Clomiphene citrate-induced perturbations during meiotic maturation and cytogenetic abnormalities in mouse oocytes in vivo and in vitro

The association between in vitro fertilization and intracytoplasmic sperm injection treatment and the risk of congenital heart defects

OBJECTIVE

Assisted reproductive technology (ART), an effective treatment modality for infertility, is associated with a higher prevalence of congenital anomalies such as congenital heart defects (CHDs). The present study aimed to evaluate data linking CHDs in infants to pregnancies resulting from in vitro fertilization (IVF) and/or intracytoplasmic sperm injection (ICSI).

METHODS

In this study, we conducted a systematic literature search on CHDs in infants following IVF/ICSI in Google Scholar, Embase, Scopus, MEDLINE, and PubMed databases from inception to February 2020. The search strategy used combinations of search keywords that included assisted reproductive technology/ART, in vitro fertilization/IVF, intracytoplasmic sperm injection/ICSI, birth defect, congenital malformation, and congenital heart defects.

RESULTS

Fifty-six studies fulfilled the inclusion criteria and were selected in the current systematic review, which assessed the association between ART and the risk of CHDs.

CONCLUSION

Children conceived by IVF/ICSI manifested an increased risk of CHDs compared with spontaneously conceived children. Further studies are needed to assess the long-term cardiovascular safety of these techniques, which is important for the counseling of patients before the use of ART.

Design and evaluation of a novel nanodrug delivery system for reducing the side effects of clomiphene citrate on endometrium

BACKGROUND

Stimulation of ovulation with clomiphene citrate can cause side effects on endometrial receptivity. Formulation with nano-size may be an alternative therapy for women with ovulatory disorders. In this study, we investigated sustained-release clomiphene citrate by using Phosal-based formulation (PBF) and evaluate its decreased side effect on the endometrial receptivity.

METHODS

In the in-vitro study, CC loaded PBF was analyzed using Zetasizer, Fourier-transform infrared spectroscopy (FTIR), and Transmission electron microscopy (TEM). In the in-vivo study, 24 female mice were randomly divided into three groups: CC (5 mg/kg), CC/PBF (5 mg/kg) and SS (1 ml) daily administered and injected with 5 IU HCG and mated after two days. At day 4.5, pregnant mice were euthanized and endometrial tissue was extracted for quantitative polymerase chain reaction (Q-PCR) analysis.

RESULTS

The optimized PBF contained Phosal 50PG/glycerol in a 2:8 ratios (w/w) and the particle size of optimum formulation was 67 ± 0.30551 nm and the release of CC from CC-containing PBF was slightly faster in the first 24 h; wherein, 29% of CC was released, and 76% of CC was released up to 120 h. The mRNA levels of leukemia inhibitory factor (LIF), leukemia inhibitory factor receptor alpha (LIFR), HOXA10, Heparin-binding epidermal growth factor (HB-EGF), and epidermal growth factor (EGF) were significantly upregulated and MUC1 and PGR mRNA levels were significantly downregulated in the CC-containing PBF-treated animals compared with only CC group (P < 0.05).

CONCLUSION

Sustained release formulation of clomiphene citrate increased its targeting efficiency and improved the impact of the CC on implantation. Graphical abstract A new Phosal Based Formulation (PBF) was designed to decrease the side effects of Clomiphene citrate (CC) on endometrium. This drug formulation could react better during implantation by increasing the expression of genes involved in implantation. The in vivo study demonstrated that the CC-containing PBF in mice has a significantly higher endometrial receptivity, compared with the suspension.

Chemically induced aneuploidy in germ cells. Part II of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

As part of the 7th International Workshops on Genotoxicity Testing held in Tokyo, Japan in November 2017, a workgroup of experts reviewed and assessed the risk of aneugens for human health. The present manuscript is one of three manuscripts from the workgroup and reports on the unanimous consensus reached on the evidence for aneugens affecting germ cells, their mechanisms of action and role in hereditary diseases. There are 24 chemicals with strong or sufficient evidence for germ cell aneugenicity providing robust support for the ability of chemicals to induce germ cell aneuploidy. Interference with microtubule dynamics or inhibition of topoisomerase II function are clear characteristics of germ cell aneugens. Although there are mechanisms of chromosome segregation that are unique to germ cells, there is currently no evidence for germ cell-specific aneugens. However, the available data are heavily skewed toward chemicals that are aneugenic in somatic cells. Development of high-throughput screening assays in suitable animal models for exploring additional targets for aneuploidy induction, such as meiosis-specific proteins, and to prioritize chemicals for the potential to be germ cell aneugens is encouraged. Evidence in animal models support that: oocytes are more sensitive than spermatocytes and somatic cells to aneugens; exposure to aneugens leads to aneuploid conceptuses; and, the frequencies of aneuploidy are similar in germ cells and zygotes. Although aneuploidy in germ cells is a significant cause of infertility and pregnancy loss in humans, there is currently limited evidence that aneugens induce hereditary diseases in human populations because the great majority of aneuploid conceptuses die in utero. Overall, the present work underscores the importance of protecting the human population from exposure to chemicals that can induce aneuploidy in germ cells that, in contrast to carcinogenicity, is directly linked to an adverse outcome.

Genotoxic and mutagenic studies of teratogens in developing rat and mouse

In this review, genotoxic and mutagenic effects of teratogenic chemical agents in both rat and mouse have been reviewed. Of these chemicals, 97 are drugs and 33 are pesticides or belong to other groups. Large literature searches were conducted to determine the effects of chemicals on chromosome abnormalities, sister chromatid exchanges, and micronucleus formation in experimental animals such as rats and mice. In addition, studies that include unscheduled DNA synthesis, DNA adduct formations, and gene mutations, which help to determine the genotoxicity or mutagenicity of chemicals, have been reviewed. It has been estimated that 46.87% of teratogenic drugs and 48.48% of teratogenic pesticides are positive in all tests. So, all of the teratogens involved in this group have genotoxic and mutagenic effects. On the other hand, 36.45% of the drugs and 21.21% of the pesticides have been found to give negative results in at least one test, with the majority of the tests giving positive results. However, only 4.16% of the drugs and 18.18% of the pesticides were determined to give negative results in the majority of the tests. Among tests with major negative results, 12.50% of the teratogenic drugs and 12.12% of the teratogenic pesticides were negative in all conducted tests.

Direct evidences for the groove binding of the Clomifene to double stranded DNA

It has been reported that the antiestrogen Tamoxifen induces liver tumors in rats and genotoxic effects in vitro through DNA interaction. So, it can be proposed that its structural analogue, Clomifene, also can bind to DNA. To test this hypothesis, the DNA binding properties of Clomifene have been studied by absorption spectroscopy, fluorescence spectroscopy, cellular uptake, cell viability, cell proliferation and molecular modeling techniques. Evidences are provided that Clomifene could interact with DNA via minor groove interaction mode. The negative ΔG value implied that the interaction occurred between DNA and Clomifene spontaneously. Also, the positive ΔH and positive ΔS values indicated that the binding of Clomifene with DNA is mainly entropy driven and the enthalpy is unfavorable parameter. This also suggests that the hydrophobic interaction plays a major role in the binding with overall binding constant of K=5.645×10⁷M^-1 at 298K. From the results of docking, it can be concluded that Hydrogen bonds is also one of the most important interactions. The increase in entropy of system after binding might be due to the destruction of the DNA structure.

Cardiovascular and metabolic profiles of offspring conceived by assisted reproductive technologies: a systematic review and meta-analysis

OBJECTIVE

To evaluate cardiovascular and metabolic features of offspring conceived by in vitro fertilization/intracytoplasmic sperm injection (IVF-ICSI).

DESIGN

Literature review and meta-analysis.

SETTING

Not applicable.

PATIENT(S)

Offspring from IVF-ICSI versus natural conception.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Systolic and diastolic blood pressure (SBP and DBP), cardiovascular function, body mass index (BMI), and lipid and glucose profiles.

RESULT(S)

We included 19 studies that had recruited 2,112 IVF-ICSI and 4,096 naturally conceived offspring, ranging from childhood to early adulthood. The blood pressure levels of IVF-ICSI offspring were statistically significantly higher than those of naturally conceived offspring (weighted mean differences and confidence intervals: 1.88 mm Hg [95% CI, 0.27, 3.49] for SBP and 1.51 mm Hg [95% CI, 0.33, 2.70] for DBP). In addition, cardiac diastolic function was suboptimal and vessel thickness was higher among IVF-ICSI offspring. Compared with the metabolism of naturally conceived offspring, IVF-ICSI offspring displayed comparable BMI, lower low-density lipoprotein cholesterol levels, and higher fasting insulin levels.

CONCLUSION(S)

Children conceived by IVF-ICSI manifested a minor yet statistically significant increase in blood pressure without the clustering of increased BMI or impaired lipid metabolism by early adulthood. Our findings indicate a risk of cardiovascular disease among IVF-ICSI offspring, which calls for longer-term follow-ups and further investigation.

Induction of sister chromatid exchanges and cell division delays by clomiphene citrate in human lymphocytes

Objective:

Clomiphene citrate (CC) is a selective estrogen receptor modulator and is used for the treatment of in vitro fertilization, intracytoplasmic sperm injection, intrauterine insemination, and so on. In this study, sister chromatid exchanges (SCEs) and cell cycle delays were analyzed to investigate genotoxicity and cytotoxicity of CC in peripheral blood lymphocytes of healthy women.

Methods:

Human peripheral blood lymphocytes obtained from two donors were used to detect genotoxicity and cytotoxicity of CC. Lymphocytes were treated with various concentrations (0.40, 0.80, 1.60, and 3.20 µg/ml) of CC. A negative (distilled water) and a positive control (mitomycin-C = 0.20 µg/ml) were also used simultaneously with test substance-treated cultures. SCEs and cell division delays were measured from 25 cells and 100 cells perdonor, respectively.

Results:

CC significantly increased the mean SCE value at all concentrations compared with the negative control. This increase was found to be dose dependent ( r = 0.83) and at the highest concentration, nearly two times higher increase was observed than the negative control. However, replication index was not affected by the CC treatment.

Conclusion:

The present study shows that CC is genotoxic for human lymphocytes in vitro. Further investigations, especially in vivo are now needed in different test organisms to clarify the genotoxic activity of CC, which should also help to better understand genotoxic mechanism of this ovulation-stimulating drug.

Genotoxicity of the some selective estrogen receptor modulators: a review

The objective of this article is to review genotoxicological profile of the major selective estrogen receptor modulators, including clomiphene, tamoxifen, toremifene, raloxifene. These drugs have been used for infertility treatment and breast cancer prevention in high risk-women. However, some studies reported that especially tamoxifen is a genotoxic agent and is related with endometrial cancer. Our review indicate that clomiphene and tamoxifen were found as genotoxic agent in majority of the tests. However published reports showed that toremifene is a weakly genotoxic agent. The genotoxic effects of raloxifene are still poorly known. Further genotoxicity studies should be conducted especially for raloxifene.

Melatonin protects against clomiphene citrate-induced generation of hydrogen peroxide and morphological apoptotic changes in rat eggs

The present study was aimed to determine whether clomiphene citrate-induces generation of hydrogen peroxide in ovary, if so, whether melatonin could scavenge hydrogen peroxide and protect against clomiphene citrate-induced morphological apoptotic changes in rat eggs. For this purpose, forty five sexually immature female rats were given single intramuscular injection of 10 IU pregnant mare's serum gonadotropin for 48 h followed by single injections of 10 IU human chorionic gonadotropin and clomiphene citrate (10 mg/kg bw) with or without melatonin (20 mg/kg bw) for 16 h. The histology of ovary, ovulation rate, hydrogen peroxide concentration and catalase activity in ovary and morphological changes in ovulated eggs were analyzed. Co-administration of clomiphene citrate along with human chorionic gonadotropin significantly increased hydrogen peroxide concentration and inhibited catalase activity in ovary, inhibited ovulation rate and induced egg apoptosis. Supplementation of melatonin reduced hydrogen peroxide concentration and increased catalase activity in the ovary, delayed meiotic cell cycle progression in follicular oocytes as well as in ovulated eggs since extrusion of first polar body was still in progress even after ovulation and protected against clomiphene citrate-induced egg apoptosis. These results clearly suggest that the melatonin reduces oxidative stress by scavenging hydrogen peroxide produced in the ovary after clomiphene citrate treatment, slows down meiotic cell cycle progression in eggs and protects against clomiphene citrate-induced apoptosis in rat eggs.

8q deletion in MYCN-amplified neuroblastoma of a child born from assisted reproductive technology

The occurrence of pediatric cancer in children born from assisted reproductive technologies has been sporadically reported. Chromosomal characterization of the neoplasic disease in this setting is poorly described. In the present study, neuroblastoma cells from a 13-month-old infant boy born after intracytoplasmatic sperm injection were characterized by combining conventional cytogenetics, fluorescence in situ hybridization (FISH), comparative genomic hybridization, and quantitative polymerase chain reaction methods. Cytogenetic analysis of neuroblastoma (NB) metaphase spreads at the time of diagnosis revealed numerous centromere-free extrachromosomal double minutes, suggesting high MYCN amplification. Comparative genomic hybridization analysis demonstrated the amplification of 2q24 approximately pter, with additional gain of the long arm of chromosome 17. Chromosome losses involved 8q, 9q, and 11q. No deletion of 1p was found. MYCN amplification was confirmed by quantitative polymerase chain reaction and fluorescence in situ hybridization analysis. This report describes several chromosomal abnormalities that were present in NB of a child born after intracytoplasmatic sperm injection. Besides some well described and prognostic genetic findings in NB as MYCN amplification, gain on 17q and losses on 9q and 11q23, we report an unusual deletion involving 8q region in this disease. Whether this genetic abnormality may be associated to assisted reproductive technologies deserves further investigation.

Gender effects on the incidence of aneuploidy in mammalian germ cells

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

Clomiphene citrate inhibits gonadotropin-induced ovulation by reducing cyclic adenosine 3',5'-cyclic monophosphate and prostaglandin E2 levels in rat ovary

OBJECTIVE

To determine whether clomiphene citrate (CC) inhibited gonadotropin-induced ovulation by reducing cyclic adenosine 3',5'-cyclic monophosphate (cAMP) and prostaglandin E2 (PGE2) levels in ovary. If so, to determine whether E2 coadministration could protect against these effects of CC.

DESIGN

A controlled prospective study.

SETTING

Laboratory research setting in the department of reproductive biomedicine at a national research institute in India.

ANIMAL(S)

Sixty sexually immature female rats that were 24-25 days of age.

INTERVENTION(S)

The sexually immature female rats were given a single injection (10 IU i.m.) of pregnant mare serum gonadotropin. After 48 hours, the rats were given single injections of hCG (10 IU) along with CC, with or without E2, for 16 hours.

MAIN OUTCOME MEASURE(S)

Number of superovulated COCs, ovary and uterus weight, FSH and LH levels in serum, and cAMP and PGE2 levels in ovary.

RESULT(S)

The superovulatory dose of gonadotropins significantly increased ovary and uterus weights, cAMP and PGE2 levels in ovary, and serum levels of FSH and LH. Coadministration of CC (10 mg/kg body weight) significantly reduced levels of cAMP, PGE2 in the ovary, ovary and uterus weights, and ovulation rate, whereas FSH and LH levels were not significantly altered. Supplementation of E2 protected against these inhibitory effects of CC and augmented levels of FSH and LH in serum.

CONCLUSION(S)

Clomiphene citrate inhibited gonadotropin-induced ovulation by reducing cAMP and PGE2 levels in the ovary, and E2 protected against these effects of CC on gonadotropin-induced ovulation in rat.

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.

Follicle-Stimulating Hormone Affects Metaphase I Chromosome Alignment and Increases Aneuploidy in Mouse Oocytes Matured in Vitro1

Follicle-Stimulating Hormone (FSH) at a wide range of doses is routinely added to culture media during in vitro maturation (IVM) of oocytes, but the effects on oocyte health are unclear. The suggestion that superovulation may cause aneuploidy and fetal abnormalities prompted us to study the potential role of FSH in the genesis of chromosomal abnormalities during meiosis I. Mouse cumulus-oocyte complexes (COCs) isolated from the antral follicles of unprimed, sexually immature B6CBF1 mice were cultured in increasing concentrations of FSH. Following culture, matured oocytes were isolated, spread, stained with DAPI, and the numbers of chromosomes counted. Significantly increased aneuploidy, arising during the first meiotic division, was observed in metaphase II oocytes matured in higher concentrations of FSH (> or =20 ng/ml). The effect of FSH on spindle morphology and chromosome alignment during metaphase I was then explored using immunocytochemistry and three-dimensional reconstruction of confocal sections. High FSH had no effect on gross spindle morphology but did alter chromosome congression during prometaphase and metaphase, with the spread of chromosomes across the spindle at this time being significantly greater in oocytes cultured in 2000 ng/ml compared with 2 ng/ml FSH. Analysis of three-dimensional reconstructions of spindles in oocytes matured in 2000 ng/ml FSH shows that chromosomes are more scattered and farther apart than they are following maturation in 2 ng/ml FSH. These results demonstrate that exposure to high levels of FSH during IVM can accelerate nuclear maturation and induce chromosomal abnormalities and highlights the importance of the judicious use of FSH during IVM.

Is ovulation induction still a therapeutic problem in patients with polycystic ovary syndrome?

Polycystic ovary syndrome (PCOS) is one of the most common endocrine diseases affecting women of fertile age, and is characterized by hyperandrogenism, chronic anovulatory cycles and oligomenorrhea or amenorrhea. Given the repercussions of chronic anovulation on sterility, PCOS is a heavy social burden. Here we describe the procedures used to induce ovulation in PCOS patients, the surgical approach and medical treatments that are still being experimented.

Aromatase inhibitors for the treatment of infertility

Ovarian stimulation during infertility treatment is used either alone or in conjunction with intrauterine insemination and assisted reproductive technologies. At the present time, the two main medications used for ovarian stimulation include an oral antioestrogen, clomiphene citrate and injectable gonadotrophins. In spite of the high ovulation rate, the use of clomiphene citrate is associated with adverse side effects and low pregnancy rates. In clomiphene citrate failures, gonadotrophin injections are generally the next treatment option but, especially in polycystic ovarian syndrome, are associated with increased risk of severe ovarian hyperstimulation syndrome and high multiple pregnancies. Therefore, an effective oral treatment that could be used without risk of ovarian hyperstimulation syndrome and with minimal monitoring is preferred. It was hypothesised that aromatase inhibitors can be administered early in the follicular phase to induce ovulation by releasing the hypothalamus and/or pituitary from oestrogen negative feedback. The success of aromatase inhibitors in induction and augmentation of ovulation has been reported. In addition, increased intraovarian androgen levels may synergise with central effects of decreased oestrogen to enhance ovarian response to gonadotrophin stimulation. This increased sensitivity to follicle-stimulating hormone may be especially useful in poor responders. The potential future applications for aromatase inhibitors in infertility management are also discussed.

Aromatase inhibition for ovarian stimulation: future avenues for infertility management

Ovarian stimulation is applied during infertility management either alone or in conjunction with intrauterine insemination and assisted reproductive technologies. At the present time, the two main medications used for ovarian stimulation include an oral antiestrogen, clomiphene citrate, and injectable gonadotropins. In spite of the high ovulation rate with the use of clomiphene citrate, the pregnancy rate is much lower. In clomiphene citrate failures, gonadotropin injections have generally been used as the next treatment option. Treatment with gonadotropins is difficult to control and characteristically associated with increased risk of severe ovarian hyperstimulation syndrome and high multiple pregnancies. Therefore, an effective oral treatment that could be used without risk of hyperstimulation and with minimal monitoring is the preferred therapy. We hypothesize that aromatase inhibitors can be administered early in the follicular phase to induce ovulation by releasing the hypothalamus or pituitary from estrogen negative feedback. Based on this hypothesis, we have reported the success of aromatase inhibitors in induction and augmentation of ovulation in addition to improving ovarian response to gonadotropin stimulation. Moreover, there are other potential applications for aromatase inhibitors in infertility management, including improving implantation in assisted reproduction and in-vitro maturation.

Developmental capacity of Bos indicus oocytes after inhibition of meiotic resuption by 6-dimethylaminopurine

Several reports have suggested that a treatment before in vitro maturation might improve oocyte competence and increase its developmental potential. Therefore, the objectives of the present study were to establish the kinetics of IVM in Zebu oocytes, to assess the effect of 6-dimethylaminopurine (6-DMAP), a phosphorylation inhibitor, on meiotic resumption, and to verify the developmental potential of the blocked oocytes after removal of the inhibitory conditions. To establish the kinetics of in vitro maturation 1422 oocytes were obtained from Nellore cows ovaries and matured in presence and absence of gonadotropins. Samples of oocytes were taken from culture at 0, 6, 9, 12, 15, 18, 21 and 24h, and the oocytes were fixed, stained and evaluated for nuclear morphology. Germinal vesicle break down (GVBD) occurred between 6 and 12h of culture in both groups. By 21h the majority of the oocytes had reached metaphase II in presence (71%) and absence (62%) of gonadotropins. In order to examine the inhibitory effect of 6-DMAP, 585 oocytes were cultured for 12, 18 and 24h in the presence or absence of 2mM of 6-DMAP. At each time point the oocytes were evaluated for nuclear morphology. To test the reversibility of meiotic inhibition 366 oocytes were incubated for 0, 12, 18 and 24h in the presence of 6-DMAP and then were transferred to the maturation medium and cultured for further 24h. A total of 429 oocytes were used to evaluate the developmental potential after meiotic inhibition. The oocytes were cultured in the presence of 6-DMAP for 0, 12, 18 and 24h, and then were matured, fertilized and cultured in vitro. Culture of bovine oocytes in the presence of 6-DMAP up to 24h completely blocked GVBD with more than 90% of the oocytes at GV stage. The inhibitory effect of 6-DMAP was fully reversible since maturation rates were similar (P>0.05) among all treatment groups. The evaluation of embryo development after various periods of meiotic blockage showed that inhibition, regardless the time period, had no effect (P>0.05) on penetration and cleavage rates. However, the proportion of embryos at blastocyst stage was reduced after inhibition for 12 (20.2%), 18 (20.1%) and 24h (19.0%) compared with the control group (35.6%). 6-DMAP has a reversible effect on maintenance of meiotic arrest, but reduced further embryo development.