Chromosomal analysis of unfertilized human oocytes

Unfertilized human oocytes were obtained from women in an in-vitro fertilization programme. The women had a mean age of 29.4 years (range 24-35 years). Chromosomal complements could be analysed in 50 oocytes. Q-banding of the chromosomes facilitated identification of individual chromosomes: 34 oocytes (68%) had the normal haploid chromosomal complement, 14 complements were hypohaploid (28%), 1 complement was hyperhaploid (2%) and 2 had structural abnormalities (4%). (One oocyte had numerical and structural abnormalities). The 16 abnormal oocytes were obtained from 15 different women. A conservative estimate of aneuploidy in this sample is 4%; however, the frequency of aneuploidy may be higher if there is a predisposition to chromosome loss during oogenesis. This study provides information on the largest series of karyotyped unfertilized human oocytes published to date.     

Test of a semiselective screen for induced aneuploidy in germ cells of Drosophila melanogaster females with structurally normal chromosomes

A new semiselective screen (only female progeny survive) for induced aneuploidy in germ cells of Drosophila melanogaster (referred to as 20/Q56 for the X-chromosome mutation markers in the parental females) has been validated by recovering cold, colchicine and N,N-dimethylnitrosamine (DMN) induced chromosome gain and loss events in females that contain structurally normal chromosomes. In addition, the spontaneous and induced results from the 20/Q56 assay, which identifies gain events at division I and loss events at divisions I and II of meiosis, were compared with a nonselective (all progeny survive) modified mating scheme that identifies gains and losses at both divisions of oogenesis. Females with the same genotypes are treated in the two mating schemes and are then mated with males that contain different marked Y chromosomes. The spontaneous rates of chromosome gains and losses were not significantly different in the two mating schemes (these rates ranged from 0.008 to 0.022%), supporting previous reports that spontaneous aneuploidy occurs at a higher frequency at division I of meiosis in females of D. melanogaster than at division II. Both the 20/Q56 and modified screens were able to identify significant increases in aneuploidy after adult treatments with cold shock (10 degrees C and 5 degrees C), colchicine (5 ppm and 10 ppm), and DMN (100 ppm). Brood analysis (five 2-day or five 3-day broods) showed that the largest increases in aneuploidy after cold treatment occurred in the first brood, which contains a high proportion of stage 14 oocytes, whereas colchicine induced the highest frequencies in the latter broods and DMN was effective in all but the last brood. Although the 20/Q56 mating scheme identifies gain events only in division I of meiosis whereas the modified mating scheme identifies gains in both divisions, the 20/Q56 scheme is just as effective in identifying induced aneuploidy as is the modified scheme. There were no significant differences in the frequencies of induced gains or losses in the two schemes. These results also suggest that the 3 treatments induced chromosome gain events mainly at division I of oogenesis. Taken together, the results from this study suggest that the 20/Q56 mating scheme in D. melanogaster, which is semiselective and therefore less expensive and time-consuming to perform, is an appropriate test system to screen for chemical induced aneuploidy in germ cells of a higher organism.     

Increase with age of the frequency of spontaneous karyotype disorders in mice

328 oocytes from 874 studied were found suitable for counting chromosomes. 263 oocytes (80.18%) had a normal chromosome number, while aneuploidy was observed in 18.3% of oocytes (61). Among then 46 (14.02%) were with hypoploidy and 14 oocytes (4.26%) were hyperploid with 21 chromosomes and over. Structural chromosome aberrations were found in 24 oocytes (7.31%).     

Mechanisms of aneuploidy induction in human oogenesis and early embryogenesis

The mechanisms of aneuploidy induction in human oogenesis mainly involve nondisjunction arising during the first and second meiotic divisions. Nondisjunction equally affects both whole chromosomes and chromatids, in the latter case it is facilitated by "predivision" or precocious centromere division. Karyotyping and CGH studies show an excess of hypohaploidy, which is confirmed in studies of preimplantation embryos, providing evidence in favour of anaphase lag as a mechanism. Preferential involvement of the smaller autosomes has been clearly shown but the largest chromosomes are also abnormal in many cases. Overall, the rate of chromosomal imbalance in oocytes from women aged between 30 and 35 has been estimated at 11% from recent karyotyping data but accruing CGH results suggest that the true figure should be considerably higher. Clear evidence has been obtained in favour of germinal or gonadal mosaicism as a predisposing factor. Constitutional aneuploidy in embryos is most frequent for chromosomes 22, 16, 21 and 15; least frequently involved are chromosomes 14, X and Y, and 6. However, embryos of women under 37 are far more likely to be affected by mosaic aneuploidy, which is present in over 50% of 3-day-old embryos. There are two main types, diploid/aneuploid and chaotic mosaics. Chaotic mosaics arise independently of maternal age and may be related to centrosome anomalies and hence of male origin. Aneuploid mosaics most commonly arise by chromosome loss, followed by chromosome gain and least frequently by mitotic nondisjunction. All may be related to maternal age as well as to lack of specific gene products in the embryo. Partial aneuploidy as a result of chromosome breakage affects a minimum of 10% of embryos.     

Does human oocyte cryopreservation affect equally on embryo chromosome aneuploidy?

Oocytes cryopreservation as an important part of assisted reproductive technologies, which should ensure after warming not only intact oocyte morphological characteristics, but also their genetic apparatus stability. However, the meiotic spindle is very sensitive to the temperature fluctuations that can lead to unequal chromosome segregation during meiosis and as a consequence can cause embryo aneuploidy after oocyte fertilization. The aim of the study was to estimate the oocytes cryopreservation impact on human embryo chromosome aneuploidy. It has been shown that fertilization rate of the cryopreserved oocytes did not differ from fresh ones (83.1% vs 84% respectively). The number of blastocysts obtained from cryopreserved oocytes was less than that obtained from fresh oocytes, however, their morphological characteristics were better if compared the fresh oocytes. Our results showed different cryopreservation impact on aneuploidy rates of certain chromosomes in embryos obtained from cryopreserved oocytes. They had an increased aneuploidy of chromosome 13 and a decreased nondisjunction of chromosome 18 and sex chromosomes.     

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.     

Frequency of aneuploidy in in vitro-matured MII oocytes and corresponding first polar bodies in two dairy cattle (Bos taurus) breeds as determined by dual-color fluorescent in situ hybridization

The current study was undertaken to investigate the aneuploidy rates in in vitro-matured meiosis II (MII) oocytes and corresponding first polar bodies in two dairy cattle (Bos taurus) breeds by using dual-color fluorescent in situ hybridization (FISH). A total of 159 and 144 in vitro-matured MII oocytes of the Italian Friesian and Italian Brown breeds, respectively, were obtained according to the standard methods and analyzed by FISH using “Xcen” and “5” chromosome-specific painting probes, produced by chromosome microdissection and Degenerate Oligonucleotide Primer- Polymerase Chain Reaction (DOP-PCR). Oocytes with unreduced chromosome number were 10.1% and 16.7% in the two breeds, respectively. To avoid bias due to possible artifacts, the aneuploidy rates were determined by analyzing only oocytes with the corresponding polar bodies. In the Italian Friesian, 100 of 143 (69.9%) secondary MII oocytes showed clear MII plates with corresponding first polar bodies and were scored for aneuploidy detection; one oocyte was “nullisomic” for chromosome X (1.0%) and one “disomic” for chromosome 5 (1.0%). In the Italian Brown, 100 of 120 (83.3%) MII oocytes with corresponding first polar bodies were analyzed; one oocyte was nullisomic (1.0%) and one was disomic (1.0%), both for chromosome 5. Totally, 303 oocytes were analyzed, 40 of which showed an unreduced chromosome complement (13.2%); of 200 MII oocytes with the corresponding first polar bodies, the aneuploidy rate (nullisomy + disomy) for the two chromosomes scored was 2%. Assuming that each chromosome is equally involved in aneuploidy, it results that in cattle oocytes matured in vitro, at least 30% of the oocytes (1 × 30 haploid chromosomes) should be aneuploid. Premature separation of sister chromatids (PSSC) was also observed in 2% of the oocytes in the Italian Friesian breed involving chromosome 5 and in 1% of the Italian Brown breed involving the X chromosome. Estimation of the “baseline” level of aneuploidy in the in vitro-matured oocytes of the various domestic animal species and breeds is, to our opinion, a useful reference for improving the in vitro production of embryos as well as for monitoring future trends of the reproductive health of the species/breeds engaged in zootechnical productions, especially in relation to management errors and environmental hazards.     

Increased susceptibility to maternal aneuploidy demonstrated by comparative genomic hybridization analysis of human MII oocytes and first polar bodies

Single cell comparative genomic hybridization (CGH) was employed to extensively investigate 24 unfertilized or in vitromatured meiosis II oocytes and their corresponding first polar bodies (PBs), to determine how and whether all 23 chromosomes participate in female meiosis I errors and to accurately estimate the aneuploidy rate in the examined cells. Results were obtained for 15 oocytes and 16 PBs, representing 23 eggs (MII oocyte-PB complexes) donated from 15 patients (average age 32.2 years). Abnormalities were detected in ten eggs, giving an overall aneuploidy rate of 43.5%. In all, fourteen anomalies were scored, with the fertilized oocyte being at risk of monosomy in eight cases and at risk of trisomy in six; chromosomes of various sizes participated. CGH was able to give a comprehensive aneuploidy rate, as both absence of chromosomal material and the presence of extra copies were accurately scored. The aneuploidy mechanisms determined were: classical whole univalent non-disjunction; chromatid predivision prior to anaphase I, leading to metaphase II imbalance. There was also evidence of germinal mosaicism for a trisomic cell line. Three patients appeared to be predisposed to meiosis I errors, based on the presence of either multiple abnormalities in one or more of their examined cells, or of the same type of abnormality in all of their cells. Exclusion of these susceptible patients reduces the aneuploidy rate to 20%. Various hypotheses are put forward to explain these observations in order to stimulate research into the complex nature of female meiotic regulation.     

Nondisjunction, disturbances in spindle structure, and characteristics of chromosome alignment in maturing oocytes of mice heterozygous for Robertsonian translocations

To correlate the chromosomal constitution of meiotic cells with possible disturbances in spindle function and the etiology of nondisjunction, we examined the spindle apparatus and chromosome behavior in maturing oocytes and analyzed the chromosomal constitution of metaphase II-arrested oocytes of CD/Cremona mice, which are heterozygous for a large number of Robertsonian translocation chromosomes (18 heterobrachial metacentrics in addition to two acrocentric chromosomes 19 and two X chromosomes). Spreading of oocytes during prometaphase 1 revealed that nearly all oocytes of the heterozygotes contained one large ring multivalent, apart from the bivalents of the two acrocentric chromosomes 19 and the X chromosomes, indicating that proper pairing and crossing-over between the homologous chromosome arms of all heterobrachial chromosomes took place during prophase. A large proportion of in vitro-matured oocytes arrested in metaphase II exhibited numerical chromosome aberrations (26.5% hyperploids, 40.8% hypoploids, and 6.1% diploids). In addition, some of the oocytes with euploid chromosome numbers (26.5% of the total examined) appeared to be nullisomic for one chromosome and disomic for another chromosome, so that aneuploidy levels may even be higher than expected on the basis of chromosome counts alone. Although oocytes of the complex heterozygous mice seemed able initially to form a bipolar spindle during first prometaphase, metaphase I spindles were frequently asymmetrical. Chromosomes in the multivalent did not align properly at the equator, centromeres of neighboring chromosomes in the multivalent remained maloriented, and pronounced lagging of chromosomes was observed at telophase I in oocytes obtained from the Robertsonian translocation heterozygotes. Therefore, disturbance in spindle structure and chromosome behavior appear to correlate with the chromosomal constitution in these oocytes and, ultimately, with failures in proper chromosome separation. In particular, reorientation appears to be a rare event, and malorientation of chromosomes may remain uncorrected throughout prometaphase, as we could not find many typical metaphase I stages in heterozygotes. This, in turn, could be the basis for malsegregation at anaphase and may ultimately induce a high rate of nondisjunction and aneuploidy in the oocytes of CD/Cremona mice, leading to total sterility in heterozygous females.     

Efficient production of nuclear transferred rat embryos by modified methods of reconstruction

In this study we investigated spontaneous oocyte activation and developmental ability of rat embryos of the SD-OFA substrain. We also tried to improve the somatic cell nuclear transfer (SCNT) technique in the rat by optimizing methods for the production of reconstructed embryos. About 20% of oocytes extruded the second polar body after culture for 3 hr in vitro and 84% of oocytes were at the MII stage. MG132 blocked spontaneous activation but decreased efficiency of parthenogenetic activation. Pronuclear formation was more efficient in strontium-activated oocytes (66.1-80.9%) compared to roscovitine activation (24.1-54.5%). Survival rate after enucleation was significantly higher (89.4%) after slitting the zona pellucida and then pressing the oocyte with a holding pipette in medium without cytochalasin B (CB) compared to the conventional protocol using aspiration of the chromosomes after CB treatment (67.7%). Exposure of rat ova to UV light for 30 sec did not decrease their in vitro developmental capacity. Intracytoplasmic cumulus cell injection dramatically decreased survival rate of oocytes (42%). In contrast, 75.9% of oocytes could be successfully electrofused. Development to the 2-cell stage was reduced after SCNT (24.6% compared 94.6% in controls) and none from 244 reconstructed embryos developed in vitro beyond this stage. After overnight in vitro culture, 74.4% of the SCNT embryos survived and 56.1% formed pronuclei. The pregnancy rate of 33 recipients after the transfer of 695 of these cloned embryos was, however, very low (18.2%) and only six implantation sites could be detected (0.9%) without any live fetuses and offspring.     

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.     

Tamoxifen-induced alterations in meiotic maturation and cytogenetic abnormalities in mouse oocytes and 1-cell zygotes

Alterations in the rate of oocyte meiotic maturation (OM) and the timing of the metaphase-anaphase transition may predispose oocytes to premature centromere separation (PCS) and aneuploidy. Tamoxifen has the potential for perturbing the rate of OM since it can function as a calcium antagonist by binding to calmodulin and inhibiting the formation of a calcium-calmodulin complex which is needed for activating calmodulin-dependent cAMP phosphodiesterase and initiating OM. The objective of this study was to test the hypothesis that tamoxifen alters the rate of OM and predisposes oocytes to PCS and aneuploidy. Different does of tamoxifen were administered by oral gavage to female mice preovulation. Metaphase II oocyte and 1-cell zygote chromosomes were C-banded and cytogenetically analysed. Tamoxifen treatment resulted in a modest, but significant (p < 0.05), increase in oocytes with PCS. Similar frequencies of hyperploidy and oocytes with unpaired, single chromatids (SC) were found. Metaphase I, diploid and premature anaphase (PA) oocytes were not detected. Hyperploidy, polyploidy, PCS, PA and SC were not detected in zygotes. These data indicate that the levels of tamoxifen-induced PCS found in mouse oocytes did not predispose zygotes to aneuploidy. Tamoxifen did, however, reduce the proportion of females exhibiting oestrus.     

Synaptonemal complex analysis in spermatocytes and oocytes of tertiary trisomic Ts(512)31H mice with male sterility

Whole-mount preparations of silver-stained spermatocytes and oocytes from Ts(512)31H mice were examined in the electron microscope. The 5(12) chromosome was associated with the XY bivalent in the large majority of spermatocytes, whereas in about one-half of the oocytes, the 5(12) was associated with either unpaired chromosomes or heterochromatic parts of chromosomes or showed self-synapsis. There was a tendency for 5(12) chromosomes to be more fully heterochromatic in oocytes than in spermatocytes. A large proportion of oocytes (50%) and a much smaller proportion of spermatocytes exhibited various errors of chromosome pairing, but these proportions were only marginally greater than in control gametocytes from mice with normal karyotypes. It is concluded that the observed errors of pairing bear no simple relation to the almost complete breakdown of spermatogenesis and the marked impairment of oogenesis that occur in tertiary trisomic Ts(5(12))31H mice.     

X-chromosome heteromorphism and appearance of meiosis in the rat fetal ovary

Fetal rat oogenesis was examined attempting to test the hypothesis that two functional X chromosomes are required for the onset of meiosis. The presence of a Barr body in germ cells was considered to be evidence for one inactive X chromosome and the detection of leptotene oocytes as the criterion for the establishment of meiotic prophase. It was found that on Day 16 of gestation, 3.9% of the germ cells were leptotene oocytes, but the incidence of Barr body-positive oogonia persisted at 9.9%. On Day 17, the leptotene oocytes had increased to 26.6% and the Barr body-positive oogonia had decreased to 3.5%. It was concluded that X-chromosome reactivation, though occurring at some time during the onset of meiosis, was not the initiating event.     

Aneuploidy detection in porcine embryos using fluorescence in situ hybridization

In contrast to human embryos, there are very few studies published on the frequency of chromosomal aneuploidy in farm animals. The objectives of this study were to apply a three-color fluorescent in situ hybridization (FISH) method for evaluating aneuploidy in porcine embryos using chromosome-specific DNA probes, establish baseline frequencies of aneuploidy in embryos and compare the results with our previous findings of aneuploidy in spermatozoa and oocytes. The embryos were collected from superovulated gilts, which were slaughtered 48 h after insemination. FISH was performed using probes specific for the centromeric regions of porcine chromosomes 1, 10 and Y. Altogether 403 blastomeres from 114 porcine embryos were successfully investigated. Diploidy was observed in 101 (88.6%) embryos, triploidy in 2 (1.8%) embryos, mosaicism/mixoploidy in 9 (7.9%) embryos, and trisomy for chromosomes 1 or 10 in 2 (1.8%) embryos. No blastomere showed aneuploidy for chromosome Y. These findings correspond with the frequencies of aneuploidy we have found previously in porcine germ cells.     

Chromosome abnormalities in secondary pig oocytes matured in vitro

Abnormalities of chromosome segregation during in vitro maturation of oocytes cause failure of in vitro fertilization. Oocytes collected from pig ovaries after slaughter were matured in vitro (IVM) for 30-48 h. In total, 1144 secondary oocytes were studied cytogenetically. An unreduced (diploid) chromosome set was identified in 146 spreads (12.8 %). A higher proportion of diploidy was noticed in secondary oocytes matured for 40 h and longer (15.0 %) than in the groups matured for 30 and 36 h (9.0 %). Among 998 secondary oocytes with the reduced chromosome number, 612 could be analyzed in detail. Hypohaploidy (n=19-1) was identified in 22 cells (3.59 %) and a hyperhaploid (n=19+1) set of chromosomes was identified in 15 cells (2.45 %). The rate of aneuploidy, estimated by doubling the rate of hyperhaploidy was 4.9 %. It was also found that aneuploid spreads occurred more frequently in the group of oocytes matured for 40 h and longer. Small acrocentrics were mostly found as an extra chromosome in the hyperhaploid spreads. Our study indicates that to avoid an excess of chromosomally abnormal secondary oocytes, IVM duration of pig oocytes should not exceed 40 h.     

The chromosomal normality of in vitro-fertilized rabbit oocytes

The chromosomal normality of rabbit oocytes fertilized in vitro was examined. Ovum donors were superovulated with pregnant mare's serum gonadotropin and human chorionic gonadotropin (hCG). Follicular oocytes were collected laparoscopically 9-10 h after hCG treatment and incubated in vitro with spermatozoa capacitated in vivo. Of 267 aspirated oocytes, 191 (71.4%) were fertilized in vitro and developed to the 2- to 8-cell stage 24-48 h after insemination. In the chromosomal studies, 121 (63.4%) were examined. Of these, 94 (77.7%) had a normal diploid complement of chromosomes (2n = 44) and 6 (5%) showed aneuploidy. Of the remaining 21, 20 were triploid and one was tetraploid. The incidence of triploid oocytes after in vitro fertilization was higher than the rate in vivo (16.5% vs. 9.0%, respectively). These triploid oocytes were suspected to be the result of polyspermic fertilization in vitro. In addition, at the Metaphase II stage, 62 (89.9%) of 69 induced, preovulatory oocytes had a normal number of chromosomes.     

Similar rates of chromosomal aberrant secondary oocytes in two indigenous cattle (Bos taurus) breeds as determined by dual-color FISH

In vitro-matured metaphase II (MII) oocytes with corresponding first polar bodies (I pb) from two indigenous cattle (Bos taurus) breeds have been investigated to provide specific data upon the incidence of aneuploidy. A total of 165 and 140 in vitro-matured MII oocytes of the Podolian (PO) and Maremmana (MA) breeds, respectively, were analyzed by fluorescence in situ hybridization using Xcen and five chromosome-specific painting probes. Oocytes with unreduced chromosome number were 13.3% and 6.4% in the two breeds, respectively, averaging 10.2%. In the PO, out of 100 MII oocytes + I pb analyzed, two oocytes were nullisomic for chromosome 5 (2.0%) and one disomic for the same chromosome (1.0%). In the MA, out of 100 MII oocytes + I pb, one oocyte was found nullisomic for chromosome 5 (1.0%) and one was disomic for the X chromosome (1.0%). Out of 200 MII oocytes + I pb, the mean rate of aneuploidy (nullisomy + disomy) for the two chromosomes scored was 2.5%, of which 1.5% was due to nullisomy and 1.0% due to disomy. By averaging these data with those previously reported on dairy cattle, the overall incidence of aneuploidy in cattle, as a species, was 2.25%, of which 1.25% was due to nullisomy and 1.0% due to disomy. The results so far achieved indicate similar rates of aneuploidy among the four cattle breeds investigated. Interspecific comparison between cattle (Xcen-5 probes) and pig (Sus scrofa domestica) (1-10 probes) also reveal similar rates. Further studies are needed that use more probes to investigate the interchromosomal effect. Establishing a baseline level of aneuploidy for each species/breed could also be useful for improving the in vitro production of embryos destined to the embryo transfer industry as well as for monitoring future trends of the reproductive health of domestic animals in relation to management errors and/or environmental hazards.     

Trichlorfon predisposes to aneuploidy and interferes with spindle formation in in vitro maturing mouse oocytes

The pesticide trichlorfon (TCF) has been implicated in human trisomy 21, and in errors in chromosome segregation at male meiosis II in the mouse. We previously provided evidence that TCF interferes with spindle integrity and cell-cycle control during murine oogenesis. To assess the aneugenic activity of TCF in oogenesis, we presently analysed maturation, spindle assembly, and chromosome constitution in mouse oocytes maturing in vitro in the presence of 50 or 100 microg/ml TCF for 16 h or in pulse-chase experiments. TCF stimulated maturation to meiosis II at 50 microg/ml, but arrested meiosis in some oocytes at 100 microg/ml. TCF at 100 microg/ml was aneugenic causing non-disjunction of homologous chromosomes at meiosis I, a significant increase of the hyperploidy rate at metaphase II, and a significant rise in the numbers of oocytes that contained a 'diploid' set of metaphase II chromosomes (dyads). TCF elevated the rate of precocious chromatid segregation (predivision) at 50 and 100 microg/ml. Pulse-chase experiments with 100 microg/ml TCF present during the first 7 h or the last 9 h of maturation in vitro did not affect meiotic progression and induced intermediate levels of hyperploidy at metaphase II. Exposure to > or =50 microg/ml TCF throughout maturation in vitro induced severe spindle aberrations at metaphase II, and over one-third of the oocytes failed to align all chromosomes at the spindle equator (congression failure). These observations suggest that exposure to high concentrations of TCF induces non-disjunction at meiosis I of oogenesis, while lower doses may preferentially cause errors in chromosome segregation at meiosis II due to disturbances in spindle function, and chromosome congression as well as precocious separation of chromatids prior to anaphase II. The data support evidence from other studies that TCF has to be regarded as a germ cell aneugen.     

The CBA mouse as a model for age-related aneuploidy in man: studies of oocyte maturation,spindle formation and chromosome alignment during meiosis

To elucidate the possible mechanism of disturbances in chromosome segregation leading to the increase in aneuploidy in oocytes of aged females we examined the meiotic spindles of CBA/Ca mice. Employing immunofluorescence with an anti-tubulin antibody, and human scleroderma serum, as well as 4-6-diamidino-2-phenylindole (DAPI) staining of chromosomes the microtubular cytoskeleton could be visualized, and the behaviour of chromosomes and centromeres of oocytes spontaneously maturing in vitro could be studied. The morphology of spindles during the first meiotic division was not conspiciously different in oocytes from young and aged mice as far as the cytoskeletal elements were concerned. Neither multipolar spindles nor pronounced cytoplasmic asters appeared in oocytes of mice approaching the end of their reproductive life (9 months and older). Oocytes of aged females also did not exhibit any sign of premature separation of parental chromosomes at prophase, obvious malorientations of bivalents, or significant lagging of chromosomes during ana and telophase. Metaphase I with all bivalents aligned at the spindle equator appeared to be a relatively brief stage in oocyte development compared with pro-and prometaphase. Therefore, already slight disturbances occuring in the timing of the developmental programme which leads to a premature anaphase transition may be responsible for the high incidence of chromosomally unbalanced gametes in aged females, rather than non-separation and lagging of chromosomes during late ana-and telophase. In a second set of experiments we compared the metaphase II spindles of spontaneously ovulated oocytes obtained from animals at different ages. Previous studies have shown that spindle length and chromosome alignment may be altered in cells predisposed to aneuploidy. To distinguish between the significance of the chronological age of the female and the physiological age of the ovaries (as indicated by the total number of oocytes remaining) we examined the spindle apparatus in young (3–4 months old) and aged (9 months and older) mice as well as CBA females which had been unilaterally ovariectomized (uni-ovx) early in adult life and were approaching the end of their reproductive life at 6–7 months of age. Measurements of the pole-to-pole distance implied that spindle length may be related to maternal age. In oocytes of aged (9 month), uni-ovx (6 month) as well as 6-month-old sham-operated controls the metaphase II spindle was significantly shorter than in oocytes of young mice. By contrast, chromosome disorder and displacement was most pronounced in the aged and uni-ovx mice whilst most oocytes from young mice and moderately aged shamtreated controls exhibited a more regular alignment of chromosomes. These results, which are consistent with recent findings in CBA mice of an increased rate of aneuploidy in females approaching the end of their reproductive life, are discussed with respect to the hypothesis that the aetiology of aneuploidy rests on the critical timing of different events in oocyte development.