Changes in maturation-promoting activity in the cytoplasm of pig oocytes throughout maturation

Lab partners: oocytes, embryos and company. A personal view on aspects of oocyte maturation and the development of monozygotic twins

The present review addresses the oocyte and the preimplantation embryo, and is intended to highlight the underlying principle of the "nature versus/and nurture" question. Given the diversity in mammalian oocyte maturation, this review will not be comprehensive but instead will focus on the porcine oocyte. Historically, oogenesis was seen as the development of a passive cell nursed and determined by its somatic compartment. Currently, the advanced analysis of the cross-talk between the maternal environment and the oocyte shows a more balanced relationship: Granulosa cells nurse the oocyte, whereas the latter secretes diffusible factors that regulate proliferation and differentiation of the granulosa cells. Signal molecules of the granulosa cells either prevent the precocious initiation of meiotic maturation or enable oocyte maturation following hormonal stimulation. A similar question emerges in research on monozygotic twins or multiples: In Greek and medieval times, twins were not seen as the result of the common course of nature but were classified as faults. This seems still valid today for the rare and until now mainly unknown genesis of facultative monozygotic twins in mammals. Monozygotic twins are unique subjects for studies of the conceptus-maternal dialogue, the intra-pair similarity and dissimilarity, and the elucidation of the interplay between nature and nurture. In the course of in vivo collections of preimplantation sheep embryos and experiments on embryo splitting and other microsurgical interventions we recorded observations on double blastocysts within a single zona pellucida, double inner cell masses in zona-enclosed blastocysts and double germinal discs in elongating embryos. On the basis of these observations we add some pieces to the puzzle of the post-zygotic genesis of monozygotic twins and on maternal influences on the developing conceptus.

Effects of alpha lipoic acid treatment during in vitro maturation on the development of porcine somatic cell nuclear transfer embryos

Oxidative stress influences the embryo production efficiency in vitro. We investigated the effects of alpha lipoic acid (ALA) treatment during the in vitro maturation (IVM) period on the porcine somatic cell nuclear transfer (SCNT) embryo production. After IVM, maturation rates of the 12.5- and 25-μM ALA-treated groups were not significantly different from those of the 0-μM ALA-treated group. Compared to those in the 0-μM ALA-treated group, the reactive oxygen species and glutathione levels were significantly decreased and increased, respectively, in the cytoplasm of matured oocytes in the 12.5-50-μM ALA-treated groups. Apoptosis rate in cumulus cells after IVM was significantly lower in the 12.5-50-μM ALA-treated groups than in the 0-μM ALA-treated group. Blastocyst formation rate was significantly higher in parthenogenetic oocytes treated with 12.5-μM ALA than in the 0-, 25-, and 50-μM ALA-treated groups. Similarly, in SCNT embryos, the 12.5-μM ALA-treated group showed a significantly higher blastocyst formation rate than the 0-μM ALA-treated group. Apoptosis rate in SCNT blastocysts was significantly decreased by 12.5-μM ALA treatment. The results showed that treatment with 12.5-μM ALA during IVM improves porcine SCNT embryo development and partial quality.

Perfluorodecanoic acid induces meiotic defects and deterioration of mice oocytes in vitro

Perfluorodecanoic acid (PFDA) is a member of the perfluoroalkyl substances, which are toxic to organic functions. Recently, it has been found in follicular fluid, seriously interfering with reproduction. Follicular fluid provides the oocyte with necessary resources during the process of oocytes maturation. However, the effects of PFDA on the oocyte need investigation. Our study evaluated the impacts of PFDA on the meiosis and development potential of mouse oocytes by exposing oocytes to PFDA in vitro at 350, 400, and 450 μM concentrations. The results showed that exposure to PFDA resulted in the first meiotic prophase arrest by obstructing the function of the maturation-promoting factor. It also induced the dysfunction of the spindle assembly checkpoint, expedited the progression of the first meiotic process, and increased the risk of aneuploidy. The oocytes treated with PFDA had a broken cytoskeleton which also contributed to meiotic maturation failure. Besides, PFDA exposure caused mitochondria defections, increased the reactive oxygen species level in oocytes, and consequently induced oocyte apoptosis. Moreover, PFDA produced epigenetic modifications in oocytes and increased the frequency of mature oocytes with declined development potential. In summary, our data indicated that PFDA disturbs the meiotic process and induces oocyte quality deterioration.

CENP-T regulates both the G2/M transition and anaphase entry by acting through CDH1 in meiotic oocytes

Oocyte meiotic maturation failure is one of the major causes for female infertility. Meiotic resumption (the G2/M transition) and progression through metaphase I (MI) are two critical stages of oocyte meiotic maturation. Here, we report that centromere protein T (CENP-T), an internal kinetochore protein, plays a critical role in meiotic resumption of mouse oocytes. Depletion of CENP-T by siRNA injection increased the CDH1 (also known as FZR1) level, resulting in increased activity of the anaphase-promoting complex (APC)-CDH1 complex, and further leading to decreased levels of the cyclin protein CCNB1, attenuated maturation-promoting factor (MPF) activity, and finally severely compromised meiotic resumption. The impaired meiotic resumption caused by CENP-T depletion could be rescued by overexpression of exogenous CCNB1 or knockdown of endogenous CDH1. Overexpression of exogenous CENP-T resulted in decreased CDH1 levels, which accelerated the progression of G2/M transition, and accelerated meiotic cell cycle progression after germinal vesicle breakdown (GVBD). Unexpectedly, spindle organization after GVBD was not affected by the overexpression, but the distribution of chromosomes was affected. Our findings reveal a novel role for CENP-T in regulating meiotic progression by acting through CDH1.

Vertebrate Reproduction

Vertebrate reproduction requires a myriad of precisely orchestrated events-in particular, the maternal production of oocytes, the paternal production of sperm, successful fertilization, and initiation of early embryonic cell divisions. These processes are governed by a host of signaling pathways. Protein kinase and phosphatase signaling pathways involving Mos, CDK1, RSK, and PP2A regulate meiosis during maturation of the oocyte. Steroid signals-specifically testosterone-regulate spermatogenesis, as does signaling by G-protein-coupled hormone receptors. Finally, calcium signaling is essential for both sperm motility and fertilization. Altogether, this signaling symphony ensures the production of viable offspring, offering a chance of genetic immortality.

The control of meiotic maturation in mammalian oocytes

Mammalian oocytes spend the majority of their lives in a dormant state, residing in primordial follicles. This arrest, most analogous to the G2 stage of the mitotic cell cycle division, is only broken in the hours preceding ovulation, when a hormonal rise induces meiotic resumption and entry into the first meiotic division. At a molecular level, this event is triggered by CDK1 activity, and here, we examine how CDK1 is suppressed during meiotic arrest and raised for oocyte maturation. We focus on signaling: intercellular signaling between the oocyte and the somatic cells of the follicle, and spatial signaling involving the anaphase-promoting complex (APC) within the oocyte. Meiotic arrest is achieved through APC(FZR1)-mediated cyclin B1 degradation. Once meiotic resumption resumes, CDK1 levels rise, but its activity eventually needs to be suppressed for completion of the first meiotic division. This is achieved by APC(CDC20), whose activity is critically regulated by the spindle assembly checkpoint, and which induces both a loss in CDK1 activity as well as the cohesive ties holding chromosomes together.

Mitogen-activated protein kinase phosphorylation patterns in pig oocytes and cumulus cells during gonadotrophin-induced resumption of meiosis in vitro

The present study investigated the phosphorylation pattern of mitogen-activated protein kinase (MAPK) in cumulus-oocyte complexes (COCs) during spontaneous and FSH/LH-induced in vitro maturation (IVM). Both isoforms of MAPK were unphosphorylated in oocytes recovered immediately after liberation from follicles and became phosphorylated following 25 h incubation, corresponding to the time of germinal vesicle breakdown (GVBD). In contrast, MAPK was already phosphorylated in minimal amounts in cumulus cells at the time of liberation from follicles and phosphorylation of MAPK increased after 0.5 h incubation. Supplementation of medium with gonadotrophins intensified phosphorylation at 0.5 h incubation, demonstrating the early and rapid action of FSH/LH on MAPK phosphorylation. Phosphorylation of MAPK in cumulus cells peaked after 21 h of incubation, whereas MAPK was almost completely dephosphorylated at the end of incubation (45 h). During subsequent incubation in the absence of added gonadotrophins, between 5 and 10 h exposure to FSH/LH-supplemented medium was required to induce resumption of meiosis in COCs. Phosphorylation of MAPK in oocytes was prevented by the MEK inhibitor U0126, but the inhibitor reduced phosphorylation of MAPK in cumulus cells only during the first 2 h of IVM. The data support the hypothesis that two different MAPK phosphorylation events occurred following gonadotrophin stimulation, one in cumulus cells and the other in oocytes. In cumulus cells, FSH/LH induced early and rapid U0126-insensitive phosphorylation of MAPK, whereas U0126-susceptible MAPK phosphorylation took place in the oocyte itself around the time of GVBD.

Analysis of actinomycin D treated cattle oocytes and their use for somatic cell nuclear transfer

The present work aimed to evaluate the transcription and replication inhibitor, actinomycin D, for oocyte chemical enucleation. Cattle oocytes matured in vitro were treated with actinomycin D according to the following treatments: T1, control; T2=1.0 microg/ml for 16 h; T3=1.0 microg/ml for 14 h; T4=2.5 microg/ml for 14 h; T5=5.0 microg/ml for 14 h. The oocytes were denuded and activated during 24-26 h of maturation. Oocytes were fixed to determine the maturation status and for chromosome morphology evaluation. Furthermore, oocytes treated with actinomycin D were used for somatic cell nuclear transfer (SCNT). Parthenogenetic and SCNT embryos were fixed to evaluate the percentage of apoptotic nuclei by the TUNEL assay. The maturation (T1=90.4%; T2=82.3%; T3=79.1%; T4=83.4%; T5=74.7%), cleavage (T1=68.9%; T2=46.0%; T3=49.7%; T4=33.4%; T5=29.3%) and blastocyst rate at D8 (T1=41.1%; T2=1.8%; T3=1.3%; T4=0.9%; T5=0.0%) after actinomycin D treatment were significantly different. There was a significant chromosome uncoiling when treated with greater concentrations (2.5 and 5.0 microg/ml). After SCNT, the cleavage rate (61.3%) was similar to the actinomycin D-treated control group (61.3%) and less than the non-treated control (70.2%), although the blastocyst rate was greater in the SCNT group (11.8%) comparing with the treated control (3.6%) and less than the untreated control (38.0%). Treated parthenogenetic embryos had more apoptotic cells than the parthenogenetic controls (24.2% compared with 4.8%). However, the SCNT group using treated cytoplasts was similar from the SCNT control (9.3 compared with 13.0%). Actinomycin D treatment was efficient in blocking embryonic development. Moreover, it was possible to obtain reconstructed embryos that possess an apoptotic cell index indistinguishable from controls.

Development to the Blastocyst Stage of Porcine Somatic Cell Nuclear Transfer Embryos Reconstructed by the Fusion of Cumulus Cells and Cytoplasts Prepared by Gradient Centrifugation

The present study was designated to examine the possibility of producing somatic cell nuclear transfer (SCNT) embryos in pigs using oocyte cytoplasm fragments (OCFs), prepared by centrifugations, as recipient cytoplasts. In Experiment 1, in vitro matured oocytes were centrifuged at 13,000 x g for 3, 6, and 9 min to stratify the cytoplasm, and then the oocytes were freed from zona pellucida and recentrifuged at 5,000 x g for 4 sec in Percoll gradient solution to produce OCFs as the source of recipient cytoplasts. It was found that a long duration of the first centrifugation tends to produce large-sized OCFs after the second centrifugation. In Experiment 2, two or three cytoplasts without chromosomes were aggregated, and then they were fused with a cumulus cell to produce SCNT embryos. The results showed that 66.4 +/- 9.4% of the reconstructed embryos underwent premature chromosome condensation at 1 h after activation, and 85.2 +/- 7.1% and 61.6 +/- 7.0% of them had pseudopronuclei at 10 and 24 h after activation, respectively. In Experiment 3, when SCNT embryos reconstructed by the fusion of three cytoplasts and one cumulus cell, a significantly higher (p < 0.05) rate of reconstructed embryos developed to the blastocyst stage (10.6 +/- 1.8%) than that of reconstructed with two cytoplasts and one cumulus cell (5.2 +/- 1.5%). These results indicate that cytoplasts obtained by two centrifugations can support the remodeling of a transferred somatic nucleus, resulting in the development of the reconstructed porcine embryos to the blastocyst stage.

Activation of maturation promoting factor in Bufo arenarum oocytes: injection of mature cytoplasm and germinal vesicle contents

Although progesterone is the established maturation inducer in amphibians, Bufo arenarum oocytes obtained during the reproductive period (spring-summer) resume meiosis with no need of an exogenous hormonal stimulus if deprived of their enveloping follicle cells, a phenomenon called spontaneous maturation. In this species it is possible to obtain oocytes competent and incompetent to undergo spontaneous maturation according to the seasonal period in which animals are captured. Reinitiation of meiosis is regulated by maturation promoting factor (MPF), a complex of the cyclin-dependent kinase p34cdc2 and cyclin B. Although the function and molecule of MPF are common among species, the formation and activation mechanisms of MPF differ according to species. This study was undertaken to evaluate the presence of pre-MPF in Bufo arenarum oocytes incompetent to mature spontaneously and the effect of the injection of mature cytoplasm or germinal vesicle contents on the resumption of meiosis. The results of our treatment of Bufo arenarum immature oocytes incompetent to mature spontaneously with sodium metavanadate (NaVO3) and dexamethasone (DEX) indicates that these oocytes have a pre-MPF, which activates and induces germinal vesicle breakdown (GVBD) by dephosphorylation on Thr-14/Tyr-15 by cdc25 phosphatase and without cyclin B synthesis. The injection of cytoplasm containing active MPF is sufficient to activate an amplification loop that requires the activation of cdc25 and protein kinase C, the decrease in cAMP levels, and is independent of protein synthesis. However, the injection of germinal vesicle content also induces GVBD in the immature receptor oocyte, a process dependent on protein synthesis but not on cdc25 phosphatase or PKC activity.

Time course of meiotic progression after transferring primary spermatocyte into ooplasm at different stages

This study attempted to investigate the time course of meiotic progression after transferring primary spermatocyte (PS) into ooplasm at different maturing stages. In present experiments, PSs were introduced into maturing ooplasts or oocytes by electrofusion. Higher fusion rate was obtained by phytohemagglutinin (PHA) agglutination than by perivitelline space (PVS) insertion. When the ooplasms prepared at 0, 2, 5, and 8.5 hr of in vitro maturation (IVM) were used as recipients and PSs were used as donors, the reconstructed cells extruded the first polar body (PB1) approximately 8.5, 7, 5.5, and 3 hr after electrofusion, respectively. Especially, when ooplasm cultured for 8.5 hr in vitro after GV removal was fused with PS, the PB1 was emitted 7-11 hr after electrofusion. Additionally, the PB1 extrusions of GV and pro-MI oocytes fertilized with PSs were 2.5 hr earlier than control oocytes. The results suggest that (1) PSs undergo the first meiosis in different time courses when introduced into ooplasm at different maturing stages; (2) GV material plays an important role in determining the timing of PB1 extrusion; and (3) first meiotic division of GV and pro-MI oocytes can be accelerated by introducing PS.

Induction and activation of meiosis and subsequent parthenogenetic development of growing pig oocytes using calcium ionophore A23187

The pig ovary contains a large number of growing oocytes, which do not mature in vitro and cannot be readily used in various biotechnologies. This study was conducted to determine the possibility of inducing meiotic maturation in growing pig oocytes with an internal diameter of 110 microm, which had developed partial meiotic competence. Most of these oocytes spontaneously stopped maturation at the metaphase I stage (68%); a limited number proceeded to the metaphase II stage (26%). Treatment with calcium ionophore A23187 (50 microM for 5 or 10 min) after 24h in vitro culture overcame the block at the metaphase I stage, and treated growing pig oocytes matured to the metaphase II stage (66%). Oocytes in which maturation had been induced by calcium ionophore were again treated with calcium ionophore. Up to 58% of the treated oocytes were activated. Parthenogenetic development in oocytes treated with ionophore for meiosis induction and activation was very limited. The portion which reached morula stage did not exceed 8% and at most 3% developed to the blastocyst stage.

Alterations and reversibility in the chromatin, cytoskeleton and development of pig oocytes treated with roscovitine

Germinal vesicle (GV) breakdown in mammalian oocytes is regulated by the activation of maturation promoting factor (MPF). We investigated a specific cdc2 kinase inhibitor, roscovitine, to maintain pig oocytes in the GV stage. Cumulus-oocyte complexes (COCs) were aspirated from slaughterhouse ovaries and cultured for 44 hr in NCSU#23 medium containing different levels of roscovitine (0, 10, 20, 30, 40, 50 microM in Experiment 1 and 0, 40, 60, 80, 100, 120 microM in Experiment 2). The COCs were cultured for another 44 hr after removal of the chemical. Twenty oocytes in each group were fixed at 44 hr for immunocytochemical labeling of the cytoskeleton and the rest (approximately 20/group) were fixed at the end of 88 hr after culture. Results showed that the inhibition of the oocyte in the GV stage was not effective when 10-50 microM (Experiment 1) of roscovitine were used (19-34%). When oocytes were released from the inhibitor, similar proportions (70-83%) of oocytes were observed in the MII or advanced stages among treatments. However, when higher concentrations of roscovitine were used (Experiment 2), significantly greater inhibitory effect was observed at the levels of 80-120 microM with 83-91% oocytes being blocked in the GV stage when compared to the control (9%) and the 40-60 microM (27-43%) groups (P < 0.05). Although 15-21% of the oocytes showed abnormal MII morphology with aberrant meiotic spindles and/or formation of cytoplasmic microtubules, a substantial number of oocytes resumed meiosis and reached MII stage at 44 hr after removal of this chemical. In Experiment 3, different concentrations of roscovitine (0, 20, 40, and 80 microM) were tested to examine the length of intervals (0, 11, 22, 33, and 44 hr) for an effective inhibition. Results showed that the inhibitory effect was significantly more prominent at 22 hr than that at 33 and 44 hr after roscovitine treatment in all treatment groups (P < 0.05). This study demonstrated that roscovitine-treated oocytes resumed meiosis after removal of the inhibitor. This could provide flexibility for studying porcine oocyte development and embryo cloning and may have application in other species.

Molecular Mechanisms Underlying Pig Oocyte Maturation and Fertilization

Since the pig is not only an important farm animal, but also a model animal for biomedical applications, the development of reproductive technologies in this species has been very important. In vitro oocyte maturation and fertilization (IVM-IVF) are basic techniques for a number of oocyte- or embryo-related technologies. The practical aspects for pig oocyte IVM-IVF have been reviewed, while the molecular mechanisms underlying oocyte meiotic maturation and fertilization have not been well summarized, although accumulating data have been obtained in recent one decade. This review will focus on what is known about the molecular mechanisms of porcine oocyte maturation and fertilization such as first meiosis resumption, meiotic spindle assembly, second meiosis metaphase (MII) arrest during oocyte maturation, sperm-egg recognition and fusion, sperm acrosome reaction, second meiosis resumption, sperm chromatin decondensation, and pronucleus formation during fertilization, as well as the establishment of polyspermy block.

Cell coupling and maturation-promoting factor activity in in vitro-matured prepubertal and adult sheep oocytes

We examined some differences between prepubertal and adult ovine oocytes; in particular we analyzed the functional status of the cumulus-oocyte complex, protein synthesis during in vitro maturation, and because no information is available on prepubertal and adult sheep, maturation-promoting factor (MPF) fluctuations throughout meiotic progression both in prepubertal and adult sheep oocytes. After 24 h of maturation, percentages of MII oocytes were similar between prepubertal and adult animals. Electron microscopy examinations showed that prepubertal oocytes had fewer transzonal projections than adult oocytes. Methionine uptake was significantly lower in prepubertal cumulus-enclosed oocytes examined through meiotic progression. On the contrary, denuded prepubertal oocytes showed a higher methionine incorporation in the first 4 h of incubation compared with adult oocytes. We also found some differences in MPF activity between prepubertal and adult oocytes at MII stage. In fact, prepubertal MII oocytes had a significantly lower level of MPF activity than adult oocytes did and, after fusion with germinal vesicle oocytes, they were unable to induce nuclear breakdown and chromosome condensation 1-2 h post-fusion, whereas adult MII oocytes could induce these processes. Our findings show that the lesser competence of prepubertal oocytes could be due to morphological anomalies and alterations in physiological activity and that oocytes do not reach full developmental competence until puberty.

Activation of pig oocytes using calcium ionophore: effect of protein synthesis inhibitor cycloheximide

In vitro matured pig oocytes were activated using a combined treatment of calcium ionophore A 23187 with cycloheximide. The oocytes were exposed to ionophore (10, 25 or 50 microM) for 0.5, 1, 3, 5 or 7 min and then cultured with cycloheximide (0 or 10 microg/ml) for 6 h. Cycloheximide treatment significantly increased the activation rate of oocytes and the percentage of oocytes that were able to develop after activation. The highest activation rate was observed after treatment with 50 microM ionophore. The highest percentage of developing eggs was observed after combined treatment of ionophore (25 microM) with cycloheximide. The percentage of oocytes developing up to the morula and blastocyst stage was not significantly increased after cycloheximide treatment.

Protection of porcine oocytes against apoptotic cell death caused by oxidative stress during In vitro maturation: role of cumulus cells

The present study was conducted to examine the protective effect of cumulus cells on oocyte damage caused by reactive oxygen species (ROS), generated by the hypoxanthine-xanthine oxidase (XOD) system, during in vitro maturation of porcine oocytes. Cumulus-oocyte complexes (COCs) and cumulus-denuded oocytes (DOs) were cultured for 44 h in NCSU37 supplemented with cysteine, gonadotropins, 10% porcine follicular fluid, and hypoxanthine in the presence or absence of XOD. DNA cleavage and damage were analyzed using the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) method and single cell microgel electrophoresis (comet) assay, respectively, and caspase-3 activity and glutathione (GSH) content were measured in each experimental group. Exposure of DOs to ROS resulted in meiotic arrest and the increase of degenerated oocytes. These degenerated DOs underwent apoptosis, as shown by the TUNEL-positive reaction within their germinal vesicles and the activation of caspase-3. The length of DNA migration in DOs treated with XOD was significantly longer than that of untreated DOs (P: < 0.05). However, irreparable cell damage caused by ROS was not observed in COCs, and no difference was observed in the caspase-3 activity of both COCs treated with and without XOD. A significantly (P: < 0.05) high level of GSH was found in COCs after 44 h of culture, compared with that of oocytes freshly isolated from their follicles, whereas GSH content in DOs markedly decreased after treatment with or without XOD. These findings suggest that cumulus cells have a critical role in protecting oocytes against oxidative stress-induced apoptosis through the enhancement of GSH content in oocytes.

Meiosis-activating sterol-mediated resumption of meiosis in mouse oocytes in vitro is influenced by protein synthesis inhibition and cholera toxin

To explore the possible signaling pathways of meiosis-activating sterol (MAS)-induced oocyte maturation and to elucidate whether the MAS pathway involves transcription or translation, arrested immature mouse oocytes were cultured with either the protein synthesis inhibitor cycloheximide or the heteronuclear RNA inhibitors alpha-amanitin or actinomycin D, respectively. Moreover, the possible involvement of a G protein-coupled receptor mechanism in MAS-mediated oocyte maturation was explored by influencing oocyte maturation with cholera toxin (CT). MAS-induced oocyte maturation was completely blocked by the addition of 50 microg/ml cycloheximide 4 h before the addition of MAS. Simultaneous addition of MAS and the protein synthesis inhibitor also significantly reduced the meiotic resumption compared to that in MAS-treated controls. In contrast, neither of the treatment regimens to inhibit transcription of DNA to RNA was observed to have any effect on the MAS-induced resumption of meiosis. CT was observed to inhibit MAS-induced, but not spontaneous, oocyte maturation in vitro, suggesting a putative involvement of G protein-coupled receptor mechanism in the MAS mode of action. In conclusion, protein synthesis was found to be an essential requirement for maintaining the oocytes' responsiveness to MAS-induced resumption of meiosis, in contrast to transcription.

Low oxygen inhibits but complex high-glucose medium facilitates in vitro maturation of squirrel monkey oocyte-granulosa cell complexes

PURPOSE

The objectives of these in vitro maturation studies in primate cumulus-oocyte complexes (COCs) were to evaluate the effect of a reduced-oxygen environment and to compare medium with a high-glucose concentration to medium with pyruvate but no glucose.

METHODS

COCs were retrieved from squirrel monkeys stimulated with 1 mg of follicle-stimulating hormone (FSH) for 4-6 days. Experiment 1 examined maturation after 48 hr in 5% O2/5% CO2/90% N2 compared with 5% CO2/air. The medium was CMRL-1066 containing moderate glucose (5.5 mM) supplemented with 1 mM glutamine, 0.33 mM pyruvate, 0.075 IU/ml human FSH, 5 IU/ml human chorionic gonadotropin, 75 U penicillin G/ml, and 20% fetal bovine serum. Experiment 2 in 5% CO2/air, compared P-1 medium (pyruvate and lactate but no glucose) to Waymouth's medium (27.5 mM glucose), both with identical supplements.

RESULTS

Only 3 (8%) of 37 COCs matured in 5% O2, while 39 (49%) of 80 matured in ambient O2. Fourteen (22%) of 64 complexes matured in P-1 medium, compared to 47 (49%) of 96 meiosis II oocytes in Waymouth's medium (P < 0.05).

CONCLUSIONS

These are the first primate studies showing detrimental effects of reduced-oxygen culture on in vitro maturation. Additionally, maturation was enhanced with complex high-glucose medium suggesting that the predominant metabolism is aerobic glycolysis.

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.

Calcium elevation in sheep cumulus-oocyte complexes after luteinising hormone stimulation

We investigated Ca2+ levels in intact cumulus-oocyte complexes (COCs) on exposure to peak levels of luteinising hormone (LH). Specific preparations were used where cumulus corona cells were loaded with a membrane-permeant Ca(2+)-sensitive dye (FLUO-3AM), whereas the oocyte was injected directly with the nonpermeant form of the dye (FLUO-3). After exposure to LH, cumulus and corona radiata cells showed distinct rises in intracellular Ca2+ in 50-200 sec. The pattern of Ca2+ response varied in the different cells both for the duration of the transients and for their persistence. Interestingly, Ca2+ elevations were recorded in all the layers of the cumulus mass, including the innermost layer of corona cells, demonstrating the wide diffusion of LH receptors. Following the Ca2+ raise in somatic cells, an intracellular Ca2+ elevation also was recorded within the oocyte with a delay of 100-300 sec. The elevation started at the cortex of the oocyte and then spread all over the ooplasm. The addition of verapamil or manganese chloride did not prevent LH-induced Ca2+ elevation in the COC, whereas mechanical uncoupling of cumulus cells from the oocyte prevented any Ca2+ response within the oocyte. The results indicate that cumulus corona cells are capable of transducing LH message by rising intracellular Ca2+ and show that this signal is rapidly transferred into the oocyte through gap junctions. This may result from the direct diffusion of Ca2+ or its putative releaser IP3 from cumulus cells to the oocyte.

Involvement of cumulus cells stimulated by FSH in chromatin condensation and the activation of maturation-promoting factor in bovine oocytes

The effects of FSH-stimulated cumulus cells on the regulatory mechanisms of chromatin condensation and maturation-promoting factor (MPF) activation around the time of germinal vesicle breakdown (GVBD) in bovine oocytes were examined. Chromatin condensation occurred in oocytes arrested at the germinal vesicle (GV) stage by protein synthesis inhibitor, cycloheximide, but this condensation was blocked by FSH-stimulated cumulus cells. However, treatment with cyclic AMP (cAMP)-dependent protein kinase inhibitor, H-8, dramatically increased the proportion of oocytes possessing GVs with condensed bivalents. Under the condition of inhibited protein synthesis, the phosphorylation form of p34cdc2 kinase was not changed due to chromatin condensation, although the activity of histone H1 kinase was significantly increased compared with that of oocytes possessing GVs with filamentous bivalents. The cycloheximide-dependent GVBD block was overcome by okadaic acid (OA) in 48 and 13% of the oocytes in the absence and presence of FSH, respectively. An initial 6-h culture period critical for protein synthesis was necessary for OA to counteract the inhibitory effect exerted by cycloheximide on the induction of GVBD and activation of histone H1 kinase in the absence of FSH, whereas this first culture period was prolonged for 2 h in the presence of FSH. Furthermore, even in FSH-stimulated oocytes, H-8 facilitated an OA-counteracted overcome of the cycloheximide-dependent GVBD block after 2 h of initial culture for protein synthesis. From these results, it is concluded that cAMP-dependent protein kinase activity regulated by cumulus cells following FSH-stimulation requests plays a role in the complex mechanism of chromatin condensation and MPF activation leading to meiotic resumption in bovine oocytes.

Transformation of sperm nuclei into metaphase chromosomes in maturing pig oocytes penetrated in vitro

Cumulus-free pig oocytes at the germinal vesicle (GV) stage were incubated in modified Brackett & Oliphant's medium with 5% fetal calf serum and 5mM caffeine with or without cryopreserved, ejaculated spermatozoa. When oocytes were transferred into modified tissue culture medium (TCM-199B at pH 7.4) supplemented with 10 IU/ml eCG, 10 IU/ml hCG and 1 microg/ml oestradiol-17beta after 8 h of incubation with spermatozoa and cultured for 0-48 h, 86-99% of oocytes were penetrated. Most (95-100%) oocytes penetrated 0-16 h after transfer had decondensed sperm chromatin. However, 24 h after transfer 47% and 33% of penetrated oocytes contained recondensed sperm chromatin and sperm metaphase chromosomes, respectively. The proportion of penetrated oocytes containing sperm metaphase chromosomes increased after 36-48 h of transfer (51-65%). The transformation of sperm nuclei to metaphase chromosomes was obtained in 75% and 79% of anaphase I (AI) to telophase I (TI) and metaphase II (MII) oocytes, respectively, but only in 38% of metaphase I (MI) oocytes. Moreover, such transformation was observed only in 1 of 30 oocytes at the stages of GV breakdown to prometaphase I and none of 69 oocytes at the GV stage. The transformation of sperm nuclei into metaphase chromosomes was completely inhibited in oocytes penetrated by eight or more spermatozoa. Well-developed male and female pronuclei were observed in only 3 (4%) of 77 oocytes penetrated 48 h after transfer. The proportion of oocytes reaching MII was greatly inhibited by sperm penetration; only 18% of penetrated oocytes, but 87% of non-inseminated oocytes, reached MII by 48 h after transfer. None of the oocytes penetrated by seven or more spermatozoa reached MII. Most (75%) oocytes were inhibited from the transition from MI to MII even though they were cultured for 48 h. The present results indicate that: (1) the cytoplasm of maturing oocytes possesses an activity for transforming sperm nuclei into metaphase chromosomes, (2) immature pig oocytes penetrated by spermatozoa can undergo meiotic maturation to MI, and (3) the transition of such oocytes from MI to MII is inhibited, suggesting that an activity of mitogen-activated protein kinase may be retarded.

Association between p34cdc2 levels and meiotic arrest in pig oocytes during early growth

The molecules involved in determining meiotic competence were determined in porcine oocytes isolated from preantral and antral follicles of different sizes. Oocytes isolated from preantral follicles had a mean diameter of 78 microns, contained diffuse filamentous chromatin in the germinal vesicle and were incapable of progressing from the G2 to the M phase of the cycle even after 72 h in culture. Oocytes from early antral follicles had a mean diameter of 105 microns, showed a filamentous chromatin configuration and about half resumed meiosis but arrested at metaphase I (MI) when cultured. Oocytes from mid-antral (3-4 mm) and large antral follicles (5-6 mm) had mean oocyte diameters of 115 and 119 microns respectively, contained condensed chromatin around the nucleolus and progressed to metaphase II (MII) in 48% and 93% of instances respectively. Analysis of p34cdc2, the catalytic subunit of maturation promoting factor (MPF), by immunoblotting indicates that the inability of small (78 microns) oocytes to resume meiosis is due, at least in part, to inadequate levels of the catalytic subunit of MPF. On the other hand, the inability of intermediate-sized (105 microns) oocytes from antral follicles to complete the first meiotic division by progressing beyond MI appears not to be limited by levels of p34cdc2, which are maximal by this stage. We postulate that an inadequacy of molecules other than p34cdc2 limits progression of MI to MII; the acquisition of these molecules during the final stages of growth may be correlated with the formation of the perinucleolar chromatin rim in the germinal vesicle.

Molecular control of meiosis

The animal cell cycle consists of a round of chromosomal DNA replication in S-phase, followed by segregation of the replicated chromosomes into the daughter nuclei during M-phase. In most animal cells, gap phases termed G(1) and G(2) are introduced between the M- and S-phases, respectively. Meiosis is a particular example of cell division occurring in germ cells. This specialized cell cycle consists of two successive rounds of chromosome segregation that follow a round of DNA replication. Meiosis produces progeny cells with half as many chromosomes as their parents, thus making sexual reproduction possible. This review is concerned with the factors that have been implicated in the control of meiosis, although research in progress may reveal additional regulatory processes.

Requirement for protein synthesis during the onset of meiosis in bovine oocytes and its involvement in the autocatalytic amplification of maturation-promoting factor

The present study was carried out using the method of electrofusion, or treatment with okadaic acid (OA), to determine whether protein synthesis at the onset of culture was required for the meiotic resumption of bovine follicular oocytes. Germinal vesicle breakdown (GVBD) occurred in bovine oocytes at 6 hr after separation from their follicles in vitro. Following this, immature germinal vesicle (GV) oocytes, preincubated for 0, 2, 4, and 6 hr, were fused to mature oocytes. When immature oocytes, preincubated for 0 hr, were fused to mature oocytes and then cultured for 3 hr in basic medium, GVBD was observed in all fused cells, whereas in the case of cultivation in medium supplemented with the protein synthesis inhibitor (25 micrograms/ml cycloheximide; CX), 39% of the fused cells possessed an intact GV within their cytoplasm. In immature oocytes preincubated for 4 or 6 hr, however, this proportion was significantly reduced to 7% and 4%, respectively, without protein synthesis after fusion. In addition, the CX-dependent block of GVBD could be overcome in only 13% of bovine follicular oocytes by the addition of 2 microM OA, although 51% of oocytes which synthesized the protein during the first 6 hr of culture induced GVBD in subsequent culture with CX plus OA. Thus, we conclude that the initiation of GVBD in bovine oocytes requires protein synthesized at the onset of meiosis, which is related to the autocatalytic amplification of the maturation-promoting factor.

Time-dependent effects of cycloheximide and α-amanitin on meiotic resumption and progression in bovine follicular oocytes

To identify the stage during maturation at which new protein and RNA are synthesized for meiotic resumption, follicular oocytes were cultured in TCM-199 with the protein synthesis inhibitor cycloheximide or the hnRNA synthesis inhibitor alpha-amanitin. Although the meiotic resumption of cumulus-enclosed oocytes was completely blocked by the addition of 25 microg/ml cycloheximide at 4 h after the onset of culture, 23% of oocytes cultured from 5 h post cultivation in the medium with cycloheximide underwent germinal vesicle breakdown (GVBD). By further delaying the addition of cycloheximide, the proportion of oocytes which underwent GVBD increased. Addition of the inhibitor at 8 h or more post cultivation resulted in GVBD occurring in more than 87% of oocytes, though none of them were able to proceed beyond the metaphase I stage. In contrast, the addition of 50 microg/ml alpha-amanitin from the onset of culture significantly reduced the proportion of GVBD to 75% in cumulus-enclosed oocytes, while no significant reduction in the proportions of GVBD was noted in the case of its addition from 1 h of culture onward. However, denuded oocytes were almost insensitive to any treatments with alpha-amanitin. These results indicate that protein synthesis in the oocytes and RNA synthesis in the cumulus cells soon after the onset of culture are necessary for GVBD and that continuous protein synthesis following GVBD is indispensable for progression of the meiotic division in bovine oocytes.

Protein synthesis requirements during resumption of meiosis in the hamster oocyte: early nuclear and microtubule configurations

The organization of chromatin and cytoplasmic microtubules changes abruptly at M-phase entry in both mitotic and meiotic cell cycles. To determine whether the early nuclear and cytoplasmic events associated with meiotic resumption are dependent on protein synthesis, cumulus-enclosed hamster oocytes were cultured in the presence of 100 micrograms/ml puromycin or cycloheximide for 5 hr. Both control (untreated) and treated oocytes were analyzed by fluorescence microscopy after staining with Hoechst 33258 and tubulin antibodies. Freshly isolated oocytes exhibit prominent nucleoli and diffuse chromatin within the germinal vesicle as well as an interphase network of cytoplasmic microtubules. After 4-4.5 hr in culture, most oocytes were in prometaphase I of meiosis as characterized by a prominent spindle with fully condensed chromosomes and numerous cytoplasmic asters. After 5-5.5 hr in culture, microtubule asters are no longer detected in most cells, and the spindle is the only tubulin-positive structure. Incubation for 5 hr in the presence of inhibitors does not impair germinal vesicle breakdown, chromatin condensation, kinetochore microtubule assembly, or cytoplasmic aster formation in the majority of oocytes examined; however, under these conditions, a population of oocytes retains a germinal vesicle, exhibiting variable degrees of chromatin condensation and cytoplasmic aster formation. Meiotic spindle formation is inhibited in all oocytes. These effects are fully reversible upon culture of treated oocytes in drug-free medium for 5 hr. The data indicate that meiotic spindle assembly is dependent on ongoing protein synthesis in the cumulus-enclosed hamster oocyte; in contrast, chromatin condensation and aster formation are not as sensitive to protein synthesis inhibitors during meiotic resumption.