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.     

The response of germ cells of the mouse to the induction of non-disjunction by X-rays

The sensitivity of male and female pre-meiotic germ cells of the mouse to the induction of non-disjunction by low doses of X-rays, has been tested. No enhancement with 5 rad was observed over control of values in dictyate oocytes irradiated from young or aged females. In males, a 3-fold increase in overall chromosome abnormalities (aneuploids, polyploids and mosaics) was found following the treatment of germ cells sampled in the 7th week after irradiation (spermatogonia and early primary spermatocytes) with 100 rad. The increase in aneuploidy alone was not however significant at the 5% level of probability. Primary spermatocytes sampled in week 5 after irradiation were generally insensitive to the induction of chromosome abnormalities.     

Taxol-induced meiotic maturation delay, spindle defects, and aneuploidy in mouse oocytes and zygotes

To increase our understanding about the potential risks of chemically-induced aneuploidy, more information about the various mechanisms of aneuploidy induction is needed, particularly in germ cells. Most chemicals that induce aneuploidy inhibit microtubule polymerization. However, taxol alters microtubule dynamics by enhancing polymerization and stabilizing the polymer fraction. We tested the hypothesis that taxol induces meiotic delay, spindle defects, and aneuploidy in mouse oocytes and zygotes. Super-ovulated ICR mice received 0 (control), 2.5, 5.0, and 7.5 mg/kg taxol intraperitoneally immediately after HCG. Females were paired (1:1) with males for 17 h after taxol treatment. Mated females were given colchicine 25 h after taxol and their one-cell zygotes were collected 16 h later. Ovulated oocytes from non-mated females were collected 17 h after taxol. Chromosomes were C-banded for cytogenetic analyses. Oocytes were also collected from another group of similarly treated females for in situ chromatin and microtubule analyses. Taxol significantly (p<0.01) enhanced the proportion of oocytes exhibiting parthenogenetic activation, chromosomes displaced from the meiotic spindle, and sister-chromatid separation. Moreover, 7.5 mg/kg taxol significantly (p<0.01) increased the proportions of metaphase I and diploid oocytes and polyploid zygotes. A significant (p<0.01) dose response for taxol-induced hyperploidy in oocytes and zygotes was found. These results support the hypothesis that taxol-induced meiotic delay and spindle defects contribute to aneuploid mouse oocytes and zygotes.     

Effect of aging on superovulation efficiency, aneuploidy rates, and sister chromatid cohesion in mice aged up to 15 months

Human eggs are highly aneuploid, with female age being the only known risk factor. Here this aging phenomenon was further studied in Swiss CD1 mice aged between 1 and 15 mo. The mean number of eggs ± SEM recovered from mice following superovulation peaked at 22.5 ± 3.8 eggs/oviduct in 3-mo-old females, decreasing markedly between 6 and 9 mo old, and was only 2.1 ± 0.2 eggs/oviduct by 15 mo. Measurement of aneuploidy in these eggs revealed a low rate, ～3-4%, in mice aged 1 and 3 mo, rising to 12.5% by 9 mo old and to 37.5% at 12 mo. Fifteen-month-old mice had the highest rate of aneuploidy, peaking at 60%. The in situ chromosome counting technique used here allowed us to measure with accuracy the distance between the kinetochores in the sister chromatids of the eggs analyzed for aneuploidy. We observed that this distance increased in eggs from older females, from 0.38 ± 0.01 μm at 1 mo old to 0.82 ± 0.03 μm by 15 mo. Furthermore, in 3- to 12-mo-old females, aneuploid eggs had significantly larger interkinetochore distances than euploid eggs from the same age, and measurements were similar to eggs from the oldest mice. However, the association between aneuploidy and interkinetochore distance was not observed at the oldest, 15-mo age, despite such measurements being maximal. We conclude that in aging CD1 mice, a reduction in the ovulated egg number precedes a rise in aneuploidy and, furthermore, except at very advanced ages, increased interkinetochore distance is associated with aneuploidy.     

Effect of a single severe stress on behaviour of males and females in the mouse inbred strains CBA/Lac and C57BL/6J

The effect of single severe stress in the form of forced swimming on the behavior of males and females in the mouse inbred strains CBA/Lac and C57BL/6J were examined in the open field test. Measurements were carried out within two hours after the stress exposure (Trial 1) and repeated 2 hours thereafter (Trial 2). Intact males and females of the both mouse strains which tested in the open field twice too were used as control. An increased latency was found until first escape from the center both in males and females of the CBA/Lac strain within two hours after the end of forced swimming. This parameter was still high in females in the Trial2. Four out of seven behavior parameters were changed in females of the C57BL/6J strain two hours after the stress exposure, but their behavior was similar to control in the Trial 2. The males of the C57BL/6J strain demonstrated the least changed behavior in the open field test after the stress exposure with the exception of increased number of grooming in the Trial 1. Further on, a detailed analysis of repeated testing in the open field within intact and stressed mice of both strains was performed. This comparison allowed revealing hereditary and gender peculiarities in the mouse behavior after single severe stress exposure. The results are discussed in respect to the possible genetically inherent increased traitanxiety in females of C57BL/6J strain and the state of anxiety in females of CBA/Lac strain.     

Two new X-autosome Robertsonian translocations in the mouse. I. Meiotic chromosome segregation in male hemizygotes and female heterozygotes.

Two new X-autosome Robertsonian (Rb) translocations, Rb(X.9)6H and Rb(X.12)7H, were found during the course of breeding the Rb(X.2)2Ad rearrangement at Harwell. The influence of these new Rbs on meiotic chromosome segregation was investigated in hemizygous males and heterozygous females and compared to that of Rb(X.2)2Ad. Screening of metaphase II spermatocytes gave incidences of sex chromosome aneuploidy of 9.2% in Rb(X.2)6H/Y and 9.6% in Rb(X.9)2Ad/Y males; no metaphase II cells were present in the testes of the Rb(X.12)7H/Y males examined and no males with this karyotype have so far proved fertile. In breeding tests, 5% of the progeny of Rb(X.2)2Ad/Y males were sex chromosome aneuploids compared to 10% of the Rb(X.9)6H/Y offspring. The difference was not significant, however. Cytogenetic analyses of metaphase II stage oocytes showed elevated rates of hyperhaploidy (n + 1) in Rb heterozygous females over chromosomally normal mice: 4.2% for Rb(X.2)2Ad/+; 2.1% for Rb(X.9)6H/+; 2.2% for Rb(X.12)7H/+ and 1.1% for normal females. There was, however, no statistically significant difference in the rates of hyperhaploidy between the three different Rb types, nor overall between Rb/+ and normal females. Karyotypic analyses of liveborn offspring of Rb heterozygous females revealed low incidences of X0 animals but no other type of sex chromosome aneuploidy. Intercrosses of heterozygous females and hemizygous males yielded 5.5% aneuploidy for Rb(X.2)2Ad and 5.4% for Rb(X.9)6H. In heterozygous females, there was evidence from the metaphase II and breeding test data for all three rearrangements, of preferential segregation of the Rb metacentric to the polar body resulting in a deficiency of cells and progeny carrying a translocation chromosome.     

Sex-dependent frequency and type of autosomal univalency at the first meiotic metaphase in mouse germ cells

Univalents at the first meiotic metaphase in mouse spermatocytes occur mainly in the XY pair, making it difficult to compare the amounts of univalency in males and females. In this study, the amounts of autosomal univalency in male and female meiosis were compared using the model strain CBA-T6, in which univalency of the small marker autosome pair T6 has been shown to occur very frequently in spermatocytes. Mice from inbred CBA and DBA strains were also analysed. The total frequencies of univalency (sex chromosomes plus autosomes) in metaphase I spermatocytes were 45.6% in CBA, 36.9% in CBA-T6, and 37.3% in DBA males. The aneuploidy in metaphase II spermatocytes ranged from 1.4 to 3% in these strains, which was in agreement with previous findings that most primary spermatocytes with abnormal chromosome configurations are arrested in their development before metaphase II. In the CBA-T6 strain, autosomal univalency at metaphase I mostly involved chromosome pair T6; however, its frequency differed significantly between the sexes, amounting to 18.9% in spermatocytes and 4.3% in oocytes. In the CBA strain, autosomal univalents at metaphase I were seen in 7.7% of the spermatocytes and 1.4% of the oocytes and, in DBA mice, in 4.9% of the spermatocytes and 3.8% of the oocytes. However, in DBA oocytes, when univalency occurred it usually concerned a greater number of bivalents in one cell (range: 2-19 disjoined bivalents), a phenomenon very rare in males of this strain. This study shows that univalent formation differs between the male and female types of meiosis.     

Genetic variation in C57BL/6 ES cell lines and genetic instability in the Bruce4 C57BL/6 ES cell line

Genetically modified mouse strains derived from embryonic stem (ES) cells are powerful tools for gene function analysis. ES cells from the C57BL/6 mouse strain are not widely used to generate mouse models despite the advantage of a defined genetic background. We assessed genetic variation in six such ES cell lines with 275 SSLP markers. Compared to C57BL/6, Bruce4 differed at 34 SSLP markers and had significant heterozygosity on three chromosomes. BL/6#3 and Dale1 ES cell lines differed at only 3 SSLP makers. The C2 and WB6d ES cell lines differed at 6 SSLP markers. It is important to compare the efficiency of producing mouse models with available C57BL/6 ES cells relative to standard 129 mouse strain ES cells. We assessed genetic stability (the tendency of cells to become aneuploid) in 110 gene-targeted ES cell clones from the most widely used C57BL/6 ES cell line, Bruce4, and 710 targeted 129 ES cell clones. Bruce4 clones were more likely to be aneuploid and unsuitable for ES cell-mouse chimera production. Despite their tendency to aneuploidy and consequent inefficiency, use of Bruce4 ES cells can be valuable for models requiring behavioral studies and other mouse models that benefit from a defined C57BL/6 background. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Aneuploidy in Drosophila, II. Further validation of the FIX and ZESTE genetic test systems employing female Drosophila melanogaster

Two sensitive genetic systems for the detection of germline aneuploidy employing Drosophila melanogaster females were described in the first paper of this series (Zimmering et al., submitted to Mutation Research). Designated FIX and ZESTE, these systems permit the rapid and efficient detection of exceptional offspring derived from aneuploid female germ cells. The current report presents test results from a survey of 8 additional chemicals that have been analyzed in both systems. The tested chemicals include: acetonitrile, cadmium chloride, carbendazim, dimethylsulfoxide (DMSO), methylmercury(II) chloride, methoxyethyl acetate, propionitrile and water. Excluding the negative control, water, only the fungicide carbendazim failed to induce aneuploidy in either test system. Of the remaining 6 chemicals one, methylmercury(II) chloride, was positive in the FIX system but not in ZESTE, while MEA was positive in ZESTE and borderline in FIX. The results provide little evidence of germ-cell stage specificity of response to the tested chemicals. Comparison of the induced rates of aneuploidy i indicates that these can exhibit departures from simple additivity to the spontaneous rates: induced rates in the ZESTE system are generally higher and more variable than those from FIX. Possible reasons for the difference in responsiveness between FIX and ZESTE flies are discussed as is the question of the classification of those chemicals which induce chromosome loss events but not chromosome gains.     

Embryonic development and incidence of aneuploidy in two rabbit strains of different fecundity

The correlation between strain fecundity and (i) development and (ii) rate of aneuploidy was studied in rabbit preimplantation embryos obtained from 2 strains of different fecundity. Embryos were investigated at Days 3-6 (preimplantation development) or Days 2, 4 and 6 post coitum (aneuploidy). Embryonic size and cell proliferation varied on the days of investigation, but with no consistent tendency in favour of one strain. The incidence of aneuploidy did not differ significantly between embryos from the 2 strains (P greater than 0.05). The multifactorially determined criterion of prolificacy was not selectively correlated with overall differences in embryonic preimplantation growth and rate of aneuploidy.     

Nocodazole sensitivity, age-related aneuploidy, and alterations in the cell cycle during maturation of mouse oocytes

To detect age-related alterations in the formation and function of the spindle apparatus, we examined in vitro maturing oocytes obtained from young (2-4 mo) and aged (greater than 9 mo) diestrous CBA/Ca mice. Observation of cells processed for antitubulin immunofluorescence revealed that oocytes from aged females progress faster through first maturation division than those from young animals. They are also more prone to nondisjunction, as shown by a significantly higher level of aneuploidy in C-banded cells arrested at metaphase II. The ability of oocytes to recover from treatment with a microtubule inhibitor, nocodazole, and the effect of the drug on spindle integrity and chromosome segregation were also studied. In both age groups, treatment of metaphase I oocytes with 10 microM nocodazole caused rapid and complete microtubule depolymerization and chromosome scattering. Upon recovery, oocytes from both age groups were able to reestablish a spindle apparatus, proceed through anaphase, and extrude a first polar body. However, nocodazole treatment led to a dramatic increase of aneuploidy. Unexpectedly, the relative rise in hyperploids was greater in oocytes from young mice than in those from aged mice, so that the absolute percentage of hyperploid metaphase II cells was similar in both age groups after drug treatment. Concomitantly, nocodazole exposure abolished or, at least, diminished intrinsic differences in the cell cycle and anaphase trigger present in the controls (e.g., the earlier onset of chromosome separation in oocytes from aged females). It shortened the period available for spindle formation before chromosome segregation in all oocytes. Therefore, our study implies that temporal differences in the progression of oocytes through maturation, in particular, the shortening of the time available for alignment of bivalents before chromosome separation occurs in oocytes of old females, are mainly responsible for age-related rises in aneuploidy. There is no indication that (1) the spindle apparatus of oocytes from aged mammals is more labile or susceptible to disturbances than the spindle apparatus of oocytes from young individuals or that (2) an increase in the number of univalents makes oocytes from aged mammals particularly prone to nondisjunction.     

Analysis of aneuploidy rate in antral and ovulated mouse oocytes during female aging

Two forms of oocytes termed SN (surrounded nucleolus) and NSN (nonsurrounded nucleolus) differing for the spatial distribution of nuclear and nucleolar-associated chromatin have been described within the antral compartment of the ovary of a number of mammals. The biological significance of these two kind of oocytes is as yet not completely clear. In previous studies we have shown that prior to ovulation, mouse SN oocytes isolated from the antral compartment, matured and fertilized in vitro have a far better meiotic and developmental competence than NSN oocytes. Immediately after ovulation SN and NSN oocytes remaining in the antral compartment do not develop beyond the 2-cell stage. To further examine the correlation between chromatin distribution and meiotic competence of mouse antral oocytes, in the present study we have analyzed chromosome segregation at the first meiotic division in antral (SN and NSN) and in ovulated oocytes. SN and NSN oocytes were isolated before (48 h post PMSG injection) or after (15 h post–hCG injection) ovulation from ovaries of females of increasing age, they were cultured in vitro to metaphase II, and their aneuploidy rate was examined. Comparison of data obtained before and after ovulation highlights two main points: 1. Following ovulation a statistically significant increase of aneuploidy is observed in antral oocytes in most age groups and it is attributable to SN oocytes. 2. The aneuploidy rate of ovulated oocytes does not increase during female aging. We have found a correlation between chromatin distribution, hormonal status, and the incidence of aneuploidy during the oocyte first meiotic division. Mol. Reprod. Dev. 50 :305–312, 1998. © 1998 Wiley-Liss, Inc.     

Control of meiosis by somatic cells in mice: inheritance of the meiosis I error 'diploidy' and nonexpression in sensitive NMRI/Han oocytes ovulated from chimeras

NMRI mouse and Djungarian hamster females ovulate diploid and/or hyperploid oocytes with increased frequencies after gonadotrophin stimulation, suggesting that somatic cells are involved in the failures of endocrine control resulting in aneuploidy. To study the inheritance of gonadotrophin-induced aneuploidy as well as the fate of sensitive oocytes in a resistant somatic environment and vice versa, we analysed the frequency of diploid oocytes in NMRI/Han, C57BL/6J and their F1 hybrids (C57BL/6J X NMRI/Han), (NMRI/Han X C57BL/6J) as well as in NMRI/Han in equilibrium C57BL/6J chimeric females after gonadotrophin injections. Ovulated oocytes were analysed in all females for the appearance of diploidy, characterized as premature arrest of development at metaphase I. Our data suggest that the trait of induced diploidy is genetically determined and can be transmitted either maternally or paternally. A maternal effect modulated the expression of that trait. Several mechanisms acting on the feed-back control ovary-hypothalamus/pituitary, within the ovary or even within a chimeric follicle, may be responsible that 'sensitive' oocytes ovulated from chimeras are all normal haploid. These data suggest that not only oocyte maturation but also chromosome disjunction during meiosis I is controlled by somatic cells.     

Aneuploidy in male germ cells induced by alcohol ingestion

The effects of ethyl alcohol ingestion, at levels comparable to human drinking, have been evaluated by cytogenetic analysis of mouse spermatogonia and primary spermatocytes. The alcohol was administered intragastrically at different doses and for various periods of time. An increase in mitotic and first division meiotic aneuploidy was observed in direct proportion to the alcohol dose. Approximately a three-fold increase in mitotic and meiotic aneuploidy over control levels was observed in animals exposed to a daily 3 gm/kg bw dose for 6 to 28 days. The level of aneuploidy was found to return to control levels by 14 days after cessation of alcohol ingestion. Analysis of the kinetics of decrease in aneuploidy following cessation of alcohol suggests that the primal event resulting in altered chromosome numbers occurs at a stage of the cell cycle other than anaphase and therefore is not due to nondisjunction nor to anaphase lagging.     

Role of the genetic-physiological interrelationships of mother-progeny in establishing the viability and fertility of mammals. II. The weight of the embryos of BALB, CBA and DBA strain mice developing from transplanted blastocysts

The weights of embryos and their placentas obtained by blastocysts of CBA/LacY and DBA/2 mouse strains transfer in recipient females of BALB/c strain and also by blastocysts of BALB/c mouse strain transfer in recipient females of CBA/LacY strain were analysed on the 16th gestation day. It is shown that placental weights are determined by genotypic peculiarities of embryos, although comparative placental weights of different strain embryos could also depend on females fertility. The weight of allogenic embryos (developed from transferred blastocysts) significantly excessed the weight of singenic embryos, and most significant differences were noted in females with small number of embryos. The possibilities to solve some applied and fundamental problems of mammalian reproduction given by the method of interstrain and interbreed early stage embryos transfer are discussed.     

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.     

Trichlorfon effects on mouse oocytes following in vivo exposure

Trichlorfon (TCF) is a widely used pesticide, which according to some epidemiological and experimental data, is suspected of being aneugenic in human and mouse cells. In particular, in vitro studies in mouse oocytes showed the induction of aneuploidy and polyploidy at the first meiotic division and of severe morphological alterations of the second meiotic spindle. We have tested the hypothesis that an acute treatment of mice with TCF might similarly affect chromosome segregation in maturing oocytes. Superovulated MF-1 mice were intraperitoneally injected with 400mg/kg TCF or orally administered with 600mg/kg TCF either at the time of or 4h after human chorionic gonadotrophin (HCG) injection. Oocytes were harvested 17h after HCG and metaphase II chromosomes were cytogenetically analyzed. No significant increase of aneuploid or polyploid cells was detected at any treatment condition. A significant (p<0.001) decrease of metaphases showing premature chromatid separation or premature anaphase II in all TCF-treated groups with respect to controls suggested that TCF treatment may have delayed the first meiotic division. To evaluate possible effects of the pesticide upon the second meiotic division, a group of females orally treated with 600mg/kg TCF at resumption of meiosis was mated with untreated males and zygotes were collected for cytogenetic analysis. No evidence of aneuploidy induction was obtained, but the frequency of polyploid zygotes was increased fivefold over the control level (p<0.01). Such polyploid embryos might have arisen from fertilization of oocytes that were either meiotically delayed and still in metaphase I at fertilization or progressed through anaphase II without cytokinesis. These findings show that in vivo studies on aneuploidy induction in oocytes may yield results different from those obtained by in vitro experiments and that both kinds of data may be necessary for risk assessment of environmentally relevant exposures.     

Effect of maternal parity on aneuploidy in early mouse embryos.

An attempt to evaluate the incidence of chromosomal aneuploidy in mouse blastocysts recovered from females of various ages and parity levels revealed an insignificant regression coefficient for aneuploidy on age of the female and its square, and an insignificant correlation coefficient for aneuploidy with the number of previous offspring born to the dam. However, significant regression coefficients were obtained for aneuploidy on parity of the dam and its square. These results indicate that not only does aneuploidy increase with parity level, but the rate of increase accelerates as parity level increases. Possible explanations for the increase in aneuploidy and its detrimental effect on reproductive efficiency were discussed.     

Hyperthermia-induced embryonic aneuploidy in mice

Chromosomal aneuploidy, as an effect of elevated ambient temperature during early gestation, was studied in 650 eight-day embryos from 60 females of two different mouse stocks. Exposure of treatment females to 34 C and 50% relative humidity during days 5 and 6 of gestation caused an increase in embryo chromosomal hyperploidy. No embryo sex interaction with the percent hyperploidy was found.