First-meiotic-division nondisjunction in human oocytes

Whole-chromosome aneuploidy analysis in human oocytes: focus on comparative genomic hybridization

The study of aneuploidy in human oocytes, discarded from IVF cycles, has provided a better understanding of the incidence of aneuploidy of female origin and the responsible mechanisms. Comparative genomic hybridization (CGH) is an established technique that allows for the detection of aneuploidy in all chromosomes avoiding artifactual chromosome losses. In this review, results obtained using CGH in single cells (1PB and/or MII oocytes) are included. The results of oocyte aneuploidy rates obtained by CGH from discarded oocytes of IVF patients and of oocyte donors are summarized. Moreover, the mechanisms involved in the aneuploid events, e.g. whether alterations occurred due to first meiotic errors or germ-line mitotic errors are also discussed. Finally, the incidence of aneuploid oocyte production due to first meiotic errors and germ-line mitotic errors observed in oocytes coming from IVF patients and IVF oocyte donors was assessed.

Evidence that weakened centromere cohesion is a leading cause of age-related aneuploidy in oocytes

Aneuploidy arising early in development is the leading genetic cause of birth defects and developmental disabilities in humans. Most errors in chromosome number originate from the egg, and maternal age is well established as the key risk factor. Although the importance of this problem for reproductive health is widely recognized, the underlying molecular basis for age-related aneuploidy in female meiosis is unknown. Here we show that weakened chromosome cohesion is a leading cause of aneuploidy in oocytes in a natural aging mouse model. We find that sister kinetochores are farther apart at both metaphase I and II, indicating reduced centromere cohesion. Moreover, levels of the meiotic cohesin protein REC8 are severely reduced on chromosomes in oocytes from old mice. To test whether cohesion defects lead to the observed aneuploidies, we monitored chromosome segregation dynamics at anaphase I in live oocytes and counted chromosomes in the resulting metaphase II eggs. About 90% of age-related aneuploidies are best explained by weakened centromere cohesion. Together, these results demonstrate that the maternal age-associated increase in aneuploidy is often due to a failure to effectively replace cohesin proteins that are lost from chromosomes during aging.

Age-related meiotic segregation errors in mammalian oocytes are preceded by depletion of cohesin and Sgo2

BACKGROUND

The growing trend for women to postpone childbearing has resulted in a dramatic increase in the incidence of trisomic pregnancies. Maternal age-related miscarriage and birth defects are predominantly a consequence of chromosome segregation errors during the first meiotic division (MI), which involves the segregation of replicated recombined homologous chromosomes. Despite the importance to human reproductive health, the events precipitating female age-related meiotic errors are poorly understood.

RESULTS

Here we use a long-lived wild-type mouse strain to show that the ability to segregate chromosomes synchronously during anaphase of MI declines dramatically during female aging. This is preceded by depletion of chromosome-associated cohesin in association with destabilization of chiasmata, the physical linkages between homologous chromosomes, and loss of the tight association between sister centromeres. Loss of cohesin is not due to an age-related decline in the ability of the spindle checkpoint to delay separase-mediated cleavage of cohesin until entry into anaphase I. However, we find that reduced cohesin is accompanied by depletion of Sgo2, which protects centromeric cohesin during MI.

CONCLUSIONS

The data indicate that cohesin declines gradually during the long prophase arrest that precedes MI in female mammals. In aged oocytes, cohesin levels fall below the level required to stabilize chiasmata and to hold sister centromeres tightly together, leading to chromosome missegregation during MI. Cohesin loss may be amplified by a concomitant decline in the levels of the centromeric cohesin protector Sgo2. These findings indicate that cohesin is a key molecular link between female aging and chromosome missegregation during MI.

Maternal antimullerian hormone levels do not predict fetal aneuploidy

PURPOSE

To determine if diminished ovarian reserve (measured by maternal antimullerian hormone (AMH) levels), is associated with fetal aneuploidy (determined by prenatal karyotype).

METHODS

This case-control study included 213 women with singleton pregnancies who underwent both serum aneuploidy screening and invasive prenatal diagnosis. 18 patients carrying an aneuploid fetus served as cases and the remaining 195 women with a euploid fetus were controls. Serum AMH was measured using two assays: AMHbc (Beckman-Coulter) and AMHdsl (Diagnostic Systems Laboratories). Karyotypes were determined by chorionic villus sampling or amniocentesis.

RESULTS

AMHbc levels did not differ between women with an aneuploid fetus and women with a euploid fetus (p = 0.46) and did not predict aneuploidy (ROC Area = 0.57). Additionally, AMHbc values declined significantly with advancing gestational age.

CONCLUSIONS

Maternal AMH does not appear to be a marker of fetal aneuploidy in ongoing pregnancies. Contrary to previous reports, we found a significant decline in maternal AMH levels with advancing gestational age.

Metaphase II karyoplast transfer from human in-vitro matured oocytes to enucleated mature oocytes

Metaphase II karyoplast transfer is believed to be a useful method to rescue aged oocytes. This study attempted karyoplast transfer of in-vitro matured metaphase II (MII) oocytes, as a model of aged oocytes, into enucleated freshly ovulated metaphase II oocytes with visualization of their chromosomes under an inverted microscope. Recipient karyoplasts derived from immature oocytes were cultured in-vitro until first polar body extrusion. After 1-2 days culture, 52.1% extruded a polar body, 95.5% had PSC, aneuploidy was very low (4.5%) and none had structural aberrations. Donor oocytes were obtained from IVF or intracytoplasmic sperm injection (ICSI) patients. Chromosomes were easily confirmed in 92.3% and 95.0% of in-vivo and in-vitro matured oocytes respectively. Thirty-one karyoplasts were placed in the perivitelline space of enucleated donor oocytes, and 25 (80.6%) fused to form a reconstituted oocyte. Fertilization, cleavage and blastocyst formation rates following ICSI were 76.0%, 64.0% and 28.0% respectively for reconstructed oocytes and 59.2%, 48.0% and 3.1% respectively for control (in-vitro matured) oocytes. Chromosomal analysis of five embryos developed after karyoplast transfer and ICSI showed normal diploid sets of 46 chromosomes. In conclusion, this metaphase II karyoplast transfer technique can be applied to the solution of chromosomal abnormalities related to oocyte ageing.

Clinical implications of ovarian reserve testing

Serum and urinary markers of ovarian reserve, follicular phase inhibin B, follicle stimulating hormone, and antimullerian hormone levels, are physiologically associated with ovarian aging, decline with chronologic age, and appear to predict later stages of reproductive aging including the menopause transition and menopause. In infertile women, they can be used to predict low oocyte yield and treatment failure in women undergoing in vitro fertilization. These markers seem to be affected by common ovarian toxicants, such as smoking, which advance the age at menopause. Although available for commercial use, home test kits have not been shown to predict fertility or infertility in the general population. Clinical use of these markers is limited by the variety of assays, lack of definitive thresholds, and their intercycle variability in older women. Results should be conveyed with caution when highly discrepant with age, in the obese, and in women with irregular menstrual cycles. Further research is needed to assess their predictive value for determining fertility in the general population.

Effects of in vitro maturation and age on oocyte quality in the rhesus macaque Macaca mulatta

OBJECTIVE

To evaluate oocyte quality in a primate model.

DESIGN

Analysis of oocyte karyotype by chromosome spreading and oocyte spindles by confocal microscopy.

SETTING

Research laboratory, Caribbean Primate Research Center.

ANIMAL(S)

Rhesus macaques aged 6-22 years.

INTERVENTION(S)

Fourteen females underwent both Regimen A (FSH + hCG) and Regimen B (FSH only) stimulation cycles to facilitate collection of mature and immature oocytes. Immature oocytes from Regimens A and B underwent in vitro maturation (IVM) to produce metaphase II oocytes. All metaphase II oocytes underwent gradual fixation to spread chromosomes or were fixed and stained with probes specific to alpha-tubulin, actin, and DNA for visualization of the meiotic spindle using confocal microscopy.

MAIN OUTCOME MEASURE(S)

Karyotype and meiotic spindle architecture differences among in vivo matured (IVO) and IVM oocytes from young and old rhesus macaques.

RESULT(S)

In all, 4.7% of IVO oocytes (Regimen A) from young females were hyperhaploid versus 25.0% of IVM oocytes (Regimen B) from old females; 4.5% of IVO oocytes (Regimen A) from young females versus 51.5% of IVM oocytes (Regimen B) from old females displayed abnormal chromosome alignment on the metaphase spindle.

CONCLUSION(S)

IVM can induce meiotic anomalies in macaque oocytes, especially those obtained from older females. Results from this study provide possible explanations for the reported reduction in developmental competence of IVM primate oocytes.

Monozygotic twins discordant for trisomy 21 and maternal 21q inheritance: a complex series of events

We report on a monochorionic/diamniotic twin pregnancy discordant for trisomy 21. Amniocentesis (at 13(5/7) weeks) was performed following ultrasound signs of hydrops and cystic hygroma in twin 1 (T1). Prenatal karyotype showed non-mosaic trisomy 21 in T1 (47,XX,+21[7]), and low-grade mosaic trisomy 21 in twin 2 (T2) (47,XX,+21[2]/46,XX[19]). Post mortem examination of fetal skin, kidneys and lungs confirmed trisomy 21 in T1 (47,XX,+21[548]) and the placenta (47,XX,+21[200]). T2 had a normal karyotype (46,XX[648]). Analysis of microsatellite polymorphisms in multiple samples from the placenta, hand, lungs, kidneys and the umbilical cords of both twins confirmed monozygosity for all loci tested, and trisomy 21 in T1. Unexpectedly, T1 and T2 inherited different maternal alleles for markers of the most distal 4 Mbp of 21q. At least four successive events are needed to explain the genetic status of both twins and include maternal MI premature chromatids separation or maternal II meiotic nondisjunction and post-zygotic events such as, chromosome rescue, nondisjunction, an/or recombination.

Meiotic errors in human oogenesis and spermatogenesis

Chromosome anomalies are extraordinarily common in human gametes, with approximately 21% of oocytes and 9% of spermatozoa abnormal. The types of abnormalities are quite different since most abnormal oocytes are aneuploid, whereas the majority of abnormalities in spermatozoa are structural. Chromosomes 21 and 22 (the smallest chromosomes) are over-represented in aneuploid gametes in both oocytes and sperm. Chromosome 16 is also frequently observed in aneuploid oocytes, whereas the sex chromosomes are particularly predisposed to non-disjunction in human sperm. Maternal age is clearly the most significant factor in the aetiology of aneuploidy; theories about the cause of the maternal age effect are discussed. Paternal age does not have a dramatic effect on the frequency of aneuploid sperm; there is some evidence for a modest increase in the frequency of sex chromosomal aneuploidy. Meiotic recombination has a significant effect on the genesis of aneuploidy in both females and males. New techniques, which allow the analysis of recombination along the synaptonemal complex, have yielded interesting new information in healthy and infertile individuals. There is a link between infertility and the genesis of chromosome abnormalities. Future studies will unravel more of the underlying causal factors.

Oxidative stress impairs oocyte quality and melatonin protects oocytes from free radical damage and improves fertilization rate

We investigated the relationship between oxidative stress and poor oocyte quality and whether the antioxidant melatonin improves oocyte quality. Follicular fluid was sampled at oocyte retrieval during in vitro fertilization and embryo transfer (IVF-ET). Intrafollicular concentrations of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in women with high rates of degenerate oocytes were significantly higher than those with low rates of degenerate oocytes. As there was a negative correlation between intrafollicular concentrations of 8-OHdG and melatonin, 18 patients undergoing IVF-ET were given melatonin (3 mg/day), vitamin E (600 mg/day) or both melatonin and vitamin E. Intrafollicular concentrations of 8-OHdG and hexanoyl-lysine adduct were significantly reduced by these antioxidant treatments. One hundred and fifteen patients who failed to become pregnant with a low fertilization rate (< or =50%) in the previous IVF-ET cycle were divided into two groups during the next IVF-ET procedure; 56 patients with melatonin treatment (3 mg/day) and 59 patients without melatonin treatment. The fertilization rate was improved by melatonin treatment compared to the previous IVF-ET cycle. However, the fertilization rate was not significantly changed without melatonin treatment. Oocytes recovered from preovulatory follicles in mice were incubated with H2O2 for 12 hr. The percentage of mature oocytes with a first polar body was significantly reduced by addition of H2O2 (300 microm). The inhibitory effect of H2O2 was significantly blocked by simultaneous addition of melatonin. In conclusion, oxidative stress causes toxic effects on oocyte maturation and melatonin protects oocytes from oxidative stress. Melatonin is likely to improve oocyte quality and fertilization rates.

Defective cohesin is associated with age-dependent misaligned chromosomes in oocytes

Aneuploidy often results from chromosome misalignment at metaphases. Oocytes from senescence-accelerated mice (SAM) exhibit increased chromosome misalignment with age, which originates from nuclear factors. This work sought to further characterize the underlying defects of chromosome misalignments. Using immunofluorescence microscopy with specific antibodies, several specific components associated with spindles or chromosomes, including centrosomes, centromeres and cohesin complex were examined. No obvious differences were found in the distribution of centrosome focus at the spindle pole of oocytes from young and aged SAM, regardless of chromosome alignments, although cytoplasmic centrosome foci were significantly reduced in aged SAM (P < 0.0001). Oocytes from both young and aged SAM exhibited centromere-associated protein-E (CENP-E) at centromeres of all chromosomes, including misaligned chromosomes from aged SAM, demonstrating that CENP-E did not contribute to chromosome misalignments. Notably, both meiotic cohesin proteins located between sister chromatids, REC8 (recombinant 8), STAG3 (stromal antigen 3) and SMC1beta, were remarkably reduced in oocytes from aged SAM. Further, degradation of the cohesin was even more obvious in SAM than in hybrid F1 mice with age, which may explain why SAM are vulnerable to aneuploidy. This natural ageing mouse model shows that defective cohesin coincides with increased incidence of chromosome misalignment and precocious separations of sister chromatids.

Gilts and sows produce similar rate of diploid oocytes in vitro whereas the incidence of aneuploidy differs significantly

Oocytes derived from prepubertal gilts show reduced developmental competence when compared to oocytes collected from adult sows. Therefore, the aim of the study was to investigate whether gilts (4-5 months old) and adult sows (average age 3.5 years) of the same breed (Polish Landrace x Polish Large White crossbred) differ with regard to the rate of chromosomally unbalanced oocytes after IVM. COCs derived from individual pairs of slaughterhouse ovaries were matured in vitro and analyzed cytogenetically by conventional staining (Giemsa) and FISH methods (probes corresponding to centromeric regions of pig chromosomes 1 and 10). Altogether, 72 females (31 sows, 41 gilts) and 430 secondary oocytes (194 and 236 oocytes of sows and gilts, respectively) were investigated. Cytogenetic analysis revealed diploid (Giemsa, FISH) and aneuploid (FISH) spreads. The incidence of diploid oocytes was similar for sows (26.0%) and gilts (24.5%) whereas the rate of aneuploid oocytes (nullisomic/disomic) was eight times higher in gilts (10.8%) than in sows (1.3%). Diploid and aneuploid oocytes were observed in 64% of investigated females. Pig chromosome 10 was more frequently disomic/nullisomic compared to chromosome 1 suggesting, that like in human, small porcine chromosomes are often involved in the nondisjunction process. In conclusion, chromosomal imbalance significantly contributes to in vitro embryo production in the pig, since over 60% of females produced diploid or aneuploid gametes. The significantly higher rate of aneuploidy among oocytes derived from gilt ovaries may contribute to the reduced developmental competence of gametes collected from nonmature female pigs.

Gonadotropin-releasing hormone agonist/antagonist conversion with estrogen priming in low responders with prior in vitro fertilization failure

OBJECTIVE

To evaluate gonadotropin-releasing hormone (GnRH) agonist/antagonist conversion with estrogen priming (AACEP) in low responders with prior IVF failure.

DESIGN

Descriptive.

SETTING

Private practice.

PATIENT(S)

Women aged <or=42 with prior IVF attempts in which all embryos were <7 cells or >20% fragmentation on day 3 (n = 137; <38: n = 63; 38-42: n = 74). In addition to unexplained poor response to stimulation (n = 52), diagnoses included elevated follicle-stimulating hormone (FSH >9.0 mIU/mL; n = 40), advanced age (>41 years; n = 26), endometriosis (III-IV; n = 12), and decreased ovarian reserve (AFC <5; n = 7).

INTERVENTION(S)

Patients received sequential GnRH agonist, low-dose GnRH antagonist, and estradiol valerate followed by recombinant FSH, 600 IU/day (n = 72) or 750 IU/day (n = 65).

MAIN OUTCOME MEASURE(S)

Pregnancy, ongoing gestation, implantation rates.

RESULT(S)

Although women aged <38 years and those on 600 IU/day produced more mature eggs and fertilized embryos than women aged 38 to 42 years, there were no differences in peak estradiol, endometrial lining, or embryos transferred. Outcomes were similar for all patients regardless of age or FSH dosage. Ongoing gestation rates were 27% (37 out of 137) for all patients, 25% (16 out of 63) for age <38 years, and 28% (21 out of 74) for ages 38 to 42 years.

CONCLUSION(S)

Women aged <or=42 years who are candidates for oocyte donation may still achieve pregnancy using their own eggs with the AACEP protocol.

Oocyte aneuploidy mechanisms are different in two situations of increased chromosomal risk: older patients and patients with recurrent implantation failure after in vitro fertilization

OBJECTIVE

To clarify the mechanisms underlying oocyte abnormalities in meiosis: meiotic nondisjunction of a whole chromosome or premature separation of sister chromatids in two situations of increased chromosomal risk.

DESIGN

Preconception diagnosis by first polar-body analysis in two situations of increased chromosomal risk.

SETTING

Departments of reproductive biology, cytogenetics, gynecology, and obstetrics.

PATIENT(S)

First polar body analysis was proposed to 76 patients (91 cycles) for advanced age (AMA; n = 30, 36 cycles), recurrent implantation failure (RIF; >10 embryos transferred without implantation; n = 32, 36 cycles), or both (AMA + RIF; n = 14, 19 cycles), before their intracytoplasmic sperm injection procedure.

INTERVENTION(S)

First polar-body analysis using fluorescence in situ hybridization.

MAIN OUTCOME MEASURE(S)

Mechanisms and frequency of aneuploidy.

RESULT(S)

Three hundred eighty-four oocytes were analyzed by fluorescence in situ hybridization, 130 from women >38 years of age, 171 from women with RIF, and 83 from women with both indications. The oocyte abnormality rate was similar in the three groups, respectively, 38.5%, 40.4%, and 45.8%. The aneuploidy mechanisms were different for women >38 years of age who had no previous implantation failure (AMA) compared with women of whatever age who had implantation failure (P<.05 vs. RIF; P<.001 vs. AMA+RIF), with, respectively, for the AMA, RIF, and AMA+RIF groups, 72.2%, 56.6%, and 49.2% premature separation of sister chromatids and 27.8%, 43.4%, and 50.8% meiotic nondisjunction. In the two implantation-failure groups, we distinguished a subgroup (22% in the RIF group and 33% in AMA+RIF group) of patients with >2/3 abnormal oocytes, suggesting a meiosis alteration.

CONCLUSION(S)

The mechanisms accounting for oocyte aneuploidy differed in the two clinical situations of advanced maternal age and RIF. Advanced maternal-age aneuploidy was linked to a loss of sister chromatid cohesion that led to one single chromatid abnormality, whereas implantation failure is a much more heterogeneous situation.

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.

Chromosome malorientations after meiosis II arrest cause nondisjunction

This study investigated the basis of meiosis II nondisjunction. Cold arrest induced a fraction of meiosis II crane fly spermatocytes to form (n + 1) and (n - 1) daughters during recovery. Live-cell liquid crystal polarized light microscope imaging showed nondisjunction was caused by chromosome malorientation. Whereas amphitely (sister kinetochore fibers to opposite poles) is normal, cold recovery induced anaphase syntely (sister fibers to the same pole) and merotely (fibers to both poles from 1 kinetochore). Maloriented chromosomes had stable metaphase positions near the equator or between the equator and a pole. Syntelics were at the spindle periphery at metaphase; their sisters disconnected at anaphase and moved all the way to a centrosome, as their strongly birefringent kinetochore fibers shortened. The kinetochore fibers of merotelics shortened little if any during anaphase, making anaphase lag common. If one fiber of a merotelic was more birefringent than the other, the less birefringent fiber lengthened with anaphase spindle elongation, often permitting inclusion of merotelics in a daughter nucleus. Meroamphitely (near amphitely but with some merotely) caused sisters to move in opposite directions. In contrast, syntely and merosyntely (near syntely but with some merotely) resulted in nondisjunction. Anaphase malorientations were more frequent after longer arrests, with particularly long arrests required to induce syntely and merosyntely.

Nuclear transfer methods to study aging

Maternal age affects oocyte quality and early embryo development. Aberrant meiosis of oocytes and compromised early embryo development from older females could originate from defects in the nucleus, the cytoplasm, or both. Nuclear transfer has been used for decades as a tool to study nuclear-cytoplasmic interactions in early embryos, and has uncovered genomic imprinting, nuclear reprogramming, and produced animal clones. Here, we describe the technique for investigating nuclear-cyoplasmic interactions in oocytes and zygotes in female reproductive aging. Nuclear transfer can be performed efficiently and effects of the technique itself on meiosis and early embryo development are minimal as long as care is taken to minimize insult to oocytes or embryos. This protocol first focuses on use of nuclear transfer to study nucleus versus cytoplasmic origin in agingassociated meiosis defects in oocytes at the germinal vesicle (GV) stage. Then, nuclear transfer is used at the zygote stage to study nuclear and cytoplasmic abnormality and apoptosis in early development.

Reproductive ageing in women

The traditional view in respect to female reproduction is that the number of oocytes at birth is fixed and continuously declines towards the point when no more oocytes are available after menopause. In this review we briefly discuss the embryonic development of female germ cells and ovarian follicles. The ontogeny of the hypothalamic-pituitary-gonadal axis is then discussed, with a focus on pubertal transition and normal ovulatory menstrual cycles during female adult life. Biochemical markers of menopausal transition are briefly examined. We also examine the effects of age on female fertility, the contribution of chromosomal abnormalities of the oocyte to the observed decline in female fertility with age and the possible biological basis for the occurrence of such abnormalities. Finally, we consider the effects of maternal age on obstetric complications and perinatal outcome. New data that have the potential to revolutionize our understanding of mammalian oogenesis and follicular formation, and of the female reproductive ageing process, are also briefly considered.

Maternal age-related differential global expression profiles observed in human oocytes

The age-related decline in female fertility has been attributed to a variety of causes including progressive oocyte depletion, meiotic irregularities and mitochondrial dysfunction. However, additional factors could potentially be involved. To explore this possibility, comprehensive analysis of gene expression in human oocytes, discarded following IVF procedures and segregated by age, was undertaken using microarray methods. These findings indicate that the expression of oocyte genes, in a variety of major functional categories including cell cycle regulation, cytoskeletal structure, energy pathways, transcription control, and stress responses, are influenced by maternal age. These results are corroborated by a complementary extensive study using mouse oocytes.

The contradictory information on the distribution of non-disjunction and pre-division in female gametes

Valuable information on the cytogenetic constitution of female gametes has been deduced from the direct, so-called conventional analysis of oocytes remaining unfertilized in programmes of assisted reproduction. Additional, indirect conclusions have become possible by PGD of the polar bodies. Both techniques provided evidence for the co-existence of two aneuploidy-causing mechanisms during first maternal meiosis; non-disjunction (ND) of bivalents results in the loss or gain of whole chromosomes in metaphase II complements, whereas a precocious division (pre-division, PD) of univalents leads to the loss or gain of single chromatids. As to the distribution of ND and PD, however, direct oocyte chromosome studies and PGD tell surprisingly different stories. Moreover, first and second polar body analyses contradict the data derived from DNA polymorphism studies concerning the distribution of first and second meiotic division errors. An increased awareness of these problems appears necessary because important decisions are made on the basis of PGD results.

Protein phosphatase 2A protects centromeric sister chromatid cohesion during meiosis I

Segregation of homologous maternal and paternal centromeres to opposite poles during meiosis I depends on post-replicative crossing over between homologous non-sister chromatids, which creates chiasmata and therefore bivalent chromosomes. Destruction of sister chromatid cohesion along chromosome arms due to proteolytic cleavage of cohesin's Rec8 subunit by separase resolves chiasmata and thereby triggers the first meiotic division. This produces univalent chromosomes, the chromatids of which are held together by centromeric cohesin that has been protected from separase by shugoshin (Sgo1/MEI-S332) proteins. Here we show in both fission and budding yeast that Sgo1 recruits to centromeres a specific form of protein phosphatase 2A (PP2A). Its inactivation causes loss of centromeric cohesin at anaphase I and random segregation of sister centromeres at the second meiotic division. Artificial recruitment of PP2A to chromosome arms prevents Rec8 phosphorylation and hinders resolution of chiasmata. Our data are consistent with the notion that efficient cleavage of Rec8 requires phosphorylation of cohesin and that this is blocked by PP2A at meiosis I centromeres.

Arsenite induces aberrations in meiosis that can be prevented by coadministration of N-acetylcysteine in mice

OBJECTIVE

To evaluate in vitro effects of arsenite and of arsenite plus N-acetylcysteine on mouse oocyte meiosis.

DESIGN

Morphological study using mouse oocytes submitted to in vitro maturation (IVM).

SETTING

Laboratory of reproductive biology.

ANIMAL(S)

Six-week-old CD-1 mice superovulated with pregnant mare serum gonadotropin.

INTERVENTION(S)

During IVM, mouse oocytes were exposed to arsenite alone or to arsenite plus N-acetylcysteine.

MAIN OUTCOME MEASURE(S)

Meiotic anomalies were assessed using immunofluorescence microscopy and PolScope (Cambridge Research and Instrumentation, Boston, MA) imaging.

RESULT(S)

In vitro arsenite administration produced dose-dependent and time-dependent meiotic anomalies, characterized by spindle disruption or chromosome misalignment. After 12-14 hours of IVM, exposure to 2 microg/mL of arsenite for 12-14 hours or to 8 microg/mL of arsenite for 2 hours arrested oocyte maturation at the germinal vesicle or germinal-vesicle breakdown stage. Exposure to 4 microg/mL of arsenite for 2 hours arrested oocyte maturation at metaphase I stage in 95% of exposed oocytes (80% exhibiting abnormalities) after 12-14 hours in IVM. After 12-14 hours in IVM, of the oocytes exposed to 2 microg/mL of arsenite for 2 hours, only 15% reached the meiosis II stage (5% exhibiting abnormalities). After 15-17 hours in IVM, however, of the oocytes exposed to 2 microg/mL of arsenite for 2 hours, 65.2% reached the meiosis II stage (43.5% exhibiting abnormalities). Co-administration of N-acetylcysteine prevented the arsenite-induced meiotic abnormalities and the delayed IVM.

CONCLUSION(S)

In vitro arsenite exposure caused meiotic abnormalities that were prevented by co-administration of N-acetylcysteine, suggesting that arsenite-induced meiotic aberrations are mediated by reactive oxygen species.

Start me up: Cell signaling and the journey from oocyte to embryo inC. elegans

Intercellular communication plays a pivotal role in regulating and coordinating oocyte meiosis and fertilization, key triggers for embryonic development. The nematode Caenorhabaditis elegans has emerged as an important experimental paradigm for exploring these fundamental reproductive processes and their regulation. The oocytes of most animal species arrest during meiotic prophase and complete meiosis in response to intercellular signaling in the process of meiotic maturation. Oocyte meiotic maturation is defined by the transition between diakinesis and metaphase of meiosis I and is accompanied by nuclear envelope breakdown and meiotic spindle assembly. As such, the meiotic maturation process is essential for completing meiosis and a prerequisite for successful fertilization. In C. elegans, the processes of meiotic maturation, ovulation, and fertilization are temporally coupled: sperm utilize the major sperm protein as a hormone to trigger oocyte meiotic maturation, and, in turn, the maturing oocyte signals its own ovulation, leading to fertilization. The powerful genetic screens possible in C. elegans have led to the identification of several sperm cell surface proteins that are required for the interaction and fusion of gametes at fertilization. The study of these proteins provides fundamental insights into fertilization mechanisms, their role in speciation, and their potential conservation across phyla. Signaling processes sparked by fertilization are required for meiotic chromosome segregation and initiating the embryonic program. Here we review recent advances in understanding how signaling mechanisms contribute to the oocyte-to-embryo transition in C. elegans.

Cytogenetic analysis of human oocytes remaining unfertilized after intracytoplasmic sperm injection

Oocytes remaining unfertilized after intracytoplasmic sperm injection showed 12.0% aneuploidy (nondisjunction + unbalanced predivision), 3.4% structural aberrations, and 8.5% balanced predivision in fully karyotyped cells. However, the frequently observed complete or partial separation of chromatids, most probably caused by abortive activation, might complicate the evaluation of meiosis I-derived aneuploidy and questions the relevance of balanced predivision.

Frequency and distribution of chromosome abnormalities in human oocytes

It was previously shown that more than half of the human oocytes obtained from IVF patients of advanced reproductive age are aneuploid, due to meiosis I and meiosis II errors. The present paper further confirms that 61.8% of the oocytes tested by fluorescent probes specific for chromosomes 13, 16, 18, 21 and 22 are abnormal, representing predominantly chromatid errors, which are the major source of aneuploidy in the resulting embryos. Almost half of the oocytes with meiosis I errors (49.3%) are prone to sequential meiosis II errors, which may lead to aneuploidy rescue in 30.8% of the cases. Half of the detected aneuploidies (49.8%) are of complex nature with involvement of two or more chromosomes, or the same chromosome in both meiotic divisions. The aneuploidy rates for individual chromosomes are different, with a higher prevalence of chromosome 21 and 22 errors. The origin of aneuploidy for the individual chromosomes is also not random, with chromosome 16 and 22 errors originating more frequently in meiosis II, and chromosome 18, 13 and 21 errors in meiosis I. There is an age dependence not only for the overall frequency of aneuploidies, but also for each chromosome error, aneuploidies originating from meiosis I, meiosis II, and both meiosis I and meiosis II errors, as well as for different types of aneuploidies. The data further suggest the practical relevance of oocyte aneuploidy testing for detection and avoidance from transfer of the embryos deriving from aneuploid oocytes, which should contribute significantly to the pregnancy outcomes of IVF patients of advanced reproduction age.

Evidence of a high proportion of premature unbalanced separation of sister chromatids in the first polar bodies of women of advanced age

BACKGROUND

Maternal ageing is the only aetiological factor unequivocally linked to aneuploidy. Two mechanisms seem to explain these abnormalities in oocytes: non-disjunction and premature unbalanced separation of sister chromatids (PSSC). Previous studies of unfertilized oocytes argue for a major role of PSSC in the aetiology of aneuploidy for women of advanced age, but in vitro ageing of the oocytes could influence the results.

METHODS

Owing to the high prevalence of aneuploidy in women of advanced age, chromosomal screening of the first polar body just before ICSI was offered to women (from 38 years of age) included in an assisted reproduction programme.

RESULTS

Among 141 oocytes from 29 women (mean age 40 years and 2 months), 43 (30.5%) were abnormal. Sixty-five abnormalities were found and PSSC was involved in 80% of cases.

CONCLUSION

These results are in accordance with previous studies and confirm, in 'fresh' oocytes, the major role of PSSC in the aetiology of aneuploidy in women of advanced age.

Aneuploidy involving chromosome 1 in failed-fertilized human oocytes is unrelated to maternal age

PURPOSE

To study whether maternal meiotic errors in failed-fertilized oocytes involving chromosome 1 occur at frequencies similar to those involving other autosomes, and whether their frequency is affected by maternal age.

METHODS

Using fluorescence in situ hybridization (FISH), frequencies of aneusomy and chromatid pre-division involving chromosomes 1, 16, 18, and 21 were determined for 273 failed-fertilized oocytes.

RESULTS

The aneuploidy rate for chromosome 1 was 15.8%, and was neither age-dependent nor significantly different from that for chromosomes 16, 18 or 21. Only chromosome 16 exhibited an age-dependent increase in aneusomy rates. The frequency of chromatid pre-division was lower for chromosome 1 than for chromosome 18 (11.9% vs. 25.4%; p = 0.01), but not different from that for chromosomes 16 or 21.

CONCLUSION

Aneuploidy involving chromosome 1 in failed-fertilized oocytes is unrelated to maternal age and occurs at a frequency similar to that for chromosomes 16, 18, and 21.

Nouvelles données en génétique chromosomique

Novel methods allowing to analyze the human genome make it possible to assess old questions such as the molecular basis of structural chromosome anomalies and the diathesis to aneuploidy. The architecture of the human genome as unravelled by the human genome sequencing project allows to explain the recurrence of microdeletions and microduplications caused by a non allelic homologous recombination involving segmental duplications created during the evolution of primates. This structural feature of the human genome is associated with a novel class of genetic diseases called genomic disorders as opposed to genetic diseases due to gene mutations. The study of the parental and cellular origin of aneuploidy shed new light on the different mechanisms controlling meiosis in man and woman. In addition it contributes to define the role of maternal age and genetic recombination on the behavior of chromosomes during meiosis. These new data greatly contribute to our understanding of human chromosomal diseases.

SMC1beta-deficient female mice provide evidence that cohesins are a missing link in age-related nondisjunction

Mitotic chromosome segregation is facilitated by the cohesin complex, which maintains physical connections between sister chromatids until anaphase. Meiotic cell division is considerably more complex, as cohesion must be released sequentially to facilitate orderly segregation of chromosomes at both meiosis I and meiosis II. This necessitates meiosis-specific cohesin components; recent studies in rodents suggest that these influence chromosome behavior during both cell division and meiotic prophase. To elucidate the role of the meiosis-specific cohesin SMC1beta (encoded by Smc1l2) in oogenesis, we carried out meiotic studies of female SMC1beta-deficient mice. Our results provide the first direct evidence that SMC1beta acts as a chiasma binder in mammals, stabilizing sites of exchange until anaphase. Additionally, our observations support the hypothesis that deficient cohesion is an underlying cause of human age-related aneuploidy.

The occurrence of aneuploidy in human: lessons from the cytogenetic studies of human oocytes

During the last 4 decades, the cytogenetic investigation of human oocytes has never stopped to progress, according to the advents of new technologies. Both karyotyping and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes. However, these studies display a great variability in results, which may be essentially attributable to the limitations of these techniques when applied to human oocytes. The most relevant analysis have led to the estimate that 15-20% of human oocytes present chromosome abnormalities, and they have emphasized the implication of both whole chromosome nondisjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidies in human oocytes has also been clearly evidenced by recent reports based on large sample of oocytes or polar bodies, whereas most of initial studies have failed to confirm any relationship between maternal age and aneuploidy in human oocytes.

Effect of maternal age on the frequency of cytogenetic abnormalities in human oocytes

The cytogenetic investigation of human oocytes was initiated in the Sixties, and for the last four decades, this field of research has never stopped progressing as new technologies appear. Numerous karyotyping studies and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes, but also displaying a great variability in results, which may be essentially attributable to the technical limitations of these in situ methods when applied to human oocytes. Essentially, the most relevant analyses have led to the estimate that 15-20% of human oocytes display chromosome abnormalities, and they have emphasized the implication of both whole chromosome nondisjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidies has also been investigated in human oocytes. Most previous studies have failed to confirm any relationship between maternal age and aneuploidy frequency in human oocytes, whereas the more recent reports based on large samples of oocytes or polar bodies have provided evidence for a direct correlation between increased aneuploidy frequency and advanced maternal age, and have clarified the contribution of the various types of malsegregation in the maternal age-dependent aneuploidies.

Meiotic abnormalities in in vitro-matured marmoset monkey (Callithrix jacchus) oocytes: development of a non-human primate model to investigate causal factors

BACKGROUND

Meiotic abnormalities are thought to be a major causal factor of low embryo development rates, for embryos developed from in vitro-matured oocytes. A new non-human primate model, in the common marmoset, is being developed to facilitate investigation of the mechanisms involved.

METHODS

Oocytes were dissected from antral follicles from three size classes. They were allowed to mature in vitro for only 24 h, in order to focus the investigation on the rapidly maturing oocytes. Chromosome spreads were visualized with Giemsa staining, and spindles /chromosomes with fluorescently labelled anti-alpha-tubulin antibody combined with a DNA fluorochrome.

RESULTS

40% of the oocytes had reached metaphase II (MII) after 24 h. Of the MII oocytes selected for karyotyping, readable chromosomal spreads were obtained from 64%. Overall, 63% of these presented a normal haploid chromosome number of 23,X, with all abnormal karyotypes occurring in the oocytes from small follicles. For another group of MII oocytes, where meiotic spindles were visualized, only half of the MII oocytes displayed well-formed spindles and apparently correct chromosomal alignment.

CONCLUSIONS

This work provides the first information on the normal and aneuploid MII meiotic chromosome sets for the marmoset oocyte, and demonstrates a high rate of chromosomal and spindle abnormality among rapidly maturing oocytes from small antral follicles.

Karyotyping of human oocytes by cenM-FISH, a new 24-colour centromere-specific technique

BACKGROUND

Metaphase II (MII) chromosome complements are difficult to karyotype. The objective of this study was to investigate the efficiency and limitations of centromere-specific multiplex fluorescence in situ hybridization (cenM-FISH), a new 24 colour FISH technique using centromere-specific probes, to analyse the whole chromosome complement within human oocytes.

METHODS

Oocytes were donated by 34 patients undergoing ovarian stimulation and IVF. The MII oocytes were analysed by means of cenM-FISH, while the confirmation of results was performed by FISH and/or by analysing the corresponding first polar bodies using comparative genomic hybridization (CGH).

RESULTS

A total of 30 cells, corresponding to 16 oocytes and 14 first polar bodies, were successfully karyotyped by either cenM-FISH or CGH. The incidence of aneuploidy was 25%, and eight out of nine aneuploidy events were confirmed by CGH and FISH.

CONCLUSIONS

We demonstrate here for the first time that the identification of any numerical abnormality in oocytes is feasible using cenM-FISH. Despite the fact that the fixation efficiency remains low, the present results confirm the advantage of analysing the whole set of chromosomes to make an accurate estimation of the aneuploidy rate in human oocytes.

Cytogénétique des ovocytes humains : 40 ans de progrès

Chromosomal abnormalities account for the majority of pre- and post- implantation embryo wastage in humans. Most of these abnormalities result from maternal meiotic errors, which preferentially occur during the first meiotic division. Consequently, the cytogenetic analysis of human oocytes has then been considered as a highly valuable source of data for the investigation of both the occurrence and the origin of chromosomal abnormalities in human. During the last 4 decades, the cytogenetic analysis of human oocytes has never stopped progressing, according to the advents of new technologies. Both karyotyping and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes. However, these studies display a great variability in results, which may be essentially attributable to the limitations of these techniques when applied to human oocytes. The most relevant analysis have led to the estimate that 15-20% of human oocytes present chromosome abnormalities, and they have emphasized the implication of both whole chromosome non-disjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidy in human oocytes has also been clearly evidenced by recent reports based on large sample of oocytes or polar bodies.

Improvement of delivery and live birth rates after ICSI in women aged >40 years by ovarian co-stimulation with growth hormone

BACKGROUND

Growth hormone (GH) is required for ovarian follicular development, and its administration during ovarian stimulation improves pregnancy rate in cow and sheep. Data on the use of exogenous GH in human assisted reproduction treatment are inconsistent. This prospective randomized study evaluates the usefulness of GH administration in women of >40 years undergoing ovarian stimulation for assisted reproduction treatment.

METHODS

One hundred women of >40 years undergoing assisted reproduction treatment were randomized between a GH treatment group and a placebo group. Assisted reproduction treatment outcomes were evaluated.

RESULTS

In patients of the GH treatment group, a similar number of oocytes, embryos and pregnancies was achieved as compared with the placebo group. However, the patients treated with GH suffered fewer pregnancy losses, resulting in higher delivery and live birth rates. These patients also showed higher peak serum estradiol concentration and higher concentrations of GH and estradiol in pre-ovulatory follicular fluid as compared with the placebo group.

CONCLUSIONS

Administration of GH during ovarian stimulation alleviates age-related decrease in assisted reproduction treatment efficiency. This effect appears to be mainly due to an improvement of oocyte developmental potential, but GH action on the uterus cannot be excluded.

The structure and function of SMC and kleisin complexes

Protein complexes consisting of structural maintenance of chromosomes (SMC) and kleisin subunits are crucial for the faithful segregation of chromosomes during cell proliferation in prokaryotes and eukaryotes. Two of the best-studied SMC complexes are cohesin and condensin. Cohesin is required to hold sister chromatids together, which allows their bio-orientation on the mitotic spindle. Cleavage of cohesin's kleisin subunit by the separase protease then triggers the movement of sister chromatids into opposite halves of the cell during anaphase. Condensin is required to organize mitotic chromosomes into coherent structures that prevent them from getting tangled up during segregation. Here we describe the discovery of SMC complexes and discuss recent advances in determining how members of this ancient protein family may function at a mechanistic level.

Complete cytogenetic investigation of oocytes from a young cancer patient with the use of comparative genomic hybridisation reveals meiotic errors

OBJECTIVES

The complete cytogenetic investigation of human oocytes and the corresponding first polar bodies (PBs) derived from an 18-year old female cancer patient.

METHODS

A whole-genome amplification method combined with comparative genomic hybridisation (CGH) was employed for the analysis of 14 oocytes and their corresponding first PBs.

RESULTS

Chromosome abnormalities were detected in two oocyte-PB complexes. One oocyte had lost X-chromosome material (23,X,-Xcht), while its corresponding first PB showed the reciprocal gain (23,X,+Xcht). Double aneuploidy involving loss of chromatids for chromosomes X and 21 was identified in another first PB (23,X,-21cht,-Xcht). Aneuploidy was attributed to unbalanced pre-division of chromatids at meiosis I.

CONCLUSIONS

Meiotic errors in chromosome segregation can occur even in oocytes derived from young women, confirming the existence of age-independent factors contributing to aneuploidy. Such factors are of relevance to fertility, miscarriage and preimplantation aneuploidy screening for the purposes of increasing IVF success rates. The reliability of CGH in examining the whole chromosome complement of a single cell and of being able to detect chromatid anomalies is confirmed by this study.

Aneuploidy study of human oocytes first polar body comparative genomic hybridization and metaphase II fluorescence in situ hybridization analysis

BACKGROUND

The object of this study was to determine the mechanisms that produce aneuploidy in oocytes and establish which chromosomes are more prone to aneuploidy.

METHODS

A total of 54 oocytes from 36 women were analysed. The whole chromosome complement of the first polar body (1PB) was analysed by comparative genomic hybridization (CGH), while the corresponding metaphase II (MII) oocyte was analysed by fluorescence in situ hybridization (FISH) to confirm the results.

RESULTS

Matched CGH-FISH results were obtained in 42 1PB-MII doublets, of which 37 (88.1%) showed reciprocal results. The aneuploidy rate was 57.1%. Two-thirds of the aneuploidy events were chromatid abnormalities. Interestingly, the chromosomes more frequently involved in aneuploidy were chromosomes 1, 4 and 22 followed by chromosome 16. In general, small chromosomes (those equal to or smaller in size than chromosome 13) were more prone to aneuploidy (chi2-test, P=0.07); 25% of the aneuploid doublets would have been misdiagnosed as normal using FISH with probes for nine-chromosomes.

CONCLUSIONS

The combination of two different techniques, CGH and FISH, for the study of 1PB and MII allowed the identification and confirmation of any numerical chromosome abnormality, as well as helping to determine the mechanisms involved in the genesis of maternal aneuploidy.

The chromosomal analysis of human oocytes. An overview of established procedures

The cytogenetic survey of mature human oocytes has been and remains a subject of great interest because of the prevalence of aneuploidy of maternal origin in abnormal human conceptuses, and the lack of understanding about the non-disjunction processes in human meiosis. The first attempts to analyse the chromosomal content of human female gametes were made in the early 1970s, and led to limited data because of the paucity of materials and the inadequacy of the procedure used. The years to follow brought a resurgence of interest in this field, because of the development of human IVF techniques which made oocytes unfertilized in vitro available for cytogenetic analysis. Numerous studies have since been performed. However, the difficulties in obtaining good chromosome preparations and of performing accurate chromosome identification have reduced the viability of these studies, resulting in large variations in the reported incidences of chromosomal abnormalities. The further introduction of new procedures for oocyte fixation and the screening of large oocyte samples have allowed more reliable data to be obtained and to identify premature chromatid separation as a major mechanism in aneuploidy occurrence. The last decade has been privileged to witness the adaptation of molecular cytogenetic techniques to human oocytes, and thus various powerful procedures have been tried not only on female gametes, but also on polar bodies, involving sequential and multicolour fluorescent in situ hybridization (FISH) labelling, comparative genomic hybridization (CGH), spectral karyotyping and alternative methods such as primed in situ labelling (PRINS) and peptide nucleic acid (PNA) techniques. A large body of data has been obtained, but these studies also display a great variability in the frequency of abnormalities, which may be essentially attributable to the technical limitations of these in situ methods when applied to human oocytes. However, molecular cytogenetic approaches have also evidenced the co-existence of both whole chromosome non-disjunction and chromatid separation in maternal aneuploidy. In addition, the extension of these techniques to oocyte polar body materials has provided additional data on the mechanism of meiotic malsegregation. Improvements of some of these techniques have already been reported. The further development of new approaches for the in situ analysis of human meiosis will increase the impact of cytogenetic investigation of human oocytes in the understanding of aneuploidy processes in humans.

The incidence of aneuploidy in human oocytes assessed by conventional cytogenetic analysis

Human oocytes failing to fertilize during assisted reproduction are an important source of information for assessing incidence and causal mechanisms of maternal aneuploidy. This review describes the techniques of conventional oocyte chromosome analysis and evaluates the results of 59 studies comprising a total of>10,000 female gametes. The mean rate of aneuploidy (hypohaploidy + hyperhaploidy) amounts to approximately 20%, but this incidence is reduced as soon as possible artifacts introduced by the fixation technique are taken into consideration. It is therefore concluded that a realistic value for numerical abnormalities arising during first meiotic division lies between 12 and 15%. All chromosome groups are affected by aneuploidy but the actually observed frequencies exceed the expected frequencies in groups D, E, and G. Two aneuploidy-causing mechanisms have been identified in human oocytes: nondisjunction, resulting in the loss or gain of whole chromosomes, and predivision, resulting in the loss or gain of single chromatids. A brief analysis including only aneuploid complements with one extra or missing chromosome/chromatid shows a slight increase in predivision (52.9%) compared with nondisjunction (47.1%). Finally, suggestions for future studies are given since, for instance, the presentation of results and the use of cytogenetic nomenclature have not been uniform.

Nuclear Origin of Aging-Associated Meiotic Defects in Senescence-Accelerated Mice1

Factors of both cytoplasmic and nuclear origin regulate metaphase chromosome alignment and spindle checkpoint during mitosis. Most aneuploidies associated with maternal aging are believed to derive from nondisjunction and meiotic errors, such as aberrations in spindle formation and chromosome alignment at meiosis I. Senescence-accelerated mice (SAM) exhibit aging-associated meiotic defects, specifically chromosome misalignments at meiosis I and II that resemble those found in human female aging. How maternal aging disrupts meiosis remains largely unexplained. Using germinal vesicle nuclear transfer, we found that aging-associated misalignment of metaphase chromosomes is predominately associated with the nuclear factors in the SAM model. Cytoplasm of young hybrid B6C3F1 mouse oocytes could partly rescue aging-associated meiotic chromosome misalignment, whereas cytoplasm of young SAM was ineffective in preventing the meiotic defects of old SAM oocytes, which is indicative of a deficiency of SAM oocyte cytoplasm. Our results demonstrate that both nuclear and cytoplasmic factors contribute to the meiotic defects of the old SAM oocytes and that the nuclear compartment plays the predominant role in the etiology of aging-related meiotic defects.

Genetics of gametes and embryos

Chromosome analysis of oocytes, sperm and embryos has mainly relied on fluorescent in situ hybridisation (FISH) and karyotyping. FISH studies have been performed on sperm from fertile and infertile men as well as men carrying known chromosomal translocations. Molecular DNA analyses has aided in the identification and treatment of men with Y chromosome deletions. In oocytes FISH and karyotyping have identified non-disjunction of univalents and predivision of chromatids. Analysis of the chromosomes from human embryos has shown that a high proportion of embryos are mosaic or chaotic, in addition to embryos beings uniformly and abnormal. FISH and PCR have also been used clinically for preimplantation genetic diagnosis (PGD). For patients at risk of transmitting a specific genetic or chromosomal abnormality, 1-2 blastomeres are biopsied from embryos and specific genes or chromosomes analysed. Normal embryos are then transferred to the uterus.

Âge maternel et anomalies chromosomiques dans les ovocytes humains

Maternal ageing is the only etiological factor unequivocally associated with the occurrence of aneuploid conceptuses. Molecular studies of trisomies have demonstrated that the pattern of recombinaison was an important predisposing factor to meiotic nondisjunction. To complete this data, a large chromosomal study has been undertaken on 1,397 unfertilised human oocytes recovered from women participating in in vitro fertilization programmes. Conventional whole chromosome nondisjunction and premature chromatid separation were the major types of numerical abnormalities observed. A positive relationship was found between maternal age and these two types of nondisjunction, but the most significant correlation was observed with chromatid separation resulting in the presence of free chromatid in metaphase II oocyte. These data revealed that chromatid separation was an essential factor in the age-dependent occurrence of aneuploidy. This finding provided new insights into the mechanism of nondisjunction in female meiosis since disturbance in molecular chromatid cohesion by cohesins might be a causal mechanism predisposing to nondisjunction and involved in the maternal age effect.