Cell death in fetal oocytes: Many players for multiple pathways

Protein phosphatase 4 maintains the survival of primordial follicles by regulating autophagy in oocytes

In mammalian ovary, the primordial follicle pool serves as the source of developing follicles and fertilizable ova. To maintain the normal length of female reproductive life, the primordial follicles must have adequate number and be kept in a quiescent state before menopause. However, the molecular mechanisms underlying primordial follicle survival are poorly understood. Here, we provide genetic evidence showing that lacking protein phosphatase 4 (PPP4) in oocytes, a member of PP2A-like subfamily, results in infertility in female mice. A large quantity of primordial follicles has been depleted around the primordial follicle pool formation phase and the ovarian reserve is exhausted at about 7 months old. Further investigation demonstrates that depletion of PPP4 causes the abnormal activation of mTOR, which suppresses autophagy in primordial follicle oocytes. The abnormal primordial follicle oocytes are eventually erased by pregranulosa cells in the manner of lysosome invading. These results show that autophagy prevents primordial follicles over loss and PPP4-mTOR pathway governs autophagy during the primordial follicle formation and dormant period.

Gonadotropin upregulates intraovarian calpains-1 and -2 during ovarian follicular recruitment in the SD rat model

Calpain role has been shown in the cumulus cell-oocyte complexes and, corpus luteum. We investigated the association of calpains-1 and -2 in ovarian folliculogenesis using the Sprague-Dawley (SD) rat model and steroidogenesis in the human granulosa cells (hGCs). We induced PCOS in 42-day-old SD rats by letrozole oral gavage for 21 days. Premature ovarian failure (POF) was induced in 21-day-old SD rats by 4-vinylcyclohexene diepoxide (VCD). Ovulation and ovarian hyperstimulatory (OHS) syndrome were induced by pregnant mare gonadotropin (PMSG) + human chorionic gonadotropin (hCG) treatments in 21 days SD rats, respectively. Steroidogenesis is stimulated in human granulosa cells (hGCs) by forskolin and the response of 17-beta-estradiol (E2) on calpains expression was checked in hGCs. The protein expression by immunoblotting and activity by biochemical assay of calpains-1 and -2 showed an oscillating pattern in the ovarian cycle. PMSG-induced follicular recruitment showed upregulation of calpains-1 and -2, but with no change during ovarian function cessation (POF). Upregulated calpain-2 expression and calpain activity was found in the hCG +PMSG-induced ovulation. Letrozole-induced PCOS showed downregulation of calpain-1, but upregulation of calpain-2. PMSG+hCG-induced OHS led to the upregulation of calpain-1. Letrozole and metformin separately increased the expression level of calpains-1 and -2 in the hGCs during luteinization. In conclusion, the expression levels of calpains -1 and -2 are increased with ovarian follicular recruitment by PMSG and calpain-1 is decreased in the PCOS condition, and letrozole and metformin upregulate the expression of calpains-1 and -2 during luteinization in the hGCs possibly via E2 action.

Reactive oxygen species signalling in the deterioration of quality of mammalian oocytes cultured in vitro: Protective effect of antioxidants

The in vitro fertilization (IVF) is the first choice of infertile couples worldwide to plan for conception. Besides having a significant advancement in IVF procedure, the success rate is still poor. Although several approaches have been tested to improve IVF protocol, minor changes in culture conditions, physical factors and/or drug treatment generate reactive oxygen species (ROS) in oocytes. Due to large size and huge number of mitochondria, oocyte is more susceptible towards ROS-mediated signalling under in vitro culture conditions. Elevation of ROS levels destabilize maturation promoting factor (MPF) that results in meiotic exit from diplotene as well as metaphase-II (M-II) arrest in vitro. Once meiotic exit occurs, these oocytes get further arrested at metaphase-I (M-I) stage or metaphase-III (M-III)-like stage under in vitro culture conditions. The M-I as well as M-III arrested oocytes are not fit for fertilization and limits IVF outcome. Further, the generation of excess levels of ROS cause oxidative stress (OS) that initiate downstream signalling to initiate various death pathways such as apoptosis, autophagy, necroptosis and deteriorates oocyte quality under in vitro culture conditions. The increase of cellular enzymatic antioxidants and/or supplementation of exogenous antioxidants in culture medium protect ROS-induced deterioration of oocyte quality in vitro. Although a growing body of evidence suggests the ROS and OS-mediated deterioration of oocyte quality in vitro, their downstream signalling and related mechanisms remain poorly understood. Hence, this review article summarizes the existing evidences concerning ROS and OS-mediated downstream signalling during deterioration of oocyte quality in vitro. The use of various antioxidants against ROS and OS-mediated impairment of oocyte quality in vitro has also been explored in order to increase the success rate of IVF during assisted reproductive health management.

Impaired primordial follicle assembly in offspring ovaries from zearalenone-exposed mothers involves reduced mitochondrial activity and altered epigenetics in oocytes

Previous works have shown that zearalenone (ZEA), as an estrogenic pollutant, has adverse effects on mammalian folliculogenesis. In the present study, we found that prolonged exposure of female mice to ZEA around the end of pregnancy caused severe impairment of primordial follicle formation in the ovaries of newborn mice and altered the expression of many genes in oocytes as revealed by single-cell RNA sequencing (scRNA-seq). These changes were associated with morphological and molecular alterations of mitochondria, increased autophagic markers in oocytes, and epigenetic changes in the ovaries of newborn mice from ZEA-exposed mothers. The latter increased expression of HDAC2 deacetylases was leading to decreased levels of H3K9ac and H4K12ac. Most of these modifications were relieved when the expression of  Hdac2 in newborn ovaries was reduced by RNA interference during in vitro culture in the presence of ZEA. Such changes were also alleviated in offspring ovaries from mothers treated with both ZEA and the coenzyme Q10 (CoQ10), which is known to be able to restore mitochondrial activities. We concluded that impaired mitochondrial activities in oocytes caused by ZEA are at the origin of metabolic alterations that modify the expression of genes controlling autophagy and primordial follicle assembly through changes in epigenetic histones.

Inhibiting bridge integrator 2 phosphorylation leads to improved oocyte quality, ovarian health and fertility in aging and after chemotherapy in mice

Female ovaries degenerate about 20 years earlier than testes leading to reduced primordial follicle reserve and a reduction in oocyte quality. Here we found that bridge integrator 2 (BIN2) is enriched in mouse ovaries and oocytes and that global knockout of this protein improves both female fertility and oocyte quality. Quantitative ovarian proteomics and phosphoproteomics showed that Bin2 knockout led to a decrease in phosphorylated ribosomal protein S6 (p-RPS6), a component of the mammalian target of rapamycin pathway and greatly increased nicotinamide nucleotide transhydrogenase (NNT), the free-radical detoxifier. Mechanistically, we find that phosphorylation of BIN2 at Thr423 and Ser424 leads to its translocation from the membrane to the cytoplasm, subsequent phosphorylation of RPS6 and inhibition of Nnt translation. We synthesized a BIN2-penetrating peptide (BPP) designed to inhibit BIN2 phosphorylation and found that a 3-week BPP treatment improved primordial follicle reserve and oocyte quality in aging and after chemotherapy-induced premature ovarian failure without discernible side effects.

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

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

Characterization of the effects of heat stress on autophagy induction in the pig oocyte

BACKGROUND

Heat stress (HS) occurs when body heat accumulation exceeds heat dissipation and is associated with swine seasonal infertility. HS contributes to compromised oocyte integrity and reduced embryo development. Autophagy is a potential mechanism for the oocyte to mitigate the detrimental effects of HS by recycling damaged cellular components.

METHODS

To characterize the effect of HS on autophagy in oocyte maturation, we utilized an in vitro maturation (IVM) system where oocytes underwent thermal neutral (TN) conditions throughout the entire maturation period (TN/TN), HS conditions during the first half of IVM (HS/TN), or HS conditions during the second half of IVM (TN/HS).

RESULTS

To determine the effect of HS on autophagy induction within the oocyte, we compared the relative abundance and localization of autophagy-related proteins. Heat stress treatment affected the abundance of two well described markers of autophagy induction: autophagy related gene 12 (ATG12) in complex with ATG5 and the cleaved form of microtubule-associated protein 1 light chain 3 beta (LC3B-II). The HS/TN IVM treatment increased the abundance of the ATG12-ATG5 complex and exacerbated the loss of LC3B-II in oocytes. The B-cell lymphoma 2 like 1 protein (BCL2L1) can inhibit autophagy or apoptosis through its interaction with either beclin1 (BECN1) or BCL2 associated X, apoptosis regulator (BAX), respectively. We detected colocalization of BCL2L1 with BAX but not BCL2L1 with BECN1, suggesting that apoptosis is inhibited under the HS/TN treatment but not autophagy. Interestingly, low doses of the autophagy inducer, rapamycin, increased oocyte maturation.

CONCLUSIONS

Our results here suggest that HS increases autophagy induction in the oocyte during IVM, and that artificial induction of autophagy increases the maturation rate of oocytes during IVM. These data support autophagy as a potential mechanism activated in the oocyte during HS to recycle damaged cellular components and maintain developmental competence.

Visfatin protein may be responsible for suppression of proliferation and apoptosis in the infantile mice ovary

Visfatin is an important adipokines, which are expressed in different tissues including ovary of mammals. The postnatal ovary in rodents undergoes dramatic changes of intra-ovarian factors in relation to proliferation and apoptosis. There are studies which showed that gonadal visfatin changes in postnatal life. However, role of visfatin in the early postnatal period i.e. infantile period has not been studied. Therefore, the present study was aimed to explore the role of visfatin in the early postnatal ovarian functions. Furthermore, to explore the role of visfatin, the endogenous visfatin was inhibited from PND14-PND21 by FK866 with dose of 1.5 mg/kg. Our results showed gain in body weight and ovarian weight after visfatin inhibition. The inhibition of visfatin increased the ovarian proliferation (increase in PCNA, GCNA expression and BrdU incorporation) and apoptosis (increase in BAX and active caspase3 expression). Moreover, visfatin inhibition decreased the expression of antiapoptotic/survival protein, BCL2 in the ovary. These findings suggest that visfatin in the infantile ovary may suppress the proliferation and apoptosis by up-regulating BCL2 expression. An interesting finding has been observed that circulating estrogen and progesterone remain unaffected, although visfatin inhibition up-regulated ER-β and down-regulated ER-α. It may also be suggested that visfatin could regulates proliferation and apoptosis via modulating estrogen signaling. In conclusion, visfatin inhibits the proliferation and apoptosis without modulating the ovarian steroid biosynthesis and visfatin mediated BCL2 expression could also be mechanism to preserve the good quality follicle in early postnatal period.

The Role of Caspase-2 in Regulating Cell Fate

Caspase-2 is the most evolutionarily conserved member of the mammalian caspase family and has been implicated in both apoptotic and non-apoptotic signaling pathways, including tumor suppression, cell cycle regulation, and DNA repair. A myriad of signaling molecules is associated with the tight regulation of caspase-2 to mediate multiple cellular processes far beyond apoptotic cell death. This review provides a comprehensive overview of the literature pertaining to possible sophisticated molecular mechanisms underlying the multifaceted process of caspase-2 activation and to highlight its interplay between factors that promote or suppress apoptosis in a complicated regulatory network that determines the fate of a cell from its birth and throughout its life.

The regulation of autophagy in the pesticide-induced toxicity: Angel or demon?

Pesticides have become an essential tool for pest kill, weed control and microbiome inhibition for both agricultural and domestic use. However, with the massive use, pesticides can exist in soil, air and water, and sometimes even accumulate in the human or other mammals through food chains. Lots of researches have proven that pesticides possess toxicity to mammals on endocrine, neural and immune systems. Autophagy, as a conservative intracellular process, which is activated by stress-related signals, plays a pivotal role, either "angle" or "demon", in regulation of cell fate and function. Recent evidences in researches elucidated a strong link between the autophagy and the toxicity of pesticides. In this review, we summarized the previous researches which focus on the autophagy regulation in the pesticides-induced toxicity, and hope that this work can help us to discover a potential strategy for the treatment of the disease caused by pesticides.

Interplay between Caspase 9 and X-linked Inhibitor of Apoptosis Protein (XIAP) in the oocyte elimination during fetal mouse development

Mammalian female fertility is limited by the number and quality of oocytes in the ovarian reserve. The number of oocytes is finite since all germ cells cease proliferation to become oocytes in fetal life. Moreover, 70-80% of the initial oocyte population is eliminated during fetal and neonatal development, restricting the ovarian reserve. Why so many oocytes are lost during normal development remains an enigma. In Meiotic Prophase I (MPI), oocytes go through homologous chromosome synapsis and recombination, dependent on formation and subsequent repair of DNA double strand breaks (DSBs). The oocytes that have failed in DSB repair or synapsis get eliminated mainly in neonatal ovaries. However, a large oocyte population is eliminated before birth, and the cause or mechanism of this early oocyte loss is not well understood. In the current paper, we show that the oocyte loss in fetal ovaries was prevented by a deficiency of Caspase 9 (CASP9), which is the hub of the mitochondrial apoptotic pathway. Furthermore, CASP9 and its downstream effector Caspase 3 were counteracted by endogenous X-linked Inhibitor of Apoptosis (XIAP) to regulate the oocyte population; while XIAP overexpression mimicked CASP9 deficiency, XIAP deficiency accelerated oocyte loss. In the CASP9 deficiency, more oocytes were accumulated at the pachytene stage with multiple γH2AFX foci and high LINE1 expression levels, but with normal levels of synapsis and overall DSB repair. We conclude that the oocytes with LINE1 overexpression were preferentially eliminated by CASP9-dependent apoptosis in balance with XIAP during fetal ovarian development. When such oocytes were retained, however, they get eliminated by a CASP9-independent mechanism during neonatal development. Thus, the oocyte is equipped with multiple surveillance mechanisms during MPI progression to safe-guard the quality of oocytes in the ovarian reserve.

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.

Heat stress induces autophagy in pig ovaries during follicular development

Hyperthermia or heat stress (HS) occurs when heat dissipation mechanisms are overwhelmed by external and internal heat production. Hyperthermia negatively affects reproduction and potentially compromises oocyte integrity and reduces developmental competence of ensuing embryos. Autophagy is the process by which cells recycle energy through the reutilization of cellular components and is activated by a variety of stressors. Study objectives were to characterize autophagy-related proteins in the ovary following cyclical HS during the follicular phase. Twelve gilts were synchronized and subjected to cyclical HS (n = 6) or thermal neutral (n = 6) conditions for 5 days during the follicular phase. Ovarian protein abundance of Beclin 1 and microtubule associated protein light chain 3 beta II were each elevated as a result of HS (P = 0.001 and 0.003, respectively). The abundance of the autophagy related (ATG)12-ATG5 complex was decreased as a result of HS (P = 0.002). Regulation of autophagy and apoptosis occurs in tight coordination, and B-cell lymphoma (BCL)2 and BCL2L1 are involved in regulating both processes. BCL2L1 protein abundance, as detected via immunofluorescence, was increased in both the oocyte (∼1.6-fold; P < 0.01) and granulosa cells of primary follicles (∼1.4-fold P < 0.05) of HS ovaries. These results suggest that ovarian autophagy induction occurs in response to HS during the follicular phase, and that HS increases anti-apoptotic signaling in oocytes and early follicles. These data contribute to the biological understanding of how HS acts as an environmental stress to affect follicular development and negatively impact reproduction.

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.

Z-cells and oogonia/oocytes in the advanced process of autophagy are the dominant altered cells in the ovaries of hypothyroid newborn rats

Induced prenatal hypothyroidism in rat pups leads to accelerated primordial follicle assembly and premature follicular atresia with ovary failure. This work investigates the influence of maternal hypothyroidism induced with 6-n-propyl-2-thyouracil (PTU) on the number and morphology of oogonia/oocytes in newborn rat pups with light and transmission electron microscopy. Expression of apoptosis and autophagy markers in oogonia/oocytes were examined using immunohistochemistry. Hypothyroid newborn pups had a decreased number of mitotic and resting oogonia, while the number of altered oogonia/oocytes was increased. Ultrastructural observations revealed the increased presence of degenerated pachytene oocytes (Z-cells) and oogonia/oocytes undergoing autophagy, apoptosis and combined apoptosis and autophagy, in this group. The most abundant altered oogonia/oocytes in the hypothyroid group were those with morphological features of advanced autophagy and Z-cells. The percentage of TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) positive oogonia/oocytes was significantly lower in the hypothyroid group. No significant difference was recorded in the expression of caspase-3, ATG7 and LC3 possibly reflecting that these proteins were not involved in the oogonia/oocyte alteration process during prenatal rat hypothyroidism. The obtained results indicate that developmental hypothyroidism in the offspring enhances the number of Z-cells and oogonia/oocytes altered with the advanced process of autophagy.

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.

Starvation at birth impairs germ cell cyst breakdown and increases autophagy and apoptosis in mouse oocytes

The female reproductive lifespan is largely determined by the size of primordial follicle pool, which is established following germ cell cyst breakdown around birth. Almost two-third of oocytes are lost during germ cell cysts breakdown, following autophagic and apoptosis mechanisms. To investigate a possible relationship between germ cell cyst breakdown and nutrition supply, we established a starvation model in mouse pups at birth and evaluated the dynamics of cyst breakdown during nutrient deprivation. Our results showed that after 36 h of starvation between 1.5 and 3 d.p.p., indicators of metabolism both at systemic and ovarian level were significantly altered and the germ cell cyst breakdown markedly decreased. We also found that markers of oxidative stress, autophagy and apoptosis were increased and higher number of oocytes in cyst showing autophagic markers and of TUNEL-positive oocytes and somatic cells were present in the ovaries of starved pups. Moreover, the proliferation of pre-granulosa cells and the expression of the oocyte-specific transcription factor Nobox were decreased in such ovaries. Finally, we observed that the ovaries of the starved pups could recover a normal number of follicles after about 3 weeks from re-feeding. In conclusion, these data indicate that nutrient deficiency at birth can generate a number of adaptive metabolic and oxidative responses in the ovaries causing increased apoptosis both in the somatic cells and oocyte and autophagy mainly in these latter and leading to a delay of germ cell cyst breakdown and follicle assembly.

Effects of Endocrine-disrupting Chemicals on Female Reproductive Health

Endocrine-disrupting chemicals (EDCs) are increasingly prevalent in the environment and the evidence demonstrates that they affect reproductive health, has been accumulating for the last few decades. In this review of recent literature, we present evidence of the effects of estrogen-mimicking EDCs on female reproductive health especially the ovaries and uteri. As representative EDCs, data from studies with a pharmaceutical estrogen, diethylstilbestrol (DES), an organochlorine pesticide methoxychlor (MXC), a phytoestrogen (genistein), and a chemical used in plastics, bisphenol a (BPA) have been presented. We also discuss the effects of a commonly found plasticizer in the environment, a phthalate (DEHP), even though it is not a typical estrogenic EDC. Collectively, these studies show that exposures during fetal and neonatal periods cause developmental reprogramming leading to adult reproductive disease. Puberty, estrous cyclicity, ovarian follicular development, and uterine functions are all affected by exposure to these EDCs. Evidence that epigenetic modifications are involved in the progression to adult disease is also presented.

Physiologic Course of Female Reproductive Function: A Molecular Look into the Prologue of Life

The genetic, endocrine, and metabolic mechanisms underlying female reproduction are numerous and sophisticated, displaying complex functional evolution throughout a woman's lifetime. This vital course may be systematized in three subsequent stages: prenatal development of ovaries and germ cells up until in utero arrest of follicular growth and the ensuing interim suspension of gonadal function; onset of reproductive maturity through puberty, with reinitiation of both gonadal and adrenal activity; and adult functionality of the ovarian cycle which permits ovulation, a key event in female fertility, and dictates concurrent modifications in the endometrium and other ovarian hormone-sensitive tissues. Indeed, the ultimate goal of this physiologic progression is to achieve ovulation and offer an adequate environment for the installation of gestation, the consummation of female fertility. Strict regulation of these processes is important, as disruptions at any point in this evolution may equate a myriad of endocrine-metabolic disturbances for women and adverse consequences on offspring both during pregnancy and postpartum. This review offers a summary of pivotal aspects concerning the physiologic course of female reproductive function.

Influence of follicle stage on artificial ovary outcome using fibrin as a matrix

STUDY QUESTION

Do primordial-primary versus secondary follicles embedded inside a fibrin matrix have different capabilities to survive and grow after isolation and transplantation?

SUMMARY ANSWER

Mouse primordial-primary follicles showed a lower recovery rate than secondary follicles, but both were able to grow.

WHAT IS KNOWN ALREADY

Fresh isolated mouse follicles and ovarian stromal cells embedded in a fibrin matrix are capable of surviving and developing after short-term autografting.

STUDY DESIGN, SIZE, DURATION

In vivo experimental model using 11 donor Naval Medical Research Institute (NMRI) mice and 11 recipient severe combined immunodeficiency (SCID) mice. Both ovaries from all NMRI mice were mechanically disrupted and primordial-primary and secondary follicles were isolated with ovarian stromal cells. They were then encapsulated in a fibrin matrix composed of 12.5 mg/ml of fibrinogen (F12.5) and 1 IU/ml of thrombin (T1) (F12.5/T1), and grafted to the inner part of the peritoneum of SCID mice for 2 and 7 days.

PARTICIPANTS/MATERIALS, SETTING, METHODS

This study was conducted at the Gynecology Research Unit, Université Catholique de Louvain. All materials were used to conduct histological (H-E staining) and immunohistochemical (Ki67, TUNEL) analyses.

MAIN RESULTS AND THE ROLE OF CHANCE

Although all grafted fibrin clots were recovered, the follicle recovery rate on day 2 was 16 and 40% for primordial-primary and secondary follicles respectively, while on day 7, it was 6 and 28%. The secondary group showed a significantly higher recovery rate than the primordial-primary group (23%, P-value <0.001). Follicles found in both groups were viable, as demonstrated by live/dead assays, and no difference was observed in the apoptosis rate between groups, as evidenced by TUNEL. Their growth to further stages was confirmed by Ki67 immunostaining.

LIMITATIONS, REASONS FOR CAUTION

As demonstrated by our results, secondary follicles appear to be more likely to survive and develop than primordial-primary follicles in a fibrin matrix after both periods of grafting. These findings may also be attributed to the specific features of the fibrin matrix, which could benefit larger follicles, but not smaller follicles.

WIDER IMPLICATIONS OF THE FINDINGS

This study is essential to understanding possible impairment caused by factors such as the isolation procedure or fibrin matrix composition to the survival and development of different follicle stages. It therefore provides the basis for further investigations with longer periods of grafting.

STUDY FUNDING/COMPETING INTERESTS

This study was supported by grants from the Fonds National de la Recherche Scientifique de Belgique (grant Télévie No. 7.4578.14 and 7.4627.13, grant 5/4/150/5 awarded to Marie-Madeleine Dolmans), Fonds Spéciaux de Recherche, Fondation St Luc, the Foundation Against Cancer, and the Region Wallone (Convention N°6519-OVART) and donations from Mr Pietro Ferrero, Baron Frère and Viscount Philippe de Spoelberch. None of the authors have any competing interests to declare.

S100A8, An Oocyte-Specific Chemokine, Directs the Migration of Ovarian Somatic Cells During Mouse Primordial Follicle Assembly

In the mammalian ovaries, the primordial follicle pool determines the reproductive capability over the lifetime of a female. The primordial follicle is composed of two cell members, namely the oocyte and the pre-granulosa cells that encircle the oocyte. However, it is unclear what factors are involved in the reorganization of the two distinct cells into one functional unit. This study was performed to address this issue. Firstly, in an in vitro reconstruction system, dispersed ovarian cells from murine fetal ovaries at 19.0 days post coitum (dpc) reassembled into follicle-like structures, independent of the physical distance between the cells, implying that either oocytes or ovarian somatic cells (OSCs) were motile. We then carried out a series of transwell assay experiments, and determined that it was in fact 19.0 dpc OSCs (as opposed to oocytes), which exhibited a significant chemotactic response to both fetal bovine serum and oocytes themselves. We observed that S100A8, a multi-functional chemokine, may participate in the process as it is mainly expressed in oocytes within the cysts/plasmodia. S100A8 significantly promoted the number of migrating OSCs by 2.5 times in vitro, of which 66.9% were FOXL2 protein-positive cells, implying that the majority of motile OSCs were pre-granulosa cells. In addition, an S100A8-specific antibody inhibited the formation of follicle-like reconstruction cell mass in vitro. And, the primordial follicle formation was reduced when S100a8-specific siRNA was applied onto in vitro cultured 17.5 dpc ovary. Therefore, S100A8 could be a chemokine of oocyte origin, which attracts OSCs to form the primordial follicles.

Mcl-1 is a key regulator of the ovarian reserve

A majority of ovarian follicles are lost to natural death, but the disruption of factors involved in maintenance of the oocyte pool results in a further untimely follicular depletion known as premature ovarian failure. The anti-apoptotic B-cell lymphoma 2 (Bcl-2) family member myeloid cell leukemia-1 (MCL-1) has a pro-survival role in various cell types; however, its contribution to oocyte survival is unconfirmed. We present a phenotypic characterization of oocytes deficient in Mcl-1, and establish its role in maintenance of the primordial follicle (PMF) pool, growing oocyte survival and oocyte quality. Mcl-1 depletion resulted in the premature exhaustion of the ovarian reserve, characterized by early PMF loss because of activation of apoptosis. The increasingly diminished surviving cohort of growing oocytes displayed elevated markers of autophagy and mitochondrial dysfunction. Mcl-1-deficient ovulated oocytes demonstrated an increased susceptibility to cellular fragmentation with activation of the apoptotic cascade. Concomitant deletion of the pro-apoptotic Bcl-2 member Bcl-2-associated X protein (Bax) rescued the PMF phenotype and ovulated oocyte death, but did not prevent the mitochondrial dysfunction associated with Mcl-1 deficiency and could not rescue long-term breeding performance. We thus recognize MCL-1 as the essential survival factor required for conservation of the postnatal PMF pool, growing follicle survival and effective oocyte mitochondrial function.

Cell death and morphogenesis during early mouse development: are they interconnected?

Shortly after implantation the embryonic lineage transforms from a coherent ball of cells into polarized cup shaped epithelium. Recently we elucidated a previously unknown apoptosis-independent morphogenic event that reorganizes the pluripotent lineage. Polarization cues from the surrounding basement membrane rearrange the epiblast into a polarized rosette-like structure, where subsequently a central lumen is established. Thus, we provided a new model revising the current concept of apoptosis-dependent epiblast morphogenesis. Cell death however has to be tightly regulated during embryogenesis to ensure developmental success. Here, we follow the stages of early mouse development and take a glimpse at the critical signaling and morphogenic events that determine cells destiny and reshape the embryonic lineage.

Physiological consequences of heat stress in pigs

Heat stress negatively influences the global pork industry and undermines genetic, nutritional, management and pharmaceutical advances in management, feed and reproductive efficiency. Specifically, heat stress-induced economic losses result from poor sow performance, reduced and inconsistent growth, decreased carcass quality, mortality, morbidity, and processing issues caused by less rigid adipose tissue (also known as flimsy fat). When environmental conditions exceed the pig’s thermal neutral zone, nutrients are diverted from product synthesis (meat, fetus, milk) to body temperature maintenance thereby compromising efficiency. Unfortunately, genetic selection for both increased litter size and leaner phenotypes decreases pigs’ tolerance to heat, as enhanced fetal development and protein accretion results in increased basal heat production. Additionally, research has demonstrated that in utero heat stress negatively and permanently alters post-natal body temperature and body composition and both variables represent an underappreciated consequence of heat stress. Advances in management (i.e. cooling systems) have partially alleviated the negative impacts of heat stress, but productivity continues to decline during the warm summer months. The detrimental effects of heat stress on animal welfare and production will likely become more of an issue in regions most affected by continued predictions for climate change, with some models forecasting extreme summer conditions in key animal-producing areas of the globe. Therefore, heat stress is likely one of the primary factors limiting profitable animal protein production and will certainly continue to compromise food security (especially in emerging countries) and regionalise pork production in developed countries. Thus, there is an urgent need to have a better understanding of how heat stress reduces animal productivity. Defining the biology of how heat stress jeopardises animal performance is critical in developing approaches (genetic, managerial, nutritional and pharmaceutical) to ameliorate current production issues and improve animal wellbeing and performance.

Taking control of the female fertile lifespan: a key role for Bcl-2 family proteins

Precisely how the length of the female fertile lifespan is regulated is poorly understood and it is likely to involve complex factors, one of which is follicle number. Indeed, the duration of female fertility appears to be intimately linked to the number of available oocytes, which are stored in the ovary as primordial follicles. There is mounting evidence implicating the intrinsic apoptosis pathway, which is controlled by members of the B-cell lymphoma-2 (BCL-2) family, as a key regulator of the number of primordial follicles established in the ovary at birth and maintained throughout reproductive life. Consequently, the pro- and anti-apoptotic BCL-2 family proteins are emerging as key determinants of the length of the female fertile lifespan. This review discusses the relationship between the intrinsic apoptosis pathway, follicle number and length of the female fertile lifespan.

Involvement of doublesex and mab-3-related transcription factors in human female germ cell development demonstrated by xenograft and interference RNA strategies

We identified three doublesex and mab-3-related transcription factors (DMRT) that were sexually differentially expressed in human fetal gonads and present in the ovaries at the time of meiotic initiation. These were also identified in murine embryonic female germ cells. Among these, we focused on DMRTA2 (DMRT5), whose function is unknown in the developing gonads, and clarified its role in human female fetal germ cells, using an original xenograft model. Early human fetal ovaries (8-11 weeks post-fertilization) were grafted into nude mice. Grafted ovaries developed normally, with no apparent overt changes, when compared with ungrafted ovaries at equivalent developmental stages. Appropriate germ cell density, mitotic/meiotic transition, markers of meiotic progression and follicle formation were evident. Four weeks after grafting, mice were treated with siRNA, specifically targeting human DMRTA2 mRNA. DMRTA2 inhibition triggered an increase in undifferentiated FUT4-positive germ cells and a decrease in the percentage of meiotic γH2AX-positive germ cells, when compared with mice that were injected with control siRNA. Interestingly, the expression of markers associated with pre-meiotic germ cell differentiation was also impaired, as was the expression of DMRTB1 (DMRT6) and DMRTC2 (DMRT7). This study reveals, for the first time, the requirement of DMRTA2 for normal human female embryonic germ cell development. DMRTA2 appears to be necessary for proper differentiation of oogonia, prior to entry into meiosis, in the human species. Additionally, we developed a new model of organ xenografting, coupled with RNA interference, which provides a useful tool for genetic investigations of human germline development.

microRNA 376a regulates follicle assembly by targeting Pcna in fetal and neonatal mouse ovaries

In mammals, the primordial follicle pool, providing all oocytes available to a female throughout her reproductive life, is established perinatally. Dysregulation of primordial follicle assembly results in female reproductive diseases, such as premature ovarian insufficiency and infertility. Female mice lackingDicer1(Dicer), a gene required for biogenesis of microRNAs, show abnormal morphology of follicles and infertility. However, the contribution of individual microRNAs to primordial follicle assembly remains largely unknown. Here, we report that microRNA 376a (miR-376a) regulates primordial follicle assembly by modulating the expression of proliferating cell nuclear antigen (Pcna), a gene we previously reported to regulate primordial follicle assembly by regulating oocyte apoptosis in mouse ovaries. miR-376a was shown to be negatively correlated withPcnamRNA expression in fetal and neonatal mouse ovaries and to directly bind toPcnamRNA 3′ untranslated region. Cultured 18.5 days postcoitum mouse ovaries transfected with miR-376a exhibited decreasedPcnaexpression both in protein and mRNA levels. Moreover, miR-376a overexpression significantly increased primordial follicles and reduced apoptosis of oocytes, which was very similar to those in ovaries co-transfected with miR-376a and siRNAs targetingPcna. Taken together, our results demonstrate that miR-376a regulates primordial follicle assembly by modulating the expression ofPcna. To our knowledge, this is the first microRNA–target mRNA pair that has been reported to regulate mammalian primordial follicle assembly and further our understanding of the regulation of primordial follicle assembly.

Prenatal exposure to chromium induces early reproductive senescence by increasing germ cell apoptosis and advancing germ cell cyst breakdown in the F1 offspring

Hexavalent chromium (CrVI), one of the more toxic heavy metals, is widely used in more than 50 industries such as chrome plating, welding, wood processing and tanneries. As one of the world's leading producers of chromium compounds, the U.S. is facing growing challenges in protecting human health against multiple adverse effects of CrVI. CrVI is rapidly converted to CrIII intracellularly, and can induce apoptosis through different mechanisms. Our previous studies demonstrated postnatal exposure to CrVI results in a delay or arrest in follicle development and puberty. Pregnant rats were treated with 25 ppm potassium dichromate (CrVI) from gestational day (GD) 9.5 to 14.5 through drinking water, placentae were removed on GD 20, and total Cr was estimated in the placentae; ovaries were removed from the F1 offspring on postnatal day (PND)-1 and various analyses were performed. Our results show that gestational exposure to CrVI resulted in (i) increased Cr concentration in the placenta, (ii) increased germ cell apoptosis by up-regulating p53/p27-Bax-caspase-3 proteins and by increasing p53-SOD-2 co-localization; (iii) accelerated germ cell cyst (GCC) breakdown; (iv) advanced primordial follicle assembly and primary follicle transition and (v) down regulation of p-AKT, p-ERK and XIAP. As a result of the above events, CrVI induced early reproductive senescence and decrease in litter size in F1 female progeny.

Notch pathway regulates female germ cell meiosis progression and early oogenesis events in fetal mouse

A critical process of early oogenesis is the entry of mitotic oogonia into meiosis, a cell cycle switch regulated by a complex gene regulatory network. Although Notch pathway is involved in numerous important aspects of oogenesis in invertebrate species, whether it plays roles in early oogenesis events in mammals is unknown. Therefore, the rationale of the present study was to investigate the roles of Notch signaling in crucial processes of early oogenesis, such as meiosis entry and early oocyte growth. Notch receptors and ligands were localized in mouse embryonic female gonads and 2 Notch inhibitors, namely DAPT and L-685,458, were used to attenuate its signaling in an in vitro culture system of ovarian tissues from 12.5 days post coitum (dpc) fetus. The results demonstrated that the expression of Stra8, a master gene for germ cell meiosis, and its stimulation by retinoic acid (RA) were reduced after suppression of Notch signaling, and the other meiotic genes, Dazl, Dmc1, and Rec8, were abolished or markedly decreased. Furthermore, RNAi of Notch1 also markedly inhibited the expression of Stra8 and SCP3 in cultured female germ cells. The increased methylation status of CpG islands within the Stra8 promoter of the oocytes was observed in the presence of DAPT, indicating that Notch signaling is probably necessary for maintaining the epigenetic state of this gene in a way suitable for RA stimulation. Furthermore, in the presence of Notch inhibitors, progression of oocytes through meiosis I was markedly delayed. At later culture periods, the rate of oocyte growth was decreased, which impaired subsequent primordial follicle assembly in cultured ovarian tissues. Taken together, these results suggested new roles of the Notch signaling pathway in female germ cell meiosis progression and early oogenesis events in mammals.

Quantification of healthy and atretic germ cells and follicles in the developing and post-natal ovary of the South American plains vizcacha, Lagostomus maximus: evidence of continuous rise of the germinal reserve

The female germ line in mammals is subjected to massive cell death that eliminates 60-85% of the germinal reserve by birth and continues from birth to adulthood until the exhaustion of the germinal pool. Germ cell demise occurs mainly through apoptosis by means of a biased expression in favour of pro-apoptotic members of the BCL2 gene family. By contrast, the South American plains vizcacha, Lagostomus maximus, exhibits sustained expression of the anti-apoptotic BCL2 gene throughout gestation and a low incidence of germ cell apoptosis. This led to the proposal that, in the absence of death mechanisms other than apoptosis, the female germ line should increase continuously from foetal life until after birth. In this study, we quantified all healthy germ cells and follicles in the ovaries of L. maximus from early foetal life to day 60 after birth using unbiased stereological methods and detected apoptosis by labelling with TUNEL assay. The healthy germ cell population increased continuously from early-developing ovary reaching a 50 times higher population number by the end of gestation. TUNEL-positive germ cells were <0.5% of the germ cell number, except at mid-gestation (3.62%). Mitotic proliferation, entrance into prophase I stage and primordial follicle formation occurred as overlapping processes from early pregnancy to birth. Germ cell number remained constant in early post-natal life, but a remnant population of non-follicular VASA- and PCNA-positive germ cells still persisted at post-natal day 60. L. maximus is the first mammal so far described in which female germ line develops in the absence of constitutive massive germ cell elimination.

The dynamics of the primordial follicle reserve

The female germline comprises a reserve population of primordial (non-growing) follicles containing diplotene oocytes arrested in the first meiotic prophase. By convention, the reserve is established when all individual oocytes are enclosed by granulosa cells. This commonly occurs prior to or around birth, according to species. Histologically, the ‘reserve’ is the number of primordial follicles in the ovary at any given age and is ultimately depleted by degeneration and progression through folliculogenesis until exhausted. How and when the reserve reaches its peak number of follicles is determined by ovarian morphogenesis and germ cell dynamics involving i) oogonial proliferation and entry into meiosis producing an oversupply of oocytes and ii) large-scale germ cell death resulting in markedly reduced numbers surviving as the primordial follicle reserve. Our understanding of the processes maintaining the reserve comes primarily from genetically engineered mouse models, experimental activation or destruction of oocytes, and quantitative histological analysis. As the source of ovulated oocytes in postnatal life, the primordial follicle reserve requires regulation of i) its survival or maintenance, ii) suppression of development (dormancy), and iii) activation for growth and entry into folliculogenesis. The mechanisms influencing these alternate and complex inter-related phenomena remain to be fully elucidated. Drawing upon direct and indirect evidence, we discuss the controversial concept of postnatal oogenesis. This posits a rare population of oogonial stem cells that contribute new oocytes to partially compensate for the age-related decline in the primordial follicle reserve.

Relationship between numerous mast cells and early follicular development in neonatal MRL/MpJ mouse ovaries

In the neonatal mouse ovary, clusters of oocytes called nests break into smaller cysts and subsequently form individual follicles. During this period, we found numerous mast cells in the ovary of MRL/MpJ mice and investigated their appearance and morphology with follicular development. The ovarian mast cells, which were already present at postnatal day 0, tended to localize adjacent to the surface epithelium. Among 11 different mouse strains, MRL/MpJ mice possessed the greatest number of ovarian mast cells. Ovarian mast cells were also found in DBA/1, BALB/c, NZW, and DBA/2 mice but rarely in C57BL/6, NZB, AKR, C3H/He, CBA, and ICR mice. The ovarian mast cells expressed connective tissue mast cell markers, although mast cells around the surface epithelium also expressed a mucosal mast cell marker in MRL/MpJ mice. Some ovarian mast cells migrated into the oocyte nests and directly contacted the compressed and degenerated oocytes. In MRL/MpJ mice, the number of oocytes in the nest was significantly lower than in the other strains, and the number of oocytes showed a positive correlation with the number of ovarian mast cells. The gene expression of a mast cell marker also correlated with the expression of an oocyte nest marker, suggesting a link between the appearance of ovarian ? 4mast cells and early follicular development. Furthermore, the expression of follicle developmental markers was significantly higher in MRL/MpJ mice than in C57BL/6 mice. These results indicate that the appearance of ovarian mast cells is a unique phenotype of neonatal MRL/MpJ mice, and that ovarian mast cells participate in early follicular development, especially nest breakdown.

5-Azacytidine induces early stage apoptosis and promotes in vitro maturation by changing chromosomal construction in murine oocytes

As an anticancer drug, 5-azacytidine (5-AzaC) has been widely used to treat various cancers. To investigate the effect of 5-AzaC on mouse oocytes cultured in vitro, we have performed morphological and molecular biology studies to examine the behavior of chromosomes and oocyte development. In 5-AzaC-treated oocytes, chromosomes were decondensed and unstable. The mRNA levels of Caspase3, Caspase8, and Caspase9 increased with the occurrence of early stage apoptosis in oocytes following 5-AzaC treatment. Furthermore, the mRNA levels of Gdf9 and Bmp15 also increased with the corresponding morphological changes in 5-AzaC-treated oocytes. In conclusion, 5-AzaC not only induced early apoptosis through both extrinsic and intrinsic pathways, but also had a positive effect on the developmental competence of mouse oocytes during in vitro maturation. These effects may be due to changes in chromosomal construction induced by DNA hypomethylation.

Cross talk between autophagy and apoptosis in pulmonary hypertension

Endothelial cell (EC) apoptosis and apoptosis resistant proliferation have been proposed to play crucial roles in the development of featured plexiform lesions in the pathogenesis of pulmonary hypertension (PH). Subsequently, EC injury associated smooth muscle cell (SMC) proliferation facilitates vascular remodeling and eventually leads to narrowed vascular lumen, increased pulmonary vascular resistance, increased pulmonary arterial pressure, and right heart failure. The imbalance between cell death and proliferation occurs in every stage of pulmonary vascular remodeling and pathogenesis of PH, and involves every cell type in the vasculature including, but not limited to ECs, SMCs, and fibroblasts. Despite extensive studies, the detailed cellular and molecular mechanisms on how the transition from initial apoptosis of ECs to apoptosis resistant proliferation on ECs and SMCs remains unclear. Recent knowledge on autophagy, a conservative and powerful regulatory machinery existing in almost all mammalian cells, has shed light on the complex and delicate control on cell fate in the development of vascular remodeling in PH. In this review, we will discuss the recent understandings on how the cross-talk between apoptosis and autophagy regulates cell death or proliferation in PH pathogenesis, particularly in pulmonary vascular remodeling involving ECs and SMCs.

Caspase 9 is constitutively activated in mouse oocytes and plays a key role in oocyte elimination during meiotic prophase progression

In many mammalian species, more than half of the initial oocyte population is eliminated by neonatal life, thus limiting the oocyte reserve for reproduction. The cause or mechanism of this major oocyte loss remains poorly understood. We examined the apoptotic pathway involved in oocyte elimination in wild-type mouse ovaries as well as in Msh5 -/- ovaries, in which all oocytes were eliminated due to a lack of double strand break repair. Immunoblot and immunofluorescence staining showed that an initiator caspase 9 and an effector caspase 7 were constitutively activated in almost all oocytes in fetal ovaries regardless of their genotypes. In caspase 9 -/- ovaries, the total number of oocytes remained high while that in wild-type ovaries steadily declined during ovarian development. Therefore, the activation of caspase 9 was required for but did not immediately lead to oocyte demise. We found that XIAP, an endogenous inhibitor of apoptosis, was also abundant in oocytes during meiotic prophase progression. On the other hand, a cleaved form of PARP1, a target of effector caspases, was localized to the nuclei of a limited number of oocytes, and the frequency of cleaved PARP1-positive oocyte nuclei increased significantly higher before all oocytes disappeared in Msh5 -/- ovaries. We conclude that the mitochondrial apoptotic pathway mediated by caspase 9 is constitutively activated in oocytes and renders the elimination of oocytes with meiotic errors, which can be captured by the cleavage of PARP1.

The infant and pubertal human ovary: Balbiani's body-associated VASA expression, immunohistochemical detection of apoptosis-related BCL2 and BAX proteins, and DNA fragmentation

STUDY QUESTION

How do apoptosis-related BCL2 and BAX genes, known to regulate death or survival of germ cells in fetal and adult life, and germ-cell-specific VASA protein behave from birth to puberty in the human ovary?

SUMMARY ANSWER

In resting primordial follicles in both infant and pubertal ovaries, BCL2 family members and germ-cell-specific VASA behave as in fetal life. After birth, once follicles leave the resting reserve to enter the growing follicular pool, detection of apoptosis-related genes moves from the germ cell to granulosa cells and VASA expression is lost.

WHAT IS KNOWN ALREADY

In the human ovary, around 85% of the 7 × 10⁶ potential oocytes produced at mid-gestation are lost before birth, an extra 10% before puberty, and loss continues throughout reproductive life until germinal exhaustion of the ovary. Oocyte loss is mainly driven through a balanced expression of BCL2 gene family members. Apoptosis-inducing BAX gene shows a sustained expression throughout fetal and adult life, whereas apoptosis-inhibiting BCL2 is detectable during the proliferative stage of primordial germ cells and oogonia in the fetal ovary and proliferation of granulosa cells in growing follicles in the adult ovary. The germ-cell marker VASA is detectable in the fetal ovary from early oogenesis and is conspicuously expressed in primordial follicles, where in late pregnancy it is associated with the Balbiani's vitelline space. VASA expression is not detectable in the adult ovary.

STUDY DESIGN, SIZE, DURATION

This is a qualitative analysis involving infant/pubertal paraffin-embedded human ovaries screened for apoptosis-related proteins, DNA fragmentation and germ-cell identity.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Ovaries from 13 patients ranging in age from 4 to 16 years, undergoing gynaecological surgical procedures due to benign pathology, were studied. Tissues were fixed in 10% formalin, paraffin-embedded and processed for immunohistochemistry to screen the temporal and cellular localization of germ-cell-specific VASA protein and BCL2 and BAX apoptosis-related proteins. In addition, a terminal deoxynucleotidyl transferase-mediated deoxiuridinetriphosphate nick-end labelling (TUNEL) assay was performed to detect DNA fragmentation. General histology and tissue integrity were assessed by haematoxylin-eosin staining.

MAIN RESULTS AND THE ROLE OF CHANCE

VASA showed a differential pattern of expression; in the resting primordial follicle reserve in infant and pubertal ovaries, it was associated with the Balbiani's body space in the germ cell. VASA remained detectable in primary follicles leaving the resting reserve, but once follicles entered the growing pool it became undetectable. This pattern of VASA expression is the same as in the fetal ovary. BAX was expressed both in the resting primordial reserve and in the pool of growing follicles, whereas BCL2 was detected only in granulosa cells in antral follicles in the growing pool. Apoptosis-related protein expression moved from the germ cell to the somatic stratum when primordial follicles left the resting reserve to enter the pool of growing follicles, irrespective of female age. Most TUNEL-positive cells were detected in the granulosa cells of antral follicles. No TUNEL-positive cells were found in resting primordial follicles.

LIMITATIONS, REASONS FOR CAUTION

The study was limited by the qualitative nature of the immuno-histochemical analysis and the TUNEL assay. The results neither quantify the levels of germ-cell death nor exclude other concurrent cell death mechanisms that could act in the regulation of female germ-cell number.

WIDER IMPLICATIONS OF THE FINDING

This study provides missing knowledge about apoptosis and germ-cell-specific VASA expression in the human ovary between birth and puberty and the participation of BCL2 and BAX genes in the balance between death and survival throughout female germ-line development. Intracellular localization of VASA in resting follicles emerges as a possible marker with prognostic value that needs further investigation, especially in infant patients entering ovarian cryo-preservation programmes. This knowledge will be valuable in optimizing the rescue and clinical use of germ cells to restore fertility in women.

STUDY FUNDING/COMPETING INTEREST(S)

No external funding was obtained for this study. The authors have no conflicts of interest to declare.

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

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

Yeast aging and apoptosis

A concerted balance between proliferation and apoptosis is essential to the survival of multicellular organisms. Thus, apoptosis per se, although it is a destructive process leading to the death of single cells, also serves as a pro-survival mechanism pro-survival mechanism that ensures healthy organismal development and acts as a life-prolonging or anti-aging anti-aging program. The discovery that yeast also possess a functional and, in many cases, highly conserved apoptotic machinery has made it possible to study the relationships between aging and apoptosis in depth using a well-established genetic system and the powerful tools available to yeast researchers for investigating complex physiological and cytological interactions. The aging process of yeast, be it replicative replicative or chronological chronological aging, is closely related to apoptosis, although it remains unclear whether apoptosis is a causal feature of the aging process or vice versa. Nevertheless, experimental results obtained during the past several years clearly demonstrate that yeast serve as a powerful and versatile experimental system for understanding the interconnections between these two fundamentally important cellular and physiological pathways.

Apoptosis in the germ line

Apoptosis is a critical process for regulating both the size and the quality of the male and female germ lines. In this review, we examine the importance of this process during embryonic development in establishing the pool of spermatogonial stem cells and primordial follicles that will ultimately define male and female fertility. We also consider the importance of apoptosis in controlling the number and quality of germ cells that eventually determine reproductive success. The biochemical details of the apoptotic process as it affects germ cells in the mature gonad still await resolution, as do the stimuli that persuade these cells to commit to a pathway that leads to cell death. Our ability to understand and ultimately control the reproductive potential of male and female mammals depends upon a deeper understanding of these fundamental processes.

Proliferating cell nuclear antigen (PCNA) regulates primordial follicle assembly by promoting apoptosis of oocytes in fetal and neonatal mouse ovaries

Primordial follicles, providing all the oocytes available to a female throughout her reproductive life, assemble in perinatal ovaries with individual oocytes surrounded by granulosa cells. In mammals including the mouse, most oocytes die by apoptosis during primordial follicle assembly, but factors that regulate oocyte death remain largely unknown. Proliferating cell nuclear antigen (PCNA), a key regulator in many essential cellular processes, was shown to be differentially expressed during these processes in mouse ovaries using 2D-PAGE and MALDI-TOF/TOF methodology. A V-shaped expression pattern of PCNA in both oocytes and somatic cells was observed during the development of fetal and neonatal mouse ovaries, decreasing from 13.5 to 18.5 dpc and increasing from 18.5 dpc to 5 dpp. This was closely correlated with the meiotic prophase I progression from pre-leptotene to pachytene and from pachytene to diplotene when primordial follicles started to assemble. Inhibition of the increase of PCNA expression by RNA interference in cultured 18.5 dpc mouse ovaries strikingly reduced the apoptosis of oocytes, accompanied by down-regulation of known pro-apoptotic genes, e.g. Bax, caspase-3, and TNFα and TNFR2, and up-regulation of Bcl-2, a known anti-apoptotic gene. Moreover, reduced expression of PCNA was observed to significantly increase primordial follicle assembly, but these primordial follicles contained fewer granulosa cells. Similar results were obtained after down-regulation by RNA interference of Ing1b, a PCNA-binding protein in the UV-induced apoptosis regulation. Thus, our results demonstrate that PCNA regulates primordial follicle assembly by promoting apoptosis of oocytes in fetal and neonatal mouse ovaries.

Cell cycle control of germ cell differentiation

The germ cell lineage is our lifelong reservoir of reproductive stem cells and our mechanism for transmitting genes to future generations. These highly specialised cells are specified early during development and then migrate to the embryonic gonads where sex differentiation occurs. Germ cell sex differentiation is directed by the somatic gonadal environment and is characterised by two distinct cell cycle states that are maintained until after birth. In the mouse, XY germ cells in a testis cease mitotic proliferation and enter G(1)/G(0) arrest from 12.5 dpc, while XX germ cells in an ovary enter prophase I of meiosis from 13.5 dpc. This chapter discusses the factors known to control proliferation and survival of germ cells during their journey of specification to sex differentiation during development.

In situ labeling of DNA breaks and apoptosis by T7 DNA polymerase

The native T7 DNA polymerase is a fast and highly processive enzyme that can be used for in situ detection of apoptosis and various types of DNA breaks. The technique is quick and simple, and was shown to label earlier stages of apoptosis compared to the terminal transferase technique. The in situ labeling applications of T7 DNA polymerase are presented and summarized from the DNA damage detection standpoint. The detailed protocols are provided together with the discussion of their advantages and limitations.

Apoptosis: a way to maintain healthy individuals

Apoptosis, the best known form of programmed cell death, is tightly regulated by a number of sensors, signal transducers and effectors. Apoptosis is mainly active during embryonic development, when deletion of redundant cellular material is required for the correct morphogenesis of tissues and organs; moreover, it is essential for the maintenance of tissue homeostasis during cell life. Cells also activate apoptosis when they suffer from various insults, such as damage to DNA or to other cellular components, or impairment of basic processes, such as DNA replication and DNA repair. Removal of damaged cells is fundamental in maintaining the health of organisms. In addition, apoptosis induction following DNA damage is exploited to kill cancer cells. In this chapter we will review the main features of developmental and induced apoptosis.

Glucose transporters in gametes and preimplantation embryos

The oocyte, sperm and preimplantation embryo have unique metabolic needs that must be met to ensure successful pregnancy. The family of facilitative glucose transporters (GLUTs) plays a major role in providing metabolic substrates to these tissues. The variety of GLUTs expressed in these tissues allows for flexibility to adapt to a changing environment. Alterations in glucose transport and metabolism at the earliest stages of development can impact fetal development. Research into the mechanisms of normal glucose transport into cells is critical for improving outcomes in the increasingly common diabetic maternal environment. Here, we review the current understanding in the distribution and role of glucose transporters in gametes and preimplantation embryos under normal and diabetic conditions.

The primordial pool of follicles and nest breakdown in mammalian ovaries

The creation of the pool of follicles available for selection and ovulation is a multi-faceted, tightly regulated process that spans the period from embryonic development through to the first reproductive cycle of the organism. In mice, this development can occur in mere weeks, but in humans, it is sustained for years. Embryonic germ cell development involves the migration of primordial germs cells to the genital ridge, and the mitotic division of germ cell nuclei without complete cytokinesis to form a multi-nucleated syncytia, or germ cell nest. Through combined actions of germ cell apoptosis and somatic cell migration, the germ cell nuclei are packaged, with surrounding granulosa cells, into primordial follicles to form the initial follicle pool. Though often dismissed as quiescent and possibly uninteresting, this initial follicle pool is actually quite dynamic. In a very strictly controlled mechanism, a large portion of the initial primordial follicles formed is lost by atresia before cycling even begins. Remaining follicles can undergo alternate fates of continued dormancy or selection leading to follicular growth and differentiation. Together, the processes involved in the fate decisions of atresia, sustained dormancy, or activation carve out the follicle pool of puberty, the pool of available oocytes from which all future reproductive cycles of the female can choose. The formation of the initial and pubertal follicle pools can be predictably affected by exogenous treatment with hormones or molecules such as activin, demonstrating the ways the ovary controls the quality and quantity of germ cells maintained. Here, we review the biological processes involved in the formation of the initial follicle pool and the follicle pool of puberty, address the alternate models for regulating germ cell number and outline how the ovary quality-controls the germ cells produced.

Oogenesis and cell death in human prenatal ovaries: what are the criteria for oocyte selection?

Prenatal oogenesis produces hundreds of thousands of oocytes, most of which are discarded through apoptosis before birth. Despite this large-scale selection, the survivors do not constitute a perfect population, and the factors at the cellular level that result in apoptosis or survival of any individual oocyte are largely unknown. What then are the selection criteria that determine the size and quality of the ovarian reserve in women? This review focuses on new data at the cellular level, on human prenatal oogenesis, offering clues about the importance of the timing of entry to meiotic prophase I by linking the stages and progress through MPI with the presence or absence of apoptotic markers. The characteristics and responsiveness of cultured human fetal ovarian tissue at different gestational ages to growth factor supplementation and the impact of meiotic abnormalities upon apoptotic markers are discussed. Future work will require the use of a tissue culture model of prenatal oogenesis in order to investigate the fate of individual live oocytes at different stages of development.

Prepubertal primordial follicle loss in mice is not due to classical apoptotic pathways

More than half of the primordial follicles that are formed by Day 6 of postnatal life in the mouse will be eliminated from the ovary by the time of puberty. Apoptosis, a form of programmed cell death, is one mechanism by which these follicles could be actively lost. To investigate whether apoptosis is responsible for the loss of primordial follicles, follicular atresia was examined during the prepubertal period, when follicles die and are cleared from the ovary at an extremely high rate. Four hallmarks of classical apoptosis were measured in follicles present in prepubertal ovaries. The primordial follicle cohort was not positively associated with nuclear condensation or cell shrinkage, activation of caspase 3, cleavage of poly(ADP ribose) polymerase 1 (PARP1), or fragmentation of DNA. These data are consistent with a nonapoptotic pathway that is responsible for small follicle death.