1 Department of Genetics and 2 Department of Pharmacology, Case Western Reserve University, Cleveland, Ohio, USA
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Abstract |
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Key words: age-related aneuploidy/folliculogenesis/meiosis/non-disjunction
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Introduction |
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The complexity of a meiotic process in which chromosome segregation is influenced both by prenatal events and age has significantly complicated the study of human meiotic non-disjunction. Studies of human trisomy have provided considerable information on the impact of age but not the underlying molecular mechanism of error. Further, since the effect appears to be unique to the human female, studies in lower eukaryotes have provided little insight. However, recent direct studies of human oocytes by us (Volarcik et al., 1998) and by others (Battaglia et al., 1996
) have provided evidence for a meiotic phenotype that may be related to non-disjunction. Specifically, in both laboratories, immunofluorescence studies of oocytes from different-aged donors revealed an age-related increase in oocytes with gross aberrations in spindle morphology and chromosome alignment; as the chromosomes appeared unable to move to the equator of the MI spindle, we termed this condition `congression failure' (Figure 1a,b
). Because the oocyte acquires the competency to reinitiate and complete the meiotic divisions during the late stages of growth in the adult ovary, we interpreted this age-related disturbance in chromosome alignment as evidence that the maternal age effect on chromosome segregation results from a decline in the growth process of the human oocyte (Volarcik et al., 1998
).
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Because age-related non-disjunction appears to be unique to the human, designing experimental approaches to study the mechanism of human meiotic defects has proved difficult. However, the hypothesis that subtle changes in the complex interplay of signals that regulate folliculogenesis may impact the meiotic process is testable in an experimental mammal. Accordingly, we initiated meiotic studies of mouse oocytes in several situations in which folliculogenesis is disrupted. In this report, we summarize data from these studies that suggest that: (i) congression failure at MI is associated with a primary defect in either the intraovarian or extraovarian environment, but that it is not merely a symptom of oocyte immaturity; (ii) congression failure at MI is not associated with metaphase arrest or a delay in anaphase onset; and (iii) congression failure is correlated with an increase in meiotic non-disjunction. Taken together, these data provide support for the hypothesis that congression failure is a consequence of disturbances in either the paracrine or endocrine regulation of follicle growth, and that it predisposes to errors in chromosome segregation at the first meiotic division. In addition, these studies provide further evidence that the control of mammalian female meiosis differs markedly from mitotic cell division and from meiotic cell division in some animals, lacking a stringent chromosome-mediated checkpoint mechanism to delay the onset of anaphase in the event of disturbances in chromosome behaviour.
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Materials and methods |
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Oocyte collection, culture, and fixation
Oocytes at the germinal vesicle stage (GV oocytes) were liberated from antral follicles using 26 gauge needles, placed in 10 µl drops of Waymouth's medium (Gibco BRL, Gaithersburg, MD, USA) supplemented with 10% fetal bovine serum and 0.23 mmol/l sodium pyruvate overlaid with Squibb mineral oil, and incubated at 37°C in an atmosphere of 5% CO2 in air. After 2 h in culture, oocytes were scored for evidence of germinal vesicle breakdown (GVBD), indicating resumption of the first meiotic division. Only oocytes resuming meiosis within the first 2 h of culture were included in the study. To obtain oocytes at metaphase of the first meiotic division, oocytes were incubated for a total of 610 h. At the end of the culture period, oocytes were fixed, stained and analysed as described below. To obtain oocytes arrested at metaphase II, oocytes were maintained in culture for 16 h, and those exhibiting polar body extrusion were fixed for analysis.
Oocytes from all females were embedded in a fibrin clot (bovine fibrinogen type IV; Calbiochem, La Jolla, CA, USA; bovine thrombin; Sigma, St Louis, MO, USA) attached to a microscope slide as previously described (Hunt et al., 1995) and immediately fixed in 2% formaldehyde, 1% Triton X-100, 0.1 mmol/l PIPES, 5 mmol/l MgCl2 and 2.5 mmol/l EGTA, for 30 min at 37°C. Following fixation, oocytes were washed for 10 min in 0.1% normal goat serum (NGS; Gibco BRL)/phosphate-buffered saline (PBS), blocked for at least 1 h at 37°C in PBS wash solution containing 10% NGS, 0.02% sodium azide and 0.1% Triton X-100, and stored at 4°C.
Immunofluorescence staining
Oocytes were incubated with a 1:2000 dilution in PBS of a primary mouse monoclonal antibody to acetylated tubulin (Sigma), washed in 10% NGS/PBS, and detected with an fluoroscein isothiocyanate-conjugated goat anti-mouse IgG (Accurate Chemical, Westbury, NY, USA). Prior to analysis, oocytes were counter-stained with 100 ng/ml propidium iodide and a coverslip applied with 50% glycerol/4xstandard saline citrate (SSC) and 0.1 µg/ml p-phenylenediamine mounting medium and sealed with rubber cement. Oocytes were analysed on a Zeiss Axioplan epifluorescent microscope and three-dimensional images were collected on a BioRad MRC 600 confocal system. The meiotic stage of individual oocytes was classified on the basis of chromosome configuration and spindle morphology as described previously (LeMaire-Adkins et al., 1997).
Metaphase II chromosome preparations and spectral karyotyping
Chromosome preparations of MII arrested oocytes were made using a modification of a published technique (Tarkowski, 1966). Briefly, oocytes were hypotonized in 1% citrate, moved to a small drop of acidified water on a microscope slide, and fixed in situ with several drops of 3 parts methanol:1 part acetic acid. For conventional cytogenetic analysis, slides were denatured briefly (~40 s at 72°C) in 70% formamide in SSC to accentuate the centromeric region and stained with 400 mg/ml 4',6-diamidino-2-phenylindole (DAPI). For spectral karyotyping, the probe was denatured and pre-annealed according to the manufacturer's specifications (Applied Spectral Imaging, Inc., Migdal Ha'Emek, Israel). The slides were soaked in 2xSSC, dehydrated through an ethanol series, allowed to air dry, and denatured at 72°C for 45 s in 70% formamide in SSC. Immediately after being denatured, slides were dehydrated in a cold ethanol series, dried, and hybridized for 48 h at 37°C with the pre-annealed probe (SkyPaint probe mixture for mouse chromosomes; SkyPaint Kit M-10). Following hybridization, the slides were washed, counterstained, mounted according to the manufacturer's specifications, and stored in the dark until analysed.
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Results and interpretation |
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Is congression failure simply a reflection of oocyte immaturity?
To determine if defects in meiotic chromosome alignment are a symptom of oocyte immaturity, we first analysed mouse oocytes from early antral follicles. Analysis of the first wave of growing oocytes in the immature mouse ovary has revealed that the oocyte acquires the competence to undergo the meiotic divisions during the late stages of follicle growth: Oocytes isolated from pre-antral follicles from 12 day old females are incapable of undergoing any nuclear maturation. However, when follicles reach the early antral stage several days later (e.g. 1520 day old females), the oocytes have acquired partial meiotic competence; they will reinitiate meiosis when placed in culture but will arrest at metaphase I. In contrast, oocytes from slightly more mature follicles (e.g. from 22 day old females) exhibit full meiotic competence, resuming and completing the first meiotic division and arresting at metaphase of MII when released from the follicle and placed in culture (Eppig, 1994).
The majority of MI arrested oocytes isolated from the antral follicles of sexually immature (2628 day old) C57BL/6 females will spontaneously resume meiosis (as evidenced by nuclear envelope breakdown) within the first 2 h of culture. At this stage, immunostaining with an antibody to ß-tubulin reveals highly condensed chromosomes and a disorganized mass of microtubules (Figure 1c). Within several hours, the microtubules become organized into a bipolar spindle with the broad poles characteristic of MI in the female. The congression of the chromosomes to the spindle equator is contemporaneous, and the chromosomes are loosely aligned at the spindle equator by the time the poles become evident (Figure 1d
). Representative data for oocytes analysed after 8 h in culture are shown in Table Ia
. In our laboratory, these results are highly reproducible; the vast majority of oocytes from C57BL/6 females exhibit a normal MI configuration with the chromosomes aligned at the spindle equator (Figure 1e
); however, a proportion (67%) have one or two poorly aligned bivalents (e.g. Figure 1f
), and in 12% the MI spindle is fully formed but the majority of the chromosomes have not aligned at the equator (congression failure; identical in appearance to Figure 1g
). Similarly, the majority of oocytes that extrude the first polar body and enter metaphase arrest after extended time in culture exhibit a normal MII metaphase configuration, with the chromosomes tightly aligned at the spindle equator.
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Thus, the analysis of partially competent oocytes suggests that congression failure is not simply a symptom of meiotic immaturity. However, to determine if the inability to organize chromosomes on the MI spindle is symptomatic of defects in the acquisition of competence, we analysed oocytes from females produced on the LT/Sv inbred stain background. Oocytes from these females exhibit a high frequency of MI arrest, whether matured in vitro or in vivo, and have a propensity for parthenogenetic activation that results in a high incidence of ovarian teratomas (Eppig et al., 1996). Oocytes from LT/Sv females exhibit abnormal centrosomal characteristics, and minor abnormalities in pole structure are evident among those that arrest at MI (Albertini and Eppig, 1995
). Although the underlying molecular defect remains unknown, recent studies suggest that it is oocyte intrinsic (Eppig et al., 2000
), and results in a delay in the acquisition of the competence to trigger anaphase onset (Hirao and Eppig, 1999
). To determine if the centrosomal and/or cell cycle control defects in these oocytes influence meiotic chromosome behaviour, we analysed chromosome alignment at metaphase I (Table Ic
). Similar to partially competent oocytes and controls, we detected no cells exhibiting congression failure. However, the incidence of metaphase cells with outliers was high, both at the 8 h timepoint (which includes both oocytes capable of completing MI and those that will become arrested) and at the 1618 h timepoint (which includes only MI arrested oocytes). While differences in strain background preclude formal comparisons of the LT/Sv and control data (Table Iac
), it seems reasonable to conclude that this abnormality is increased in oocytes from LT/Sv females.
In general, the data from studies of partially competent oocytes from normal C57BL/6 females and oocytes with defects in the acquisition of meiotic competence from LT/Sv females are strikingly similar: although the frequency of cells with minor disturbances in chromosome alignment was slightly increased in both, gross disturbances in the congression of the chromosomes to the spindle equator was not a feature of either. Hence, these results provide experimental evidence that congression failure is not simply a reflection of oocyte immaturity.
Is congression failure a reflection of disturbances in the regulation of folliculogenesis?
To determine if congression failure is a symptom of compromised oocyte growth, we initiated meiotic studies of two different mouse mutants with defects in the process of folliculogenesis. We chose for these studies a mutant in which the primary defect is extraovarian (the LHßCTP transgenic mouse) and one in which the primary defect is intraovarian (the XYPOS sex-reversed female). Due to the complexity of the interplay between signals from the hypothalamus, pituitary and ovary, a primary defect at one level ultimately influences regulation at all levels. However, to minimize the influence of secondary effects, all studies were conducted on oocytes derived from the first wave of follicles that initiate growth in the immature ovary.
The LH ßCTP transgenic female
A genetically engineered modification of the LH ß-subunit results in LH hypersecretion in the LHßCTP mouse (Risma et al., 1995). Chronically elevated LH levels cause early onset of puberty and the rapid development of ovarian abnormalities, culminating in enlarged, cystic and haemorrhagic ovaries and infertility due to anovulation (Risma et al., 1995
, 1997
; Mann et al., 1999
). Despite the extensive ovarian pathology, recent studies have demonstrated that oocytes from LHßCTP females are developmentally competent and can give rise to liveborn when transferred to pseudopregnant recipients (Mann et al., 1999
). From the standpoint of oocyte growth, these females are particularly interesting; hypersecretion of LH not only results in altered FSH:LH ratios, but also in altered testosterone:estradiol ratios (Risma et al., 1995
, 1997
). Thus, oocyte growth occurs in a highly abnormal endocrine environment. To determine if this affects the meiotic process, we analysed chromosome alignment at metaphase I and among metaphase II arrested oocytes (Table Id
). In contrast to the results from control oocytes, partially competent oocytes and oocytes from LT/Sv females, we observed a high level of congression failure at both MI (i.e. 16 out of 88 cells, or 18.2%) and MII metaphase (i.e. 22 out of 57 cells, or 38.6%) among oocytes from LHßCTP females (Table Id
). The overall level of meiotic abnormalities was high; however, we noted considerable variation among siblings but an apparent correlation between the level of congression failure and the extent of progression of the ovarian pathology (e.g. congression failure was highest in oocytes from females exhibiting uterine hypertrophy and ovarian cysts). Because the LHßCTP females were not on an inbred background, the influence of genetic factors could not be ruled out. Hence, to eliminate genetic variability, the transgene was transferred to the C57BL/6 inbred strain by repeated backcrossing. The analysis of oocytes progressing through the first meiotic division revealed a similar level of congression failure (data not shown). Further, although variation within litters was reduced, variation among litters was correlated with the extent of ovarian pathology. Thus, the meiotic abnormalities observed among oocytes from LHßCTP females produced on both a heterogeneous and an inbred background provide strong evidence that a primary defect that influences the endocrine control of folliculogenesis can impact the meiotic process. In this instance, the endocrine abnormalities do not preclude normal development of the oocyte, as evidenced by the fact that liveborn young can be obtained by embryo transfer. However, the high frequency of congression failure raises the possibility that the genetic quality of the oocytes from these females may be compromised.
The XYPOS sex-reversed female
As an independent assessment of the meiotic impact of altered oocyte growth, we initiated meiotic studies of the XYPOS sex-reversed female. These females are the result of an inappropriate interaction between the testis-determining gene, Sry, and an autosomal gene or genes involved in sex determination that results when a Y chromosome of Mus musculus domesticus origin (e.g. YPOS or YDOM) is placed onto the inbred C57BL/6 inbred strain background (Eicher et al., 1982). Most oocytes in the sex-reversed ovary become atretic before the female reaches sexual maturity, and the resultant XY females are sterile. Previous studies have demonstrated, however, that the oocytes are capable of ovulation and fertilization but are incapable of development after fertilization (Merchant-Larios et al., 1994
). The primary defect is believed to be oocyte-intrinsic (Amleh and Taketo, 1998
; Amleh et al., 2000
), and from the standpoint of oocyte growth, oocytes from these females are interesting because they display subtle defects in oocytesomatic cell communication at the late stages of follicle growth (Vanderhyden et al., 1997
). Since XO females produced on the same genetic background are fertile, inappropriate expression of a gene or genes from the Y chromosome must be responsible for the disturbances in oocytesomatic cell communication in the developing XY oocyte.
Analysis of oocytes from XYPOS females at both MI and MII metaphase revealed an extraordinarily high level of congression failure: >50% of the oocytes analysed after 6 h in culture had formed a bipolar spindle but exhibited no evidence of chromosome organization at the spindle equator (Table Ie). The frequency of congression failure dropped somewhat among oocytes analysed after 10 h in culture; however, attempts to analyse oocytes at this timepoint were hampered by the fact that a large proportion of oocytes from XYPOS females had already initiated anaphase after 10 h in culture (see below). Further, the incidence of congression failure was also high among MII arrested oocytes (Table Ie
). Indeed, the incidence of aberrations at MII metaphase was considerably higher than evidenced by the frequency of congression failure since gross aberrations in spindle formation precluded the scoring of a significant number of oocytes (45/115). Thus, the high frequency of meiotic abnormalities observed among oocytes from XYPOS females suggests that a primary defect that affects oocytesomatic cell communication can impact the meiotic process.
Does congression failure delay anaphase onset?
During mitotic cell division, the presence of a misaligned chromosome delays the onset of anaphase (Burke 2000; Shah and Cleveland, 2000
). Hence, we wondered if abnormalities in chromosome congression during the first meiotic division would induce a similar delay. To assess this, we focused on the XYPOS female since the incidence of abnormalities at MI was high and, unlike the LHßCTP female, not complicated by phenotypic and genetic variability. We approached the question in two ways. First, we compared the kinetics of the first meiotic division in oocytes from XYPOS and control females for evidence of meiotic delay. Surprisingly, not only were we unable to detect a subset of cells that were delayed or arrested at MI, but comparison studies of the number of oocytes that had progressed beyond metaphase I after only 6 or 8 h in culture revealed a significant difference between oocytes from XYPOS and control females, suggesting an acceleration in anaphase onset among oocytes from XYPOS females (Figure 2
). We next asked whether the incidence of congression failure was elevated among the latest subset of oocytes to initiate anaphase. Such an increase would suggest that defects in chromosome alignment interfered with the onset of anaphase, resulting in a `pile up' of such cells. As seen in Table Ie
, our data suggested the opposite; the number of cells exhibiting congression failure at metaphase I was lower among cells analysed after 10 h by comparison with cells analysed after 6 h in culture. Thus, the results of our analysis of oocytes from XYPOS females are consistent with the conclusion that alterations in oocyte growth predispose to congression failure at MI, and that this gross disturbance in meiotic chromosome behaviour does not affect the timing of anaphase onset.
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Does congression failure result in increased aneuploidy?
Our previous observations on human oocytes from unstimulated ovaries led us to speculate that failure of normal chromosome alignment might be associated with human age-related non-disjunction (Volarcik et al., 1998). If so, we would predict that: (i) cells exhibiting congression failure are capable of initiating anaphase; and (ii) failure to properly align chromosomes at the spindle equator predisposes to non-disjunction at anaphase I. As detailed above, the results of our analyses of oocytes from XYPOS females provided no evidence for a delay in anaphase onset in the presence of congression failure. Thus, our observations are consistent with the first of the two predictions.
To test the second prediction, i.e. that congression failure predisposes to chromosome missegregation at anaphase, we analysed chromosome preparations of MII arrested oocytes from XYPOS females. Previous studies of MII arrested oocytes from XYPOS females suggested an increase in aneuploidy levels (Hunt and LeMaire, 1992). However, since the X and Y chromosomes frequently fail to recombine (Hunt and LeMaire, 1992
), this could simply reflect sex chromosome non-disjunction or premature sister chromatid separation (PSCS) of a univalent X or Y chromosome at MI. To determine whether this was the cause of the increased aneuploidy in oocytes from XYPOS females or whetheras we would predictall chromosomes are susceptible to meiotic non-disjunction, we initiated a cytogenetic analysis of MII arrested oocytes from XYPOS and control females.
Initially, we analysed DAPI-stained preparations, scoring for autosomal but not sex chromosome abnormalities and, to avoid artefactual loss, for hyperploidy but not hypoploidy. However, the analysis of oocytes from XYPOS females was complicated by the fact that several oocytes had multiple chromatid aberrations (e.g. Figure 3). Thus, we analysed an additional set of oocytes using spectral karyotyping (SKY) methodology. This allowed us to identify individual chromosomes and chromatids, and to distinguish single chromatids resulting from an MI segregation error (i.e. due to PSCS at MI) from chromosomes that had simply `fallen apart' at MII (i.e. due to the premature release of centromere cohesion between sister chromatids).
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Thus, in two different situations in which we observed an increased frequency of congression failure at MI, the segregation of homologous chromosomes at anaphase I was also impaired. Further, the estimated incidence of such segregation errors (obtained by doubling the frequency of hyperploidy to estimate total non-disjunction, and adding to this the frequency of premature sister chromatid segregation events) was 32% for XYPOS females and 28% for LHßCTP females, values similar to the observed frequency of MI congression failure for the two types of females (Table Id,e). Thus, these data support the hypothesis that the congression failure phenotype is predictive of subsequent errors in chromosome segregation.
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Discussion |
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The combined data from our studies are consistent with the conclusion that congression failure is not a symptom of meiotic immaturity but, rather, a reflection of oocyte growth in an altered environment. Importantly, this abnormal meiotic phenotype can be provoked by different mechanisms: e.g. defects in oocytesomatic cell communication in the XYPOS female and the altered endocrine status of the LHßCTP female both result in a high incidence of congression failure at MI. Moreover, the degree of meiotic disturbance appears to be correlated with the progression of the abnormal endocrine phenotype in the LHßCTP mouse. Thus, the available evidence suggests that, although the primary defect may vary, congression failure results from changes that influence the final stages of oocyte growth and maturation. Accordingly, we predict that meiotic studies of several additional currently available mouse mutants [e.g. knockouts for 5-reductase type I (Mahendroo and Russell, 1999
) and Bmp1 (Yan et al., 2001
)] will also reveal an increased level of congression failure.
The ability to recapitulate congression failure in mouse mutants has allowed us to determine if cells exhibiting this phenotype are capable of initiating anaphase. Our results provide no evidence that gross aberrations in the alignment of the chromosomes on the MI spindle delay the onset of anaphase. This is in striking contrast to mitotic cells (Burke, 2000; Shah and Cleveland, 2000
) and male meiotic cells (Eaker et al., 2001
) where the actions of the spindle assembly checkpoint mechanism cause the delay or arrest of cells with unaligned chromosomes at metaphase. More importantly, our data provide evidence that the failure to delay or arrest such cells leads to aneuploidy. We previously hypothesized that this difference in cell cycle control is a feature of mammalian female meiosis and that it contributes to the high incidence of human female-derived aneuploidy (LeMaire-Adkins et al., 1997
). Our current data strongly support this conclusion, and suggest that changes in the endocrine environmentsuch as those that occur in the human female in the decade preceding menopausemay influence meiotic chromosome behaviour, creating age-related increase in aneuploidy.
If this interpretation is correct, understanding the molecular basis of congression failure will provide important insight to human age-related non-disjunction. Obviously a complex effect that has multiple aetiologies and involves both the somatic and germ cell component of the developing follicle poses a number of questions. Of these, one of the most interesting is the reason why chromosomes fail to congress on the meiotic spindle.
The first meiotic division is unique in many respects; however, the formation of the MI spindle and the alignment of chromosomes is particularly unusual. In mammals, the female meiotic spindle is formed through the action of multiple microtubule organizing centres rather than from a pair centrosomes and, as a result, the MI spindle has a characteristic barrel shape. Previous studies have suggested that bivalents become oriented on the meiotic spindle very early, such that the movement of the chromosomes to the equator has already commenced by the time that organized spindle poles become evident (Woods et al., 1999). However, although the chromosomes rapidly congress to the vicinity of the spindle equator, stable microtubule/kinetochore connections are not formed immediately, and a classic tight metaphase configuration is not achieved until just prior to anaphase onset (Brunet et al., 1998
, 1999
). Hence, the configuration that we describe as `normal metaphase' with chromosomes loosely aligned at the spindle equator is, in actuality, an extended prometaphase. Given these features, it seems likely that the observation of one or two outlying chromosomes in an otherwise normal cell (e.g. Figure 1f
) represents normal oscillation of the chromosomes near the equatorial plate. In contrast, the inability to loosely align the chromosomes at the spindle equator, the meiotic phenotype that we have termed congression failure, is diagnostic of meiotic abnormalities that predispose to non-disjunction.
Our previous studies demonstrated that the meiotic chromosomes are not passive players, and that their ability to make bipolar attachments to the organizing spindle is essential for the formation of a stable MI spindle (Woods et al., 1999). Since the spindle appears normal in many cells exhibiting congression failure, it seems likely that the defect is not one of spindle formation, but rather of chromosome movement. The initial movement of the chromosomes to the vicinity of the equatorial plate of the spindle has been postulated to result from pushing forces exerted by motor proteins (Brunet et al., 1998
, 1999
). Interestingly, both microinjection of kinetochore antibodies into mouse oocytes (Simerly et al., 1990
) and immunodepletion of the kinetochore-associated motor protein, CENP-E, in Xenopus egg extracts (Wood et al., 1997
) produce a congression failure phenotype. Accordingly we postulate that the congression failure observed in oocytes from reproductively-aged human donors and in mouse oocytes from XYPOS and LHßCTP mutant females results from disturbances in the control of the final maturation phase of the oocyte that affect the transcription or post-translational modification of one or more microtubule motor proteins. The extended duration of MI in female mammals has been hypothesized to reflect a period of slow, progressive maturation of the kinetochores through post-translational modification (Brunet et al., 1999
), hence it is possible that congression failure is a reflection of disturbances in this maturation process.
An important question not addressed by our studies is whether the congression failure defect is a meiotic `lethal', i.e. does the severity of the defect preclude such cells from forming functional oocytes capable of fertilization and development? Previous studies of human oocytes provide indirect evidence against this: age-related meiotic non-disjunction is clearly associated with an increase in chromosomally abnormal human conceptions. Thus, the finding that congression failure is an age-related phenotype in human oocytes argues that it is not merely a symptom of a degenerative oocyte.
Understanding a spindle defect with multiple aetiologies in a system as complex as the mammalian follicle is clearly a challenge. However, a recent model of paracrine signalling provides a mechanism whereby both intraovarian and extraovarian factors might affect oocytesomatic cell communication (Albertini et al., 2001). Moreover, the role of microtubules in the signalling between the oocyte and its granulosa cells provides a potential link to spindle-specific defects. Thus, it seems likely to us that the oocytegranulosa cell communication is a key component of the congression failure defect.
The fact that congression failure causes no obvious delay in anaphase onset and is correlated with increased non-disjunction indicates to us that cell cycle control mechanisms differ in the oocyte. It remains unclear, however, whether this is a generalized feature of female meiosis or whether congression failure occurs only when the normal cell cycle control mechanisms become non-functional. Future studies using these and other mouse models will be invaluable in understanding the control of female meiosis and, specifically, in understanding the mechanism responsible for the age-related disturbances in meiotic chromosome behaviour that we (Volarcik et al., 1998) and others (Battaglia et al., 1996
) have observed in human oocytes.
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Acknowledgements |
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Notes |
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References |
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Submitted on August 13, 2001; accepted on January 10, 2002.