Maturation of the mouse oocyte in vitro. I. Sequence and timing of nuclear progression

Assessment of active translation in cumulus-enclosed and denuded oocytes during standard in vitro maturation and early embryo development

STUDY QUESTION

Which actively translated maternal transcripts are differentially regulated between clinically relevant in vitro and in vivo maturation (IVM) conditions in mouse oocytes and zygotes?

SUMMARY ANSWER

Our findings uncovered significant differences in the global transcriptome as well as alterations in the translation of specific transcripts encoding components of energy production, cell cycle regulation, and protein synthesis in oocytes and RNA metabolism in zygotes.

WHAT IS KNOWN ALREADY

Properly regulated translation of stored maternal transcripts is a crucial factor for successful development of oocytes and early embryos, particularly due to the transcriptionally silent phase of meiosis.

STUDY DESIGN, SIZE, DURATION

This is a basic science study utilizing an ICR mouse model, best suited for studying in vivo maturation. In the treatment group, fully grown germinal vesicle oocytes from stimulated ovaries were in vitro matured to the metaphase II (MII) stage either as denuded without gonadotropins (IVM DO), or as cumulus-oocyte complexes (IVM COC) in the presence of 0.075 IU/ml recombinant FSH (rFSH) and 0.075 IU/ml recombinant hCG (rhCG). To account for changes in developmental competence, IVM COC from non-stimulated ovaries (IVM COC-) were included. In vivo matured MII oocytes (IVO) from stimulated ovaries were used as a control after ovulation triggering with rhCG. To simulate standard IVM conditions, we supplemented media with amino acids, vitamins, and bovine serum albumin. Accordingly, in vitro pronuclear zygotes (IMZ) were generated by IVF from IVM DO, and were compared to in vivo pronuclear zygotes (IVZ). All experiments were performed in quadruplicates with samples collected for both polyribosome fractionation and total transcriptome analysis. Samples were collected over three consecutive months.

PARTICIPANTS/MATERIALS, SETTING, METHODS

All ICR mice were bred under legal permission for animal experimentation (no. MZE-24154/2021-18134) obtained from the Ministry of Agriculture of the Czech Republic. Actively translated (polyribosome occupied) maternal transcripts were detected in in vitro and in vivo matured mouse oocytes and zygotes by density gradient ultracentrifugation, followed by RNA isolation and high-throughput RNA sequencing. Bioinformatic analysis was performed and subsequent data validation was done by western blotting, radioactive isotope, and mitotracker dye labelling.

MAIN RESULTS AND THE ROLE OF CHANCE

Gene expression analysis of acquired polysome-derived high-throughput RNA sequencing data revealed significant changes (RPKM ≥ 0.2; P ≤ 0.005) in translation between in vitro and in vivo matured oocytes and respectively produced pronuclear zygotes. Surprisingly, the comparison between IVM DO and IVM COC RNA-seq data of both fractionated and total transcriptome showed very few transcripts with more than a 2-fold difference. Data validation by radioactive isotope labelling revealed a decrease in global translation bof20% in IVM DO and COC samples in comparison to IVO samples. Moreover, IVM conditions compromised oocyte energy metabolism, which was demonstrated by both changes in polysome recruitment of each of 13 mt-protein-coding transcripts as well as by validation using mitotracker red staining.

LARGE SCALE DATA

The data discussed in this publication have been deposited in NCBI's Gene Expression Omnibus and are accessible through GEO Series accession number GSE241633 (https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE241633).

LIMITATIONS, REASONS FOR CAUTION

It is extremely complicated to achieve in vivo consistency in animal model systems such as porcine or bovine. To achieve a high reproducibility of in vivo stimulations, the ICR mouse model was selected. However, careful interpretation of our findings with regard to assisted reproductive techniques has to be made by taking into consideration intra-species differences between the mouse model and humans. Also, the sole effect of the cumulus cells' contribution could not be adequately addressed by comparing IVM COC and IVM DO, because the IVM DO were matured without gonadotropin supplementation.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings confirmed the inferiority of standard IVM technology compared with the in vivo approach. It also pointed at compromised biological processes employed in the critical translational regulation of in vitro matured MII oocytes and pronuclear zygotes. By highlighting the importance of proper translational regulation during in vitro oocyte maturation, this study should prompt further clinical investigations in the context of translation.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the Czech Grant Agency (22-27301S), Charles University Grant Agency (372621), Ministry of Education, Youth and Sports (EXCELLENCE CZ.02.1.01/0.0/0.0/15_003/0000460 OP RDE), and Institutional Research Concept RVO67985904. No competing interest is declared.

The second polar body contributes to the fate asymmetry in the mouse embryo

The polar bodies (PBs) are extruded microcells during oocyte meiosis and generally regarded as inessentials for embryonic development. Therefore, PBs have been widely used as important materials for pre-implantation genetic diagnosis in human. Here we report that the second PB (PB2) in the mouse zygote may play roles in cell-fate specification and post-implantation development. A subset of mRNAs encoding pluripotency-related factors are enriched in PB2. Nascent proteins may be synthesized in PB2 after fertilization and transport from PB2 to the zygote before the two-cell stage. The PB2-attached blastomere (pbB) at the two-cell stage, compared to the other blastomere (npbB), likely contributes more descendants to the inner cell mass (ICM) lineage in the blastocyst. Removal of PB2 from the zygote or transient blockage of material exchange between PB2 and the zygote by nocodazole treatment appears to cause a loss of the ICM fate bias of pbB. PB2 removal or nocodazole treatment also results in abnormal post-implantation development. Injection of PB2 lysate into pbB of PB2-removed two-cell-stage embryos may reset the cell-fate preference and rescue post-implantation development. Our data collectively suggest that PB2 would demarcate the earliest cell-fate asymmetry of the mouse zygote and be required for post-implantation development.

Isolation and in vitro Culture of Mouse Oocytes

Females are endowed at birth with a fixed reserve of oocytes, which declines both in quantity and quality with advancing age. Understanding the molecular mechanisms regulating oocyte quality is crucial for improving the chances of pregnancy success in fertility clinics. In vitro culture systems enable researchers to analyse important molecular and genetic regulators of oocyte maturation and fertilisation. Here, we describe in detail a highly reproducible technique for the isolation and culture of fully grown mouse oocytes. We include the considerations and precautionary measures required for minimising the detrimental effects of in vitro culture conditions. This technique forms the starting point for a wide range of experimental approaches such as post-transcriptional gene silencing, immunocytochemistry, Western blotting, high-resolution 4D time-lapse imaging, and in vitro fertilization, which are instrumental in dissecting the molecular determinants of oocyte quality. Hence, this protocol serves as a useful, practical guide for any oocyte researcher beginning experiments aimed at investigating important oocyte molecular factors. Graphic abstract: A step-by-step protocol for the isolation and in vitro culture of oocytes from mice.

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.

In Vitro Culture of Mouse Oocytes for Meiotic Maturation

In vitro maturation of mouse oocytes, as well as of many other mammalian species, is used commonly in experimental embryology. This model system gives the possibility to easily obtain maturing oocytes at desired stages of the maturation process. Here we describe how to establish the in vitro culture of maturating mouse oocytes. We focus on the use of oocytes maturation inhibitors to synchronize oocytes at the GVBD stage and on the procedure to distinguish between transcribing and nontranscribing GV-arrested oocytes.

Effects of Subacute Administration of Co-Trimoxazole and Folic Acid on Ovarian Tissue in Adult Female Rats

BACKGROUND

Previous studies have reported the antifertility activities of sulfonamides. This study was designed to evaluate the effects of co-trimoxazole and its co-administration with folic acid on ovarian tissue in female rats.

METHODS

A total of 54 rats were randomly divided into 9 groups (n=6). Group I served as the control and group II (vehicle) received saline. Other groups, III to IX, received co-trimoxazole (30, 60, and 120 mg/kg; i.p.), folic acid (1 mg/kg; i.p.) or their combination for 14 days, respectively. The oocytes were obtained from each group at the end of the 14th days and scored for maturational status as germinal vesicle (GV), metaphase I (MI), or metaphase II (MII). The number of primordial follicle (PrF), primary follicle (PF), and secondary follicle in formalin-fixed ovaries were counted under light microscopy. The data were analyzed by one-way ANOVA followed by post-hoc Dunnet test using SPSS statistical software (version 17.0). Results were considered statistically significant at P<0.05.

RESULTS

Co-trimoxazole (60 and 120 mg/kg) treatment for 14 days caused a significant decrease in the number of GV (P=0.02, P<0.001), MI and MII (P=0.03, P<0.001), a significant increase in structural abnormalities, including PrF, PF and secondary follicle (P<0.001) as well as congestion, inflammation and necrosis of ovarian tissue compared to the vehicle group. Folic acid co-administration with co-trimoxazole reversed partially all these parameters compared to the co-trimoxazole group (P<0.001).

CONCLUSION

The data showed the adverse effects of co-trimoxazole on the ovarian maturational status and tissue structure which was reversed partially by folic acid co-administration in rats.

Early germinal vesicle breakdown is a predictor of high preimplantation developmental competent oocytes in mice

The preselection of highly developmentally competent oocytes for in vitro maturation (IVM) is crucial for improving assisted reproductive technology. Although several intrinsic markers of oocyte quality are known to be closely related to the onset of nuclear maturation (germinal vesicle break down, GVBD), a direct comparison between GVBD timing and oocyte quality has never been reported. In this study, we established a non-invasive oocyte evaluation method based on GVBD timing for preselecting more developmental competent oocytes in mice. Because the O2 concentration during IVM may affect the nuclear kinetics, all experiments were performed under two distinct O2 concentrations: 20% and 5% O2. First, we determined the time course of changes in nuclear maturation and preimplantation developmental competence of in vitro-matured oocytes to estimate GVBD timing in high developmental competent oocytes. Two-thirds of oocytes that underwent GVBD in early IVM seemed to mainly contribute to the blastocyst yield. To confirm this result, we compared the preimplantation developmental competence of the early and late GVBD oocytes. Cleavage and blastocyst formation rates of early GVBD oocytes (80.2% and 52.7% under 20% O2, respectively, and 67.6% and 47.3% under 5% O2, respectively) were almost double those of late GVBD oocytes (44.8% and 26.0% under 20% O2, respectively, and 40.4% and 17.9% under 5% O2, respectively). With no observable alterations by checking the timing of GVBD in preimplantation developmental competence, oocyte evaluation based on GVBD timing can be used as an efficient and non-invasive preselection method for high developmental competent oocytes.

A Motor-Gradient and Clustering Model of the Centripetal Motility of MTOCs in Meiosis I of Mouse Oocytes

Asters nucleated by Microtubule (MT) organizing centers (MTOCs) converge on chromosomes during spindle assembly in mouse oocytes undergoing meiosis I. Time-lapse imaging suggests that this centripetal motion is driven by a biased 'search-and-capture' mechanism. Here, we develop a model of a random walk in a drift field to test the nature of the bias and the spatio-temporal dynamics of the search process. The model is used to optimize the spatial field of drift in simulations, by comparison to experimental motility statistics. In a second step, this optimized gradient is used to determine the location of immobilized dynein motors and MT polymerization parameters, since these are hypothesized to generate the gradient of forces needed to move MTOCs. We compare these scenarios to self-organized mechanisms by which asters have been hypothesized to find the cell-center- MT pushing at the cell-boundary and clustering motor complexes. By minimizing the error between simulation outputs and experiments, we find a model of "pulling" by a gradient of dynein motors alone can drive the centripetal motility. Interestingly, models of passive MT based "pushing" at the cortex, clustering by cross-linking motors and MT-dynamic instability gradients alone, by themselves do not result in the observed motility. The model predicts the sensitivity of the results to motor density and stall force, but not MTs per aster. A hybrid model combining a chromatin-centered immobilized dynein gradient, diffusible minus-end directed clustering motors and pushing at the cell cortex, is required to comprehensively explain the available data. The model makes experimentally testable predictions of a spatial bias and self-organized mechanisms by which MT asters can find the center of a large cell.

The adverse outcome pathway (AOP) for chemical binding to tubulin in oocytes leading to aneuploid offspring

The Organisation for Economic Co-operation and Development (OECD) has launched the Adverse Outcome Pathway (AOP) Programme to advance knowledge of pathways of toxicity and improve the use of mechanistic information in risk assessment. An AOP links a molecular initiating event (MIE) to an adverse outcome (AO) through intermediate key events (KE). Here, we present the scientific evidence in support of an AOP whereby chemicals that bind to tubulin cause microtubule depolymerization resulting in spindle disorganization followed by altered chromosome alignment and segregation and the generation of aneuploidy in female germ cells, ultimately leading to aneuploidy in the offspring. Aneuploidy, an abnormal number of chromosomes that is not an exact multiple of the haploid number, is a well-known cause of human disease and represents a major cause of infertility, pregnancy failure, and serious genetic disorders in the offspring. Among chemicals that induce aneuploidy in female germ cells, a large majority impairs microtubule dynamics and spindle function. Colchicine, a prototypical chemical that binds to tubulin and causes microtubule depolymerization, is used here to illustrate the AOP. This AOP is specific to female germ cells exposed during the periovulation period. Although the majority of the data come from rodent studies, the available evidence suggests that the MIE and KEs are conserved across species and would occur in human oocytes. The development of AOPs related to mutagenicity in germ cells is expected to aid the identification of potential hazards to germ cell genomic integrity and support regulatory efforts to protect population health.

Role of cortisol and superoxide dismutase in psychological stress induced anovulation

Stress has been identified as a potential trigger for reproductive dysfunctions, but the psycho-physiological pathway behind the effect of stress on ovulation remains unexplored. The present research work highlights the plausible mechanism of psychological stress on ovulation in mice by targeting superoxide dismutase (SOD), an enzyme involved in ovulation. For this, three consecutive studies were carried out. The first study aimed to determine the effect of psychological stress induced change in cortisol level, behavioral parameters and normal estrous cyclicity. The effect on mRNA expression of SOD subtypes, follicular growth in histological sections of ovaries and the difference in oocyte quality and number, upon superovulation were assessed in the subsequent studies. The results indicate that psychological stress model causes an increase in cortisol level (p⩽0.05) with development of anhedonia, depression and anxiety. An irregular estrous cycle was observed in stressed mice with an upregulation in mRNA expression of SOD subtypes. Histological sections revealed an increase in atretic antral follicle with an impaired follicular development. Moreover, immature oocytes were obtained from superovulated stressed mice. The study concludes that psychological stress results in anovulation which may be due to increase in cortisol level and SOD activity in stressed mice.

Conditional inactivation of Upstream Binding Factor reveals its epigenetic functions and the existence of a somatic nucleolar precursor body

Upstream Binding Factor (UBF) is a unique multi-HMGB-box protein first identified as a co-factor in RNA polymerase I (RPI/PolI) transcription. However, its poor DNA sequence selectivity and its ability to generate nucleosome-like nucleoprotein complexes suggest a more generalized role in chromatin structure. We previously showed that extensive depletion of UBF reduced the number of actively transcribed ribosomal RNA (rRNA) genes, but had little effect on rRNA synthesis rates or cell proliferation, leaving open the question of its requirement for RPI transcription. Using gene deletion in mouse, we now show that UBF is essential for embryo development beyond morula. Conditional deletion in cell cultures reveals that UBF is also essential for transcription of the rRNA genes and that it defines the active chromatin conformation of both gene and enhancer sequences. Loss of UBF prevents formation of the SL1/TIF1B pre-initiation complex and recruitment of the RPI-Rrn3/TIF1A complex. It is also accompanied by recruitment of H3K9me₃, canonical histone H1 and HP1α, but not by de novo DNA methylation. Further, genes retain penta-acetyl H4 and H2A.Z, suggesting that even in the absence of UBF the rRNA genes can maintain a potentially active state. In contrast to canonical histone H1, binding of H1.4 is dependent on UBF, strongly suggesting that it plays a positive role in gene activity. Unexpectedly, arrest of rRNA synthesis does not suppress transcription of the 5S, tRNA or snRNA genes, nor expression of the several hundred mRNA genes implicated in ribosome biogenesis. Thus, rRNA gene activity does not coordinate global gene expression for ribosome biogenesis. Loss of UBF also unexpectedly induced the formation in cells of a large sub-nuclear structure resembling the nucleolar precursor body (NPB) of oocytes and early embryos. These somatic NPBs contain rRNA synthesis and processing factors but do not associate with the rRNA gene loci (NORs).

Upstream Binding Factor (UBF) is multi-HMGB-box protein found in all vertebrates. Although this protein has been implicated in transcription of the ribosomal RNA (rRNA) gene in vitro, little is known of its function in vivo. We previously found that UBF creates a nucleosome-like structure on DNA, and that this structure is remodeled by MAP-kinase phosphorylation. Using conditional gene deletion in mouse and mouse cells we show that UBF defines the active chromatin domains of the rRNA genes and is essential for transcription of these genes. Using this system we show that, contrary to expectation, rRNA gene activity does not coordinate ribosome production. We further show that in the complete absence of rRNA synthesis a somatic nucleolar precursor body is formed. Our data show that UBF determines a dynamic transition between the active and inactive rRNA gene states that is independent of changes in DNA methylation.

Oocyte differentiation is genetically dissociable from meiosis in mice

Oogenesis is the process by which ovarian germ cells undertake meiosis and differentiate to become eggs. In mice, Stra8 is required for the chromosomal events of meiosis to occur, but its role in differentiation remains unknown. Here we report Stra8-deficient ovarian germ cells that grow and differentiate into oocyte-like cells that synthesize zonae pellucidae, organize surrounding somatic cells into follicles, are ovulated in response to hormonal stimulation, undergo asymmetric cell division to produce a polar body and cleave to form two-cell embryos upon fertilization. These events occur without premeiotic chromosomal replication, sister chromatid cohesion, synapsis or recombination. Thus, oocyte growth and differentiation are genetically dissociable from the chromosomal events of meiosis. These findings open to study the independent contributions of meiosis and oocyte differentiation to the making of a functional egg.

Effect of preantral mouse follicle culture period on meiotic maturation and developmental competence of oocytes

Purpose

The aim of this study is to determine the optimal culture period for meiotic maturation and developmental competence of in vitro-grown mouse oocytes.

Methods

Early preantral follicles with diameter of 100-130 μm were collected mechanically from day 14 mouse ovaries and cultured for 8, 10, and 12 days. The diameters of follicles and oocytes were measured, and chromatin configuration in oocytes was observed. We also examined meiotic maturation by human chorionic gonadotropin (hCG)/epidermal growth factor (EGF) stimulation, developmental competence of fertilized oocytes to blastocysts, and apoptosis in blastocysts.

Results

The follicular diameter increased significantly from days 4 to 10, and the diameter of day 12 oocytes was significantly larger than day 8 or earlier oocytes. Chromatin configuration around the nucleolus was transformed from "nonsurrounded (immature)" to "surrounded (mature)" after 10 days. Furthermore, MII rate of day 10 and 12 oocytes was significantly higher than that of day 8 oocytes. The blastocyst rate of day 10 oocytes was higher than that of day 8 or 12 oocytes. The blastocyst apoptotic rate of day 12 oocytes was higher than that of day 10 oocytes.

Conclusions

Long culture periods of in vitro-grown oocytes affect meiotic maturation, developmental competence to blastocysts, and apoptosis.

The CDC14A phosphatase regulates oocyte maturation in mouse

The final steps of oogenesis occur during oocyte maturation that generates fertilization-competent haploid eggs capable of supporting embryonic development. Cyclin-dependent kinase 1 (CDK1) drives oocyte maturation and its activity and actions on substrates are tightly regulated. CDC14 is a dual-specificity phosphatase that reduces CDK1 activity and reverses the actions of CDK1 during mitosis. In budding yeast, Cdc14 is essential for meiosis, but it is not known whether its mammalian homolog CDC14A is required for meiosis in females. Here, we report that CDC14A is concentrated in the nucleus of meiotically incompetent mouse oocytes but is dispersed throughout meiotically competent oocytes. During meiotic progression CDC14A has no specific sub-cellular localization except between metaphase of meiosis I (Met I) and metaphase of meiosis II (Met II) when it co-localizes with the central portion of the meiotic spindle. Overexpression of CDC14A generally delays meiotic progression after resumption of meiosis whereas microinjection of oocytes with an antibody against CDC14A specifically delays exit from Met I. Each of these perturbations generates eggs with chromosome alignment abnormalities and eggs that were injected with the CDC14A antibody had an elevated incidence of aneuploidy. Collectively, these data suggest that CDC14A regulates oocyte maturation and functions to promote the meiosis I-to-meiosis II transition as its homolog does in budding yeast.

Essential CDK1-inhibitory role for separase during meiosis I in vertebrate oocytes

Separase not only triggers anaphase of meiosis I by proteolytic cleavage of cohesin on chromosome arms, but in vitro vertebrate separase also acts as a direct inhibitor of cyclin-dependent kinase 1 (Cdk1) on liberation from the inhibitory protein, securin. Blocking separase-Cdk1 complex formation by microinjection of anti-separase antibodies prevents polar-body extrusion in vertebrate oocytes. Importantly, proper meiotic maturation is rescued by chemical inhibition of Cdk1 or expression of Cdk1-binding separase fragments lacking cohesin-cleaving activity.

N-cadherin mediated cell contact inhibits germinal vesicle breakdown in mouse oocytes maintained in vitro

The effect of granulosa cell contact on the ability of zona-free oocytes to undergo germinal vesicle breakdown (GVBD) was assessed using a granulosa cell co-culture system. Oocytes contacted granulosa cells in a site-specific manner such that their GV was away from the granulosa cells. Also contact with granulosa cells reduced the percentage of oocytes undergoing GVBD from about 40% to 15%. GVBD was inhibited by contact with granulosa cells but not a granulosa cell-secreted product, since oocytes cultured in the same culture, that were adjacent to the granulosa cell monolayer underwent GVBD at the same rate as controls. Similarly, media collected from granulosa cell cultures did not attenuate the rate of GVBD. The ability of granulosa cell contact to inhibit GVBD was equal to that of db-cAMP. Moreover, the ability of granulosa cells to inhibit GVBD was not mimicked by spontaneously immortalized granulosa cells. This cell specificity appeared to be related to N-cadherin, since granulosa cells and oocytes express N-cadherin and a N-cadherin antibody attenuates the effect of granulosa cell contact. The mechanism through which N-cadherin mediated cell contact maintains meiotic arrest is unknown. It is possible that homophilic N-cadherin binding between the granulosa cells and oocyte acts through a junxtacrine mechanism, which in part may lead in the activation fibroblast growth factor (FGF) receptors that are expressed by the oocyte. The involvement of FGF receptors is supported by the observations that FGF and a N-cadherin peptide known to activate FGF receptors inhibit GVBD.

Studies on Ca2+-channel distribution in maturation arrested mouse oocyte

The present study was carried out to identify the existence of voltage-dependent Ca2+-channels (P/Q-, N-, and L-type) and their distributional differences in germinal vesicle (GV) and GV breakdown (GVBD)-arrested mouse oocytes which includes GVBD to telophase I of meiosis I and matured oocytes (MII, metaphase of meiosis II) by using the immunocytochemical method and a confocal laser scanning microscope. (1) Comparison between follicular oocytes (GV) and GV-arrested oocytes after 17 hr of in vitro culture. In follicular oocytes, P/Q-, N-, L (anti-alpha1C anti-alpha1D)-type Ca2+-channels showed both localized and uniform staining. In contrast, GV-arrested oocytes, after in vitro culture for 17 hr, showed no presence of Ca2+-channels in most oocytes. (2) Comparison between GVBD oocytes after culture in vitro for 3 hr and GVBD-arrested oocytes after culture in vitro for 17 hr. In GVBD oocytes, P/Q-, N-, L (anti-1C, anti-alpha1D)-type Ca2+-channels showed both localized and uniform staining. In contrast, in GVBD-arrested oocytes, none of the three types of Ca2+-channels were identified in 72-86% of oocytes. The present study demonstrates that in most GVBD-arrested oocytes that do not mature to MII, there is no Ca2+-channel identified. Therefore, most of the GVBD-arrested oocytes seem to have defects in Ca2+-channel expression/translation. Also, distributional changes of Ca2+-channels take place depending on the maturation progress in GV oocytes and MII stage oocytes (ovulated and 17 hr cultured MII stage oocytes). In addition, we found evidence that a functional voltage-dependent Ca2+-channel (L-type) exists in mouse oocytes (ovulated and cultured MII staged oocytes by a confocal laser scanning microscope).

Changes in the amount of cytoplasmic inclusions in mouse oocytes during meiotic maturation in vivo and in vitro

Background and aims: The changes in cytoplasmic inclusions during meiotic maturation have only been examined in porcine oocytes. In the present study, the amount and the number of cytoplasmic inclusions (glycogen granules, lipid droplets and fibrous structures) were examined in mouse oocytes in the process of in vivo and in vitro maturation. For those inclusions that changed in amount during maturation, we also examined their content in oocytes treated with olomoucine, an inhibitor of cyclin-dependent kinase, in order to clarify the relationship between nuclear maturation and changes in the inclusions. Methods: Nuclear maturation in the oocytes cultured for various periods and those collected from antral follicles and oviducts was examined after staining with aceto-orcein. For the demonstration of glycogen granules and lipid droplets, oocytes were stained with periodic acid-Schiff or Sudan IV. Fibrous structures in the oocytes were observed under an electron microscope. Results: The amount of glycogen granules, Sudanophilic lipid droplets and fibrous structures did not change in the oocytes matured in vivo and in vitro, whereas the number of the lipid droplets increased during maturation. In the oocytes treated with olomoucine, the resumption of nuclear maturation was inhibited, whereas the increase in the number of Sudanophilic lipid droplets was not inhibited. Conclusion: Present findings suggest that the increase in the number of Sudanophilic lipid droplets occurs in the cytoplasm of mouse oocytes during maturation, regardless of in vivo or in vitro maturation, and that such the change in the inclusion is not related to nuclear maturation. (Reprod Med Biol 2004; 3: 231-236).

Changes in the activities of hydroxysteroid dehydrogenases in mouse oocytes during meiotic maturation

The activities of hydroxysteroid dehydrogenases (HSDs) were histochemically demonstrated in mouse oocytes in the process of maturation in vivo and in vitro, and the changes in steroid metabolism during meiotic maturation and also the relationship between nuclear maturation and changes in steroid metabolism in the cytoplasm were examined. In mouse oocytes 0 h after human chorionic gonadotrophin (hCG) injection, the activities of Delta(5)-3beta-HSD (with DHA, pregnenolone and 17alpha-hydroxypregnenolone as the substrates), 17beta-HSD (estradiol-17beta and testosterone) and 20beta-HSD (17alpha-hydroxyprogesterone and 20beta-hydroxyprogesterone) were observed in 87 to 97% of those, but that of 20alpha-HSD (20alpha-hydroxyprogesterone) was not. The percentages of oocytes showing the activities of Delta(5)-3beta-HSD, 17beta-HSD and 20beta-HSD did not change during maturation in vivo or in vitro. Oocytes with 20alpha-HSD activity appeared 4 h after the hCG injection or after culture for 4 h and the rates of those reached 92 and 100%, respectively, 14 h after the hCG injection or after culture for 14 h. In oocytes cultured for 8 h with olomoucine or 3-isobutyl-1-methylxanthine, nuclei were almost all in the germinal vesicle stage, and activity of 20alpha-HSD was observed in 84 and 89% of the treated oocytes, respectively. On the other hand, 81% of control oocytes showed 20alpha-HSD activity, with no significant difference from the rate for the olomoucine- or 3-isobutyl-1-methylxanthine-treated oocytes. The present findings suggested that the metabolic abilities of progesterone, 17alpha-hydroxyprogesterone, 17alpha,20beta-dihydroxyprogesterone, 20beta-hydroxyprogesterone, androgen and estradiol-17beta in the cytoplasm are constantly present in mouse oocytes in the process of maturation in vivo and in vitro. The results also suggested that the metabolic ability of 20alpha-hydroxyprogesterone in mouse oocytes increases during maturation, but the change in the metabolic ability of such a steroid is not related to nuclear maturation.

Ca2+-promoted cyclin B1 degradation in mouse oocytes requires the establishment of a metaphase arrest

CDK1-cyclin B1 is a universal cell cycle kinase required for mitotic/meiotic cell cycle entry and its activity needs to decline for mitotic/meiotic exit. During their maturation, mouse oocytes proceed through meiosis I and arrest at second meiotic metaphase with high CDK1-cyclin B1 activity. Meiotic arrest is achieved by the action of a cytostatic factor (CSF), which reduces cyclin B1 degradation. Meiotic arrest is broken by a Ca2+ signal from the sperm that accelerates it. Here we visualised degradation of cyclin B1::GFP in oocytes and found that its degradation rate was the same for both meiotic divisions. Ca2+ was the necessary and sufficient trigger for cyclin B1 destruction during meiosis II; but it played no role during meiosis I and furthermore could not accelerate cyclin B1 destruction during this time. The ability of Ca2+ to trigger cyclin B1 destruction developed in oocytes following a restabilisation of cyclin B1 levels at about 12 h of culture. This was independent of actual first polar body extrusion. Thus, in metaphase I arrested oocytes, Ca2+ would induce cyclin B1 destruction and the first polar body would be extruded. In contrast to some reports in lower species, we found no evidence that oocyte activation was associated with an increase in 26S proteasome activity. We therefore conclude that Ca2+ mediates cyclin B1 degradation by increasing the activity of an E3 ubiquitin ligase. However, this stimulation occurs only in the presence of the ubiquitin ligase inhibitor CSF. We propose a model in which Ca2+ directly stimulates destruction of CSF during mammalian fertilisation.

Fate of the first polar bodies in mouse oocytes

Both nuclear transfer and intracytoplasmic sperm injection (ICSI) practice necessitates studies on the spatial relationship between the MII spindle and the first polar bodies (FPB). Although recent observations have shown that the FPB position does not predict accurately the location of the meiotic spindle in metaphase II oocytes of monkey, hamster, and human, detailed studies on FPB deviation and its affecting factors are lacking. Since polar bodies can be used for genetic testing and oocyte quality grading, their life span under different conditions should be studied. The timing of formation and degeneration and the position relative to the MII spindle of the FPB and the factors affecting FPB deviation and degeneration during in vivo and in vitro aging of both in vivo and in vitro matured mouse oocytes were investigated in this study. Mice of the Kun-ming breed were used, and the intact and degenerated FPB were identified through microscopic morphology in combination with propidium iodide (PI) exclusion test and the chromosomes visualized by Hoechst staining. Results are summarized as follows: (i) oocytes started FPB extrusion at 8 hr after the onset of in vivo or in vitro maturation, but the number of FPB reached maximum much later in vitro (14 hr of culture) than in vivo (10 hr post hCG). (ii) Some FPB began to degenerate before ovulation and around 70% became degenerated within 6 hr after maximal nuclear maturation both in vivo and in vitro; they disappeared faster during in vivo than in vitro aging but turned from intact to degenerated at a similar tempo. (iii) Some FPB began to deviate from the MII spindle 10 hr after hCG injection or in vitro culture and the distance between FPB and the spindle increased with time during both in vivo and in vitro aging. (iv) FPB deviated more slowly in the in vitro matured oocytes than in in vivo matured. (v) Denudation performed after FPB extrusion markedly enhanced its deviation. (vi) The perivitelline space (PVS) increased with time during maturation and aging in vivo and in vitro and the values of PVS and the percentages of FPB adjacent to the spindle were significantly negatively correlated. (vii) Cytochalasin B and colchicine had no effect on FPB deviation. (viii) None of the more than 3,500 FPBs observed was found to be dividing or have divided into two cells at any time points before or after ovulation or in vitro maturation. Our results were consistent with the possibility that the displacement of the FPB was a time- and PVS-dependent process, indicating that PVS would increase with time and its formation and enlargement would facilitate the lateral displacement of the degenerating FPB.

Kinetics of meiotic progression, M-phase promoting factor (MPF) and mitogen-activated protein kinase (MAP kinase) activities during in vitro maturation of porcine and bovine oocytes: species specific differences in the length of the meiotic stages

The kinetics of nuclear maturation, M-phase promoting factor (MPF) and mitogen-activated protein kinase (MAP kinase) activities during in vitro maturation of porcine and bovine oocytes were examined. A further objective was to determine the duration of the meiotic stages during the maturation process. Porcine and bovine cumulus-oocyte complexes (COCs) were incubated in TCM 199 supplemented with 20% (v/v) heat inactivated fetal calf serum (FCS), 0.05microg/ml gentamycin, 0.02mg/ml insulin, 2.5microg/ml FSH and 5microg/ml LH. COCs were removed from the culture media in hourly intervals starting immediately after recovery from the follicle until 24 (bovine) or 48h (porcine) of culture. Oocytes were either fixed to evaluate the maturation status or the activity of MPF, assessed by its histone H1 kinase activity, and MAP kinase were determined by a radioactive assay simultaneously. In oocytes of both species, the MPF activity oscillated during the culture period with two maxima corresponding with the two metaphases: between 27-32 and after 46h (porcine) and between 6-9 and after 22h (bovine). There was a temporary decline in activity after 33-38 (porcine) and after 19h (bovine), which corresponded with anaphase I and telophase I. MAP kinase activity increased during the whole culture period and reached maximum levels after 47 (porcine) and after 22h (bovine). In porcine oocytes, the MAP kinase was activated before GVBD and MPF activation. In bovine oocytes, MPF and MAP kinase were activated at approximately the same time as the GVBD (8-9h of incubation). In average porcine, oocytes remain 23.4h in the germinal vesicle (GV) stage (13h in GV I, 5.7h in GV II, 3.2h in GV III and 1.5h in GV IV), 0.9h in diakinese, 9.6h in the metaphase I, 2.8h in anaphase I and 1.9h in telophase I of the first meiotic division. In bovine oocytes, the temporal distribution of the meiotic stages were 8.5h for the GV stage, 1.2h for diakinese, 8.3h for metaphase I, 1.6h for anaphase I and 1.9h for telophase I. These results indicate that the duration of the meiotic stages differs between the species and that MAP kinase is activated before MPF and GVBD in porcine oocytes.

Confocal microscopy of germinal vesicle-stage equine oocytes

The objectives were to compare cumulus type with nucleus form in equine cumulus oocyte complexes (COCs), to define the percentage of germinal vesicle (GV)-stage oocytes within a population of mares, and to further define GV nucleus shapes of equine oocytes. Cumulus types were as follows: 1) compact (56/208, 26.9%), 2) slightly expanded (37/208, 17.8%), 3) moderately expanded (27/208, 13.0%), 4) greatly expanded (15/208, 7.2%), or 5) denuded (73/208, 35.1%). One hundred thirty of 208 COCs (62.5%) were GV-stage, 21/208 (10.1%) were condensed chromatin-stage, 8/208 (3.8%) were polar body-stage, 40/208 (19.2%) were negative (nonstaining), and 9/208 (4.3%) were fragmented. Cumulus types were associated with nucleus forms because higher proportions (P < 0.05) of GV-stage oocytes occurred in compact (42/56, 75.0%), slightly expanded (30/37, 81.1%), moderately expanded (16/27, 59.3%), or denuded (40/73, 54.8%) COCs than in greatly expanded (2/15, 13.3%) COCs. In contrast, lower proportions (P<0.05) of condensed chromatin-stage oocytes occurred in compact (3/56, 5.4%), slightly expanded (0/37, 0.0%), moderately expanded (3/27, 11.1%) or denuded (9/73, 12.3%) COCs than in greatly expanded (6/15, 40.0%) COCs, and lower proportions (P < 0.05) of polar body-stage oocytes occurred in compact (0/56, 0.0%) or denuded (2/73, 2.7%) COCs than in greatly expanded (3/15, 20.0%) COCs. Germinal vesicle-stage equine oocytes had 4 distinct shapes, with higher proportions (P<0.05) having large-regular (54/130, 41.5%) than scattered (10/130, 7.7%), small-round (29/130, 22.3%), or large-irregular (37/130, 28.5%) shapes. Lower proportions (P<0.05) of large-regular GVs occurred in compact (11/42, 26.2%) COCs than in slightly expanded (15/30, 50.0%), or moderately expanded (12/16, 75.0%) COCs. Therefore oocytes with the large-regular GV shape are probably more advanced in development.

Fator de crescimento derivado das plaquetas, retinol e insulina na regulação da maturação nuclear de oócitos bovinos e suas conseqüências no desenvolvimento embrionário

O objetivo deste trabalho foi verificar as ações do fator de crescimento derivado das plaquetas (PDGF; P), da insulina (I), do retinol (R) e de suas associações (PI, PIR, IR e PR) na maturação nuclear (MN) de oócitos bovinos e suas conseqüências no desenvolvimento embrionário (DE). O meio básico para maturação dos oócitos nos diferentes tratamentos foi o TCM-199 modificado acrescido de PVA (controle). No DE, foram utilizados os grupos R, PIR, IR, um controle negativo (PVA) e um controle positivo, contendo soro fetal bovino e gonadotrofinas (SFBHOR). Os fatores P, I, R e suas associações não aceleraram a MN em 7h mas sim após 18h (P<0,001), com exceção dos tratamentos R e PR, nos quais as percentagens de metáfase II foram, respectivamente, de 4,7% e 8,3%, similares à obtida no grupo-controle (0,0%). Considerando um nível de significância de P<0,0001 em comparação ao grupo-controle, os maiores índices de metáfase II foram obtidos na presença das associações IR (19,0%) e PIR (21,3%). No DE, R (18,3%), PIR (13,9%) e IR (10,6%) incrementaram os índices de clivagem, comparados ao PVA (0,0%; P<0,001), porém não atingiram os índices do grupo SFBHOR (53,8%; P<0,001). Conclui-se que insulina e PDGF aceleram a MN e suas ações são potencializadas pelo retinol. Os índices de clivagem de oócitos maturados na presença de R, IR e PIR são superiores aos do PVA, mas significativamente inferiores aos maturados em SFBHOR.

A role for the MEK-MAPK pathway in okadaic acid-induced meiotic resumption of incompetent growing mouse oocytes

Fully grown competent mouse oocytes spontaneously resume meiosis in vitro when released from their follicular environment, in contrast to growing incompetent oocytes, which remain blocked in prophase I. The cell cycle regulators, maturation promoting factor (MPF; [p34(cdc2)/cyclin B kinase]) and mitogen-activated protein (MAP) kinases (p42(MAPK) and p44(MAPK)), are implicated in meiotic competence acquisition. Incompetent oocytes contain levels of p42(MAPK), p44(MAPK), and cyclin B proteins that are comparable to those in competent oocytes, but their level of p34(cdc2) is markedly lower. Okadaic acid (OA), an inhibitor of phosphatases 1 and 2A, induces meiotic resumption of incompetent oocytes. The kinetics and the percentage of germinal vesicle breakdown depends on whether or not oocytes have been cultured before OA treatment. We show that the fast kinetics and the high percentage of germinal vesicle breakdown induced by OA following 2 days in culture is neither the result of an accumulation of p34(cdc2) protein, nor to the activation of MPF in incompetent oocytes, but rather by the premature activation of MAP kinases. Indeed, a specific inhibitor of MAPK kinase (MEK) activity, PD98059, inhibits activation of MAP kinases and meiotic resumption. Altogether, these results indicate that the MEK-MAPK pathway is implicated in OA-induced meiotic resumption of incompetent mouse oocytes, and that the MEK-MAPK pathway can induce meiotic resumption in the absence of MPF activation.

Functional and molecular reorganization of the nucleolar apparatus in maturing mouse oocytes

In mammalian preovulatory oocytes, rRNA synthesis is down-regulated until egg fertilization and zygotic genome reactivation, but the underlying regulatory mechanisms of this phenomenon are poorly characterized. We examined the molecular organization of the rRNA synthesis and processing machineries in fully grown mouse oocytes in relation to ongoing rDNA transcription and oocyte progression throughout meiosis. We show that, at the germinal vesicle stage, the two RNA polymerase I (RNA pol I) subunits, RPA116 and PAF53/RPA53, and the nucleolar upstream binding factor (UBF) remain present irrespective of ongoing rDNA transcription and colocalize in stoichiometric amounts within discrete foci at the periphery of the nucleolus-like bodies. These foci are spatially associated with the early pre-rRNA processing protein fibrillarin and in part with the pre-ribosome assembly factor B23/nucleophosmin. After germinal vesicle breakdown, the RNA pol I complex disassembles in a step-wise manner from chromosomes, while UBF remains associated with chromosomes until late prometaphase I. Dislodging of UBF, but not of RNA pol I, is impaired by the phosphatase inhibitor okadaic acid, thus strengthening the idea of a relationship between UBF dynamics and protein phosphorylation. Since neither RNA pol I, UBF, fibrillarin, nor B23 is detected at metaphase II, i.e., the normal stage of fertilization, we conclude that these nucleolar proteins are not transported to fertilized eggs by maternal chromosomes. Together, these data demonstrate an essential difference in the dynamics of the major nucleolar proteins during mitosis and meiosis.

Effect of protein synthesis inhibition before or during in vitro maturation on subsequent development of bovine oocytes

The overall objective of this study was to assess the effect of maintaining meiotic arrest in bovine oocytes in vitro on developmental competence. In Experiment 1 the effect of inhibition of meiotic resumption using cycloheximide (CX), on subsequent was examined. Immature cumulus oocyte complexes (COCs, n = 804) were cultured in the absence (24 h) or presence of CX for 6, 12, 18 or 24 h. The control was inseminated 24 h later, while CX-treated oocytes were cultured for a further 24 h before insemination. In Experiment 2 the effect of exposing the oocyte (n = 1239) during meiotic arrest to putative stimulatory substances (pFSH and FCS) was examined. In Experiment 3, to study the importance of protein synthesis during maturation, synthesis was blocked for a 6-h period at various times (6, 12, 18 h) after start of culture (n = 1117). In Experiment 1, there was no difference in cleavage rate between treatments. However, the percentage of 5 to 8 cell embryos at 72 h post insemination was significantly lower after CX treatment (64 vs 42 to 51%; P < 0.05). This was reflected in a lower rate of blastocysts at Day 6 (9 to 15 vs 31%, P < 0.002). While the blastocyst rate at Day 8 was lower in CX-treated oocytes, the effect was only significant when CX was present for longer than 12 h. A marked decrease in development was noted following inhibition for 18 h or more compared with the control (17 to 19 vs 40%; P < 0.0002). In Experiment 2, addition of either FSH or FCS to oocytes in the presence of CX had no effect on any of the parameters studied, even though there was a positive effect in control oocytes. In Experiment 3, treatment with CX after the oocytes had matured for varying periods resulted in decreased blastocyst rates at Days 6 and 8 of culture. The most significant drop in development occurred when oocytes were cultured for 12 h before exposure to CX (15 vs 40%; P < 0.0001). In conclusion, CX-blocked oocytes retained their developmental competence, although final blastocyst yields were reduced.

Haploid maternal genome derived from early diplotene oocytes can substitute for the female pronucleus in preimplantation mouse development

We describe the preimplantation development of mouse embryos that have received the haploid maternal genome derived from early diplotene nuclei of primordial oocytes (PO). Two generations of recipient egg-cells were used. Induction of two meiotic divisions of the PO nucleus and the reduction of the number of chromosomes to the haploid level were achieved in preovulatory oocytes (primary recipients). The developmental potential of the obtained haploid genome was examined in zygotes (secondary recipients). The nuclei of PO obtained from newborn mice were transferred by cell electrofusion to in vitro maturing (IVM) and enucleated preovulatory mouse oocytes. The reconstructed oocytes which had completed maturation, i.e., reached metaphase II, were artificially activated (8% ethanol + CHX). Activated oocytes were used as donors of haploid pronuclei of PO origin which were transferred (by karyoplast fusion) to partially enucleated zygotes containing only the male pronucleus. Thus, reconstituted zygotes were transplanted to the ligated oviducts of the cycling mice and 27% of them developed to the blastocyst stage. Our experiments demonstrate that 1) the nucleus of PO can be induced to premature meiotic divisions in an IVM enucleated preovulatory oocyte; 2) in the presence of a normal male pronucleus, the haploid pronucleus of PO origin can substitute for a female pronucleus during preimplantation development.

Culture conditions affect meiotic regulation in cumulus cell-enclosed mouse oocytes

To test the hypothesis that culture conditions influence meiotic regulation in mouse oocytes, we have examined the effects of six culture media, four organic buffers, and pH on spontaneous maturation, the maintenance of meiotic arrest and ligand-induced maturation in cumulus cell-enclosed oocytes from hormonally primed immature mice. The media tested were Eagle's minimum essential medium (MEM), Ham's F-10 (F-10), M199, M16, Waymouth's MB 752/1 (MB 752/1), and Leibovitz's L-15 (L-15). All six media supported > or = 94% spontaneous germinal vesicle breakdown (GVB) during a 17-18 hr incubation period, but polar body formation was lower in M199 and MB 752/1 than in the other media. The incidence of polar bodies could be increased in these two media by the addition of pyruvate. With the exception of M16 and MB 752/1, 4 mM hypoxanthine maintained a significant number of cumulus cell-enclosed oocytes in meiotic arrest. Inhibition could be restored by the addition of glutamine to M16 and pyruvate to MB 752/1. Follicle-stimulating hormone (FSH) and epidermal growth factor (EGF) stimulated GVB in those media in which hypoxanthine was inhibitory. dbcAMP was able to maintain meiotic arrest in all of the media, but was least effective in M16. FSH stimulated GVB in all dbcAMP-arrested groups except L-15, and FSH became stimulatory in L-15 when the pyruvate level was reduced to 0.23 mM and galactose was replaced with 5.5 mM glucose. When MEM was buffered principally with the organic buffers MOPS, HEPES, DIPSO, or PIPES (at 20 mM), high frequencies of GVB and polar body formation were observed in inhibitor-free medium. dbcAMP suppressed GVB in all groups; hypoxanthine also maintained meiotic arrest in all buffering conditions, although this effect was nominal in PIPES-buffered medium. FSH and EGF stimulated GVB in all dbcAMP- and hypoxanthine-treated groups. When the concentration of HEPES was increased from 20 mM to 25 mM, a more pronounced suppressive effect on maturation in both dbcAMP- and hypoxanthine-supplemented groups was observed in the absence of FSH. But whereas HEPES reduced the induction of maturation by FSH in dbcAMP-arrested oocytes, this buffer had no effect on FSH action in hypoxanthine-treated oocytes. When MEM was buffered with HEPES and the pH was adjusted to 6.8, 7.0, 7.2, or 7.4, a dramatic effect of pH on meiotic maturation was observed. pH had no significant effect on hypoxanthine salvage by oocyte-cumulus cell complexes, but FSH-induced de novo purine synthesis was significantly augmented by increased pH, in parallel with increased induction of GVB. The results of this study demonstrate that the use of different culture media, or minor changes in culture conditions, can lead to significant variation in (1) the spontaneous maturation of oocytes, (2) the ability of meiotic inhibitors to suppress GVB, or (3) the efficacy of meiosis-inducing ligands. Furthermore, such observations provide a unique opportunity to examine specific molecules and metabolic pathways that can account for this variation and thereby gain valuable insights into the mechanisms involved in meiotic regulation.

Maturation of hamster oocytes under chemically defined conditions and sperm penetration through the zona pellucida

This study aimed to achieve high frequencies of nuclear maturation and penetrability through the zona pellucida of hamster oocytes cultured under protein-free conditions. Completion of nuclear maturation by cumulus-intact, immature oocytes (79% metaphase II stage) was depressed (37%; p < 0.05) by adding four amino acids (glutamine, isoleucine, methionine and phenylalanine) reported necessary for nuclear maturation of cumulus-free oocytes. Following in vitro maturation, cumulus cells were removed and oocytes were inseminated with capacitated sperm, but after 6 h sperm:egg co-incubation, only 24% of in vitro matured oocytes were penetrated compared with 60% of in vivo matured oocytes (p < 0.05). Time required for zona lysis by alpha-chymotrypsin was not significantly different among in vitro and in vivo matured oocytes and 1-cell embryos. Addition to the maturation medium of soybean trypsin inhibitor or fetuin, both known to inhibit the zona reaction in vitro, did not improve penetrability of in vitro matured oocytes, implying that in hamsters, unlike other rodent species, a premature zona reaction is unlikely to be responsible for inhibiting sperm penetration. When oocytes were incubated with 20% periovulatory oviductal fluid (OF) for another 3 h after maturation, penetration was significantly improved (60% vs 37% with and without OF, respectively; p < 0.05), but was not equivalent to penetration of in vivo matured follicular oocytes similarly treated with OF (84%, p < 0.05). However, zona penetration was further improved by increasing sperm concentration from 1.0 x 10(4) (66%) to 5.0 or 10.0 x 10(4) sperm/ml (89%, p < 0.05). This study shows that nuclear maturation of hamster oocytes can occur in chemically defined medium, and indicates that a deficiency in the zona of in vitro matured oocytes can be overcome by preincubation with OF and insemination at high sperm concentration.

Acute exposure of female hamsters to carbendazim (MBC) during meiosis results in aneuploid oocytes with subsequent arrest of embryonic cleavage and implantation

A single oral dose of the fungicide and microtubule poison, MBC, administered to female hamsters at proestrus, results in infertility and early pregnancy loss (1). To characterize the site and mode of action of this effect, direct assessments of oocyte chromosomes, fertilization, and preimplantation embryo development were made. Female hamsters were given a single dose of MBC (1,000 mg/kg) on the afternoon of proestrus (to coincide with meiotic maturation of the oocytes) and either killed shortly after ovulation (day 1) to recover oocytes, or bred and killed on gestation day (gd) 1 to 5 of pregnancy to assess fertilization and preimplantation embryo development and enumerate early implantation sites. Chromosome analysis in unfertilized oocytes revealed an MBC-induced increase in aneuploidy (37 vs. 14% in controls). When animals were bred after dosing, MBC had no effect on the number of oocytes recovered or fertilized. However, significant increases were found in the proportion of embryos that failed to reach the expected stage of development, namely, the eight-cell stage on the afternoon of gd 3, the morula stage by the morning of gd 4, and the blastocyst stage by the afternoon of gd 4 (a time when some embryos have implanted). The mean number of implantation sites, revealed by Evans Blue staining, was also significantly lower in treated females on the afternoon of gd 4 and the morning of gd 5. These simple direct assessments elucidated a mechanism of MBC-induced early pregnancy loss, induction of aneuploidy in oocytes. They also ruled out an effect on fertilization, but demonstrated a subsequent arrest of preimplantation embryonic development accompained by a decrease in the likelihood of implantation.

First meiosis of early dictyate nuclei from primordial oocytes in mature and activated mouse oocytes

Nuclei of diplotene (dictyate) primordial oocytes (PO) were transferred to metaphase II oocytes and to activated mouse oocytes using cell fusion techniques. In a metaphase II oocyte, the PO nucleus condenses within 2-3 h to bivalents which become arranged on the first meiotic spindle. After oocyte activation, homologous chromosomes segregate between the oocyte and the first polar body, and a diploid pronucleus-like nucleus reforms from the one set of dyads. This nucleus condenses in the first embryonic mitosis into 40 'somatic' chromosomes which coexist in the common metaphase plate with 20 somatic chromosomes originating from the female pronucleus. Shortening of the time between fusion and activation of about 1 h prevents bivalent differentiation. The PO nucleus condenses only partially and reforms, after oocyte activation, a pronucleus-like nucleus. This nucleus gives rise at the first embryonic mitosis to 20 bivalents which coexist with 20 somatic chromosomes originating from the female pronucleus. A PO nucleus introduced into an activated egg completes the first cell cycle as an intact interphase nucleus. It never condenses in the first embryonic mitosis into bivalents, and undergoes only initial condensation (preceding bivalent differentiation). These results indicate that: (1) condensation into bivalents, meiotic spindle formation and first meiotic division can be greatly accelerated by the introduction of an early diplotene (dictyate) oocyte nucleus into a metaphase II oocyte, and (2) depending on whether the diplotene nucleus enters the first embryonic (mitotic) cell cycle after just initiating or after completing the first meiosis, it gives rise at the first cleavage division to meiotic (bivalents) or 'somatic' chromosomes respectively.

Pattern and frequency of nocodazole induced meiotic nondisjunction in oocytes of mice carrying the 'tobacco mouse' metacentric Rb(16.17)7Bnr

Oocytes from (C3H/HeH x 101/H)F1 and Rb(16.17)7Bnr homozygous females were exposed to a range of doses of nocodazole in vitro. The spindle poison caused a dose dependent increase in metaphase I (MI) arrest and hyperploidy. A concentration of 0.03 microgram/ml was found to induce a maximum hyperploid frequency of 3.1% and 11.6% respectively without a high level of MI arrest. Between 0.03 and 0.05 microgram/ml MI arrest increased substantially and reached a frequency of approximately 90%. In a further experiment oocytes from Rb7 homozygous, heterozygous and 3H1 females were exposed to 0.03 microgram/ml nocodazole 4, 6 or 8 h after the onset of maturation. The phase at which the spindle was inhibited resulted in a specific pattern of nondisjunction which in turn was dependent on whether the female carried an Rb metacentric. 3H1 oocytes gave a normally distributed pattern of increase in aneuploid frequency (over the spontaneous value) centering around a 6 h application. This was thought to be due to the interaction of chromosomes with the microtubules of the spindle during attachment and/or alignment. In contrast both Rb homozygotes and heterozygotes gave the same biphasic response, with a high frequency of aneuploidy in the oocytes when nocodazole was applied 4 and 8 h after the onset of maturation. In Rb homozygotes we demonstrated that the Rb bivalent underwent nondisjunction more frequently than the average acrocentric, when nocodazole was administered early.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium and meiotic maturation of the mammalian oocyte

The role of calcium in the regulation of both the meiotic and mitotic cell cycles has been the subject of considerable investigation in the nonmammalian field. In contrast, the mechanisms for signalling meiotic maturation in the mammalian oocyte are not as well documented nor as clearly defined. In the mammalian oocyte, calcium is associated with both spontaneous and hormone-induced meiotic maturation. A transient release of endogenously stored calcium precedes germinal vesicle breakdown and can override cyclic AMP maintained meiotic arrest; it thus may signal the resumption of meiosis. Additionally, extracellular calcium is apparently required for meiotic progression past metaphase I. The time sequence for meiotic resumption and progression is very varied between species. The timing of cell cycle protein synthesis during meiosis suggests that cyclins may be expressed in oocytes of some species much earlier in their development than in others. A generic model is proposed for the mechanism for triggering meiotic resumption in the mammalian oocyte. In this model, the critical components of meiotic resumption involve the temporal relationship of cyclin synthesis and the subsequent activation of the MPF complex by the calcium signal generated, which accounts for differences among species.

Pregnancies established from water buffalo () blastocysts derived from in vitro matured, in vitro fertilized oocytes and co-cultured with cumulus and oviductal cells

Buffalo ovaries were collected immediately after slaughter and were transported to laboratory in sterile saline at 37 degrees C. Follicular oocytes with the cumulus mass aspirated from 2 to 6 mm in diameter follicles were cultured in TCM-199 medium supplemented with 10% buffalo estrus serum (BES) in 5% CO(2) at 38.5 degrees C. After 20 to 24 h of incubation, the oocytes were inseminated with precapacitated frozen thawed spermatozoa for 6 h. The fertilization rate was 78.15% of the matured oocytes. Over an in vitro culture period of 3 to 9 d, 4.02% of the inseminated oocytes developed to the morula stage when cultured with cumulus cells alone and 17.83% when cumulus cells plus oviductal epithelial cells were used. The percentage of developed blastocysts was very low (0.57%) when the oocytes were co-cultured with cumulus cells from the original oocytes. However, 8% of the inseminated oocytes that were denuded 3 d after insemination developed to the blastocyst stage when they were co-cultured with cumulus and oviductal epithelial cells. Sixteen early/expanded blastocysts were transferred non-surgically to 16 recipients. Four of the 16 recipients became pregnant, of which 2 delivered normal buffalo male calves.

Biochemical studies on goat oocytes: Timing of nuclear progression, effect of protein inhibitor and pattern of polypeptide synthesis during in vitro maturation

Temporal progression of nuclear events of goat oocytes matured in vitro was studied by adding a specific inhibitor to the culture medium at different time points, to investigate protein synthesis requirements and its pattern during in vitro maturation. Goat cumulus-oocyte complexes (COCs) were matured in vitro in TCM 199, fixed at different time intervals and stained with orcein to assess nuclear changes. The germinal vesicle (GV) stage was found to be present at 0 h, chromosomal condensation stage was observed at 8 h, metaphase I at 12 to 14 h, and metaphase II was begun after 16 h of maturation and was nearly completed at 24 h. Protein synthesis inhibitor, cycloheximide, blocked oocyte maturation at germinal vesicle breakdown(GVBD), if added to the maturation medium between 0 to 4 h, suggesting that protein synthesis is required for GVBD. The transition from metaphase I to metaphase II was also protein synthesis-dependent, as observed when cycloheximide was used between 8 to 10 h of culture. When cycloheximide was added from 12 h of culture onwards, nuclear progression to metaphase II was progressively restored, but many chromosomal abnormalities were noted. Changes in the protein synthesis pattern were studied by radiolabeling of oocytes with [(35)S]-methionine at 0, 7, 12 and 24 h of culture, corresponding with GV, GVBD, metaphase I and metaphase II stages. A polypeptide of 28.1 KDa appeared as a major band at the GV stage, and its size decreased greatly and disappeared after the GVBD stage. Three new polypeptides (35, 36.5 and 39 KDa) appeared at GVBD and were detectable at metaphase II. In conclusion, the synthesis of proteins is required for the maintenance and transition of goat oocytes from GV to metaphase II during in vitro maturation.

Variation of mouse oocyte sensitivity to griseofulvin-induced aneuploidy and meiotic delay during the first meiotic division

The effects of varying the time of chemical treatment on the induction of aneuploidy and meiotic delay in metaphase II (MII) oocytes were studied by administering 1,500 mg/kg griseofulvin (GF) at 0, 2, 4, 6, or 8 hr after an injection of human chorionic gonadotrophin (HCG). The results show that the oocytes have a different sensitivity to GF-induced aneuploidy and meiotic delay during the course of meiotic maturation. Although not restricted to a particular period of meiotic maturation, the frequency of aneuploidy was highest (P < 0.05) when GF was given at 2, 4, or 6 hr after HCG. The maximum frequency of hyperploidy (42.4%) occurred at the 4-hr treatment time. Also, GF treatment resulted in the induction of meiotic delay as demonstrated by ovulated metaphase I (MI) and polyploid MII oocytes. The meiotic delay data depict a period of relative resistance between two periods of sensitivity in that the percentages of ovulated MI oocytes were 53.3, 21.3, 3.5, 6.7, and 25.7 when GF was given at 0, 2, 4, 6, and 8 hr after HCG, respectively. Also, at these treatment times the percentages of polyploid oocytes were 0.6, 1.7, 7.7, 20.1, and 15.4, respectively. Therefore, the oocytes seem to be more sensitive to GF-induced meiotic delay during the periods preceding and following meiotic spindle assembly. In conclusion, the results demonstrate that the time of chemical treatment influences the frequency of aneuploidy and the degree of meiotic delay. Also, the results emphasize that to thoroughly characterize the aneugenic potential of a specific chemical several treatment times may be needed.

Competent mouse oocytes isolated from antral follicles exhibit different chromatin organization and follow different maturation dynamics

After labelling DNA with the specific vital fluorophore Hoechst 33342, oocytes, isolated by puncture from antral follicles in adult mice, have two essentially different configurations of their nuclear fluorescence images. These have been called SN (where the nucleolus is surrounded by chromatin) and NSN (where the nucleolus is not surrounded by chromatin). Intermediate configurations are also found, although with a lower frequency. The proportion of each class is on the average equal and depends neither on the presence of cumulus cells nor on the age of the mouse. Electron microscopy confirms several ultrastructural differences between these two nuclear configurations, namely, the structure of the nucleolus, which is vacuolated in NSN-type and compact in SN-type oocytes. Using video-enhanced fluorescence microscopy at low level of excitation light, we could follow directly in vitro the meiotic maturation of both classes, without impairing their viability. We show that in germinal vessicle (GV) state, the chromatin does not change from one configuration into the other and that both classes are able to mature to metaphase II, although the maturation has slightly different characteristics.

Electrical maturation of the murine oocyte: an increase in calcium current coincides with acquisition of meiotic competence

We have used the whole-cell recording technique to compare three stages of primary and secondary oocytes from F1 hybrid mice (C57BL/6J x SJL/J): neonatal germinal vesicle (NGV) stage primary oocytes from 10- to 20-day-old, prepubescent mice; mature germinal vesicle (MGV) stage primary oocytes from 12-week-old, post-pubescent, superovulated mice; first polar body (FPB) stage secondary oocytes from 12-week-old, post-pubescent mice during the normal oestrus cycle or following superovulation. NGV, MGV and FPB oocytes all exhibit two voltage-dependent currents: an inward, rapidly activating/inactivating current, and an outward, slowly activating/non-inactivating current. In 1.5 mmol/l external Ca the average peak inward current is -2.9, -12.4 and -13.8 microA/cm2 in NGV, MGV and FPB oocytes, respectively. In 20 mmol/l Ca these currents increase and the reversal potential shifts to the right. The outward current decreases slightly with growth and development: at 40 mV test potentials, NGV oocytes have average outward currents of 8.9 microA/cm2, and MGV and FPB oocytes have currents of 5.0 and 5.5 microA/cm2, respectively. Thus, MGV oocytes express FPB current patterns. The reversal potentials, kinetics and pharmacology of the currents indicate that Ca channels carry the inward current and K channels carry the outward current. During growth in vivo a gradual depolarisation accompanies maturation. Resting potentials ranged from -45 to -30 mV in NGV oocytes to -35 to -17 mV in MGV oocytes to -20 mV to -3 mV in FPB oocytes. These data suggest that a selective increase occurs in the number of Ca channels during oocyte growth. This increase precedes nuclear maturation and coincides with the acquisition of meiotic competence.

Parental methylation patterns of a transgenic locus in adult somatic tissues are imprinted during gametogenesis

The methylation status of a mouse metallothionein-I/human transthyretin fusion gene was studied during gametogenesis in transgenic mice. In the adult tissues of this mouse line, the promoter region of the transgene on chromosome 11 is methylated when it is maternally inherited and undermethylated when it is paternally inherited. Germ cells from various developmental stages of gametogenesis were isolated, and their DNAs were assayed using methylation-sensitive restriction endonucleases and the polymerase chain reaction. Only low to nonexistent levels of transgene methylation were detected in germ cells from 14.5-day-old male and female fetuses irrespective of the parental origin of the transgene. This undermethylated state persisted in oocytes from newborn females as well as in testicular spermatogenic cells and sperm. By contrast, the transgene promoter was completely methylated in fully grown oocytes arrested at the first meiotic prophase. The endogenous metallothionein-I gene promoter, located on a different chromosome, remained undermethylated at all stages examined, consistent with previous findings reported for a typical CpG island. Taken together, the results suggest that parental-specific adult patterns of transgene methylation are established during gametogenesis.

Cell cycle progression of parthenogenetically activated mouse oocytes to interphase is dependent on the level of internal calcium

Nuclear maturation of the mouse oocyte becomes arrested in metaphase of the second meiotic division (MII). Fertilization or parthenogenetic activation induces meiotic completion, chromosomal decondensation and formation of a pronucleus. This completion of meiosis is probably triggered by a transient increase in cytosolic calcium ions. When activated just after ovulation by a low concentration of the calcium ionophore A23187, the majority of the mouse oocytes go through a metaphase to anaphase transition and extrude their second polar body but they do not proceed into interphase; instead their chromatids remain condensed and a microtubular metaphase spindle reforms (metaphase III). However, a high percentage of these oocytes will undergo a true parthenogenetic activation assessed by the formation of a pronucleus, when exposed to a higher concentration of the calcium ionophore. The capacity of the mouse oocyte to pass into metaphase III is lost with increasing time post-ovulation. Direct measurement of intracellular calcium with Fura-2 reveals higher levels of cytosolic calcium in aged oocytes and/or using higher concentrations of calcium ionophore for activation. It is concluded that the internal free calcium level determines the transition to interphase.

Use of the fungicide carbendazim as a model compound to determine the impact of acute chemical exposure during oocyte maturation and fertilization on pregnancy outcome in the hamster

Early pregnancy loss due to acute chemical exposure is difficult to detect and essentially impossible to characterize in humans. Here we use a hamster animal model to identify early pregnancy loss due to an acute chemical exposure to the female during the perifertilization interval. The fungicide carbendazim (methyl 1H-benzimidazole-2-carbamate), a microtubule poison with antimitotic activity, was selected as a model compound because it would be expected to perturb microtubule-dependent events occurring in the oocyte during meiotic maturation and fertilization. Such effects would likely lead to aneuploidy in the zygote with subsequent early pregnancy loss. Female hamsters were given a single oral dose of carbendazim during meiosis I (the afternoon of proestrus) prior to breeding, or during meiosis II (the morning of estrus) following overnight breeding. Pregnancy outcome was assessed on Day 15 (the afternoon before parturition). When given during during meiosis I, carbendazim treatment (750 or 1000 mg/kg body weight) significantly reduced the percentage of pregnant hamsters. In those animals that became pregnant, the average number of live pups was significantly lower at all dosages of carbendazim used (250, 500, 750, and 1000 mg/kg), an effect attributable to both preimplantation and early postimplantation losses. When given early on the morning of estrus, shortly before and during fertilization (0500 or 0600 hr), carbendazim treatment (1000 mg/kg) produced a similar decrease in litter size. This effect disappeared when carbendazim was administered at a slightly later time (0800 or 0900 hr), after the microtubule-dependent events of fertilization have occurred. These results demonstrate that a single exposure to a microtubule poison such as carbendazim at critical times, coincident with microtubule-dependent meiotic events, can result in very early pregnancy loss. Such loss was readily measurable in this animal model and serves as the basis for further mechanistic studies which would be impossible to conduct in humans.

Griseofulvin-induced aneuploidy and meiotic delay in female mouse germ cells. I. Cytogenetic analysis of metaphase II oocytes

Griseofulvin (GF) was tested in female mouse germ cells for the induction of aneuploidy and meiotic arrest. Superovulated mice were orally treated with 200, 666, 1332 or 2000 mg/kg in olive oil at the time of human chorionic gonadotrophin (HCG) injection and were sacrificed 18 h later. A dose-dependent increase in the frequency of metaphase I (M I) arrested oocytes was observed (maximum of 70%). Aneuploidy was not significantly induced. Also, the kinetics of meiotic progression up to the metaphase II (M II) stage was studied in untreated mice in order to correlate the time of treatment with the time of the first meiotic division. The results demonstrate that the majority of cells was treated with GF approximately 8 h before the M I stage. A second series of experiments were performed to test GF effects at a different treatment time. Doses of 200, 666 or 2000 mg/kg were administered 2 h post HCG. As in the first series of experiments, the animals were sacrificed 18 h post HCG. The results, compared with those obtained in the first experimental series, showed an inverse trend for meiotic arrest and aneuploidy induction. The frequency of M I arrested oocytes dropped from a maximum of 70% to a maximum of 20%, while, at the latest treatment time, a dose-dependent increase in the frequency of hyperploid oocytes was observed up to 56% aberrant cells at 2000 mg/kg. Altogether the results suggest that the arrest of meiotic division and the induction of aneuploidy by GF are caused by interaction with different targets or different developmental stages of the same target. In conclusion, GF has been shown to induce aneuploidy during the first meiotic division in a dose-related manner, together with other effects such as polyploidy, developmental delay and meiotic arrest. Also, these findings demonstrate that the sensitivity of the oocyte target(s) may be restricted to a specific time period and that a correct experimental protocol is critical for assessing the aneugenic activity of a chemical.

Regulation of nuclear membrane assembly and maintenance during in vitro maturation of mouse oocytes: role of pyruvate and protein synthesis

In the absence of a suitable energy source, mouse oocytes cultured in vitro resume, but fail to complete, meiotic maturation. However, little is known about the underlying mechanisms leading to this meiotic failure. We utilized pyruvate-deficient medium to test for the role of pyruvate throughout the meiotic maturation process. Germinal vesicle-stage (GV) oocytes underwent germinal vesicle breakdown (GVBD), but failed to form a polar body when cultured continuously in pyruvate-free medium. However, when GV oocytes were preincubated for 4 h in pyruvate-free medium containing dibutyryl cyclic adenosine monophosphate (dbcAMP) and then cultured in pyruvate-free medium, GVBD was markedly inhibited. Preincubation of GV oocytes in dbcAMP and cycloheximide, followed by culture in cycloheximide only, also inhibited GVBD. A longer preincubation period was required in the cycloheximide-dbcAMP case (12 h) than in pyruvate-free-dbcAMP medium situation (4 h). Strikingly, reassembly of the nuclear membrane without polar body formation was observed following GVBD in oocytes continuously cultured in pyruvate-free medium. The reassembled nuclear membrane increased in size with continued culture, and it surrounded partially-decondensed chromatin. Nuclear membrane reassembly also occurred in oocytes which had undergone GVBD during continuous culture in medium containing only cycloheximide. Reformation of nuclear membranes after GVBD was confirmed by electron-microscopic analyses of oocytes cultured in pyruvate-free medium or in the presence of cycloheximide.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental regulation of heat-shock response in mouse oogenesis: identification of differentially responsive oocyte classes during Graafian follicle development

The response to heat (hs response) of dictyate mouse oocytes at various differentiation stages was analyzed in vitro, by determining patterns of oocyte heat-shock (hs) gene expression and heat-shock protein (HSP) synthesis, under both normal conditions and after an hs. Growing oocytes constitutively synthesized HSP89 and HSC70, and, in contrast to preovulatory oocytes which do not display an hs response, displayed a heat-elicited, transcription-dependent synthesis of two HSP68 isoforms, but not of other inducible HSPs. To determine the developmental schedule of hs response disappearance during oogenesis, fully grown oocytes from Graafian follicles were morphologically sorted into three discrete classes related to the follicle development, namely, loosely associated with granulosa cells (LA oocytes, from small Graafian follicles), intermediately associated with granulosa cells (IA oocytes, from medium-sized Graafian follicles), and cumulus-associated (CA oocytes, from mature follicles). LA oocytes displayed an hs response qualitatively similar to, but smaller in extent than, that of growing oocytes, and were able to resume and complete spontaneous meiotic maturation in vitro at a high rate after hs. We conclude that hs response of mouse dictyate oocytes is maximal during growth period, significantly declines with acquisition of full oocyte size and antrum formation within the follicle, and is finally shut off with oocyte/follicle terminal differentiation.