Investigation of effects of pregnant mare serum gonadotropin (PMSG) on the chromosomal complement of CD-1 mouse embryos

Analysis of bovine blastocysts indicates ovarian stimulation does not induce chromosome errors, nor discordance between inner-cell mass and trophectoderm lineages

Contemporary systems for oocyte retrieval and culture of both cattle and human embryos are suboptimal with respect to pregnancy outcomes following transfer. In humans, chromosome abnormalities are the leading cause of early pregnancy loss in assisted reproduction. Consequently, pre-implantation genetic testing for aneuploidy (PGT-A) is widespread and there is considerable interest in its application to identify suitable cattle IVP embryos for transfer. Here we report on the nature and extent of chromosomal abnormalities following transvaginal follicular aspiration (OPU) and IVP in cattle. Nine sexually mature Holstein heifers underwent nine sequential cycles of OPU-IVP (six non-stimulated and three stimulated cycles), generating 459 blastocysts from 783 oocytes. We adopted a SNP-array approach normally employed in genomic evaluations but reanalysed (Turner et al., 2019; Theriogenology125: 249) to detect levels of meiotic aneuploidy. Specifically, we asked whether ovarian stimulation increased the level of aneuploidy in either trophectoderm (TE) or inner-cell mass (ICM) lineages of blastocysts generated from OPU-IVP cycles. The proportion of Day 8 blastocysts of inseminated was greater (P < 0.001) for stimulated than non-stimulated cycles (0.712 ± 0.0288 vs. 0.466 ± 0.0360), but the overall proportion aneuploidy was similar for both groups (0.241 ± 0.0231). Most abnormalities consisted of meiotic trisomies. Twenty in vivo derived blastocysts recovered from the same donors were all euploid, thus indicating that 24 h of maturation is primarily responsible for aneuploidy induction. Chromosomal errors in OPU-IVP blastocysts decreased (P < 0.001) proportionately as stage/grade improved (from 0.373 for expanded Grade 2 to 0.128 for hatching Grade 1 blastocysts). Importantly, there was a high degree of concordance in the incidence of aneuploidy between TE and ICM lineages. Proportionately, 0.94 were "perfectly concordant" (i.e. identical result in both); 0.01 were imperfectly concordant (differing abnormalities detected); 0.05 were discordant; of which 0.03 detected a potentially lethal TE abnormality (false positives), leaving only 0.02 false negatives. These data support the use of TE biopsies for PGT-A in embryos undergoing genomic evaluation in cattle breeding. Finally, we report chromosome-specific errors and a high degree of variability in the incidence of aneuploidy between donors, suggesting a genetic contribution that merits further investigation.

High Dosages of Equine Chorionic Gonadotropin Exert Adverse Effects on the Developmental Competence of IVF-Derived Mouse Embryos and Cause Oxidative Stress-Induced Aneuploidy

Gonadotropins play vital roles in the regulation of female reproductive ability and fertility. Our study aimed to determine the effects of superovulation induced by increasing doses of equine chorionic gonadotropin [eCG; also referred to as pregnant mare serum gonadotropin (PMSG)] on the developmental competence of mouse embryos and on aneuploidy formation during in vitro fertilization (IVF). eCG dose-dependently enhanced the oocyte yield from each mouse. Administration of 15 IU eCG significantly reduced the fertilization rate and the formation of four-cell embryos and blastocysts and increased the risk of chromosome aneuploidy. The IVF-derived blastocysts in the 15 IU eCG treatment group had the fewest total cells, inner cell mass (ICM) cells and trophectoderm (TE) cells. Moreover, more blastocysts and fewer apoptotic cells were observed in the 0, 5, and 10 IU eCG treatment groups than in the 15 IU eCG treatment group. We also investigated reactive oxygen species (ROS) levels and variations in several variables: mitochondrial membrane potential (MMP); active mitochondria; mitochondrial superoxide production; adenosine triphosphate (ATP) content; spindle structures; chromosome karyotypes; microfilament distribution; and the expression of Aurora B [an important component of the chromosomal passenger complex (CPC)], the spindle assembly checkpoint (SAC) protein mitotic arrest deficient 2 like 1 (MAD2L1), and the DNA damage response (DDR) protein γH2AX. Injection of 15 IU eCG increased ROS levels, rapidly reduced MMP, increased active mitochondria numbers and mitochondrial superoxide production, reduced ATP content, increased abnormal spindle formation rates, and induced abnormalities in chromosome number and microfilament distribution, suggesting that a high dose of eCG might alter developmental competence and exert negative effects on IVF-obtained mouse embryos. Additionally, the appearance of γH2AX and the significantly increased expression of Aurora B and MAD2L1 suggested that administration of relatively high doses of eCG caused Aurora B-mediated SAC activation triggered by ROS-induced DNA damage in early mouse IVF-derived embryos for self-correction of aneuploidy formation. These findings improve our understanding of the application of gonadotropins and provide a theoretical basis for gonadotropin treatment.

A rapid and efficient method for the collection of highly developmental murine immature oocytes using cilostazol, a phosphodiesterase 3A inhibitor

AIMS

The present study aims to define maturation, yield, health, and ease of collection of murine immature oocytes recovered using the conventional method or from mice treated with cilostazol.

MAIN METHODS

The conventional method included the superovulation of mice and the recovery of germinal vesicle (GV) or metaphase (MI) oocytes from preovulatory follicles. The cilostazol method included the oral treatment of superovulated mice with 7.5 mg cilostazol once or twice to result in the ovulation of MI or GV oocytes, respectively.

KEY FINDINGS

The cilostazol method resulted in >95% of GV or MI oocytes with a diameter range of 60-90 μm or 50.1-70 μm in comparison to <60.0% of GV or MI oocytes resulting from the conventional method, respectively (P < 0.0001). The cilostazol method resulted in GV oocytes having higher levels of co-occurrence of peripheral cortical granules (CG) and chromatin configuration of surrounded nucleolus and MI oocytes having higher levels of co-occurrence of normally organized spindles/chromosomes and peripheral CG with free domains than did the conventional method (P < 0.001). The cilostazol method was more time and labor efficient and resulted in higher oocyte yields of normal morphology than did the conventional method (P < 0.01).

SIGNIFICANCE

The presented method provides not only oocytes with uniform size and synchronized developmental maturation but also a technique of oocyte collection that is efficient and resourceful. It is possible that not all immature oocytes resulting from the conventional method are from preovulatory follicles nor have been developed adequately and consequently ovulated as opposed to the presented method.

Expression and Contribution of NLRP3 Inflammasome During the Follicular Development Induced by PMSG

Follicular development and following ovulation induced by luteinizing hormone (LH) surge are critical for ovarian functions, but the molecular mechanism regulating ovarian ovulation attracts more attention and remains mainly unknown. Recent researches on the nucleotide leukin rich polypeptide 3 (NLRP3) inflammasome shred light on it. Given pregnant mare serum gonadotropin (PMSG) can not only trigger the follicular development, but also induce the following ovulation, the present study therefore examined that expression and localization of NLRP3 inflammasome through immunohistochemistry and Western blotting during the follicular development induced by PMSG. The results showed expressions of NLRP3 and the adaptor protein apoptosis-associated speck-like protein (ASC) significantly increased in the outside of intrafollicular fluid, further analysis found that caspase-1 was activated and IL-1β production was also upregulated after 52 h-treatment of PMSG. Furthermore, a significant increase of ovulation-related genes, hypoxia inducible factor (HIF)-1α and endothelin (ET)-1, was found after 52 h-treatment of PMSG. To our knowledge, it is the first time to clearly indicated the activation of NLRP3 inflammasome may contribute to the ovulation of PMSG-treated ovaries, which will help to further clarify the ovulatory mechanism in mammals.

Differential impacts of gonadotrophins, IVF and embryo culture on mouse blastocyst development

RESEARCH QUESTION

Conception via assisted reproductive technology (ART) increases the risk of type 2 diabetes and cardiovascular disease in adulthood. Underlying differences between ART-conceived and in-vivo-conceived embryos that contribute to this increased risk are, however, not known.

DESIGN

This study examined the developmental characteristics of mouse blastocysts derived from ART- compared with in-vivo-conceived embryos. To determine the effect of ovarian stimulation versus IVF versus in-vitro embryo culture on phenotype, six distinct groups of blastocysts were generated. Female mice were naturally cycling or treated with high or mild doses of gonadotrophin, followed by natural mating or IVF under clinical conditions. Embryo morphokinetics were assessed by continuous time-lapse monitoring. Cell lineage allocation to the inner cell mass (Oct4+) or trophectoderm (Cdx2+) was determined by immunohistochemistry, and mitochondrial DNA (mtDNA) copy number was measured by quantitative PCR.

RESULTS

Ovarian stimulation increased embryo number but reduced the percentage of blastocysts. Morphokinetic analysis showed that gonadotrophin treatment led to advanced development (P < 0.05) due to earlier post-pronuclear breakdown. The blastocyst rate was reduced in IVF embryos compared with those fertilized in vivo before culture (P < 0.001). Morphokinetics showed that embryo development was slower in all the IVF groups (P < <0.05), due to a delay from the 3-cell stage. A reduced total and trophectoderm cell number was observed in all groups of cultured blastocysts compared with naturally conceived blastocysts (P < 0.01). Gonadotrophin treatment did not affect the blastocyst mtDNA copy number; however, IVF embryos exhibited reduced mtDNA copy number compared with naturally conceived embryos.

CONCLUSION

Ovarian stimulation, IVF and in-vitro culture differentially impair blastocyst developmental kinetics, differentiation and mtDNA copy number.

Effects of ovarian hyperstimulation on mitochondria in oocytes and early embryos

A mouse model was used to compare the number and function of mitochondria in oocytes and embryos obtained by superovulation and in a natural cycle (control group). The superovulation group had a higher number of total oocytes, MII oocytes, embryos with two pronuclei, 2-cell embryos and blastocysts than the control group (P<0.05 for all). The superovulation group had high proportion of MII oocytes with low number of mitochondrial (mt) DNA copies. The average number of mtDNA copies, ATP level and mitochondrial membrane potential ( Collapse

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Improvement in in vitro fertilization outcome following in vivo synchronization of oocyte maturation in mice

Synchronization of oocyte maturation in vitro has been shown to produce higher in vitro fertilization (IVF) rates than those observed in oocytes matured in vitro without synchronization. However, the increased IVF rates never exceeded those observed in oocytes matured in vivo without synchronization. This study was therefore designed to define the effect of in vivo synchronization of oocyte maturation on IVF rates. Mice were superovulated and orally treated with 7.5 mg cilostazol (CLZ), a phosphodiesterase 3A (PDE3A) inhibitor, to induce ovulation of immature oocytes at different stages depending on frequency and time of administration of CLZ. Mice treated with CLZ ovulated germinal vesicle (GV) or metaphase I (MI) oocytes that underwent maturation in vitro or in vivo (i.e. in the oviduct) followed by IVF. Superovulated control mice ovulated mature oocytes that underwent IVF directly upon collection. Ovulated MI oocytes matured in vitro or in vivo had similar maturation rates but significantly higher IVF rates, 2-4 cell embryos, than those observed in control oocytes. Ovulated GV oocytes matured in vitro showed similar maturation rates but significantly higher IVF rates than those observed in control oocytes. However, ovulated GV oocytes matured in vivo had significantly lower IVF rates than those noted in control oocytes. It is concluded that CLZ is able to synchronize oocyte maturation and improve IVF rates in superovulated mice. CLZ may be capable of showing similar effects in humans, especially since temporal arrest of human oocyte maturation with other PDE3A inhibitors in vitro was found to improve oocyte competence level. The capability of a clinically approved PDE3A inhibitor to improve oocyte fertilization rates in mice at doses extrapolated from human therapeutic doses suggests the potential scenario of the inclusion of CLZ in superovulation programs. This may improve IVF outcomes in infertile patients.

Increased incidence of mosaicism detected by FISH in murine blastocyst cultured in vitro

The majority of in-vitro-derived human preimplantation embryos are chromosomally abnormal but whether the same pattern exists in vivo is unknown. This would be impossible to demonstrate in humans. Hence we chose murine embryos to study this difference owing to their ease of manipulation and compared the incidence of mosaicism between in-vivo- and in-vitro-cultured embryos. Two groups of embryos were analysed. Group A (in vitro) were obtained 48h following superovulation and cultured in vitro until the blastocyst stage. Fluorescent in-situ hybridization (FISH) was performed at different stages that included the cleavage, morula and blastocyst stage. Group B (in vivo) were obtained on day 2 or day 5 and FISH was performed immediately without culture. There was an increase in chromosomal mosaicism seen from the cleavage stage up to the blastocyst stage in the in-vitro culture group. Overall chromosomal abnormality from day 3 to day 5 was found to be 30% (28/94) in group A. The incidence of chromosomal abnormalities in blastocysts from group B was significantly lower than group A blastocysts (8% (3/40) and 31% (20/64) respectively; P<0.05). These data show that in-vitro cultured embryos had a significantly higher incidence of mosaicisim in comparison with the in-vivo group. Cultured human embryos show high levels of chromosomal abnormalities but whether this is a pattern seen in all embryos or is the result of culture is unknown. To study this pattern we used mouse embryos and carried out chromosome analysis by fluorescent in-situ hybridization. We compared embryos that were cultured (in vitro) with those that were not (in vivo, i.e. grown exclusively in the mouse). We found that cultured embryos showed significantly higher chromosomal abnormalities as compared with in vivo embryos. This suggests that certain culture conditions are responsible for the high level of chromosomal abnormalities seen in these embryos, which should be investigated further.

Repeated ovarian stimulations induce oxidative damage and mitochondrial DNA mutations in mouse ovaries

Superovulation by injection of exogenous gonadotropin is the elementary method to produce in vivo-derived embryos for embryo transfer in women. Increased oocyte aneuploidy, embryo mortality, fetal growth retardation, and congenital abnormalities have been studied at higher-dose stimulations. Ovarian and oocyte biological aging possibly may have adverse implications for human oocyte competence with repeated hyperstimulation. In this study, we found that reduced competence for the human oocyte has been associated with degenerative embryo upsurge during embryo culture and failure to develop into the blastocyst stage in the three, four, five, and six stimulation cycles. On the other hand, the numbers of ovulated oocytes were decreased in the groups with more ovarian stimulation. More aggregated mitochondria were found in the cytoplasm of the repetitively stimulated embryos. Higher amounts of oxidative damage including 8-OH-dG, lipoperoxides, and carbonyl proteins were also revealed in the ovaries with more cycle numbers of ovarian stimulation. Higher proportions of mtDNA mutations were also found. The detected molecular size of the mutated band was approximately 675 bp. Increased amounts of carbonyl proteins were also revealed after repeated stimulation. An understanding of the relationship between oocyte competence and ovarian responses to stimulation in the mouse may provide insights into the origin of oocyte defects and the biology of ooplasmic aging that could be of clinical relevance in the diagnosis and treatment of human infertility.

Adenosine triphosphate synthesis, mitochondrial number and activity, and pyruvate uptake in oocytes after gonadotropin injections

OBJECTIVE

To determine the effects of gonadotropin injection on the energy generation of mature oocytes.

DESIGN

Randomized prospective study.

SETTING

Gamete and stem cell biotechnology laboratory at Seoul National University in Korea.

ANIMAL(S)

Twelve- to 15-week-old golden hamsters (Mesocricetus auratus).

INTERVENTION(S)

Injections of pregnant mare serum gonadotropin (PMSG; 5 or 15 IU), of hCG (5 or 15 IU), or of PMSG and hCG (15 IU of each; PMSG + hCG group) were administered to female hamsters.

MAIN OUTCOME MEASURE(S)

Adenosine triphosphate (ATP) synthesis, mitochondrial population number and activity, and pyruvate uptake were measured.

RESULT(S)

Significant (P<.05) differences were found in the ATP levels; compared with the control (no injection), a dramatic increase was detected after injections of 15 IU of hCG or of 15 IU of PMSG and 15 IU of hCG. In the same treatments, the mitochondrial population (mitochondrial DNA copy number) significantly increased, whereas mitochondrial activity measured by the ratio of activated to less-activated mitochondria did not change. A significant increase in pyruvate uptake was detected after the injections of 15 IU of PMSG and 15 IU of hCG.

CONCLUSION(S)

The change in ATP synthesis activity was a major cause for the adverse effect of gonadotropins on oocyte development in the hamster. The injections of 15 IU of hCG, or of 15 IU of PMSG and 15 IU of hCG, dramatically increased the ATP level, the mitochondrial population number, and pyruvate uptake.

Influence of ovarian hyperstimulation and ovulation induction on the cytoskeletal dynamics and developmental competence of oocytes

This study was undertaken to determine the effects of gonadotrophin on cytoskeletal dynamics and embryo development and its role in improving the retrieval of developmentally competent oocytes. Female golden hamsters were injected with human chorionic gonadotrophin (hCG; 5-, 7.5- or 15-IU) on the day 4 of estrus, pregnant mare serum gonadotrophin (PMSG; 5-, 7.5- or 15-IU) on the day 1 of estrus, or 15-IU hCG at 56 hr post-15-IU PMSG injection in any cycle except estrus. Increasing the hCG dose decreased not only retrieval rate of 2-cell embryo but development to blastocyst after subsequent in vitro culture. Whereas, although increasing the PMSG dose induced increasing the number of 2-cell embryo and blastocyst, 15-IU PMSG injection caused retardation of development to blastocyst. No 2-cell embryos were retrieved by injecting both PMSG and hCG. The injections of 15-IU hCG and 7.5- or 15-IU PMSG inhibited the proliferation of trophectodermal and inner cell mass cells, respectively. Gonadotrophin injection didn't influence microtubular spindle formation, but 5- or 15-IU hCG, 15-IU PMSG, or PMSG and hCG injections induced aberrant cortical granule (CG) and microfilament distribution. After 15-IU hCG or PMSG and hCG injections, fewer oocytes had enriched cortical actin domains, and the expression of alpha-, beta- and gamma-actin genes was greatly increased. In conclusion, a high dose of gonadotrophins alters the microfilament and CG distribution, which in turn reduces the developmental competence of oocytes. Injecting a reduced dose of PMSG to initiate ovarian hyperstimulation without triggering ovulation contributes to the efficient retrieval of developmentally competent oocytes.

The role of aromatase inhibitors in ameliorating deleterious effects of ovarian stimulation on outcome of infertility treatment

Clinical utilization of ovulation stimulation to facilitate the ability of a couple to conceive has not only provided a valuable therapeutic approach, but has also yielded extensive information on the physiology of ovarian follicular recruitment, endometrial receptivity and early embryo competency. One of the consequences of the use of fertility enhancing agents for ovarian stimulation has been the creation of a hyperestrogenic state, which may influence each of these parameters. Use of aromatase inhibitors reduces hyperestrogenism inevitably attained during ovarian stimulation. In addition, the adjunct use of aromatase inhibitors during ovarian stimulation reduces amount of gonadotropins required for optimum stimulation. The unique approach of reducing hyperestrogenism, as well as lowering amount of gonadotropins without affecting the number of mature ovarian follicles is an exciting strategy that could result in improvement in the treatment outcome by ameliorating the deleterious effects of the ovarian stimulation on follicular development, endometrial receptivity, as well as oocyte and embryo quality.

Immunization against endogenous inhibin increases normal oocyte/embryo production in adult mice

The objectives of the present study were to induce super-ovulation in adult mice by immunoneutralization of endogenous inhibin and to investigate embryo development in vivo and in vitro. Adult female mice of the ddY strain at 3 mo of age were superovulated with a single ip injection of inhibin antiserum (inhibin-AS; 50, 100, or 200 micro L) at 12:00 h on metestrus of the 4-d estrous cycle. The control group was treated with 100 micro L normal goat serum (control serum). Five mice were sacrificed every 6 h and blood samples were collected for hormonal assay. Individual female mice were bred in the afternoon of proestrus with individual fertile males. Embryos were collected at 08.00 h on d 2 post-coitus and cultured in KSOM solution. Treatment with inhibin-AS significantly increased the concentrations of plasma FSH. Furthermore, there was a significant elevation in plasma concentrations of estradiol-17beta and progesterone in the inhibin-immunized groups in comparison with controls. The superovulated oocytes that were fertilized normally in vivo were able to develop normally to blastocysts in vitro. The number of oocytes and blastocysts developed in animals treated with inhibin-AS was significantly higher than that for mice treated with control serum. Moreover, the rate of fertilization and the rate of blastocyst development were similar in inhibin-AS treated and control groups. These results indicate that immunoneutralization of endogenous inhibin induces superovulation in adult mice without additional treatment with human chorionic gonadotropin (hCG). In addition, the superovulated oocytes obtained by administration of inhibin-AS have normal embryonic developmental competence.

Administration of increasing amounts of gonadotrophin compromises preimplantation development of parthenogenetic mouse embryos

The aim of the present study was to examine the effect of ovarian stimulation with increasing amounts of pregnant mare's serum gonadotrophin (PMSG) on preimplantation development of diploid parthenogenetic embryos in vitro. Administration of 5, 10 and 20 IU PMSG significantly increased the number of oocytes obtained per mouse in a dose-dependent manner. The amount of PMSG administered did not alter the proportion of degenerate oocytes. However, there was a significant decrease in the proportion of 8-cell/compacted embryos after 53 h of culture with the administration of increasing amounts of PMSG. Proportion of embryos reaching at the blastocyst stage after 79 h of culture was reduced significantly in both the 10 and 20 IU PMSG groups. Reduced blastocyst development after 96 h of culture, however, was significant only in the 20 IU PMSG group. Total blastocyst, trophectoderm and inner cell mass numbers were also reduced significantly with the administration of 20 IU PMSG. It is concluded on the basis of these observations that preimplantation development of diploid parthenogenetic oocytes, which depends virtually entirely on maternal molecules accumulated during oogenesis along with gene products derived from the maternal genome, is compromised with the administration of increasing amounts of PMSG.

Effects of 4-hydroxyandrostenedione and hyperstimulation with pregnant mare serum gonadotrophin on early embryonic development in rats

A possible role of high oestradiol levels in mediating the adverse effects of hyperstimulation with pregnant mare serum gonadotrophin (PMSG) on early embryonic development in the rat was investigated using an aromatase inhibitor, 4-hydroxyandrostenedione (4-OHA), to inhibit endogenous oestradiol production. Three experiments were conducted in this study. In the first, varying doses of 4-OHA were administered either concurrently with human chorionic gonadotropin (hCG) to pro-oestrus female rats hyperstimulated at early di-oestrus stage with 20 IU PMSG or alone into nonhyperstimulated pro-oestrus females. At high doses of 1000, 2000, or 5000 µg/rat, 4-OHA substantially improved the survival of embryos in hyperstimulated females, while low doses of 100 and 500 µg/rat were ineffective. The protective effect of 4-OHA on embryo count was optimum at 2000 µg. When administered alone, only the highest dose of 5000 µg/rat 4-OHA increased embryo count. In the second experiment, higher doses of PMSG were studied (30 or 40 IU), with or without 5000 µg/rat 4-OHA given at the time of hCG injection. PMSG proved to be more detrimental with increasing dose, and 5000 µg/rat 4-OHA was able to rescue embryos from death in the 30, but not 40, PMSG group. In the third experiment, the influence of the timing of 4-OHA treatment on its ability to improve the embryo count in hyperstimulated females was examined by introducing 4-OHA 24 h earlier, rather than at the time of hCG treatment. The results showed the importance of timing of 4-OHA administration, as 5000 µg/rat 4-OHA was able to restore embryo survival in the 40 PMSG hyperstimulated group only when it was administered 24 h before hCG injection. Together, these results highlighted that 4-OHA, when administered at the appropriate time and dose, could reverse the negative effects of hyperstimulation from PMSG on early embryonic development. This may be due to its potent aromatase inhibiting properties that lead to the suppression of oestrogen production, thereby alleviating the supraphysiological level of oestradiol, which is typically present in PMSG-treated females. Interestingly, 4-OHA treatment on its own was able to positively influence embryo count when given at a high dose of 5000 µg/rat, and this may be associated with its weak androgenic properties. In conclusion, this study supports the hypothesis that excessive oestradiol is responsible for the negative effects of hyperstimulation with PMSG on early embryonic development.Key words: 4-hydroxyandrostenedione, embryonic development, PMSG, rat.

Differential effects of repeated ovarian stimulation on cytoplasmic and spindle organization in metaphase II mouse oocytes matured in vivo and in vitro

The effects of four rounds of ovarian stimulation spaced 1-6 weeks apart on the normality of metaphase II (MII) spindle formation, chromosomal alignment and cytoplasmic organization were examined in intact ovulated mouse oocytes and at MII for oocytes obtained at the germinal vesicle stage from the same ovaries and matured in vitro. The terminal deoxynucleotidyl transferase-mediated dUDP nick-end labelling assay was used to identify DNA strand breaks in chromosomes, and histological studies of ovaries between and at each round of ovarian stimulation were performed. The results demonstrate a progressive and significant increase in the frequency of spindle defects with each round of ovarian stimulation, including those spaced weeks apart. Oocytes with spindle defects were also characterized by the occurrence of detached chromosomes and cytoplasmic asters. In contrast, in-vitro matured oocytes derived from the same ovaries were normal. No evidence of DNA strand breaks with repeated rounds of ovarian stimulation was detected in ovulated or in-vitro matured oocytes. The development and persistence of nodules of hypertrophied granulosa in regions where follicular growth occurs suggest that a progressively increasing proportion of oocytes in the ovulatory pathway may experience an intrafollicular milieu that has negative consequences for competence. The results are discussed with respect to ovarian and oocyte biological ageing and possible adverse implications for human oocyte competence with repeated hyperstimulation.