products. Previous studies have shown that exposure to BPS has toxic effects on various aspects of mammalian, but there are few reports about reproductive toxicity. In order to investigate the effects of maternal BPS exposure on the reproductive of F1 and F2 female mice, the pregnant mice were orally administered with different dosages of BPS only once every day from 12.5 to 15.5 days post-coitus (dpc). The results showed that maternal BPS exposure to 2 μg per kg of body weight per day (2 μg/kg) and 10 μg/kg accelerated the meiotic prophase I (MPI) of F1 female mice and the expression of the genes related to meiotic were increased. Further studies showed that maternal BPS exposure resulted in a significant increase in the percentage of oocytes enclosed in primordial follicles in the 3 days post-partum (3 dpp) ovaries of F1 female mice. And at the time of 21 days post-partum (21 dpp) in F1 female mice, the number of antral follicles were significantly lower compare to controls. In the study of five-week female mice of F1, we found that BPS disturbed the folliculogenesis, and the maturation rates and fertilization rates of oocytes were significantly decreased. Of note, maternal BPS exposure disrupted H3K4 and H3K9 tri-methylation levels in F1 ovaries. Maternal BPS exposure only affected the cyst breakdown in F2 female mice. Taken together, our results suggest that, maternal BPS exposure impaired the process of meiosis and oogenesis of F1 and F2 offspring, resulting in abnormal follicular development and serious damage to the reproduction.

Meiosis occurs normally in the fetal ovary of mice lacking all retinoic acid receptors

Gametes are generated through a specialized cell differentiation process, meiosis, which, in ovaries of most mammals, is initiated during fetal life. All-trans retinoic acid (ATRA) is considered as the molecular signal triggering meiosis initiation. In the present study, we analyzed female fetuses ubiquitously lacking all ATRA nuclear receptors (RAR), obtained through a tamoxifen-inducible cre recombinase-mediated gene targeting approach. Unexpectedly, mutant oocytes robustly expressed meiotic genes, including the meiotic gatekeeper STRA8. In addition, ovaries from mutant fetuses grafted into adult recipient females yielded offspring bearing null alleles for all Rar genes. Thus, our results show that RAR are fully dispensable for meiotic initiation, as well as for the production of functional oocytes. Assuming that the effects of ATRA all rely on RAR, our study goes against the current model according to which meiosis is triggered by endogenous ATRA in the developing ovary. It therefore revives the search for the meiosis-inducing substance.

Melatonin alleviates meiotic defects in fetal mouse oocytes induced by Di (2-ethylhexyl) phthalate in vitro

Di (2-ethylhexyl) phthalate (DEHP), an estrogen-like compound that is a ubiquitous environmental contaminant, has been reported to adversely affect human and mammalian reproduction. Many studies have found that exposure to DEHP during pregnancy perturbs female germ cell meiosis and is detrimental to oogenesis. Previous studies have demonstrated that melatonin (MLT) is beneficial to reproductive endocrinology, oogenesis, and embryonic development as the ability to antioxidative and antiapoptotic. However, whether the meiotic defect of germ cells exposed to DEHP could be rescued by MLT is not clear. Here, we cultured 12.5 days post coitum (dpc) fetal mouse ovaries for 6 days, exposed them to 100 μM DEHP with or without 1 μM MLT in vitro.. The results showed that DEHP exposure induced the abnormal formation of DNA double-strand breaks (DSBs), and inhibited the repair of DSBs during meiotic recombination. In addition, we found defective oocytes were prone to undergo apoptosis. Notably, this defect could be remarkably ameliorated by the addition of MLT via a reduction of the levels of reactive oxygen species and an inhibition of apoptosis. In conclusion, our data revealed that MLT had a protective action against the meiotic deterioration of fetal oocytes induced by DEHP in the mouse in vitro.

Protective effects of melatonin against nicotine-induced disorder of mouse early folliculogenesis

In this paper, we show that neonatal mice injected for five consecutive days with nicotine display impaired germ cell cyst breakdown and primordial follicle assembly resulting in decreased ovarian reserve lasting until sex maturation age. The effects of nicotine on the pups ovaries were associated with decreased expression of oocyte specific genes such as Nobox, Lhx8, Figlα and Sohlh2. Moreover, the ovaries of pups injected with nicotine showed increased level of cell oxidative stress and autophagic markers (upregulation of AMPKα-1, increased ratio LC3-II/LC3-I, downregulation of AKT and mTOR). Noteworthy, all these effects were counteracted by the administration of the hormone melatonin in 1 μM. In vitro culture of 0 dpp ovaries for 5 days in the presence of 10 μM nicotine reproduced its effect on germ cell cyst breakdown and primordial follicle assembly, furthermore it also revealing about 20% reduction of somatic cells proliferation, and these effects was prevented when melatonin was added to the medium. Taken together these results show that nicotine exposure can adversely affect the establishment of the ovarian reserve in the mouse likely by locally inducing cellular stress altering the primordial follicle assembly and that melatonin, however, is able to counteract such effects.

Retinoic acid signaling in ovarian folliculogenesis and steroidogenesis

Retinoids are essential for reproduction. Most research has focused on the role of retinoic acid signaling in the regulation of meiosis during early fetal germ cell development. However, less attention has been paid to the possible effects of retinoic acid signaling in adult female gonads. Retinoic acid, its receptors, and the key enzymes required for retinoic acid synthesis are expressed in the ovaries and they are involved in the regulation of folliculogenesis and steroidogenesis. Exposure to compounds that can interfere with normal retinoic acid signaling is associated with adverse ovarian outcomes, including altered steroidogenesis and reduction in indicators of ovarian reserve in women and laboratory animal models. These observations call for more attention to retinoids as regulators of adult ovarian physiology and as possible targets of endocrine disruption by environmental chemicals. In this review, we summarize the current knowledge of retinoids in folliculogenesis and steroidogenesis in post-pubertal mammalian ovaries.

Establishment and depletion of the ovarian reserve: physiology and impact of environmental chemicals

The reproductive life span in women starts at puberty and ends at menopause, following the exhaustion of the follicle stockpile termed the ovarian reserve. Increasing data from experimental animal models and epidemiological studies indicate that exposure to a number of ubiquitously distributed reproductively toxic environmental chemicals (RTECs) can contribute to earlier menopause and even premature ovarian failure. However, the causative relationship between environmental chemical exposure and earlier menopause in women remains poorly understood. The present work, is an attempt to review the current evidence regarding the effects of RTECs on the main ovarian activities in mammals, focusing on how such compounds can affect the ovarian reserve at any stages of ovarian development. We found that in rodents, strong evidence exists that in utero, neonatal, prepubescent and even adult exposure to RTECs leads to impaired functioning of the ovary and a shortening of the reproductive lifespan. Regarding human, data from cross-sectional surveys suggest that human exposure to certain environmental chemicals can compromise a woman's reproductive health and in some cases, correlate with earlier menopause. In conclusion, evidences exist that exposure to RTECs can compromise a woman's reproductive health. However, human exposures may date back to the developmental stage, while the adverse effects are usually diagnosed decades later, thus making it difficult to determine the association between RTECs exposure and human reproductive health. Therefore, epidemiological surveys and more experimental investigation on humans, or alternatively primates, are needed to determine the direct and indirect effects caused by RTECs exposure on the ovary function, and to characterize their action mechanisms.

Gestational diabetes promotes germ cell cCyst breakdown and primordial follicle formation in newborn mice via the AKT signaling pathway

Type 1 diabetes (T1D) is a common disease in which pancreatic β cells are impaired due to auto-immunity, pregnancy in women with it is associated with increased risk of neonatal morbidity, mortality. However, the effects of gestational diabetes on the reproduction of newborn offspring are still poorly understood. Here, we determined the cyst breakdown and primordial follicle formation in neonatal offspring born by streptozotocin (STZ)-induced diabetic or non-diabetic female mice, and found that the germ cell cyst breakdown was promoted in neonatal offspring of STZ -induced diabetic mice at postnatal Day 1, which sequentially accelerated the primordial follicle formation. Further investigation revealed that, the expression level of PI3K and p-AKT were significantly increased in ovaries of offspring born by T1D mice. These results indicated that STZ -induced gestational diabetes promotes germ cell cyst breakdown and primordial follicle formation by regulating the PI3K/AKT signaling pathway in the newborn offspring. In addition, this effect can be rescued by an insulin supplement. Taken together, our results uncover the intergenerational effects of gestational diabetes on neonatal offspring folliculogenesis, and provide an experimental model for treating gestational diabetes and its complications in neonatal offspring.

Sexually dimorphic germ cell identity in mammals

Germ cells are the stem cells of the species. Thus, it is critical that we have a good understanding of how they are specified, how the somatic cells instruct and support them, how they commit to one or other sex, and how they ultimately develop into functional gametes. Here, we focus on specifics of how sexual fate is determined during fetal life. Because the majority of relevant experimental work has been done using the mouse model, we focus on that species. We review evidence regarding the identity of instructive signals from the somatic cells, and the molecular responses that occur in germ cells in response to those extrinsic signals. In this way we aim to clarify progress to date regarding the mechanisms underlying the mitotic to meiosis switch in germ cells of the fetal ovary, and those involved in adopting and securing male fate in germ cells of the fetal testis.

Effects of activin A on the transcriptome of mouse oogenesis in vitro

From the previous research, it has been supported that activin A (ActA) is conducive to ovarian development in vitro. In the present paper, with the aim to identify the molecular pathways through which ActA can influence processes of the fetal and early postnatal oogenesis, we analyzed the transcriptome of embryonic ovaries (12.5 days postcoitum) in vitro cultured with or without ActA for 6 days, as well as the produced oocytes for 28 days, and further compared the gene expression profile with their in vivo counterparts. With the confirmation of designed test, we found that the addition of ActA to the ovary culture tended, generally, to align oocyte gene expression to the in vivo condition, in particular of a number of genes involved in meiosis and epigenetic modifications of histones. In particular, we identified DNA recombination during the oocyte meiotic prophase I and lysine trimethylation of the histone H3K27 during the oocyte growth phase as molecular pathways modulated by ActA.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

Nicotine exposure impairs germ cell development in human fetal ovaries cultured in vitro

In the present paper, we found that human fetal ovaries (at ~16 weeks) express the transcripts for several subunits of the nicotinic acetylcholine receptor (nAChR). Exposure to the drug in vitro resulted in the marked increase of apoptosis in the ovaries in a time and dose-dependent manner. Evidence that adverse nicotine effects are potentially due to an increased level of reactive oxygen species (ROS) and consequent DNA damage, both in the ovarian somatic cells and germ cells, are reported. After 4 days of culture, exposure to 1 mM and 10 mM nicotine caused a 50% and 75% decrease, respectively, in the number of oogonia/oocytes present in the fetal ovaries. These results represent the first indication that nicotine may directly cause apoptosis in cells of the fetal human ovary and may lead to a reduction of the ovarian reserve oocytes and consequent precocious menopause in mothers smoking during pregnancy.

MicroRNA dynamics at the onset of primordial germ and somatic cell sex differentiation during mouse embryonic gonad development

In mammals, commitment and specification of germ cell lines involves complex programs that include sex differentiation, control of proliferation, and meiotic initiation. Regulation of these processes is genetically controlled by fine-tuned mechanisms of gene regulation in which microRNAs (miRNAs) are involved. We have characterized, by small-RNA-seq and bioinformatics analyses, the miRNA expression patterns of male and female mouse primordial germ cells (PGCs) and gonadal somatic cells at embryonic stages E11.5, E12.5, and E13.5. Differential expression analyses revealed differences in the regulation of key miRNA clusters such as miR-199-214, miR-182-183-96, and miR-34c-5p, whose targets have defined roles during gonadal sexual determination in both germ and somatic cells. Extensive analyses of miRNA sequences revealed an increase in noncanonical isoforms on PGCs at E12.5 and dramatic changes of 3' isomiR expression and 3' nontemplate nucleotide additions in female PGCs at E13.5. Additionally, RT-qPCR analyses of genes encoding proteins involved in miRNA biogenesis and 3' nucleotide addition uncovered sexually and developmentally specific expression, characterized by the decay of Drosha, Dgcr8, and Xpo5 expression along gonadal development. These results demonstrate that miRNAs, their isomiRs, and miRNA machinery are differentially regulated and participate actively in gonadal sexual differentiation in both PGCs and gonadal somatic cells.

Sex Determination in the Mammalian Germline

Sexual reproduction crucially depends on the production of sperm in males and oocytes in females. Both types of gamete arise from the same precursor, the germ cells. We review the events that characterize the development of germ cells during fetal life as they commit to, and prepare for, oogenesis or spermatogenesis. In females, fetal germ cells enter meiosis, whereas in males they delay meiosis and instead lose pluripotency, activate an irreversible program of prospermatogonial differentiation, and temporarily cease dividing. Both pathways involve sex-specific molecular signals from the somatic cells of the developing gonads and a suite of intrinsic receptors, signal transducers, transcription factors, RNA stability factors, and epigenetic modulators that act in complex, interconnected positive and negative regulatory networks. Understanding these networks is important in the contexts of the etiology, diagnosis, and treatment of infertility and gonadal cancers, and in efforts to augment human and animal fertility using stem cell approaches.

Mechanisms controlling germline cyst breakdown and primordial follicle formation

In fetal females, oogonia proliferate immediately after sex determination. The progress of mitosis in oogonia proceeds so rapidly that the incompletely divided cytoplasm of the sister cells forms cysts. The oogonia will then initiate meiosis and arrest at the diplotene stage of meiosis I, becoming oocytes. Within each germline cyst, oocytes with Balbiani bodies will survive after cyst breakdown (CBD). After CBD, each oocyte is enclosed by pre-granulosa cells to form a primordial follicle (PF). Notably, the PF pool formed perinatally will be the sole lifelong oocyte source of a female. Thus, elucidating the mechanisms of CBD and PF formation is not only meaningful for solving mysteries related to ovarian development but also contributes to the preservation of reproduction. However, the mechanisms that regulate these phenomena are largely unknown. This review summarizes the progress of cellular and molecular research on these processes in mice and humans.

The role of germ cell loss during primordial follicle assembly: a review of current advances

In most female mammals, early germline development begins with the appearance of primordial germ cells (PGCs), and develops to form mature oocytes following several vital processes. It remains well accepted that significant germ cell apoptosis and oocyte loss takes place around the time of birth. The transition of the ovarian environment from fetal to neonatal, coincides with the loss of germ cells and the timing of follicle formation. All told it is common to lose approximately two thirds of germ cells during this transition period. The current consensus is that germ cell loss can be attributed, at least in part, to programmed cell death (PCD). Recently, autophagy has been implicated as playing a part in germ cell loss during the time of parturition. In this review, we discuss the major opinions and mechanisms of mammalian ovarian PCD during the process of germ cell loss. We also pay close attention to the function of autophagy in germ cell loss, and speculate that autophagy may also serve as a critical and necessary process during the establishment of primordial follicle pool.