Genetics of oocyte ageing

Chronological age enhances aging phenomena and protein nitration in oocyte

Background

The average age of childbearing has increased over the years contributing to infertility, miscarriages, and chromosomal abnormalities largely invoked by an age-related decline in oocyte quality. In this study, we investigate the role of nitric oxide (NO) insufficiency and protein nitration in oocyte chronological aging.

Methods

Mouse oocytes were retrieved from young breeders (YB, 8-14 weeks [w]), retired breeders (RB, 48-52w) and old animals (OA, 80-84w) at 13.5 and 17 hours after ovulation trigger. They were assessed for zona pellucida dissolution time (ZPDT); ooplasmic microtubule dynamics (OMD); cortical granule (CG) status and spindle morphology (SM), as markers of oocyte quality. Sibling oocytes from RB were exposed to NO supplementation and assessed for aging phenomena (AP). All oocyte cumulus complexes were subjected to fluorescence nitrotyrosine (NT) immunocytochemistry and confocal microscopy to assess morphology and protein nitration.

Results

At 13.5 h from hCG trigger, oocytes from RB compared to YB had significantly increased ZPDT (37.8 ± 11.9 vs 22.1 ± 4.1 seconds [s]), OMD (46.9 vs 0%), CG loss (39.4 vs 0%), and decreased normal SM (30.3 vs 81.3%), indicating premature AP that worsened among oocytes from RB at 17 hours post-hCG trigger. When exposed to SNAP, RB AP significantly decreased (ZPDT: 35.1 ± 5.5 vs 46.3 ± 8.9s, OMD: 13.3 vs 75.0% and CG loss: 50.0 vs 93.3%) and SM improved (80.0 vs 14.3%). The incidence of NT positivity was significantly higher in cumulus cells (13.5 h, 46.7 ± 4.5 vs 3.4 ± 0.7%; 17 h, 82.2 ± 2.9 vs 23.3 ± 3.6%) and oocytes (13.5 h, 57.1 vs 0%; 17 h, 100.0 vs 55.5%) from RB compared to YB. Oocytes retrieved decreased with advancing age (29.8 ± 4.1 per animal in the YB group compared to 10.2 ± 2.1 in RB and 4.0 ± 1.6 in OA). Oocytes from OA displayed increased ZPDT, major CG loss, increased OMD and spindle abnormalities, as well as pronuclear formation, confirming spontaneous meiosis to interphase transition.

Conclusions

Oocytes undergo zona pellucida hardening, altered spindle and ooplasmic microtubules, and premature cortical granule release, indicative of spontaneous meiosis-interphase transition, as a function of chronological aging. These changes are also associated with NO insufficiency and protein nitration and may be alleviated through supplementation with an NO-donor.

Why is oocyte aneuploidy increased with maternal aging?

One of the main causes of pregnancy failure and fetus abortion is oocyte aneuploidy, which is increased with maternal aging. Numerous possible causes of oocyte aneuploidy in aged women have been proposed, including cross-over formation defect, cohesin loss, spindle deformation, spindle assembly checkpoint malfunction, microtubule-kinetochore attachment failure, kinetochore mis-orientation, mitochondria dysfunction-induced increases in reactive oxygen species, protein over-acetylation, and DNA damage. However, it still needs to be answered if these aneuploidization factors have inherent relations, and how to prevent chromosome aneuploidy in aged oocytes. Epidemiologically, oocyte aneuploidy has been found to be weakly associated with higher homocysteine concentrations, obesity, ionizing radiation and even seasonality. In this review, we summarize the research progress and present an integrated view of oocyte aneuploidization.

Maternal age in the epidemiology of common autosomal trisomies

The birth prevalence rate of each common autosomal trisomy generally increases with advancing maternal age and there is a substantial fetal loss rate between late first trimester and term. The literature is reviewed in order to provide the best estimates of these rates, taking account where possible of biases due to prenatal diagnosis and selective termination of pregnancy. There is an almost exponential increase in Down syndrome birth prevalence between ages 15 and 45 but at older ages the curve flattens. There is no evidence of the claimed relatively high birth prevalence at extremely low ages. Gestation-specific intra-uterine fetal loss rates are estimated by follow-up of women declining termination of pregnancy after prenatal diagnosis, comparison of observed rates with those expected from birth prevalence and comparison of age-specific curves developed for prenatal diagnosis and birth. Down syndrome fetal loss rates reduce with gestation and increase with maternal age. Edwards and Patau syndrome birth prevalence is approximately 1/8 and 1/13 that of Down syndrome overall, although the ratio differs according to maternal age, particularly for Patau syndrome where it reduces steadily from 1/9 to 1/19. Fetal loss rates are higher for Edwards and Patau syndromes than for Down syndrome.

Can cytoplasmic donation rescue aged oocytes?

BACKGROUND

The pregnancy and delivery rates following assisted reproductive technology (ART) start to decrease and that the miscarriage rate increases rapidly from 35 years old. The miscarriage rate exceeds 50% at 43 years old. The number of aneuploid fetuses in miscarriages increases according to female age, reaching more than 90% when women are over 40 years old.

METHODS

Different cytoplasmic donation technologies used to rescue aged oocytes with high percentage of aneuploidy were analyzed, and their efficacy compared.

MAIN FINDINGS RESULTS

Germinal vesicle transfer (GVT) might be superior to spindle chromosome transfer (ST) theoretically from the point of higher capability of rescuing the disjunction at meiosis I which cannot be helped by ST. However, actually, in vitro maturation (IVM) of oocyte after GVT has not yet been totally completed. ST among other nuclear donations showed the higher possibility to rescue them, due to the fact it does not require in vitro maturation and it has an ethical advantage over pronuclear transfer (PNT) which requires the destruction of an embryo.

CONCLUSION

Spindle chromosome transfer has the potential to rescue aged oocytes to some extent, but we have to continue the basic study further to establish the clinical application of cytoplasmic donation to rescue aged oocytes.

Ovarian ageing: the role of mitochondria in oocytes and follicles

BACKGROUND

There is a great inter-individual variability of ovarian ageing, and almost 20% of patients consulting for infertility show signs of premature ovarian ageing. This feature, taken together with delayed childbearing in modern society, leads to the emergence of age-related ovarian dysfunction concomitantly with the desire for pregnancy. Assisted reproductive technology is frequently inefficacious in cases of ovarian ageing, thus raising the economic, medical and societal costs of the procedures.

OBJECTIVE AND RATIONAL

Ovarian ageing is characterized by quantitative and qualitative alteration of the ovarian oocyte reserve. Mitochondria play a central role in follicular atresia and could be the main target of the ooplasmic factors determining oocyte quality adversely affected by ageing. Indeed, the oocyte is the richest cell of the body in mitochondria and depends largely on these organelles to acquire competence for fertilization and early embryonic development. Moreover, the oocyte ensures the uniparental transmission and stability of the mitochondrial genome across the generations. This review focuses on the role played by mitochondria in ovarian ageing and on the possible consequences over the generations.

SEARCH METHODS

PubMed was used to search the MEDLINE database for peer-reviewed original articles and reviews concerning mitochondria and ovarian ageing, in animal and human species. Searches were performed using keywords belonging to three groups: 'mitochondria' or 'mitochondrial DNA'; 'ovarian reserve', 'oocyte', 'ovary' or 'cumulus cells'; and 'ageing' or 'ovarian ageing'. These keywords were combined with other search phrases relevant to the topic. References from these articles were used to obtain additional articles.

OUTCOMES

There is a close relationship, in mammalian models and humans, between mitochondria and the decline of oocyte quality with ageing. Qualitatively, ageing-related mitochondrial (mt) DNA instability, which leads to the accumulation of mtDNA mutations in the oocyte, plays a key role in the deterioration of oocyte quality in terms of competence and of the risk of transmitting mitochondrial abnormalities to the offspring. In contrast, some mtDNA haplogroups are protective against the decline of ovarian reserve. Quantitatively, mitochondrial biogenesis is crucial during oogenesis for constituting a mitochondrial pool sufficiently large to allow normal early embryonic development and to avoid the untimely activation of mitochondrial biogenesis. Ovarian ageing also seriously affects the dynamic nature of mitochondrial biogenesis in the surrounding granulosa cells that may provide interesting alternative biomarkers of oocyte quality.

WIDER IMPLICATIONS

A fuller understanding of the involvement of mitochondria in cases of infertility linked to ovarian ageing would contribute to a better management of the disorder in the future.

Influence of sperm fertilising concentration, sperm selection method and sperm capacitation procedure on the incidence of numerical chromosomal abnormalities in IVF early bovine embryos

The occurrence of numerical chromosomal aberrations, widely described as a major cause of mortality in in vitro-produced (IVP) embryos, has been linked to several factors. In the present study we investigated the effect of sperm fertilising concentration and semen handling (sperm selection and capacitation) before IVF on the rate of numerical chromosomal abnormalities in bovine embryos. In all, 466 IVP cattle embryos were karyotyped throughout three sequential experiments, analysing the effects of sperm fertilising concentration (0.1, 1.0 or 10×10(6) spermatozoa mL(-1)), selection method (unselected or Percoll-selected spermatozoa) and capacitation medium (bovine serum albumin (BSA), heparin or their combination). The percentage of normal (diploid) and aberrant (haploid, polyploid or aneuploid) embryos was noted in each experiment. The rate of numerical chromosomal abnormalities was mainly affected by sperm fertilising concentration (P<0.01) and, to a lesser extent, by the sperm capacitation medium (P<0.05). Polyploidy and haploidy rates were only affected by sperm fertilising concentration (P<0.05). Interestingly, the sperm selection technique used in the present study did not reduce the incidence of chromosome abnormalities in IVP cattle embryos (P>0.05). Finally, aneuploidy rates were not affected during the experiments (P>0.05), which suggests that they are not related to sperm-related factors. On the basis of these results, we conclude that sperm fertilising concentration is the 'paternal' key factor that affects the rate of numerical chromosomal abnormalities in IVP bovine embryos. By making small adjustments to fertilising protocols, the rate of cytogenetically aberrant embryos can be markedly reduced.

Relationship between diminished ovarian reserve and mitochondrial biogenesis in cumulus cells

STUDY QUESTION

What part do mitochondria play in cases of diminished ovarian reserve (DOR)?

SUMMARY ANSWER

Mitochondrial biogenesis in cumulus cells may be linked with impaired oocyte competence in patients with DOR.

WHAT IS KNOWN ALREADY

DOR, one of the causes of infertility even in young women, is characterized by the depletion of the ovarian pool associated with a decline in oocyte competence. Mitochondria, which play a role in oocyte quality, could be involved in the pathogenesis of DOR. The study of cumulus cells offers an interesting non-invasive approach for evaluating oocyte quality and the metabolic processes on which it depends. If mitochondrial dysfunction is involved in DOR, it is likely to have an impact on the functioning of cumulus cells.

STUDY DESIGN, SIZE, DURATION

This is an observational study of 74 immature oocyte-cumulus complexes retrieved from 47 women undergoing in vitro fertilization with intracytoplasmic sperm injection at the University Hospital of Angers, France, from March 2013 to March 2014. The women were divided into two groups: one group included 26 women with DOR, and the other, which included 21 women with a normal ovarian reserve (NOR), served as a control group.

PARTICIPANTS/MATERIALS, SETTINGS, METHODS

The oocyte mitochondrial content and the average mitochondrial content of the cumulus cells were assessed by mitochondrial (mt)DNA quantification using a quantitative real-time PCR technique. Microfluidic-based quantitative RT-PCR assays were used to quantify the expression of 13 genes involved in mitochondrial functions such as apoptosis and antioxidant activity or in mitochondrial biogenesis. We used orthogonal partial least-squares discriminant analysis (OPLS-DA) to distinguish between the DOR group and the NOR group of patients, and an OPLS model to predict the value of the oocyte mtDNA content that could be used as a critical marker of oocyte quality.

MAIN RESULTS AND THE ROLE OF CHANCE

The OPLS-DA model showed a good predictive capability (Q2 = 0.543). Using the variable importance in projection (VIP) metric we found three mitochondrial variables distinguishing the DOR group from the NOR group of patients, i.e. the oocyte mtDNA content (VIP = 0.92), the cumulus cell mtDNA content (VIP = 0.95) and the expression in cumulus cells of peroxisome proliferator-activated receptor γ coactivator 1 alpha (PPARGC-1A) (VIP = 1.10), all of which were lower in the DOR group than in the NOR group of patients. The OPLS model was able to satisfactorily predict the oocyte mtDNA content in only the NOR group of patients (Q2 = 0.506). We found four new variables positively linked to the oocyte mitochondrial mass, i.e. the cumulus cell mtDNA content (VIP = 1.19), and the expression in cumulus cells of three factors of mitochondrial biogenesis: polymerase gamma (POLG) (VIP = 2.13), optic atrophy 1 (OPA1) (VIP = 1.89) and the transcription factor associated with mitochondria (TFAM) (VIP = 1.32).

LIMITATIONS, REASONS OF CAUTION

This is a descriptive study. Because of ethical concerns in human clinical practice, this study has been performed only on immature oocytes and corresponding cumulus cells, which are usually discarded during in vitro fertilization procedures.

WIDER IMPLICATIONS OF THE FINDINGS

Cumulus cells may govern mitochondrial biogenesis, creating an adequate oocyte mitochondrial pool to promote embryonic development. The alteration of this process in patients with DOR may account for the impairment of oocyte quality. This suggests that some mitochondrial characteristics of cumulus cells may serve as indicators of oocyte competence and that oocyte quality may be improved by products enhancing mitochondrial biogenesis.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by a grant from the University Hospital of Angers, France: 'Appel d'offre interne à la recherche 2014'.

TRIAL REGISTRATION NUMBER

N/A.

The impact of mitochondrial function/dysfunction on IVF and new treatment possibilities for infertility

Mitochondria play vital roles in oocyte functions and they are critical indicators of oocyte quality which is important for fertilization and development into viable offspring. Quality-compromised oocytes are correlated with infertility, developmental disorders, reduced blastocyst cell number and embryo loss in which mitochondrial dysfunctions play a significant role. Increasingly, women affected by metabolic disorders such as diabetes or obesity and oocyte aging are seeking treatment in IVF clinics to overcome the effects of adverse metabolic conditions on mitochondrial functions and new treatments have become available to restore oocyte quality. The past decade has seen enormous advances in potential therapies to restore oocyte quality and includes dietary components and transfer of mitochondria from cells with mitochondrial integrity into mitochondria-impaired oocytes. New technologies have opened up new possibilities for therapeutic advances which will increase the success rates for IVF of oocytes from women with compromised oocyte quality.

Analysis of factors influencing morphokinetic characteristics of embryos in ART cycles

In this article, some factors were evaluated for their impact on embryo morphokinetics during assisted reproductive technology (ART) cycles. We detected significant differences in the fourth cell division time (t5) of embryos obtained after controlled ovarian stimulation in long GnRH agonists and GnRH antagonist protocols. We also found that higher gonadotropin dose may slow down the development of embryos. However, both male and female age, the number of oocytes and number of normal forms of sperm in the ejaculate did not affect the kinetic parameters of embryo development. Further research is needed to identify all the spectrum of factors, which can affect the rate of embryo development.

Elevated day 3 follicle-stimulating hormone in younger women: is gonadotropin stimulation/intrauterine insemination a good option?

OBJECTIVE

The objective of the study was to determine the fecundity of young women (<35 years) with an elevated day 3 follicle stimulating hormone (FSH) undergoing gonadotropin-stimulation/intrauterine insemination.

STUDY DESIGN

This was a retrospective study. The study was conducted at an academic fertility center. A total of 1396 gonadotropin stimulation/intrauterine insemination cycles from 563 women were stratified by day 3 FSH levels (<10 vs ≥10 U/L) and outcomes were compared. Gonadotropin dose, treatment duration, peak estradiol (E2), number of preovulatory follicles (total, large, and medium size), E2/follicle, endometrial thickness, spontaneous abortion, clinical and multiple pregnancy rates were measured. The statistics included a Student t test, a χ(2), regression, and a discrete survival analysis.

RESULTS

An elevated day 3 FSH was found in 10.2% of the women, despite favorable age (31.9 ± 2.5 years). Women with a day 3 FSH of 10 U/L or greater when compared with women with a normal day 3 level required significantly more medication (1058.9 ± 1106.0 vs 632.7 ± 477.5 IU, P < .0001) were triggered a day earlier (10.6 ± 2.4 vs 11.5 ± 2.9 days, P = .0006) and had E2 levels (on the day of and the day prior to human chorionic gonadotropin administration) that were significantly higher (529.5 ± 244.3 vs 450.0 ± 244.2 and 359.6 ± 141.7 vs 306.8 ± 160.9 pg/mL, respectively, P < .05). Clinical pregnancy rates were comparable among the groups (14.6 vs 14%, respectively, P > .05). Spontaneous abortion and multiple pregnancy rates were higher among women with an FSH of 10U/L or greater but not significantly so (27.8% vs 12.0%, 22.2% vs 13.8% for FSH of ≥10 vs FSH < or >10 U/L, P > .05).

CONCLUSION

Women younger than 35 years with an elevated day 3 FSH, when treated aggressively with gonadotropins have pregnancy rates comparable with those of women with a normal baseline FSH. To achieve this outcome, they need higher doses of medication to stimulate the production of a larger preovulatory follicular cohort.

The frequency of precocious segregation of sister chromatids in mouse female meiosis I is affected by genetic background

Mammalian female gametes frequently suffer from numerical chromosomal aberrations, the main cause of miscarriages and severe developmental defects. The underlying mechanisms responsible for the development of aneuploidy in oocytes are still not completely understood and remain a subject of extensive research. From studies focused on prevalence of aneuploidy in mouse oocytes, it has become obvious that reported rates of aneuploidy are strongly dependent on the method used for chromosome counting. In addition, it seems likely that differences between mouse strains could influence the frequency of aneuploidy as well; however, up till now, such a comparison has not been available. Therefore, in our study, we measured the levels of aneuploidy which has resulted from missegregation in meiosis I, in oocytes of three commonly used mouse strains-CD-1, C3H/HeJ, and C57BL/6. Our results revealed that, although the overall chromosomal numerical aberration rates were similar in all three strains, a different number of oocytes in each strain contained prematurely segregated sister chromatids (PSSC). This indicates that a predisposition for this type of chromosome segregation error in oocyte meiosis I is dependent on genetic background.

Post-ovulatory ageing of mouse oocytes affects the distribution of specific spindle-associated proteins and Akt expression levels

The aim of this study has been to determine the effects of in vivo post-ovulatory ageing (POA) on the distribution of spindle-associated proteins, histone H3/H4 post-translational modifications and on v-akt murine thymoma viral oncogene homolog 1 (Akt) expression levels. To this end, oocytes were retrieved 13, 29 and 33h after human chorionic gonadotrophin (hCG) treatment. The presence and distribution at the meiotic spindle of acetylated tubulin, γ-tubulin, polo kinase-1 and Ser473/Thr308 phosphorylated Akt (pAkt) as well as histone H3 and H4 acetylation and phosphorylation levels were assayed via immunofluorescence. Akt expression levels were determined via reverse transcription-polymerase chain reaction and western blotting analyses. Spindles from oocytes recovered 13h and 29h after hCG treatment showed similar levels of acetylated tubulin but ageing induced: (1) translocation of γ-tubulin from spindle poles to microtubules, (2) absence of Thr308- and Ser473-pAkt in 76% and 30% of oocytes, respectively, and (3) a significant reduction in phosphorylation levels of serine 10 on histone 3. At 29h, a significant decrease in Akt mRNA, but not in pAkt or Akt protein levels, was recorded. By contrast, protein content significantly decreased 33h after hCG. We conclude that POA impairs oocyte viability and fertilisability by altering the expression levels and spindle distribution of proteins that are implicated in cell survival and chromosome segregation. Together, these events could play a role in oocyte apoptosis.

Is later better or worse? Association of advanced parental age with offspring cognitive ability among half a million young Swedish men

Parental ages are increasing in the developed world, and postponed parenthood may have a negative association with the cognitive ability of offspring. There is, however, inconclusive evidence regarding the impact of both maternal and paternal ages. We have been able to reduce or eliminate unobserved confounding by using methods that account for fixed parental characteristics shared by brothers. Associations between parental age and intelligence quotient (IQ) among 565,433 Swedish males (birth cohorts 1951 to 1976) were analyzed, with IQ measured at conscription examinations (given between ages 17 and 20 years). When we accounted for the IQ time trend by adjusting for birth year, advanced paternal age showed no association with offspring IQ; however, maternal ages above 30 years were inversely associated with offspring IQ. For example, maternal ages 40-44 years were associated with an offspring IQ that was 0.07 standard deviations lower than that for maternal ages 25-29 years (P < 0.001). However, the IQ trend more than offset the impact of age, as without birth year adjustment, advanced maternal age was positively associated with IQ. Although the results confirmed that maternal age was negatively associated with offspring IQ, the association was small enough that delaying parenthood resulted in higher offspring IQ scores because of the positive IQ test score trend.

The Kunming mouse: as a model for age-related decline in female fertility in human

To ascertain whether the Kunming (KM) mouse is an available model for age-related decline in female fertility in human or not, oocytes from young (6–8 weeks), middle-aged (9 months) and aged (12 months) female mice were compared with respect to number of oocytes, frequency of in-vitro maturation (IVM) and in-vitro fertilization (IVF), and meiotic chromosome segregation and alignment. The mean number of pups born per mouse decreased significantly from the young to the middle-aged and the aged mice. The mean number of ovarian follicles, ovarian germinal vesicle oocytes and ovulated MII oocytes decreased significantly with maternal age. The rate of IVM in oocytes from young mice (73.9%) was less significantly than that in oocytes from middle-aged and aged mice (86.1% and 84.4%, respectively). Immunocytochemical analysis showed that ageing caused a significantly higher rate (49.3%) of chromosome misalignment than that (15.7%) of the young mice. The presence of premature chromatids was also significantly higher in MII oocytes of aged mice as compared with young mice (37.8 versus 8.3%). Pronuclear formation was delayed in oocytes of middle-aged and aged females (35.5 and 42.3% respectively in 5 h of IVF) as compared with young mice (88.1%). The study suggests that KM mouse exhibits an age-related decline in female fertility. Significant reduction of germinal vesicle (GV) and MII oocytes and significant increase of metaphase chromosome misalignment and premature chromatid segregation after meiotic maturation of oocytes, similar to human, presumably contribute to the decline in aged KM mice.

Maternal age and offspring adult health: evidence from the health and retirement study

Advanced maternal age is associated with negative offspring health outcomes. This interpretation often relies on physiological processes related to aging, such as decreasing oocyte quality. We use a large, population-based sample of American adults to analyze how selection and lifespan overlap between generations influence the maternal age-offspring adult health association. We find that offspring born to mothers younger than age 25 or older than 35 have worse outcomes with respect to mortality, self-rated health, height, obesity, and the number of diagnosed conditions than those born to mothers aged 25-34. Controls for maternal education and age at which the child lost the mother eliminate the effect for advanced maternal age up to age 45. The association between young maternal age and negative offspring outcomes is robust to these controls. Our findings suggest that the advanced maternal age-offspring adult health association reflects selection and factors related to lifespan overlap. These may include shared frailty or parental investment but are not directly related to the physiological health of the mother during conception, fetal development, or birth. The results for young maternal age add to the evidence suggesting that children born to young mothers might be better off if the parents waited a few years.

Three-dimensional quantitative analysis of chromosomes in the oocytes of aging mice during meiosis I in vitro

The mechanism of senescence is very complicated and can involve formation of chromosome abnormalities and a decline in female fertility. In this study, 3-D visualization of fluorescently labeled chromosomes in oocytes from aging and pubertal mice during in vitro maturation was done with a two-photon laser scanning microscope. Differences between aging and pubertal groups at various maturation stages were analyzed quantitatively in terms of chromosomal morphology, shape, and spatial arrangement. Compared with the pubertal group, the chromosomal morphology of oocytes from aging mice changed: both the mean volume and the mean surface area of chromosomes increased by approximately 20% (P < 0.05) at prometaphase and metaphase of meiosis I (considered to be the weakly condensed folded form of the chromosomes). Furthermore, at these stages, the shape of the chromosomal array became rounder (roundness factor increased by approximately 10%; P < 0.001) and the adhesion among chromosomes became more severe (P < 0.001) at approximately the same stages. Additionally, trends over time for both chromosomal morphology and shape were quite distinct between oocytes from aging and pubertal mice. Interestingly, trends for mean distance were similar; therefore, aging did not seem to influence chromosome movement toward the metaphase plate. These morphologic results should be useful to study age-related degradation of oocyte quality and to interpret results derived from molecular biology.

What is the optimal condition for fertilization of IVM oocytes?

Application of in vitro maturation (IVM) is recently increasing for human infertility, especially to rescue patients of polycystic ovarian syndrome and ovarian hyperstimulation syndrome. To increase the application of IVM oocytes for embryo production and the efficiency of successful production of babies using IVM oocytes, quality control of oocytes and achievement of fertilization in the most suitable condition may be very important. In this paper, suitable conditions for fertilization of IVM oocytes will be discussed with recent knowledge about IVM and in vitro fertilization of oocytes in domestic animals. Currently, human oocytes are collected mainly from patients' ovaries 36 h following mild gonadotropin stimulation and used for IVM for 24-26 h. However, asynchronous progression of those oocytes to reach the metaphase-II stage may have occurred during the IVM culture. In the oocytes that have already progressed to the metaphase-II stage, sudden aging such as reduction in maturation promoting factor and MAP kinases will start to occur. Application of specific inhibitors of phosphodiesterase to control intracellular cAMP (cyclic adenosine monophosphate) level may be effective to synchronize timings of the germinal vesicle breakdown and consequently the meiotic progression of oocytes, and to improve the developmental competence. Furthermore, treatment of aging oocytes with caffeine appears to rescue them from reductions in maturation promoting factor and MAP kinases and to improve the developmental competence. Assessment methods to select oocytes with good quality may also be important to improve the successful rates.

Molecular basis of maternal age-related increase in oocyte aneuploidy

Aneuploidy is one of the most common and serious pregnancy complications in humans. Most conceptuses with autosomal aneuploidy die in utero, resulting in early pregnancy loss. However, some fetuses with aneuploidy survive to term but suffer from disorders associated with congenital anomalies and mental retardation, such as Down syndrome with trisomy 21. Three general characteristics of this condition are well acknowledged: (i) in most cases the extra chromosome is of maternal origin; (ii) most cases are derived from a malsegregation event in meiosis I; and (iii) the frequency of these errors increases with maternal age. The basis for the age-dependent increase in meiosis I errors has been a long-standing enigma. Many investigators have addressed the nature of this biological phenomenon through genomic analyses of extra chromosome 21 using polymorphic markers to determine the frequency or location of crossovers that should ensure faithful chromosome segregation. Cytogenetic analyses of in vitro unfertilized oocytes have also been performed. However, no definitive conclusions regarding meiosis I errors have yet been reached from such studies. Recent findings in conditional knock-out mice for meiosis-specific cohesin have shed further light on this issue. The present review focuses on the current understanding of age-related aneuploidy and provides an overview of the mechanisms involved. We refer to recent data to illustrate some of the new paradigms that have arisen in this field.

Aneuploid human embryonic stem cells: origins and potential for modeling chromosomal disorders

Chromosomal aneuploidies are widely recognized genetic disorders in humans that often lead to spontaneous abortion. Aneuploid fetuses that survive to term commonly exhibit impaired developmental growth and mental retardation in addition to multiple congenital malformations. Preimplantation genetic screening is used to detect chromosomal aneuploidies in early embryos. Human embryonic stem cell (ESC) cell lines generated from aneuploid embryos created a unique repository of cell lines. The spectrum of aneuploidies in these ESC lines reflects the range of common embryonic chromosomal aberrations and significantly differs from the spectrum of aneuploid human ESC lines generated by cell adaptation in culture. The aneuploid human ESC lines represent an excellent model to study human chromosomal abnormalities especially in the early stages of development.

Beta- and gamma-cytoplasmic actins are required for meiosis in mouse oocytes

In mammals, female meiosis consists of two asymmetric cell divisions, which generate a large haploid oocyte and two small polar bodies. Asymmetric partitioning of the cytoplasm results from migration of the meiotic spindle toward the cortex and requires actin filaments. However, the subcellular localization and the role of the existing two cytoplasmic actin (CYA) isoforms, beta and gamma, have not been characterized. We show that beta- and gamma-CYA are differentially distributed in the maturing oocyte from late metaphase I as well as in preimplantation embryos. Gamma-CYA is preferentially enriched in oocyte cortices and is absent from all cell-cell contact areas from metaphase II until the blastocyst stage. Beta-CYA is enriched in contractile structures, at cytokinesis, at cell-cell contacts, and around the forming blastocoel. Alteration of beta- or gamma-CYA function by isoform-specific antibody microinjection suggests that gamma-CYA holds a major and specific role in the establishment and/or maintenance of asymmetry in meiosis I and in the maintenance of overall cortical integrity. In contrast, beta- and gamma-CYA, together, appear to participate in the formation and the cortical anchorage of the second meiotic spindle in waiting for fertilization. Finally, differences in gamma-CYA expression are amongst the earliest markers of cell fate determination in development.

Evidence that carbonyl stress by methylglyoxal exposure induces DNA damage and spindle aberrations, affects mitochondrial integrity in mammalian oocytes and contributes to oocyte ageing

BACKGROUND

Highly reactive carbonyl compounds formed during glycolysis, such as methylglyoxal (MG), can lead to the formation of 'advanced glycation end products' (AGE) and carbonyl stress. Toxic AGEs are suspected to accumulate and play a role in reducing quality and developmental potential of mammalian oocytes of aged females and in PCOS and diabetic patients. Whether and how MG and AGE affect young and aged oocytes at the cellular level is unknown.

METHODS

The study consists of three parts. In Part A expression of MG-detoxifying enzymes glyoxalases 1 and 2 was analysed by RT-PCR at different stages of maturation in denuded oocytes (DO), cumulus-enclosed oocytes (CEO) and metaphase (M)II oocytes of the CD-1 mouse to obtain information on stage-specific susceptibility to carbonyl stress. DO and CEO from young and aged females and from stimulated cycles were exposed to MG during maturation in vitro to assess also age-related changes in sensitivity to carbonyl stress induced by MG. Induction of apoptosis by MG on in vitro maturing DO was assessed by terminal deoxynucleotidyl transferase-mediated dUDP nick-end labelling test. In Part B of the study, DO from large antral follicles of ovaries of adult, young MF-1 mice in late diestrous were exposed to MG to assess direct influences of MG and AGEs formed during continuous exposure to MG on rate and kinetics of maturation to MII, on DNA integrity (by γ-H2AX staining) in the germinal vesicle (GV) stage, and on spindle formation and chromosome alignment (by tubulin and pericentrin immunofluorescence and polarization microscopy), and chromosome segregation (by C-banding) during in vitro maturation. Since MG and AGEs can affect functionality of mitochondria in Part C, mitochondrial distribution and membrane potential was studied using JC-1 probe. Expression of a redox-sensitive mito-Grx1-roGFP2 protein in mitochondria of maturing oocytes by confocal laser scanning microscopy was employed to determine the inner mitochondrial glutathion (GSH)/glutathion disulfide (GSSG)-dependent redox potential.

RESULTS

Part A revealed that mRNA for glyoxalases decreases during meiotic maturation. Importantly, cumulus from aged mice in CEO obtained from stimulated cycles does not protect oocytes efficiently from MG-induced meiotic arrest during in vitro maturation. Part B showed that the MG-induced meiotic delay or arrest is associated with significant rises in spindle aberrations, chromosome congression failure and aberrant telophase I in oocytes. MG exposure of meiotically arrested GV-stage oocytes significantly increases the numbers of γ-H2AX spots in the nucleus suggesting increased DNA damage, while MG exposure during maturation affects chromatin condensation and induces chromosome lagging at anaphase I. Moreover, Part C revealed that carbonyl stress by chronic exposure to MG is associated with delays in changes in mitochondrial distribution and altered inner-mitochondrial GSH/GSSG redox potential, which might be particularly relevant for cytoskeletal dynamics as well as processes after fertilization. Sensitivity to a meiotic block by MG appears dependent on the genetic background.

CONCLUSIONS

The sensitivity to carbonyl stress by MG appears to increase with maternal age. Since MG-exposure induces DNA damage, meiotic delay, spindle aberrations, anaphase I lagging and epimutation, aged oocytes are particularly at risk for such disturbances in the absence of efficient protection by cumulus. Furthermore, disturbances in mitochondrial distribution and redox regulation may be especially critical for fertilization and developmental competence of oocytes exposed to MG and carbonyl stress before or during maturation, for instance, in aged females, or in PCOS or diabetic patients, in agreement with recent suggestions of correlations between poor follicular and embryonic development, lower pregnancy rate and presence of toxic AGEs in serum, irrespective of age.

Chromosomal and cytoplasmic context determines predisposition to maternal age-related aneuploidy: brief overview and update on MCAK in mammalian oocytes

It has been known for more than half a century that the risk of conceiving a child with trisomy increases with advanced maternal age. However, the origin of the high susceptibility to nondisjunction of whole chromosomes and precocious separation of sister chromatids, leading to aneuploidy in aged oocytes and embryos derived from them, cannot be traced back to a single disturbance and mechanism. Instead, analysis of recombination patterns of meiotic chromosomes of spread oocytes from embryonal ovary, and of origins and exchange patterns of extra chromosomes in trisomies, as well as morphological and molecular studies of oocytes and somatic cells from young and aged females, show chromosome-specific risk patterns and cellular aberrations related to the chronological age of the female. In addition, analysis of the function of meiotic- and cell-cycle-regulating genes in oogenesis, and the study of the spindle and chromosomal status of maturing oocytes, suggest that several events contribute synergistically to errors in chromosome segregation in aged oocytes in a chromosome-specific fashion. For instance, loss of cohesion may differentially predispose chromosomes with distal or pericentromeric chiasmata to nondisjunction. Studies on expression in young and aged oocytes from human or model organisms, like the mouse, indicate that the presence and functionality/activity of gene products involved in cell-cycle regulation, spindle formation and organelle integrity may be altered in aged oocytes, thus contributing to a high risk of error in chromosome segregation in meiosis I and II. Genes that are often altered in aged mouse oocytes include MCAK (mitotic-centromere-associated protein), a microtubule depolymerase, and AURKB (Aurora kinase B), a protein of the chromosomal passenger complex that has many targets and can also phosphorylate and regulate MCAK localization and activity. Therefore we explored the role of MCAK in maturing mouse oocytes by immunofluorescence, overexpression of a MCAK–EGFP (enhanced green fluorescent protein) fusion protein, knockdown of MCAK by RNAi (RNA interference) and inhibition of AURKB. The observations suggest that MCAK is involved in spindle regulation, chromosome congression and cell-cycle control, and that reductions in mRNA and protein in a context of permissive SAC (spindle assembly checkpoint) predispose to aneuploidy. Failure to recruit MCAK to centromeres and low expression patterns, as well as disturbances in regulation of enzyme localization and activity, e.g. due to alterations in activity of AURKB, may therefore contribute to maternal age-related rises in aneuploidy in mammalian oocytes.

Aging changes the chromatin configuration and histone methylation of mouse oocytes at germinal vesicle stage

Aging decreases the fertility of mammalian females. In old oocytes at metaphase II stage (MII) there are alterations of the chromatin configuration and chromatin modifications such as histone acetylation. Recent data indicate that alterations of histone acetylation at MII initially arise at germinal vesicle stage (GV). Therefore, we hypothesized that the chromatin configuration and histone methylation could also change in old GV oocytes. In agreement with our hypothesis, young GV oocytes had non-surrounded nucleolus (NSN) and surrounded nucleolus (SN) chromatin configurations, while old GV oocytes also had chromatin configurations that could not be classified as NSN or SN. Regarding histone methylation, young GV and MII oocytes showed dimethylation of lysines 4, 9, 36 and 79 in histone 3 (H3K4me2, H3K9me2, H3K36me2, H3K79me2), lysine 20 in histone H4 (H4K20me2) and trimethylation of lysine 9 in histone 3 (H3K9me3) while a significant percentage of old GV and MII oocytes lacked H3K9me3, H3K36me2, H3K79me2 and H4K20me2. The percentage of old oocytes lacking histone methylation was similar at GV and MII suggesting that alterations of histone methylation in old MII oocytes initially arise at GV. Besides, the expression of the histone methylation-related factors Cbx1 and Sirt1 was also found to change in old GV oocytes. In conclusion, our study reports changes of chromatin configuration and histone methylation in old GV oocytes, which could be very useful for further understanding of human infertility caused by aging.

Miscarriage risk for IVF pregnancies in poor responders to ovarian hyperstimulation

The increasing miscarriage rate with advancing female age is attributed to a decline in oocyte quality. A poor response to ovarian hyperstimulation is often an expression of a decrease in oocyte quantity. Although oocyte quality and quantity both decrease as a result of ovarian ageing, it is unclear whether these two processes are related to each other. To investigate the relationship between oocyte quantity and quality, we compared miscarriage rates between IVF treated women with a poor and normal response, respectively. Data were studied from a retrospective nationwide cohort of Dutch women undergoing IVF treatment from 1983 to 1995. Women achieving an ongoing pregnancy after their first complete IVF cycle (n=1468) were compared with those experiencing miscarriage (n=357) with respect to their ovarian response. Logistic regression analysis showed a statistically significant association between poor response (fewer than four retrieved oocytes) and miscarriage (P=0.001). Due to interaction, this association became stronger with increasing female age. Among women < 36 years, miscarriage rates between poor and normal responders did not differ, whereas among women 36 years poor responders had a statistically significant increased miscarriage rate compared with normal responders (P=0.001). These results support the hypothesis of a relationship between quantitative ovarian reserve and oocyte quality.

The functional significance of centrosomes in mammalian meiosis, fertilization, development, nuclear transfer, and stem cell differentiation

Centrosomes had been discovered in germ cells and germ cells continue to provide excellent but also challenging material in which to study complex centrosomal dynamics. The present review highlights the importance of centrosomes for meiotic spindle integrity and the susceptibility of meiotic spindle centrosomes to aging and drugs or toxic agents which may be associated with female infertility, aneuploidy, and developmental abnormalities. We discuss cell and molecular aspects of centrosomes during fertilization, a critical stage in which centrosomes play crucial roles in precisely organizing the sperm aster that allows apposition of male and female genomes followed by formation of the zygote aster that is important for the formation of the bipolar spindle apparatus during cell division. Development of an embryo involves sequential cell divisions in which centrosomes play a critical role in establishing asymmetry that allows differentiation of cells and targeted signal transductions for the developing embryo. Asymmetric centrosome dynamics are also critical for stem cell division to maintain one daughter cell as a stem cell while the other daughter cell undergoes centrosome growth in preparation for differentiation. This review also discusses the complex interactions of somatic cell centrosomes with the recipient oocyte in reconstructed (cloned) embryos in which centrosome remodeling is crucial to fulfill functions that are carried out by the zygote centrosome in fertilized eggs. We close our discussion with a look at centrosome dysfunctions and implications for male fertility and assisted reproduction.

Mitochondrial functions on oocytes and preimplantation embryos

Oocyte quality has long been considered as a main limiting factor for in vitro fertilization (IVF). In the past decade, extensive observations demonstrated that the mitochondrion plays a vital role in the oocyte cytoplasm, for it can provide adenosine triphosphate (ATP) for fertilization and preimplantation embryo development and also act as stores of intracellular calcium and proapoptotic factors. During the oocyte maturation, mitochondria are characterized by distinct changes of their distribution pattern from being homogeneous to heterogeneous, which is correlated with the cumulus apoptosis. Oocyte quality decreases with the increasing maternal age. Recent studies have shown that low quality oocytes have some age-related dysfunctions, which include the decrease in mitochondrial membrane potential, increase of mitochondrial DNA (mtDNA) damages, chromosomal aneuploidies, the incidence of apoptosis, and changes in mitochondrial gene expression. All these dysfunctions may cause a high level of developmental retardation and arrest of preimplantation embryos. It has been suggested that these mitochondrial changes may arise from excessive reactive oxygen species (ROS) that is closely associated with the oxidative energy production or calcium overload, which may trigger permeability transition pore opening and subsequent apoptosis. Therefore, mitochondria can be seen as signs for oocyte quality evaluation, and it is possible that the oocyte quality can be improved by enhancing the physical function of mitochondria. Here we reviewed recent advances in mitochondrial functions on oocytes.

Evidence that a defective spindle assembly checkpoint is not the primary cause of maternal age-associated aneuploidy in mouse eggs

Advanced maternal age is unequivocally associated with increased aneuploidy in human eggs and infertility, but the molecular basis for this phenomenon is unknown. An age-dependent deterioration of the spindle assembly checkpoint (SAC) has been proposed as a probable cause of aneuploidy. Accurate chromosome segregation depends on correct chromosome attachment to spindle microtubules, and the SAC provides time for this process by delaying anaphase onset until all chromosomes are stably attached. If SAC function decreases with age, oocytes from reproductively old mice would enter anaphase of meiosis I (AI) prematurely, leading to chromosome segregation errors and aneuploid eggs. Although intuitively appealing, this hypothesis is largely untested. We used a natural reproductive aging mouse model to determine if a defective SAC is the primary cause of aneuploidy in eggs. We tracked the progress of individual oocytes from young and old mice through meiosis I by time-lapse microscopy and counted chromosomes in the resulting eggs. This data set allowed us to correlate the timing of AI onset with aneuploidy in individual oocytes. We found that oocytes from old mice do not enter AI prematurely compared to young counterparts despite a 4-fold increase in the incidence of aneuploidy. Moreover, we did not observe a correlation between the timing of AI onset and aneuploidy in individual oocytes. When SAC function was challenged with a low concentration of the spindle toxin nocodazole, oocytes from both young and old mice arrested at meiosis I, which is indicative of a functional checkpoint. These findings indicate that a defective SAC is unlikely the primary cause of aneuploidy associated with maternal age.

Apoptosis maintains oocyte quality in aging Caenorhabditis elegans females

In women, oocytes arrest development at the end of prophase of meiosis I and remain quiescent for years. Over time, the quality and quantity of these oocytes decreases, resulting in fewer pregnancies and an increased occurrence of birth defects. We used the nematode Caenorhabditis elegans to study how oocyte quality is regulated during aging. To assay quality, we determine the fraction of oocytes that produce viable eggs after fertilization. Our results show that oocyte quality declines in aging nematodes, as in humans. This decline affects oocytes arrested in late prophase, waiting for a signal to mature, and also oocytes that develop later in life. Furthermore, mutations that block all cell deaths result in a severe, early decline in oocyte quality, and this effect increases with age. However, mutations that block only somatic cell deaths or DNA-damage-induced deaths do not lower oocyte quality. Two lines of evidence imply that most developmentally programmed germ cell deaths promote the proper allocation of resources among oocytes, rather than eliminate oocytes with damaged chromosomes. First, oocyte quality is lowered by mutations that do not prevent germ cell deaths but do block the engulfment and recycling of cell corpses. Second, the decrease in quality caused by apoptosis mutants is mirrored by a decrease in the size of many mature oocytes. We conclude that competition for resources is a serious problem in aging germ lines, and that apoptosis helps alleviate this problem.

Neonatal exposure to genistein disrupts ability of female mouse reproductive tract to support preimplantation embryo development and implantation

Female mice treated neonatally with the phytoestrogen genistein (50 mg/kg/day) have multioocyte follicles, lack regular estrous cyclicity, and are infertile even after superovulation. To determine the cause of their infertility, we examined oocyte developmental competence and timing of embryo loss. Eggs obtained by superovulation of genistein-treated or control females were equally capable of being fertilized in vitro and cultured to the blastocyst stage. However, if eggs were fertilized in vivo, retrieved at the pronucleus stage, and cultured, there was a significant reduction in the percentage of embryos from genistein-treated females reaching the blastocyst stage. When these blastocysts were transferred to pseudopregnant recipients, the number of live pups produced was similar to that in controls. Preimplantation embryo development in vivo was examined by flushing embryos from the oviduct and/or uterus. Similar numbers of one-cell and two-cell embryos were obtained from genistein-treated and control females. However, significantly fewer embryos (<50%) were obtained from genistein-treated females on postcoital Days 3 and 4. To determine if neonatal genistein treatment altered the ability of the uterus to support implantation, blastocysts from control donors were transferred to control and genistein-treated pseudopregnant recipients. These experiments demonstrated that genistein-treated females are not capable of supporting normal implantation of control embryos. Taken together, these results suggest that oocytes from mice treated neonatally with genistein are developmentally competent; however, the oviductal environment and the uterus have abnormalities that contribute to the observed reproductive failure.

Nitric oxide extends the oocyte temporal window for optimal fertilization

Deteriorating oocyte quality is a critical hurdle in the management of infertility, especially one associated with advancing age. In this study, we explore the role of nitric oxide (NO) on the sustenance of oocyte quality postovulation. Sibling oocytes from superovulated mice were subjected to intracytoplasmic sperm injection (ICSI) with cauda-epididymal spermatozoa following exposure to either the NO donor, S-nitroso-N-acetylpenicillamine (SNAP, 0.23 microM/min), an NO synthase (NOS) inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME, 1 mM), or an inhibitor of soluble guanylyl cyclase (sGC), 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ, 100 microM); while their sibling oocytes were subjected to ICSI either before (young) or after culture for the corresponding period of time (old). Outcomes of normal fertilization, cleavage, and development to the morula and blastocyst stages were compared. Embryos from each subgroup were also subjected to TUNEL assay for apoptosis. A significant deterioration in the ability of the oocytes to undergo normal fertilization and development to morula and blastocyst stages occurred among oocytes aged in culture medium compared to their sibling cohorts subjected to ICSI immediately after ovulation (P<0.05). This deterioration was prevented in oocytes exposed to SNAP. In contrast, exposure to L-NAME or ODQ resulted in a significant compromise in fertilization and development to the morula and blastocyst stages (P<0.05). Finally, apoptosis was noted in embryos derived from aged oocytes and those exposed to L-NAME or ODQ, but not in embryos derived from young oocytes or oocytes exposed to SNAP. Thus, NO is essential for sustenance of oocyte quality postovulation.

Oocyte senescence: a firm link to age-related female subfertility

In women optimal fertility is maintained until 30 years of age and then decreases sharply. Therefore, the trend to delay childbearing until the fourth decade of life has contributed highly to the decline in total fertility rate observed in Western countries in recent decades. Much evidence supports the idea that age-related subfertility is mostly related to oocyte senescence. Based on the finding that maternal age negatively affects the storage of oocyte transcripts involved in major cellular functions, the present paper reviews the nuclear and cytoplasmic failure of the aged oocyte which can exert a negative influence on its developmental competence. Recognizing the potential role of oocyte-based technologies for improving clinical outcome for women with age-related etiologies of infertility, the importance of basic research aimed to increase knowledge of the aged oocyte and its microenvironment is also highlighted in order to set up new therapeutic strategies.

Cellular and molecular aspects of ovarian follicle ageing

It is well established that age-related decline of the biological capacity of a woman to reproduce is primarily related to the poor developmental potential of her gametes. This renders female ageing the most significant determinant of success in IVF. Starting with a reference picture of the main molecular and cellular failures of aged oocytes, granulosa cells and follicular microenvironment, this review focuses on age-related biochemical mechanisms underlying these changes. According to the most relevant concept of ageing, age-associated malfuction results from physiological accumulation of irreparable damage to biomolecules as an unavoidable side effect of normal metabolism. More than a decade after the free radical theory of ovarian ageing, biological and clinical research supporting the involvement of oxidative injuries in follicle ageing is discussed. Looking for the aetiology of oxidative stress, we consider the effect of ageing on ovarian and follicular vascularization. Then, we propose a potential role of advanced glycation end-products known to be involved in the physiological ageing of most tissues and organs. We conclude that future investigation of age-related molecular damage in the different ovarian components will be imperative in order to evaluate the possibility to save or rescue the developmental potential of aged oocytes.

Elucidating the origin of chromosomal aberrations in IVF embryos by preimplantation genetic analysis

Preimplantation genetic screening (PGS) has been proposed as a method for improving success rates in patients with repeated IVF failures. This approach is based on the hypothesis that such failures are the result of aneuploid embryos. It has been suggested that FISH analysis of blastomeres removed from preimplantation embryos represent the chromosomal constitution of the entire embryo. However, it is not yet clear whether it also represents the chromosomal constitution of the implanted embryo. PGS reanalysis on day 5 of embryos designated as "aneuploid" on day 3 may demonstrate a high rate of mosaicism for chromosomal aberration. Some of these mosaic embryos are capable of developing into normal embryos by "self-correction". Others, however, may accumulate additional chromosomal anomalies. It is therefore concluded that the chromosomal constitution of a preimplantation embryo may evolve during early cleavages. Meiotic and post zygotic mitotic errors may account for these chromosomal aberrations. This review will focus on elucidating the origin of chromosomal changes during preimplantation embryo development by studying their chromosomal constitution at different stages.

Human female meiosis: what makes a good egg go bad?

Critical events of oogenesis occur during three distinct developmental stages: meiotic initiation in the fetal ovary, follicle formation in the perinatal period, and oocyte growth and maturation in the adult. Evidence from studies in humans and mice suggests that the genetic quality of the egg may be influenced by events at each of these stages. Recent experimental studies add additional complexity, suggesting that environmental influences might adversely affect all three stages. Thus, understanding the molecular control of oogenesis during these critical developmental windows will not only contribute to an understanding of human aneuploidy, but also provide a means of assessing potential effects of environmental exposures on human reproductive health.

Spindle formation, chromosome segregation and the spindle checkpoint in mammalian oocytes and susceptibility to meiotic error

The spindle assembly checkpoint (SAC) monitors attachment to microtubules and tension on chromosomes in mitosis and meiosis. It represents a surveillance mechanism that halts cells in M-phase in the presence of unattached chromosomes, associated with accumulation of checkpoint components, in particular, Mad2, at the kinetochores. A complex between the anaphase promoting factor/cylosome (APC/C), its accessory protein Cdc20 and proteins of the SAC renders APC/C inactive, usually until all chromosomes are properly assembled at the spindle equator (chromosome congression) and under tension from spindle fibres. Upon release from the SAC the APC/C can target proteins like cyclin B and securin for degradation by the proteasome. Securin degradation causes activation of separase proteolytic enzyme, and in mitosis cleavage of cohesin proteins at the centromeres and arms of sister chromatids. In meiosis I only the cohesin proteins at the sister chromatid arms are cleaved. This requires meiosis specific components and tight regulation by kinase and phosphatase activities. There is no S-phase between meiotic divisions. Second meiosis resembles mitosis. Mammalian oocytes arrest constitutively at metaphase II in presence of aligned chromosomes, which is due to the activity of the cytostatic factor (CSF). The SAC has been identified in spermatogenesis and oogenesis, but gender-differences may contribute to sex-specific differential responses to aneugens. The age-related reduction in expression of components of the SAC in mammalian oocytes may act synergistically with spindle and other cell organelles' dysfunction, and a partial loss of cohesion between sister chromatids to predispose oocytes to errors in chromosome segregation. This might affect dose-response to aneugens. In view of the tendency to have children at advanced maternal ages it appears relevant to pursue studies on consequences of ageing on the susceptibility of human oocytes to the induction of meiotic error by aneugens and establish models to assess risks to human health by environmental exposures.

Gender differences in the induction of chromosomal aberrations and gene mutations in rodent germ cells

Germ cell mutagenicity testing provides experimental data to quantify genetic risk for exposed human populations. The majority of tests are performed with exposure of males, and female data are relatively rare. The reason for this paucity lies in the differences between male and female germ cell biology. Male germ cells are produced throughout reproductive life and all developmental stages can be ascertained by appropriate breeding schemes. In contrast, the female germ cell pool is limited, meiosis begins during embryogenesis and oocytes are arrested over long periods of time until maturation processes start for small numbers of oocytes during the oestrus cycle in mature females. The literature data are reviewed to point out possible gender differences of germ cells to exogenous agents such as chemicals or ionizing radiation. From the limited information, it can be concluded that male germ cells are more sensitive than female germ cells to the induction of chromosomal aberrations and gene mutations. However, exceptions are described which shed doubt on the extrapolation of experimental data from male rodents to the genetic risk of the human population. Furthermore, the female genome may be more sensitive to mutation induction during peri-conceptional stages compared to the male genome of the zygote. With few exceptions, germ cell experiments have been carried out under high acute exposure to optimize the effects and to compensate for the limited sample size in animal experiments. Human exposure to environmental agents, on the other hand, is usually chronic and involves low doses. Under these conditions, gender differences may become apparent that have not been studied so far. Additionally, data are reviewed that suggest a false impression of safety when responses are negative under high acute exposure of male rodents while a mutational response is induced by low chronic exposure. The classical (morphological) germ cell mutation tests are not performed anymore because they are animal and time consuming. Nevertheless, information is needed to place genetic risk extrapolations on more solid grounds and thereby to prevent an increased genetic burden to future generations. It is pointed out that modern molecular methodologies are available now to experimentally address the open questions.

Gender effects on the incidence of aneuploidy in mammalian germ cells

Aneuploidy occurs in 0.3% of newborns, 4% of stillbirths, and more than 35% of all human spontaneous abortions. Human gametogenesis is uniquely and gender-specific susceptible to errors in chromosome segregation. Overall, between 1% and 4% of sperm and as many as 20% of human oocytes have been estimated by molecular cytogenetic analysis to be aneuploid. Maternal age remains the paramount aetiological factor associated with human aneuploidy. The majority of extra chromosomes in trisomic offspring appears to be of maternal origin resulting from nondisjunction of homologous chromosomes during the first meiotic division. Differences in the recombination patterns between male and female meiosis may partly account for the striking gender- and chromosome-specific differences in the genesis of human aneuploidy, especially in aged oocytes. Nondisjunction of entire chromosomes during meiosis I as well as premature separation of sister chromatids or homologues prior to meiotic anaphase can contribute to aneuploidy. During meiosis, checkpoints at meiotic prophase and the spindle checkpoint at M-phase can induce meiotic arrest and/or cell death in case of disturbances in pairing/recombination or spindle attachment of chromosomes. It has been suggested that gender differences in aneuploidy may result from more permissive checkpoints in females than males. Furthermore, age-related loss of chromosome cohesion in oocytes as a cause of aneuploidy may be female-specific. Comparative data about the susceptibility of human male and female germ cells to aneuploidy-causing chemicals is lacking. Increases of aneuploidy frequency in sperm have been shown after exposure to therapeutic drugs, occupational agents and lifestyle factors. Conversely, data on oocyte aneuploidy caused by exogenous agents is limited because of the small numbers of oocytes available for analysis combined with potential maternal age effects. The vast majority of animal studies on aneuploidy induction in germ cells represent cause and effect data. Specific studies designed to evaluate possible gender differences in induction of germ cell aneuploidy have not been found. However, the comparison of rodent data available from different laboratories suggests that oocytes are more sensitive than male germ cells when exposed to chemicals that effect the meiotic spindle. Only recently, in vitro experiments, analyses of transgenic animals and knockdown of expression of meiotic genes have started to address the molecular mechanisms underlying chromosome missegregation in mammalian germ cells whereby striking differences between genders could be shown. Such information is needed to clarify the extent and the mechanisms of gender effects, including possible differential susceptibility to environmental agents.

Moderately elevated levels of basal follicle-stimulating hormone in young patients predict low ovarian response, but should not be used to disqualify patients from attempting in vitro fertilization

OBJECTIVE

To evaluate and compare IVF outcomes of patients within different age categories who had a normal basal FSH level with outcomes of patients with an elevated day 3 FSH level.

DESIGN

Retrospective analysis.

SETTING

Large, private IVF center.

PATIENT(S)

We analyzed 2,708 patients. Of these, 2,477 had normal basal FSH levels, and 231 had elevated basal FSH levels (> or =13.03 IU/L). Patients were segregated into various age groups.

INTERVENTION

Follow-up.

MAIN OUTCOME MEASURE(S)

Outcomes of IVF overall, including cancellation rates, oocyte yield, and fertilization, implantation, and clinical pregnancy rates (PRs).

RESULT(S)

Cancellation rates were significantly higher in patients with elevated day 3 FSH levels compared with patients with normal FSH levels in all age groups. A significantly lower oocyte yield was observed in patients with elevated basal FSH. Fertilization rates were not affected by FSH levels. A significant decrease in the number of embryos available for transfer in patients > or =38 with an elevated day 3 FSH level was found. Implantation and clinical PRs were lower in patients >40 years of age who had an elevated day 3 FSH level when compared to same age patients with a normal day 3 FSH level. Loss rates were not significantly different.

CONCLUSION(S)

Young women with an elevated basal FSH level should be counseled differently than older women, and should be given adequate counseling and granted the opportunity to undergo an IVF cycle.

Nuclear and cytoplasmic maturation of in vitro matured human oocytes after temporary nuclear arrest by phosphodiesterase 3-inhibitor

BACKGROUND

The use of hormones for controlled ovarian stimulation results in follicular heterogeneity, with oocytes at diverse stages of nuclear and cytoplasmic development. This study evaluated the impact of temporary nuclear arrest by a specific phosphodiesterase 3-inhibitor (PDE3-I), cilostamide, on nuclear and cytoplasmic maturation of cumulus-free germinal vesicle (GV) human oocytes from controlled ovarian stimulated cycles.

METHODS

GV oocytes (n = 234) were cultured in: (i) medium without the inhibitor (control); (ii) medium supplemented with 1 microM cilostamide and (iii) medium supplemented with 10 microM cilostamide. Oocytes in groups (ii) and (iii) were exposed to cilostamide for 24 h. The PDE3-I was subsequently removed by transfer of oocytes to fresh in vitro maturation (IVM) medium and the reversibility of GV arrest was assessed during IVM culture for maximum 48 h.

RESULTS

Cilostamide (1 and 10 microM) could maintain >80% of the oocytes at the GV stage, without affecting subsequent maturation to metaphase II. Oocytes exposed to 1 microM cilostamide were more likely to have normal bipolar spindles with aligned chromosomes than control oocytes (P < 0.05). When GV chromatin configurations before and after arrest were compared, a significantly higher proportion of oocytes had acquired a nucleolus completely surrounded by a rim of highly condensed chromatin (P < 0.05).

CONCLUSIONS

Temporary nuclear arrest of human GV oocytes with PDE3-I proved to be beneficial for obtaining normal spindle and chromosome configurations after IVM. It resulted also in synchronization within the population of GV oocytes.

Influence of follicular fluid meiosis-activating sterol on aneuploidy rate and precocious chromatid segregation in aged mouse oocytes

BACKGROUND

Follicular fluid meiosis-activating sterol (FF-MAS) protects young oocytes from precocious chromatid separation (predivision). Reduced expression of cohesion and checkpoint proteins and predivision has been hypothesized to occur in age-related aneuploidy in oocytes.

METHODS

To know whether FF-MAS also protects aged oocytes from predivision and from age-related non-disjunction, we analysed chromosome constitution in mouse oocytes matured spontaneously with or without 10 microM FF-MAS and in hypoxanthine (HX)-arrested young and aged oocytes induced to resume maturation by FF-MAS. Messenger RNA for checkpoint protein MAD2 and cohesion protein SMC1beta was compared between oocytes matured with or without FF-MAS.

RESULTS

Aged oocytes possessed many bivalents with single distal chiasma at meiosis I. Predivision was especially high in aged oocytes cultured sub-optimally to metaphase II in alpha-minimum essential medium (alpha-MEM). FF-MAS reduced predivision significantly (P < 0.001) but neither reduced non-disjunction nor induced aneuploidy in aged oocytes. Polyploidy was high in FF-MAS-stimulated maturation, in particular in the aged oocytes (P > 0.001). Relative levels of Smc1beta mRNA appeared increased by maturation in FF-MAS, and mitochondrial clustering was restored.

CONCLUSIONS

Sister chromatids of aged oocytes appear to be highly susceptible to precocious chromatid separation, especially when maturation is under sub-optimal conditions, e.g. in the absence of cumulus and FF-MAS. This may relate to some loss of chromatid cohesion during ageing. FF-MAS protects aged oocytes from predivision during maturation, possibly by supporting Smc1beta expression, thus reducing risks of meiotic errors, but it cannot prevent age-related non-disjunction. Aged oocytes appear prone to loss of co-ordination between nuclear maturation and cytokinesis suggesting age-related relaxed cell cycle control.

Perinatal exposures and breast cancer risk in the Western New York Exposures and Breast Cancer (WEB) Study

BACKGROUND

There is increasing evidence that early life exposures, such as birth weight, infant feeding practices, birth rank and maternal age at delivery may play a role in breast carcinogenesis.

METHODS

We conducted a case-control study of women aged 35-80 in Western New York (Western New York Exposure and Breast Cancer Study, the WEB Study, 1996-2001). The study included 845 women diagnosed with primary, incident, histologically confirmed breast cancer, and 1538 controls frequency-matched to cases on age, race, and county of residence. We conducted extensive in-person interviews including self-reported birth weight, history of having been breastfed, birth rank, and maternal age at delivery.

RESULTS

Birth weight was significantly associated with pre- but not post-menopausal breast cancer risk. Compared to women whose birth weight was 5.5-7 pounds, we found an increased risk associated with a birth weight greater than 8.5 pounds (OR 1.84, 95%CI: 1.12-3.02). Risk was also increased for pre- but not post-menopausal women who had not been breastfed (OR 1.78, 95%CI: 1.21-2.60). Birth order and maternal age at delivery were not significantly associated with breast cancer risk.

CONCLUSIONS

Our findings are consistent with other studies showing breast cancer risk associated with birth weight for pre- but not post-menopausal breast cancer. As we found in an earlier study, having been breastfed was associated with decreased risk. These findings add to the accumulating evidence that early life events impact women's subsequent breast cancer risk.

Light retardance by human oocyte spindle is positively related to pronuclear score after ICSI

Disturbed spindle assembly increases risks of chromosome mal-segregation. Non-invasive polarization microscopy (PolScope) was employed in two centres to assess spindle integrity for the first time quantitatively in human oocytes from consenting patients undergoing intracytoplasmic sperm injection (ICSI) with respect to pronuclear (PN) score after fertilization. In one centre oocytes were selected before ICSI, in another selection was after ICSI according to PN score. In both centres, mean retardance of light by birefringent spindles in oocytes forming a pre-embryo with good PN score after ICSI was significantly higher compared with spindles in oocytes developing into a lower PN score pre-embryo with limited developmental potential (P < 0.001). Transfers involving oocytes with high retardance and at least one good PN score embryo resulted more frequently in a conception than transfers from oocytes with spindles of lower mean retardance and lower PN score embryos. There was a trend for an inverse relationship between age and magnitude of retardance in a small oocyte cohort. The study suggests that quantitative evaluation of mean retardance of light by the oocyte spindle predicts oocyte health, is related to PN score of the embryo and may be especially useful to assess oocyte quality in countries with legal restrictions to select after fertilization.