Metaphase I arrest of starfish oocytes induced via the MAP kinase pathway is released by an increase of intracellular pH

Reinitiation of meiosis in oocytes usually occurs as a two-step process during which release from the prophase block is followed by an arrest in metaphase of the first or second meiotic division [metaphase I (MI) or metaphase II (MII)]. The mechanism of MI arrest in meiosis is poorly understood, although it is a widely observed phenomenon in invertebrates. The blockage of fully grown starfish oocytes in prophase of meiosis I is released by the hormone 1-methyladenine. It has been believed that meiosis of starfish oocytes proceeds completely without MI or MII arrest, even when fertilization does not occur. Here we show that MI arrest of starfish oocytes occurs in the ovary after germinal vesicle breakdown. This arrest is maintained both by the Mos/MEK/MAP kinase pathway and the blockage of an increase of intracellular pH in the ovary before spawning. Immediately after spawning into seawater, activation of Na+/H+ antiporters via a heterotrimeric G protein coupling to a 1-methyladenine receptor in the oocyte leads to an intracellular pH increase that can overcome the MI arrest even in the presence of active MAP kinase.     

Fertilization and InsP3-induced Ca2+ release stimulate a persistent increase in the rate of degradation of cyclin B1 specifically in mature mouse oocytes

Vertebrate oocytes proceed through meiosis I before undergoing a cytostatic factor (CSF)-mediated arrest at metaphase of meiosis II. Exit from MII arrest is stimulated by a sperm-induced increase in intracellular Ca2+. This increase in Ca2+ results in the destruction of cyclin B1, the regulatory subunit of cdk1 that leads to inactivation of maturation promoting factor (MPF) and egg activation. Progression through meiosis I also involves cyclin B1 destruction, but it is not known whether Ca2+ can activate the destruction machinery during MI. We have investigated Ca2+ -induced cyclin destruction in MI and MII by using a cyclin B1-GFP fusion protein and measurement of intracellular Ca2+. We find no evidence for a role for Ca2+ in MI since oocytes progress through MI in the absence of detectable Ca2+ transients. Furthermore, Ca2+ increases induced by photorelease of InsP3 stimulate a persistent destruction of cyclin B1-GFP in MII but not MI stage oocytes. In addition to a steady decrease in cyclin B1-GFP fluorescence, the increase in Ca2+ stimulated a transient decrease in fluorescence in both MI and MII stage oocytes. Similar transient decreases in fluorescence imposed on a more persistent fluorescence decrease were detected in cyclin-GFP-injected eggs undergoing fertilization-induced Ca2+ oscillations. The transient decreases in fluorescence were not a result of cyclin B1 destruction since transients persisted in the presence of a proteasome inhibitor and were detected in controls injected with eGFP and in untreated oocytes. We conclude that increases in cytosolic Ca2+ induce transient changes in autofluorescence and that the pattern of cyclin B1 degradation at fertilization is not stepwise but exponential. Furthermore, this Ca2+ -induced increase in degradation of cyclin B1 requires factors specific to mature oocytes, and that to overcome arrest at MII, Ca2+ acts to release the CSF-mediated brake on cyclin B1 destruction.     

Involvement of mitogen-activating protein kinase and intracellular pH in the duration of the metaphase I (MI) pause of starfish oocytes after spawning

The metaphase I (MI) arrest of starfish oocytes is released after spawning. In this study using starfish Asterina pectinifera , the duration of MI after spawning was ~20 min and ~30 min in fertilized and unfertilized oocytes, respectively. This prolongation of MI in unfertilized oocytes, referred to as the MI pause, was maintained by mitogen-activating protein kinase (MAPK) as well as low intracellular pH (~7.0). Contrary to previous reports, MI arrest was not maintained by MAPK, since it was inactive in the oocytes arrested at MI in the ovary and activated immediately after spawning. Also, cyclin B was not degraded at pH 6.7 in the cell-free preparation without MAPK activity, whereas it was degraded at pH 7.0, suggesting that MI arrest was solely maintained by lower pH (< 7.0). Normal development occurred when the spawned oocytes were fertilized before the first polar body formation, whereas fertilization after the first polar body formation increased the rate of abnormal development. Thus, due to MI pause and MI arrest, the probability for fertilization before the polar body formation might be increased, leading to normal development.     

Emi1 Class of Proteins Regulate Entry into Meiosis and the Meiosis I to Meiosis II Transition in Xenopus Oocytes

Xenopus oocytes are arrested at the G2/prophase boundary of meiosis I and enter meiosis in response to progesterone. A hallmark of meiosis is the absence of DNA replication between the successive cell division phases meiosis I (MI) and meiosis II (MII). After the MI-MII transition, Xenopus eggs are locked in metaphase II by the cytostatic factor (CSF) arrest to prevent parthenogenesis. Early Mitotic Inhibitor 1 (Emi1) maintains CSF arrest by inhibiting the ability of the Anaphase Promoting Complex (APC) to direct the destruction of cyclin B. To investigate whether Emi1 has an earlier role in meiosis, we injected Xenopus oocytes with neutralizing antibodies against Emi1 at G2/prophase and during the MI-MII transition. Progesterone-treated G2/prophase oocytes injected with anti-Emi1 antibody fail to activate Maturation Promoting Factor (MPF), a complex of cdc2/cyclin B, and the MAPK pathway, and do not undergo germinal vesicle breakdown (GVBD). Injection of purified ?90 cyclin B protein or blocking anti-Emi1 antibody with purified Emi1 protein rescues these meiotic processes in Emi1-neutralized oocytes. Acute inhibition of Emi1 in progesterone treated oocytes immediately after GVBD causes rapid loss of cdc2 activity with simultaneous loss of cyclin B levels and inactivation of the MAPK pathway. These oocytes decondense their chromosomes and enter a DNA replication phase instead of progressing to MII. Prior ablation of Cdc20, addition of methyl-ubiquitin, or addition of indestructible ?90 cyclin B rescues the MI-MII transition in Emi1 inhibited oocytes.     

Comparative proteomic analysis of proteins involved in oocyte meiotic maturation in mice

After birth, oocytes stay at the diplotene stage in prophase of meiosis I. Meiosis resumes about 1 day before ovulation, and arrests in metaphase II (MII) after ovulation. The mature, MII oocytes are then ready for fertilization and to provide materials for early embryonic development. Proteomic characterization of oocytes can help identify proteins that are important for female meiotic maturation and early embryonic development. In this study, we compared the proteomic profiles between the germinal vesicle and MII mouse oocytes by two-dimensional electrophoresis; 95 differentially expressed protein spots corresponding to 63 proteins were identified. Many of these proteins are known to be essential for oocyte meiosis and early embryonic development, such as adenylosuccinate synthetase, nucleoplasmin-2, and protein-arginine deiminase type-6. Of the 12 proteins that were identified and are highly expressed in oocytes, a novel protein, E330034G19Rik, was found to be oocyte-specific. According to analysis by bioinformatics, it may regulate chromosome segregation during meiosis or cleavage. An in-depth study of these proteins will help us better understand the mechanisms of oocyte meiotic maturation, fertilization, and early embryogenesis. It will also help us understand the mechanisms of diseases that stem from abnormal oocyte maturation, such as polycystic ovary syndrome and premature ovary failure.     

Protein kinase C activity regulates the onset of anaphase I in mouse oocytes

The metaphase-to-anaphase I transition is a key step in the completion of meiosis I. In mouse oocytes, competence to exit metaphase I (MI) is developmentally regulated and typically not acquired until the preovulatory stage. The possible role of protein kinase C (PKC) in regulating this critical transition was assessed in both normal oocytes isolated from small antral follicles (18-day-old B6SJLF1 mice), which have not yet developed the capacity to progress to metaphase II (MII), and also oocytes defective in their ability to exit MI despite development to the preovulatory stage (24-day-old CX8 recombinant inbred strains). In both systems, transient suppression of endogenous PKC activity by treatment with a PKC-specific inhibitor, bisindolylmaleimide I (BIM), promoted the onset of anaphase I in a dose-dependent manner, while activation of PKC with the phorbol ester TPA blocked progression to MII. Following a 2-h incubation with BIM, the majority of oocytes progressed to, and arrested at, MII. The resulting MII oocytes were fertilizable in vitro, showing similar cleavage and blastocyst development rates between BIM treated and untreated controls. Transferred embryos resulted in the development of pups to term in both groups. These data demonstrate that PKC plays an important role in regulating the onset of anaphase I in mouse oocytes. Moreover, it is concluded that oocytes isolated from small antral follicles become blocked at MI due to a PKC-mediated signal, suggesting that acquisition of competence to complete meiosis I involves, in part, the control of PKC activity. Similarly, failure to regulate PKC activity at the preovulatory stage likely promotes arrest at MI.     

In vitro culture of mouse GV oocytes and preantral follicles isolated from ovarian tissues cryopreserved by vitrification

Cryopreservation of ovarian tissues containing many immature oocytes occurs in both gamete/embryo research and clinical medicine. Using vitrification, we studied factors related to meiosis after cryopreservation using the COCs (cumulus oocyte complexes) and preantral follicles obtained from cryopreserved ovarian tissues. COCs were isolated and cultured for 17 approximately 19 hr. Thereafter, Metaphase II stage (MII stage) oocytes and fertilized oocytes after IVF were observed at a rate of 76.5% and 60.0%, respectively. Preantral follicles (100 approximately 130 microm in diameter) were isolated and cultured in alpha MEM containing hFSH, ITS, and FBS. HCG and EGF were added to the media to stimulate ovulation on the 12th day of culture. The survival rates of the follicles obtained from the frozen/thawed ovaries were 66.4%. After 12 days of culture, the diameter of the follicles isolated from fresh (620.2 +/- 11.3 microm) and frozen/thawed ovaries (518.7 +/- 15.1 microm) differed as did the estradiol concentrations (3474.2 +/- 159 pg/ml vs. 1508.2 +/- 134 pg/ml). After in vitro ovulation, MII stage oocytes were observed in 84.5% of the fresh group and 60.5% of the frozen/thawed group while the fertilization rate was 74.2% and 53.5%, respectively. These studies demonstrate that cryopreservation of mouse ovarian tissues by vitrification did not affect the oocyte's ability to undergo meiosis. Thus, this technique may become a powerful tool for the preservation of the female gamete.     

The effect of pregnant mare's serum and chorionic gonadotropic hormone on the capacity of bovine oocytes, matured in vitro, for parthenogenetic activation by cycloheximide]

The effect of chorionic gonadotropic hormone (HG) and pregnant mare serum (PMS) on the capacity of cattle oocytes for spontaneous maturation and parthenogenetic formation of the female pronucleus was studied in situ and in vitro. HG added to the culture medium had no effect, irrespective of the dosage. However, perfusion of cow ovaries with HG at a dose of 2 micrograms/ml for 1 h abolished the block of meiosis I in 80% oocytes. About 25% oocytes reached metaphase II (MII) and telophase II (TII); 30% oocytes acquired the capacity to form female pronucleus upon the action of cycloheximide (25 Mg/ml). Perfusion of cow ovaries with PMS at a dose of 5 MU/ml for 1 h resulted in elimination of the block of meiosis I in 95% oocytes; 26% oocytes reached MII and TII, and about 17% oocytes acquired the capacity to form female pronucleus.     

A test of the production line hypothesis of mammalian oogenesis

Summary Germ cells in female mammals become committed to meiosis and enter its prophase sequentially in fetal life and, according to the Production Line Hypothesis, the oocytes thus generated are released after puberty as mature ova in the same sequence as that of meiotic entry in fetu. This hypothesis in its original and complete form has a subordinate proposition that concerns chiasma (Xma) frequency; it postulates that Xma number would decrease with fetal age. Consequently, univalents would increase, leading to errors of chromosome disjunction at the first meiotic division (MI), and thus to maternal age-dependent numerical chromosome anomalies. By using an in vitro/in vivo approach, we radioactively labelled the DNA of germ cells at premeiotic synthesis as they sequentially entered meiosis, while the fetal ovaries were in culture. At the end of this in vitro phase, pachytene/diplotene (P/D) stages were studied to determine their labelled fraction. The ovaries were then transplanted to spayed females and, after the in vivo phase, mature ova were harvested and the proportion of labelled first and second meiotic metaphases (MI/MII) determined. By marking the germ cells with label while in vitro during periods equivalent to early and late gestation, and by comparing the observed proportions of labelled MI/MII with those of oocytes labelled at P/D, we concluded that, in the mouse, ova do not mature at random for release, but are formed according to a production line system in which the time of release after puberty is related to the time of entry into meiosis in fetu.     

Metaphase I arrest and spontaneous parthenogenetic activation of strain LTXBO oocytes: chimeric reaggregated ovaries establish primary lesion in oocytes

Oocytes of strain LT mice, and related strains such as LTXBO, exhibit a high incidence of arrest in the progression of meiosis at metaphase I (MI) and in spontaneous parthenogenetic activation. Activation of these oocytes within the ovary leads to the formation of ovarian teratomas. In this study, the role of the oocyte's companion granulosa cells, the cumulus cells, was investigated using fully grown oocytes matured in vitro after isolation from LTXBO mice. Results showed that the role of cumulus cells in MI arrest is dichotomous. Cumulus cells temporarily helped to sustain MI arrest, but they also promoted a delayed progression to metaphase II. Cumulus cells also promoted parthenogenetic activation that occurred in association with the delayed progression to metaphase II. Next, the question of whether the lesion(s) promoting MI arrest and spontaneous activation is due to defects in the somatic cells or is intrinsic to the oocyte was addressed using chimeric reaggregated ovaries. An improved method for completely exchanging the germ cell and the somatic cell compartments of ovaries from newborn mice is described. These chimeric reaggregated ovaries, grafted beneath the renal capsule of SCID mice, allowed the complete development of LTXBO oocytes to occur in association with somatic cells from control (B6SJLF(1)) ovaries and development of control oocytes in association with LTXBO somatic cells. Oocyte growth and follicular development appeared generally normal in reaggregated ovaries. High incidences of MI arrest and spontaneous activation of LTXBO oocytes occurred regardless of the genotype of the somatic cells. Moreover, there was a low incidence of MI arrest and spontaneous activation of control oocytes, even though they underwent complete development and maturation associated with LTXBO somatic cells. It is concluded that the phenotypes of MI arrest and parthenogenetic activation in LTXBO oocytes are defects caused by lesions intrinsic to the oocyte. Nevertheless, the oocyte's companion somatic cells play crucial roles in the expression of these lesions.     

Checkpoint kinase 1 is essential for meiotic cell cycle regulation in mouse oocytes

Checkpoint kinase 1 (Chk1) plays key roles in all currently defined cell cycle checkpoints, but its functions in mouse oocyte meiosis remain unclear. In this study, we report the expression, localization and functions of Chk1 in mouse oocyte meiosis. Chk1 was expressed from germinal vesicle (GV) to metaphase II (MII) stages and localized to the spindle from pro-metaphase I (pro-MI) to MII stages in mouse oocytes. Chk1 depletion facilitated the G₂/M transition while Chk1 overexpression inhibited the G₂/M transition as indicated by germinal vesicle breakdown (GVBD), through regulation of Cdh1 and Cyclin B1. Chk1 depletion did not affect meiotic cell cycle progression after GVBD, but its overexpression after GVBD activated the spindle assembly checkpoint and prevented homologous chromosome segregation, thus arresting oocytes at pro-MI or metaphase I (MI) stages. These results suggest that Chk1 is indispensable for prophase I arrest and functions in G₂/M checkpoint regulation in meiotic oocytes. Moreover, Chk1 overexpression affects meiotic spindle assembly checkpoint regulation and thus chromosome segregation.     

Pronuclear synchronization and nuclear morphology of mature and in vitro matured oocytes in the rat: An ultrastructural study

Summary The objective of this study was to evaluate synchronous and asynchronous pronucleus (PN) formation and the related patterns of juxtapositional nucleolus (n) formation in immature (prophase I [PI] and metaphase I [MI]) and mature (metaphase II [MII]) oocytes after fertilization, both ultrastructurally and at the level of light microscope. A single dose of 15 IU gonadotrophin was injected subcutaneously to twenty four 26-wk-old, female Wistar rats to induce ovulation. Human chorionic gonadotrophin (4 IU) was administered 40 h later, and after 4–6 h the ovaries were dissected, and the oocytes were aspirated. A total of 214 rat oocytes were classified according to a maturation index as follows: group I, 80 PI oocytes; group II, 50 MI oocytes; and group III, 84 MII oocytes. Immature oocytes were in vitro matured for 18–36 h. Spermatozoa were acquired by microepididymal sperm aspiration and processed using swim-up technique. Intracytoplasmic sperm injection was performed on mature oocytes after 2 h of incubation and on in vitro matured (IVM) oocytes 4 h after maturation. Pronuclear synchronization [both pronucleases (PNs) centrally located, equal sized, with equal numbers and sizes of juxtapositional nucleoli (Nn)] was observed in fertilized oocytes. Asynchronous PN formation (diversity between male and female PNs, related to dimensions, localization, and the number of Nn) in groups I, II, and III was found in 75, 86, and 47% of preembryos, respectively. There was a significant difference of synchronous pronuclear formation between mature and IVM oocytes (P<0.05). In IVM oocytes, asynchronous PN formation is high, and juxtapositional pronucleolar patterns are observed to be low by transmission electron microscope (TEM).     

Mitogen-activated protein kinase activity during goat oocyte maturation and the acquisition of meiotic competence

Changes in MPF and MAPK activities during meiotic maturation of goat oocytes were investigated. Detection of MPF activity occurred concomitantly with GVBD, increased at MI, decreased during anaphase-telophase I transition, and increased thereafter in MII oocytes. The appearance of MAPK activity was delayed compared to MPF activity. MAPK activity increased after GVBD and persisted during the MI-MII transition. Whether MAPK was implicated in goat oocyte meiotic competence was also investigated by using oocytes from different follicle size categories that arrest at specific stages of the maturation process (GV, GVBD, MI, and MII). Results indicate that the ability of goat oocytes to resume meiosis is not directly related to the presence of Erk2. The ability to phosphorylate MAPK is acquired by the oocyte during follicular growth after the ability to resume meiosis. GVBD-arrested oocytes exhibited a high level of MPF activity after 27 hr of culture. However, 28% of oocytes from this group contained inactive MAPK, and 72% exhibited high MAPK activity. In addition, 29% of GVBD-arrested oocytes contained a residual interphasic network without recruitment of microtubules around the condensed chromosomes; 71% of GVBD-arrested oocytes displayed recruitment of microtubules near the condensed chromosomes and contained asters of microtubules distributed throughout the cytoplasm. These results indicate that oocytes arrested at GVBD were not exactly at the same point in the meiotic cell cycle progression, and suggest that MAPK could be implicated in the regulation of microtubule organization. The data presented here suggest that in goat oocytes, MAPK is not implicated in the early events of meiosis resumption, but rather in post-GVBD events such as spindle formation and MII arrest. © 1996 Wiley-Liss Inc.     

Modulation of cell cycle control during oocyte‐to‐embryo transitions

Ex ovo omnia—all animals come from eggs—this statement made in 1651 by the English physician William Harvey marks a seminal break with the doctrine that all essential characteristics of offspring are contributed by their fathers, while mothers contribute only a material substrate. More than 360 years later, we now have a comprehensive understanding of how haploid gametes are generated during meiosis to allow the formation of diploid offspring when sperm and egg cells fuse. In most species, immature oocytes are arrested in prophase I and this arrest is maintained for few days (fruit flies) or for decades (humans). After completion of the first meiotic division, most vertebrate eggs arrest again at metaphase of meiosis II. Upon fertilization, this second meiotic arrest point is released and embryos enter highly specialized early embryonic divisions. In this review, we discuss how the standard somatic cell cycle is modulated to meet the specific requirements of different developmental stages. Specifically, we focus on cell cycle regulation in mature vertebrate eggs arrested at metaphase II (MII‐arrest), the first mitotic cell cycle, and early embryonic divisions.     

Degradation of Ccnb3 is essential for maintenance of MII arrest in oocyte

Before fertilization, ovulated mammalian oocytes are arrested at the metaphase of second meiosis (MII), which is maintained by the so-called cytostatic factor (CSF). It is well known that the continuous synthesis and accumulation of cyclin B is critical for maintaining the CSF-mediated MII arrest. Recent studies by us and others have shown that Ccnb3 is required for the metaphase-to-anaphase transition during the first meiosis of mouse oocytes, but whether Ccnb3 plays a role in MII arrest and exit remains unknown. Here, we showed that the protein level of Ccnb3 gradually decreased during oocyte meiotic maturation, and exogenous expression of Ccnb3 led to release of MII arrest, degradation of securin, separation of sister chromatids, extrusion of the second polar body (PB2), and finally entry into interphase. These phenotypes could be rescued by inhibition of Wee1B or CDK2. Our results indicate that Ccnb3 plays a critical regulatory role in MII arrest and exit in mouse oocytes.     

Mouse Oocyte Maturation: Meiotic Checkpoints

Mouse oocytes at different stages of maturation were fused together and the ensuing cell cycle events were analyzed with the objective of identifying checkpoints in meiosis. Fusion of maturing oocytes just undergoing germinal vesicle breakdown (GVBD) induces PCC (premature chromosome condensation) but no spindle formation in immature (GV) partner oocytes. On the other hand, fusion of metaphase I (MI) oocytes containing spindles to GV oocytes induces both PCC and spindle formation in the immature partner. Thus, while molecules required for condensation are present throughout metaphase, those involved in spindle formation are absent in early M-phase. Oocytes cultured for 6 h—early metaphase I (i.e., 2 h before the onset of anaphase I)—and then fused to anaphase-telophase I (A-TI) fusion partners block meiotic progression in the more advanced oocytes and induce chromatin dispersal on the spindle. By contrast, oocytes cultured for 8 h (late MI) before fusion to A-TI partners are driven into anaphase by signals from the more advanced oocytes and thereafter advance in synchrony to telophase I. When early (10 h) or late (12 h) metaphase II oocytes were fused to A-TI partners the signals generated from early MII oocytes block the anaphase to telophase I transition and induce a dispersal of A-TI chromosomes along the spindle. On the other hand, late MII oocytes respond to A-TI signals by exiting from the MII block and undergoing the A-TII transition. Moreover, the oocytes in late MI are not arrested in this stage and progress without any delay through A-TI to MII when fused to metaphase II partners. The signals from the less-developed partner force the MII oocyte through A-TII to MIII. In total, these studies demonstrate that the metaphase period is divided into at least three distinct phases and that a checkpoint in late metaphase controls the progress of meiosis in mammalian oocytes.     

H3 Thr3 phosphorylation is crucial for meiotic resumption and anaphase onset in oocyte meiosis

Haspin-catalyzed histone H3 threonine 3 (Thr3) phosphorylation facilitates chromosomal passenger complex (CPC) docking at centromeres, regulating indirectly chromosome behavior during somatic mitosis. It is not fully known about the expression and function of H3 with phosphorylated Thr3 (H3T3-P) during meiosis in oocytes. In this study, we investigated the expression and sub-cellular distribution of H3T3-P, as well as its function in mouse oocytes during meiotic division. Western blot analysis revealed that H3T3-P expression was only detected after germinal vesicle breakdown (GVBD), and gradually increased to peak level at metaphase I (MI), but sharply decreased at metaphase II (MII). Immunofluorescence showed H3T3-P was only brightly labeled on chromosomes after GVBD, with relatively high concentration across the whole chromosome axis from pro-metaphase I (pro-MI) to MI. Specially, H3T3-P distribution was exclusively limited to the local space between sister centromeres at MII stage. Haspin inhibitor, 5-iodotubercidin (5-ITu), dose- and time-dependently blocked H3T3-P expression in mouse oocytes. H3T3-P inhibition delayed the resumption of meiosis (GVBD) and chromatin condensation. Moreover, the loss of H3T3-P speeded up the meiotic transition to MII of pro-MI oocytes in spite of the presence of non-aligned chromosomes, even reversed MI-arrest induced with Nocodazole. The inhibition of H3T3-P expression distinguishably damaged MAD1 recruitment on centromeres, which indicates the spindle assembly checkpoint was impaired in function, logically explaining the premature onset of anaphase I. Therefore, Haspin-catalyzed histone H3 phosphorylation is essential for chromatin condensation and the following timely transition from meiosis I to meiosis II in mouse oocytes during meiotic division.     

Meiotic metaphase arrest in animal oocytes: its mechanisms and biological significance

Metaphase arrest in meiosis I or II before fertilization is a common and unique feature of oogenesis in many animal species. How and why oocytes from many species are arrested at metaphase, rather than after the completion of meiosis, has long remained a mystery. This article reviews recent advances in our understanding of the mechanisms and biological significance of meiotic metaphase arrest in animal oocytes.     

Stability of cyclin B protein during meiotic maturation and the first mitotic cell division in mouse oocytes

This report examines in detail the metabolism of the cyclin protein B1 during meiotic maturation and following the activation of mature mouse oocytes using immunoprecipitation of the radiolabelled protein. The net synthesis of cyclin B increases progressively during meiotic maturation, reaching its maximum levels at least 1 h before oocytes exit into metaphase of meiosis II (MII). This increase correlates with the rise in cdc2 kinase activity reported previously and suggests an association between the length of the first meiotic M phase (MI) and the net synthesis of cyclin B, that seems to regulate the time required for the cdc2 kinase to reach its maximum activity. Moreover, no marked degradation of cyclin B was observed before the MI to MII transition and that which occurs does so independently of the presence of microtubules, which are essential for cyclin degradation during metaphase II arrest and exit of oocytes into interphase of the first mitotic cell cycle. Cyclin B is degraded rapidly during the transitions MI to MII, MII to the first mitotic interphase and MII to an abortive third metaphase state (MIII). However, whilst its degradation was incomplete during the MI to MII transition, virtually no cyclin B protein was detected following both the MII to interphase and MII to MIII transitions. Thus, the decision of oocytes to exit into MIII, or interphase is not controlled at the level of cyclin B degradation. Lastly, in aging, non-activated oocytes, the net synthesis of cyclin B declines. Whereas, in activated eggs cultured in parallel although the rate of net synthesis declines initially, it is effectively ‘rescued’ being two-fold greater than in non-activated oocytes of an equivalent age. This gradual fall in the net synthesis of cyclin B observed in aging oocytes may contribute to the increasing ease with which they become activated, compared to recently ovulated oocytes.     

In vitro maturation,fertilization and development of domestic cat oocytes recovered from ovaries collected at three stages of the reproductive cycle

This study was conducted to examine the effect of the donor cat's reproductive cycle stage on in vitro maturation (IVM), in vitro fertilization (IVF), and in vitro development of oocytes recovered from ovaries that were collected and stored at 35 degrees C for a short period (1-6 h). Based on the presence or absence of follicles and corpora lutea, the ovarian pairs collected were classified into inactive, follicular, or luteal stages. Nuclear status of 161 cumulus-oocyte complexes (COCs) were examined immediately after recovery; 91.3% of the oocytes were found to be at the immature germinal vesicle (GV) stage, and 3.7% of the oocytes were at metaphase II (MII) stage. The percentage of the oocytes at the GV stage was significantly lower in the follicular stage than in the inactive stage (P < 0.01). Of the oocytes from the follicular stage, 9.1% were at MII stage. After culture for 24 h, however, the proportions of oocytes that reached metaphase I and MII were not different among the reproductive cycle stages of the ovaries collected (P > 0.05). After co-incubation with sperm, 63.1% of oocytes were fertilized, but there were no significant differences among the reproductive cycle stages of the ovaries with respect to the proportions of normal and polyspermic fertilization. However, the number of oocytes reaching cleavage stage and development to the morula and blastocyst stages from follicular stage ovaries were significantly lower (P < 0.05) than those obtained from inactive and luteal stage ovaries. These results indicate that the reproductive cycle stage of donor cat ovaries, stored at 35 degrees C, has no apparent effects on the frequencies of maturation and fertilization of oocytes, but influences developmental competence of the oocytes following IVM or IVF.