Analysis of the mechanism(s) of metaphase I arrest in strain LT mouse oocytes: participation of MOS

Mapk activation drives male and female mouse teratocarcinomas from late PGCs

Germ cell tumors (GCTs) are rare tumors that can develop in both sexes, peaking in adolescents. To understand the mechanisms that underlie germ cell transformation, we established a GCT mouse model carrying germ cell-specific BRafV600E mutation with or without heterozygous Pten deletion. Both male and female mice developed monolateral teratocarcinomas containing embryonal carcinoma (EC) cells that showed an aggressive phenotype and metastatic ability. Germ cell transformation started in fetal gonads and progressed after birth leading to gonadal invasion. Early postnatal testes showed foci of tumor transformation, while ovaries showed increased number of follicles, multi-ovular follicles (MOFs) and scattered metaphase I oocytes containing follicles. Our results indicate that Mapk over-activation in fetal germ cells of both sexes can expand their proliferative window leading to neoplastic transformation and metastatic behavior.

Suppression of chemically induced and spontaneous mouse oocyte activation by AMP-activated protein kinase

Oocyte activation is an important process triggered by fertilization that initiates embryonic development. However, parthenogenetic activation can occur either spontaneously or with chemical treatments. The LT/Sv mouse strain is genetically predisposed to spontaneous activation. LT oocytes have a cell cycle defect and are ovulated at the metaphase I stage instead of metaphase II. A thorough understanding of the female meiosis defects in this strain remains elusive. We have reported that AMP-activated protein kinase (PRKA) has an important role in stimulating meiotic resumption and promoting completion of meiosis I while suppressing premature parthenogenetic activation. Here we show that early activation of PRKA during the oocyte maturation period blocked chemically induced activation in B6SJL oocytes and spontaneous activation in LT/SvEiJ oocytes. This inhibitory effect was associated with high levels of MAPK1/3 activity. Furthermore, stimulation of PRKA partially rescued the meiotic defects of LT/Sv mouse oocytes in concert with correction of abnormal spindle pole localization of PRKA and loss of prolonged spindle assembly checkpoint activity. Altogether, these results confirm a role for PRKA in helping sustain the MII arrest in mature oocytes and suggest that dysfunctional PRKA contributes to meiotic defects in LT/SvEiJ oocytes.

Spindle assembly checkpoint-related meiotic defect in oocytes from LT/Sv mice has cytoplasmic origin and diminishes in older females

The spindle assembly checkpoint (SAC) ensures proper segregation of chromosomes by delaying anaphase onset until all kinetochores are properly attached to the spindle microtubules. Oocytes from the mouse strain LT/Sv arrest at the first meiotic metaphase (MI) due to, as reported recently, enormously prolonged activity of the SAC. We compared the dynamics of cyclin B1-GFP degradation, the process which is a measure of the SAC activity, in chromosomal and achromosomal halves of LT/Sv oocytes. In chromosome-containing oocyte halves arrested at MI, cyclin B1-GFP was not degraded indicating active SAC. However, in the halves lacking chromosomes, which is a condition precluding the SAC function, degradation always occurred confirming that MI arrest in LT/Sv oocytes is SAC dependent. Transferring the germinal vesicle (GV) from LT/Sv oocytes into the enucleated oocytes from wild-type mice resulted in the progression through meiosis one, indicating that a SAC-activating defect in LT/Sv oocytes is cytoplasmic, yet can be rescued by foreign cytoplasm. These results may help to define the etiology of the human infertility related to the oocyte MI arrest, indicating the involvement of the SAC as likely candidate, and point to GV transfer as the possible therapy. Finally, we found that majority of oocytes isolated from old LT/Sv mice complete the first meiosis. Reciprocal transfers of the GV between the oocytes from young and old LT/Sv females suggest that the factor(s) responsible for the reversal of the phenotype in oocytes from old mice is located both in the GV and in the cytoplasm.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

Spindle assembly checkpoint-related failure perturbs early embryonic divisions and reduces reproductive performance of LT/Sv mice

The phenotype of the LT/Sv strain of mice is manifested by abnormalities in oocyte meiotic cell-cycle, spontaneous parthenogenetic activation, teratomas formation, and frequent occurrence of embryonic triploidy. These abnormalities lead to the low rate of reproductive success. Recently, metaphase I arrest of LT/Sv oocytes has been attributed to the inability to timely inactivate the spindle assembly checkpoint (SAC). As differences in meiotic and mitotic SAC functioning were described, it remains obscure whether this abnormality is limited to the meiosis or also impinges on the mitotic divisions of LT/Sv embryos. Here, we show that a failure to inactivate SAC affects mitoses during preimplantation development of LT/Sv embryos. This is manifested by the prolonged localization of MAD2L1 on kinetochores of mitotic chromosomes and abnormally lengthened early embryonic M-phases. Moreover, LT/Sv embryos exhibit elevated frequency of abnormal chromosome separation during the first mitotic division. These abnormalities participate in severe impairment of preimplantation development and significantly decrease the reproductive success of this strain of mice. Thus, the common meiosis and mitosis SAC-related failure participates in a complex LT/Sv phenotype.

Metaphase I arrest in LT/Sv mouse oocytes involves the spindle assembly checkpoint

During meiotic maturation, the majority of oocytes from LT/Sv mice arrest at metaphase I. However, anaphase may be induced through parthenogenetic activation. If this happens within the ovary, it often results in the development of ovarian teratomas. Here, we show that the induction of first meiotic anaphase in LT/Sv oocytes results in incorrect chromosome segregation. In search of the molecular basis of this complex phenotype, we analyzed the localization/destruction of cohesins, as well as the function of the components of the spindle assembly checkpoint (SAC). Both localization and removal of meiotic cohesin REC8 from chromosomes are unperturbed. In contrast, there is prolonged localization of SAC proteins BUB1 and MAD2L1 (MAD2) at the metaphase I kinetochores in mutant oocytes compared with the wild-type. Interfering with BUB1 function through expression of a dominant-negative mutant protein resulted in the increase of the number of LT/Sv oocytes completing the first meiosis, which indicates SAC involvement in metaphase I arrest. These data show for the first time that there is a direct link between the SAC function and the heritable meiotic incompetence of a mammalian oocyte.

Allocation of Gamma-Tubulin Between Oocyte Cortex and Meiotic Spindle Influences Asymmetric Cytokinesis in the Mouse Oocyte1

In oocytes, asymmetric cytokinesis represents a conserved strategy for karyokinesis during meiosis to retain ooplasmic maternal factors needed after fertilization. Given the role of gamma-tubulin in cell cycle progression and microtubule dynamics, this study focused on gamma-tubulin as a key regulator of asymmetric cytokinesis in mouse oocytes. Gamma-tubulin properties were studied using multiple-label digital imaging, Western blots, quantitative RT-PCR, and microinjection strategies in mouse oocytes matured in vivo (IVO) or in vitro (IVM). Quantitative image analysis established that IVO oocytes extrude smaller first polar bodies (PBs), contain smaller spindles, and have more cytoplasmic microtubule organizing centers (MTOCs) relative to IVM oocytes. Maturation in culture was shown to alter gamma-tubulin distribution, as evidenced by incorporation throughout the meiotic spindle and within the first PB. Western blot analysis confirmed that total gamma-tubulin content remained elevated in IVM oocytes compared with IVO oocytes. Analysis of gamma-tubulin mRNA during maturation revealed fluctuations in IVO oocytes, whereas IVM oocytes maintained relatively stable at lower levels for the time points examined (0-16 h). Selective reduction of gamma-tubulin mRNA by injection of siRNA diminished both spindle and PB size, whereas overexpression of enhanced green fluorescent protein gamma-tubulin had the opposite effect. Together, these studies reinforce the notion that limiting gamma-tubulin availability during meiotic maturation ensures coordination of karyokinesis and cytokinesis and conservation of gamma-tubulin as an embryonic reserve.

Changes in endoplasmic reticulum structure during mouse oocyte maturation are controlled by the cytoskeleton and cytoplasmic dynein

Oocyte maturation in mouse is associated with a dramatic reorganisation of the endoplasmic reticulum (ER) from a network of cytoplasmic accumulations in the germinal vesicle-stage oocyte (GV) to a network of distinctive cortical clusters in the metaphase II egg (MII). Multiple lines of evidence suggest that this redistribution of the ER is important to prepare the oocyte for the generation of repetitive Ca2+ transients which trigger egg activation at fertilisation. The aim of the current study was therefore to investigate the timecourse and mechanism of ER reorganisation during oocyte maturation. The ER is first restructured at the time of GV-breakdown (GVBD) into a dense network of membranes which envelop and invade the developing meiotic spindle. GVBD is essential for the initiation of ER reorganisation, since ER structure does not change in GV-arrested oocytes. ER reorganisation is also prevented by the microtubule inhibitor nocodazole and by the inhibition of cytoplasmic dynein, a microtubule-associated motor protein. ER redistribution at GVBD is therefore dynein-driven and cell cycle-dependent. Following GVBD the dense network of ER surrounds the spindle during its migration to the oocyte cortex. Cortical clusters of ER are formed close to the time of, but independently of the metaphase I-metaphase II transition. Formation of the characteristic ER clusters is prevented by the depolymerisation of microfilaments, but not of microtubules. These experiments reveal that ER reorganisation during oocyte maturation is a complex multi-step process involving distinct microtubule- and microfilament-dependent phases and indicate a role for dynein in the cytoplasmic changes which prepare the oocyte for fertilisation.

Selective degradation of maternal and embryonic transcripts in in vitro produced bovine oocytes and embryos using sequence specific double-stranded RNA

RNA interference (RNAi) has been used for selective degradation of an mRNA transcript or inhibiting its translation to a functional protein in various species. Here, we applied the RNAi approach to suppress the expression of the maternal transcript C-mos and embryonic transcripts Oct-4 in bovine oocytes and embryos respectively, using microinjection of sequence-specific double-stranded RNA (dsRNA). For this, 435 bp C-mos and 341 bp Oct-4 dsRNA were synthesized and microinjected into the cytoplasm of immature oocytes and zygotes respectively. In experiment 1, immature oocytes were categorized into three groups: those injected with C-mos dsRNA, RNase-free water and uninjected controls. In experiment 2, in vitro produced zygotes were categorized into three groups: those injected with Oct-4 dsRNA, RNase-free water and uninjected controls. The developmental phenotypes, the level of mRNA and protein expression were investigated after treatment in both experiments. Microinjection of C-mos dsRNA has resulted in 70% reduction of C-mos transcript after maturation compared to the water-injected and uninjected controls (P<0.01). Microinjection of zygotes with Oct-4 dsRNA has resulted in 72% reduction in transcript abundance at the blastocyst stage compared to the uninjected control zygotes (P<0.01). Moreover, a significant reduction in the number of inner cell mass (ICM) cells was observed in Oct-4 dsRNA-injected embryos compared to the other groups. From oocytes injected with C-mos dsRNA, 60% showed the extrusion of the first polar body compared to 50% in water-injected and 44% in uninjected controls. Moreover, only oocytes injected with C-mos dsRNA showed spontaneous activation. In conclusion, our results demonstrated that sequence-specific dsRNA can be used to knockdown maternal or embryonic transcripts in bovine embryogenesis.

Genetic influences on ovulation of primary oocytes in LT/Sv strain mice

A high proportion of LT/Sv strain oocytes arrest in meiotic metaphase I (MI) and are ovulated as diploid primary oocytes rather than haploid secondary oocytes. (Mus musculus castaneus x LT/SvKau)F1 x LT/SvKau backcross females were analysed for the proportion of oocytes that arrested in MI and typed by PCR for a panel of microsatellite DNA sequences (simple sequence repeat polymorphisms) that differed between strain LT/SvKau and M. m. castaneus. This provided a whole genome scan of 86 genetic markers distributed over all 19 autosomes and the X chromosome, and revealed genetic linkage of the MI arrest phenotype to markers on chromosomes 1 and 9. Identification of these two chromosomal regions should facilitate the identification of genes involved in mammalian oocyte maturation and the control of meiosis.

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.

A binding site for germ cell nuclear factor within c-mos regulatory sequences

The proto-oncogene c-mos is specifically expressed in male and female germ cells. Previous studies have shown that the orphan nuclear receptor chicken ovalbumin upstream promoter transcription factor (COUP-TF) contributes to the repression of c-mos in somatic cells by binding to an inverted hexamer repeat within the c-mos regulatory region. In the present study, we demonstrate that another nuclear receptor superfamily member, germ cell nuclear factor (GCNF), binds to a sequence overlapping the c-mos COUP-TF binding site. Electrophoretic mobility shift assays with recombinant GCNF and both wild-type and mutant c-mos oligonucleotides demonstrated the binding of GCNF to an extended half site, CCAAGTTCA, which overlaps the first hexamer of the COUP-TF binding site. Transient transfection assays in NIH 3T3 cells further demonstrated that GCNF fused to a VP16 activation domain stimulated transcription from reporter constructs containing the c-mos GCNF binding site. Since GCNF is expressed in male and female germ cells at the same stages of development at which c-mos is transcribed, these results suggest that GCNF may serve as a regulator of c-mos transcription. Mol. Reprod. Dev. 67: 55-64, 2004.

Cell-cycle control during meiotic maturation

The meiotic cell cycle, which is comprised of two consecutive M-phases, is crucial for the production of haploid germ cells. Although both mitotic and meiotic M-phases share cyclin-B-Cdc2/CDK1 as a key controller, there are meiosis-specific modulations in the regulation of cyclin-B-Cdc2. Recent insights indicate that a common pattern in these modulations can be found by considering the particular activities of mitogen-activated protein kinase (MAPK) during meiosis. The G(2)-phase arrest of meiosis I is released via specific, MAPK-independent signalling that leads to cyclin-B-Cdc2 activation; thereafter, however, the meiotic process is under the control of interplay between MAPK and cyclin-B-Cdc2.

Requirement of Cks2 for the first metaphase/anaphase transition of mammalian meiosis

We generated mice lacking Cks2, one of two mammalian homologs of the yeast Cdk1-binding proteins, Suc1 and Cks1, and found them to be viable but sterile in both sexes. Sterility is due to failure of both male and female germ cells to progress past the first meiotic metaphase. The chromosomal events up through the end of prophase I are normal in both CKS2-/- males and females, suggesting that the phenotype is due directly to failure to enter anaphase and not a consequence of a checkpoint-mediated metaphase I arrest.

Follicular factors regulating oocyte maturation and quality

Maturation of the oocyte can be divided into two different aspects: nuclear maturation and cytoplasmic maturation. The spontaneous nature of nuclear maturation in oocytes removed from the follicle and cultured in vitro was observed in mammals as early as 1935. However, oocytes cultured in basic conditions are deficient in some cytoplasmic factors and are, therefore, developmentally incompetent. Data from large domestic species indicate that although oocytes matured in vitro in supplemented media can develop after fertilization, they require the presence of follicular factors during culture to ensure their developmental competence. The importance of follicular maturation on the capacity of oocytes to achieve fertilization and early embryonic development can be studied by reproducing some important events in vitro. Follicular supplementation may be in the form of follicular fluid, granulosa cells or follicle-conditioned media, and there is evidence that the maturational status of the follicle used for co-culture influences subsequent male pronuclear formation. Studies in this laboratory on the prolific Chinese Meishan pig, which has significantly higher early embryonic survival than conventional European breeds, indicate crucial differences in the pattern of follicle development and, therefore, the intrafollicular environment in which the oocytes are nurtured. It is suggested that this produces oocytes of improved 'quality' and this hypothesis is supported by experiments both in vivo and in vitro. Ultimately, it is hoped that these studies on large domestic animals will lead to identification of the follicular factors that influence oocyte quality.

Evidence that protein kinase C (PKC) participates in the meiosis I to meiosis II transition in mouse oocytes

Oocytes from LTXBO mice exhibit a delayed entry into anaphase I and frequently enter interphase after the first meiotic division. This unique oocyte model was used to test the hypothesis that protein kinase C (PKC) may regulate the meiosis I-to-meiosis II transition. PKC activity was detected in LTXBO oocytes at prophase I and increased with meiotic maturation, with the highest (P < 0.05) activity observed at late metaphase I (MI). Treatment of late MI-stage oocytes with the PKC inhibitor, bisindolylmaleimide I (BIM), transiently reduced (P < 0.05) M-phase-promoting factor (MPF) activity and promoted (P < 0.05) progression to metaphase II (MII), while mitogen-activated protein kinase (MAPK) activity remained elevated during the MI-to-MII transition. Confocal microscopy analysis of LTXBO oocytes during this transition showed PKC-delta associated with the meiotic spindle and then with the chromosomes at MII. Inhibition of PKC activity also prevented untimely entry into interphase, but only when PKC activity was reduced in oocytes before the progression to MII and thus indicates that the transition into interphase is directly associated with the delayed triggering of anaphase I. Moreover, the defect(s) that initiate activation occur upstream of MAPK, as suppression of PKC activity failed to prevent activation by Mos(tm1Ev)/ Mos(tm1Ev) LTXBO oocytes expressing no detectable MAPK activity. In summary, PKC participates in the regulatory mechanisms that delay entry into anaphase I in LTXBO oocytes, and the disruption promotes untimely entry into interphase. Thus, loss of regulatory control over PKC activity during oocyte maturation disrupts the critical MI-to-MII transition, leading to a precocious exit from meiosis.

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.

Analysis of the mechanism(s) of metaphase I-arrest in strain LT mouse oocytes: delay in the acquisition of competence to undergo the metaphase I/anaphase transition

Fully grown oocytes of most laboratory mice progress without interruption from the germinal vesicle (GV) stage to metaphase II, where meiosis is arrested until fertilization. In contrast, many oocytes of strain LT mice arrest precociously at metaphase I and often undergo subsequent spontaneous parthenogenetic activation. Cytostatic factor (CSF), which prevents the degradation of cyclin B and maintains high maturation-promoting factor (MPF) activity, is required for maintenance of metaphase I-arrest in LT oocytes, similar to its requirement for maintaining metaphase II-arrest in normal oocytes. However, CSF does not instigate metaphase I-arrest since a temporary metaphase I-arrest occurs in MOS-null LT oocytes. This paper addresses the mechanism(s) that may instigate metaphase I-arrest and tests the hypothesis that there may be one or more defects in LT oocytes that delay their acquisition of competence to trigger the cascade of processes that normally drive entry into and progression through anaphase I. To test this hypothesis, MPF activity was artificially abrogated by treating oocytes with a general protein kinase inhibitor, 6-DMAP, at various times during the progression of meiosis I. This allowed a comparison of the time at which LT and normal oocytes become competent to undergo the metaphase I/anaphase transition even if oocytes were arrested at metaphase I when 6-DMAP-treatment was begun. There were no differences between LT and control oocytes in the kinetics of MPF suppression by 6-DMAP. However, it was found that LT oocytes do not acquire competence to undergo the metaphase I/anaphase transition in response to 6-DMAP until 50-60 min after normal oocytes. A similar delay was observed in strain CX8-4 oocytes, which also have a high incidence of metaphase I-arrest, but not in strain CX8-11 oocytes, which exhibit a low incidence of metaphase I-arrest. MOS-null LT oocytes also exhibit a delay in acquisition of competence to undergo the metaphase I/anaphase transition. Thus, a delay in competence to undergo the metaphase I/anaphase transition in response to 6-DMAP-treatment correlates with metaphase I-arrest. It is therefore hypothesized that the observed delay in acquisition of competence to enter anaphase I may instigate the sustained metaphase I-arrest in LT oocytes by allowing CSF activity to rise to a level that prevents cyclin B degradation and maintains high MPF activity before anaphase can be initiated by normal triggering mechanisms.

Cytostatic activity develops during meiosis I in oocytes of LT/Sv mice

Oocytes of wild-type mice are ovulated as the secondary oocytes arrested at metaphase of the second meiotic division. Their fertilization or parthenogenetic activation triggers the completion of the second meiotic division followed by the first embryonic interphase. Oocytes of the LT/Sv strain of mice are ovulated either at the first meiotic metaphase (M I) as primary oocytes or in the second meiotic metaphase (M II) as secondary oocytes. We show here that during in vitro maturation a high proportion of LT/Sv oocytes progresses normally only until metaphase I. In these oocytes MAP kinase activates shortly after histone H1 kinase (MPF) activation and germinal vesicle breakdown. However, MAP kinase activation is slightly earlier than in oocytes from wild-type F1 (CBA/H x C57Bl/10) mice. The first meiotic spindle of these oocytes forms similarly to wild-type oocytes. During aging, however, it increases in size and finally degenerates. In those oocytes which do not remain in metaphase I the extrusion of first polar bodies is highly delayed and starts about 15 h after germinal vesicle breakdown. Most of the oocytes enter interphase directly after first polar body extrusion. Fusion between metaphase I LT/Sv oocytes and wild-type mitotic one-cell embryos results in prolonged M-phase arrest of hybrids in a proportion similar to control LT/Sv oocytes and control hybrids made by fusion of two M I LT/Sv oocytes. This indicates that LT/Sv oocytes develop cytostatic factor during metaphase I. Eventually, anaphase occurs spontaneously and the hybrids extrude the polar body and form pronuclei in a proportion similar as in controls. In hybrids between LT/Sv metaphase I oocytes and wild-type metaphase II oocytes (which contain cytostatic factor) anaphase I proceeds at the time observed in control LT/Sv oocytes and hybrids between two M I LT/Sv oocytes, and is followed by the parthenogenetic activation and formation of interphase nuclei. Also the great majority of hybrids between M I and M II wild-type oocytes undergoes the anaphase but further arrests in a subsequent M-phase. These observations suggest that an internally triggered anaphase I occurs despite the presence of the cytostatic activity both in LT/Sv and wild-type M I oocytes. Anaphase I triggering mechanism must therefore either inactivate or override the CSF activity. The comparison between spontaneous and induced activation of metaphase I LT/Sv oocytes shows that mechanisms involved in anaphase I triggering are altered in these oocytes. Thus, the prolongation of metaphase I in LT/Sv oocytes seems to be determined by delayed anaphase I triggering and not provoked directly by the cytostatic activity.