Older women are more likely to experience a miscarriage or give birth to a child who has a developmental disorder. This occurs because age increases the chances that a woman’s egg cells will have the wrong number of chromosomes. If a sperm fertilizes an egg with too many or too few copies of a chromosome, the resulting embryo will have the wrong number of copies for many genes. Many of these embryos fail to develop and die, but some are born with developmental conditions like Down's syndrome and Turner syndrome.

New egg cells develop from immature egg cells that are present in a woman from birth. In an immature egg cell, chromosomes that came from the woman’s father are paired up with the matching chromosomes from the woman’s mother and the handle-like structures on each chromosome (called the kinetochores) are fused. Just before the immature egg cell divides, a molecular machine called ‘the spindle’ attaches to the chromosome handles. The spindle then separates these pairs of chromosomes such that each new cell receives only one copy of each chromosome. However, while it is known that this process sometimes goes wrong, it is not clear why mistakes happen more often in older women.

Now, Zielinska et al. used powerful microscopes to observe cell division in over 200 preserved or living immature egg cells donated by women between the ages of 23 and 46. First, the experiments examined over 1,000 chromosomes in preserved immature egg cells that were about to divide. This revealed that the chromosome handles that were supposed to be fused had often disconnected in women over 35 years old. Chromosome pairs without correctly fused handles were also prone to rotating during the division process, and sometimes the pairs simply fell apart too soon.

Further experiments with living immature egg cells then revealed that the spindle struggled to grip and separate the chromosomes correctly, possibly because the chromosome handles were not properly fused. These events increased the likelihood of a new egg cell receiving too many or too few chromosomes. Finally, Zielinska et al. found that immature egg cells lack a robust control mechanism that can detect when these problems occur.

Together these findings help to explain why miscarriages and chromosome abnormalities are more common in the children of older women. Research building on these findings may in the future help women in their late 30s and early 40s to increase their chances of having a family.

Once during every menstrual cycle, an oocyte segregates half of its chromosomes into a small cell, the polar body, to prepare for fertilization (Petronczki et al., 2003, Clift and Schuh, 2013). Segregating the chromosomes accurately is vital for the development of healthy human embryos upon fertilization. Surprisingly, oocytes segregate their chromosomes much more inaccurately than do human mitotic cells (Nagaoka et al., 2012, Knouse et al., 2014), human spermatocytes (Pacchierotti et al., 2007, Templado et al., 2011) or mouse oocytes (Pacchierotti et al., 2007, Danylevska et al., 2014, Hassold and Hunt, 2001). The accuracy of chromosome segregation drops even further with increasing age (Nagaoka et al., 2012, Chiang et al., 2010, Fragouli et al., 2011, Kuliev et al., 2011). This may be one of the reasons behind an increase in miscarriages, trisomic pregnancies and infertility with advanced maternal age.

Notably, eggs from young women are also often aneuploid (Pacchierotti et al., 2007, Obradors et al., 2011). Why chromosomes missegregate so frequently, even in oocytes from young women, is still poorly understood. We showed previously that human oocytes assemble a spindle by an unusually long and error-prone mechanism that is likely to sensitize human oocytes of all ages to abnormal kinetochore-microtubule attachments and chromosome segregation errors (Holubcova et al., 2015). However, it is still unclear why chromosome segregation errors in oocytes become more frequent as women get older.

Research in mouse oocytes suggests that changes in chromosome architecture contribute to this age-dependent increase in aneuploidy (Lister et al., 2010, Chiang et al., 2011, Yun et al., 2014, Sakakibara et al., 2015). The architecture of chromosomes differs in meiosis and mitosis. During the first meiotic division, the oocyte segregates entire chromosomes instead of sister chromatids as is the case in mitosis. To facilitate this, the chromosome pairs are linked with each other through meiotic recombination (Nagaoka et al., 2012, Kong et al., 2004, Baudat et al., 2013). The sister kinetochores of each chromosome become unified so that they function as a single kinetochore (Watanabe, 2012). This prevents the undesirable bi-orientation of sister kinetochores during meiosis I (Watanabe, 2012, Yokobayashi and Watanabe, 2005, Kim et al., 2015). The resulting unit of two linked chromosomes with two functional kinetochores is called a bivalent chromosome. The cohesion complex has a crucial role in maintaining the integrity of the bivalents: it holds the homologous chromosomes together and promotes the close association of the sister kinetochores (Petronczki et al., 2003, Watanabe, 2012).

Recent work in mouse oocytes suggests that the cohesin complex is gradually lost from oocyte chromosomes as mice get older (Lister et al., 2010, Tachibana-Konwalski et al., 2010, Chiang et al., 2010). This loss of cohesin has been suggested to lead to an increase in sister kinetochore distances and the precocious dissociation of bivalents into individual chromosomes (Yun et al., 2014, Sakakibara et al., 2015), often referred to as univalents in meiosis. Whether such a loss of cohesin is also relevant in human oocytes is still unclear (Garcia-Cruz et al., 2010, Tsutsumi et al., 2014, Handyside et al., 2012).

The aim of this study was to investigate whether age-related changes in chromosome architecture also contribute to the maternal age effect in human oocytes. To this end, we analyzed the organization of chromosomes and how they interact with the spindle in over 200 live and fixed human oocytes from donors aged between 23 and 46 years.

We first investigated the chromosome organization in oocytes from young women (≤30 years) by recording high resolution z-stacks (300 nm in x and y, 650 nm in z) of 1,051 chromosomes and their kinetochores in 23 oocytes. Unexpectedly, 60% of sister kinetochores appeared as two discrete spots when observed by light microscopy (Figure 1A,B). This is in contrast to oocytes from young mice, whose sister kinetochores generally appear as a single spot (Chiang et al., 2010, Lister et al., 2010, Shomper et al., 2014). The sister kinetochores were spaced up to 1.7 µm apart (Figure 2C) and sometimes separated by prominent gaps (Figure 1A,B). Such bivalents with separated kinetochores were a frequent phenomenon, observed throughout the entire cohort of oocyte donors (Figure 1C; Figure 1—figure supplement 1,2), including very young donors (Video 1).

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We then analyzed how the split kinetochores interact with the spindle. Around 90% of split kinetochores were attached to two separate kinetochore fibers (Figure 1D,E). The two kinetochore fibers did not always run in parallel, but often linked the sister kinetochores to distant positions at the spindle poles (Figure 1D,F). Thus, consistent with the high degree of separation, the spindle recognizes most split sister kinetochores in human oocytes as independent units (Figure 1—figure supplement 3,4).

The fraction of split kinetochores in human oocytes increased with maternal age. Around 75% of all kinetochores were split in oocytes from women between 30 and 35. This increased to 87% in women over 35 (Figure 2A–E; Videos 1, 2). The fraction of sister kinetochores that was separated from each other by a prominent gap also increased dramatically with advanced maternal age (Figure 2D). Sister kinetochores also sometimes become distinguishable in oocytes from very old mice (Chiang et al., 2010, Lister et al., 2010). But in contrast to human oocytes, their sister kinetochores are not separated by a prominent gap. Consistent with the remarkably large distances between sister kinetochores during meiosis I (Figure 2C, Figure 2—figure supplement 1) and meiosis II (Duncan et al., 2012) in human oocytes, sister kinetochores were also separated by very large gaps during meiosis II (Figure 2—figure supplement 2; Videos 3, 4).

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Together, these data show that the majority of bivalent chromosomes in human oocytes feature three or four functionally distinct kinetochores. Each of these kinetochores is able to establish its own kinetochore fiber. Thus, human oocytes question the paradigm that sister kinetochores are unified during meiosis I.

We then went on to analyze whether the split nature of sister kinetochores in human oocytes could affect chromosome segregation. First, we assessed if having three or four functionally distinct kinetochores per bivalent increased the probability of abnormal kinetochore-microtubule attachments. During meiosis I, the sister kinetochores of each chromosome should be attached to a single spindle pole only (amphitelic attachment). In this way, they can be segregated accurately after anaphase onset. If a pair of sister kinetochores is attached to opposite spindle poles instead (merotelic attachment), the chromosome will be pulled from both sides of the spindle upon anaphase onset. Such pulling will cause a chromosome to lag behind during anaphase (Cimini et al., 2001, Lane et al., 2012). This can lead to aneuploidy when the ingressing cytokinetic furrow partitions chromosomes inaccurately between the egg and the polar body. To assess how the bivalent chromosomes were attached to the spindle, we briefly placed the oocytes on ice before fixation. This selectively preserves the more stable microtubules that interact with kinetochores (Zhai et al., 1995). We then stained the oocytes for microtubules, chromosomes and kinetochores.

Separated sister kinetochores were more likely to be merotelically attached to the spindle than unified sister kinetochores. Interestingly, we observed that the degree of separation correlated with the probability of a pair being merotelically attached: the fraction of merotelic attachments increased from 12% in unified kinetochore pairs to 17% when the sister kinetochores were distinct but still linked, and up to 28% in completely separated kinetochore pairs (Figure 2F,H). This could potentially be due to improved accessibility from the opposite spindle pole, when two tightly joined sister kinetochores are compared with two separated sister kinetochores.

The number of merotelic kinetochore-microtubule attachments also increased with maternal age, from around 7% in women under 30 to around 21% in women over 35 (Figure 2G). This is consistent with the increased fraction of separated sister kinetochores with advanced maternal age. Separated kinetochores were more likely to be merotelically attached in oocytes from older women than in oocytes from young women (Figure 2—figure supplement 3). This could be explained by the increase in the degree of separation between sister kinetochores as women get older (Figure 2C), as well as additional age-related defects that could contribute to aberrant attachments.

The remarkably large distances between some sister kinetochores raised an unexpected problem: a stable arrangement of bivalent chromosomes on the spindle could theoretically also be achieved if the split sister kinetochores are facing towards opposite poles instead of the same spindle pole (Figure 3A scheme, middle row; fully-rotated). Such a chromosome would be rotated on the spindle by 90 degrees and could not segregate accurately during anaphase.

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To our surprise, a large fraction of oocytes had one or more rotated bivalent chromosomes, in which the sister kinetochores of both pairs faced opposite spindle poles (Figure 3A–C, middle row; Figure 3D; Figure 3—figure supplement 1; Videos 5, 6). Given that the orientation of sister kinetochores in these rotated bivalents is inverted, we called these bivalents ‘inverted bivalents’. We also often observed half-inverted bivalents, in which only one pair of the sister kinetochores was oriented towards opposite spindle poles (Figure 3A–C bottom row, half-inverted; Figure 3—figure supplement 1D, right panel), while the other pair was correctly oriented, with both sister kinetochores facing a single spindle pole (Figure 3D). Bivalent rotation was also evident from the distribution of Shugoshin-1, which marked the pairs of sister kinetochores (Figure 3C, Figure 3—figure supplement 1; Videos 5,6). Interestingly, Shugoshin-1 associated with a wider domain than in meiosis I mouse oocytes (Lee et al., 2008). It will be interesting to investigate how meiotic cohesion is achieved in human oocytes and whether the unexpected kinetochore configurations that we observed could be linked to species-specific differences in this process.

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In total, more than 2% of all bivalents were half or fully inverted (Figure 3E). Importantly, kinetochores of inverted bivalents were linked to the spindle through cold-resistant and thus stable microtubules (Figure 3—figure supplement 2A,C). This indicates that the altered orientation of kinetochores in inverted bivalents does not completely compromise their ability to capture microtubules. Moreover, the incidence of inverted bivalents increased with maternal age, from 12.5% in oocytes from women under 30 to 40% in oocytes from women over 35 years old (Figure 3F; Figure 3—figure supplement 1C). This is consistent with the increased separation of sister kinetochores with maternal age, which is expected to promote the rotation (Figure 3—figure supplement 2B,D).

Having four instead of only two independent microtubule attachment sites also allowed bivalents to be twisted along their axis (Figure 4A). In around 20% of bivalents with split sister kinetochores, the chromosomes were twisted, as judged by the observation that their sister kinetochore pairs were oriented perpendicular instead of parallel to each other (Figure 4B). Such twisting is likely to exert additional forces on arm cohesion, which is possibly already weakened in oocytes at the time of meiotic resumption (Duncan et al., 2012, Chiang et al., 2010). Arm cohesion was indeed reduced in a large fraction of chromosome bivalents (Figure 5A,C,D). In total, 10% of all bivalents were weakly associated as evident from a gap in the bivalents’ center. In some cases, the chromosomes were separated by a gap of a few 100 nm only. In other cases, gaps of up to 3.2 µm between two chromosomes were visible (Figure 5A). Despite the gap, the chromosome pairs were under tension, with each pair of sister kinetochores facing the correct spindle pole. This suggests that chromosomes with reduced arm cohesion still behave as functional bivalents.

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Insufficient arm cohesion was also evident from the complete disintegration of 1.2% of bivalents into univalents (Figure 5B–F). 95% of these univalents aligned in the center of the spindle, with their sister kinetochores facing opposite spindle poles, suggesting that they could segregate equally into two sister chromatids upon anaphase onset (Figure 5E). The fraction of bivalents affected by loss of arm cohesion increased significantly with maternal age. In women under 30, 5% of all chromosome pairs were separated by gaps or completely disintegrated into univalents. This increased to 11% of chromosome pairs in women over 35 years old (Figure 5D). A recent study also reported the presence of univalents in oocytes from older women (Sakakibara et al., 2015). Our data suggests that the premature dissociation of bivalents into univalents also contributes to aneuploidy in oocytes from young women (Figure 5F). It also provides further evidence from a larger number of oocytes that an increase in univalents is a likely contributor to the maternal age effect, because more than 40% of oocytes from women over 35 featured univalents (Figure 5F).

Having more than two functional kinetochores on each bivalent chromosome is expected to hinder the correct attachment of chromosomes to the spindle. Live imaging in human oocytes showed that around 16.3 ± 2.3 hr were needed to align the chromosomes in the center of the spindle upon nuclear envelope breakdown (Holubcova et al., 2015). The period of chromosome congression and alignment coincided in time with spindle instability and reorganization (Figure 6—figure supplement 1). It is conceivable that the reorganization of the spindle is required to attach the bivalent chromosomes correctly to the spindle, consistent with a previous study in mouse oocytes (Kitajima et al., 2011).

Difficulties in correctly attaching bivalents to the spindle would only lead to aneuploidy if oocytes are allowed to progress into anaphase in the presence of abnormal kinetochore-microtubule attachments. Thus, a decisive question in understanding how aneuploidy arises is whether human oocytes have effective control mechanisms that monitor progression into anaphase. In mitosis, the spindle assembly checkpoint delays progression into anaphase until all chromosomes are correctly attached to the spindle (Musacchio and Salmon, 2007). Whether human oocytes have similar mechanisms is unclear. Live imaging revealed that all oocytes without chromosome alignment defects progressed into anaphase. Similarly, all oocytes with mild alignment defects and 92% of oocytes with severe misalignment progressed into anaphase (Figure 6A,B; Videos 7, 8). Thus, the presence of chromosome alignment defects did not preclude progression into anaphase. In addition, the onset of anaphase was not severely delayed when misaligned or lagging chromosomes were present (Figure 6C,D). Less prominent delays might however be masked by some variability in anaphase timing between oocytes. Collectively, these results suggest that human oocytes progress into anaphase with normal efficiency and without major delays if chromosomes are misaligned.

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Our findings suggest possible answers to the two key questions related to human oocyte meiosis. Firstly, why is chromosome segregation in human oocytes in general more error-prone than in mitosis, even in oocytes from young women? Our previous work established that the spindles in human oocytes form over a lengthy and error-prone process, during which the spindle undergoes extensive reorganization. Chromosomes frequently remain merotellically attached to the spindle close to anaphase onset. These merotelic attachments could be favoured by the high instability and morphology of the spindle in human oocytes (Holubcova et al., 2015). The data presented in this study suggest that merotelic attachments are also promoted by the split nature of sister kinetochores in human oocytes.

It is conceivable that the specialized architecture of bivalents in human oocytes and spindle reorganization are directly linked: the fact that the four sister kinetochores of a bivalent chromosome can interact with microtubules independently may make it more challenging for the oocyte to correctly attach the bivalents to the spindle. Thus, more extensive spindle reorganization may be required to correct attachment errors that arise during spindle assembly. Conversely, repeated rounds of microtubule attachment and detachment during spindle reorganization may further exacerbate splitting of sister kinetochores and cause strain on chromosome arm cohesion, promoting the twisting of bivalents and the precocious dissociation of bivalents into univalents. In this way, spindle reorganization could contribute to chromosome segregation defects in human oocytes.

Secondly, what causes the maternal age effect? We show that bivalents disintegrate precociously into univalents as women get older. Univalents were observed in more than 40% of oocytes from women over 35, suggesting that they are a major contributor to the maternal age effect, consistent with a recent study (Sakakibara et al., 2015). We also demonstrate that the degree of sister kinetochore separation increases with advanced maternal age. Sister kinetochore separation correlated with merotelic kinetochore-microtubule attachment. In addition, it allowed bivalents to rotate on the spindle with each of their sister-kinetochores facing towards opposite spindle poles. This is unexpected, because it is generally believed that mechanisms exist that prevent the biorientation of sister kinetochores in meiosis I. We observed half- or fully-inverted bivalents in a large fraction of oocytes. It is possible that some of these abnormal attachments are corrected by the time of anaphase onset. However, it is unclear whether mechanisms exist in the oocyte that could detect these inverted bivalents and correct their attachment. Our observation that human oocytes progress into anaphase efficiently even if misaligned chromosomes are present suggests that oocytes could in principle also progress into anaphase with inverted bivalents (Figure 7). Such inverted bivalents could lag behind during anaphase if centromeric cohesion is still intact in one or both of the two pairs of sister kinetochores. If centromeric cohesion is compromised in one of the two sister kinetochores, the bivalent could disintegrate into one univalent and two sister chromatids. If centromeric cohesion is compromised in both pairs of sister kinetochores, the bivalent could disintegrate into four individual sister chromatids upon anaphase onset, and two non-sister chromatids of different primary parental origin would remain in the egg (Figure 7). Interestingly, a recent genetic study has reported that precisely this segregation outcome, which was called reverse segregation, seems to be a major cause of chromosome segregation errors during meiosis I in human oocytes (Ottolini et al., 2015). The mechanisms behind this reverse segregation pattern have remained unclear. Our study indicates that inverted bivalents are likely to contribute to this segregation pattern.

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In addition, the precocious separation of bivalents into univalents is likely to contribute to this pattern in oocytes from both young and older women. The vast majority of univalents bioriented on the MI spindle. If the sister chromatids segregated equally to the two spindle poles upon anaphase onset, this would also result in the reverse segregation pattern that was previously observed.

Together, these data establish a mechanistic framework whereby the separated nature of sister kinetochores in human oocytes and the precocious dissociation of bivalents into univalents, combined with spindle instability (Holubcova et al., 2015) and unrestrained progression into anaphase, prime oocytes from women of all ages for aneuploidy.

1. 1. Baudat F
   2. Imai Y
   3. de Massy B

   (2013) Meiotic recombination in mammals: localization and regulation

   Nature Reviews Genetics 14:794–806.

   https://doi.org/10.1038/nrg3573

   • Google Scholar
2. 1. Chiang T
   2. Duncan FE
   3. Schindler K
   4. Schultz RM
   5. Lampson MA

   (2010) Evidence that weakened centromere cohesion is a leading cause of age-related aneuploidy in oocytes

   Current Biology 20:1522–1528.

   https://doi.org/10.1016/j.cub.2010.06.069

   • Google Scholar
3. 1. Chiang T
   2. Schultz RM
   3. Lampson MA

   (2011) Age-dependent susceptibility of chromosome cohesion to premature separase activation in mouse oocytes

   Biology of Reproduction 85:1279–1283.

   https://doi.org/10.1095/biolreprod.111.094094

   • Google Scholar
4. 1. Cimini D
   2. Howell B
   3. Maddox P
   4. Khodjakov A
   5. Degrassi F
   6. Salmon ED

   (2001) Merotelic kinetochore orientation is a major mechanism of aneuploidy in mitotic mammalian tissue cells

   The Journal of Cell Biology 153:517–528.

   https://doi.org/10.1083/jcb.153.3.517

   • Google Scholar
5. 1. Clift D
   2. Schuh M

   (2013) Restarting life: fertilization and the transition from meiosis to mitosis

   Nature Reviews Molecular Cell Biology 14:549–562.

   https://doi.org/10.1038/nrm3643

   • Google Scholar
6. 1. Danylevska A
   2. Kovacovicova K
   3. Awadova T
   4. Anger M

   (2014) The frequency of precocious segregation of sister chromatids in mouse female meiosis I is affected by genetic background

   Chromosome Research 22:365–373.

   https://doi.org/10.1007/s10577-014-9428-6

   • Google Scholar
7. 1. Duncan FE
   2. Hornick JE
   3. Lampson MA
   4. Schultz RM
   5. Shea LD
   6. Woodruff TK

   (2012) Chromosome cohesion decreases in human eggs with advanced maternal age

   Aging Cell 11:1121–1124.

   https://doi.org/10.1111/j.1474-9726.2012.00866.x

   • Google Scholar
8. 1. Faddy M
   2. Gosden R
   3. Ahuja K
   4. Elder K

   (2011) Egg sharing for assisted conception: a window on oocyte quality

   Reproductive BioMedicine Online 22:88–93.

   https://doi.org/10.1016/j.rbmo.2010.08.009

   • Google Scholar
9. 1. Fragouli E
   2. Alfarawati S
   3. Goodall N.-n.
   4. Sanchez-Garcia JF
   5. Colls P
   6. Wells D

   (2011) The cytogenetics of polar bodies: insights into female meiosis and the diagnosis of aneuploidy

   Molecular Human Reproduction 17:286–295.

   https://doi.org/10.1093/molehr/gar024

   • Google Scholar
10. 1. Garcia-Cruz R
    2. Brieno MA
    3. Roig I
    4. Grossmann M
    5. Velilla E
    6. Pujol A
    7. Cabero L
    8. Pessarrodona A
    9. Barbero JL
    10. Garcia Caldes M

    (2010) Dynamics of cohesin proteins REC8, STAG3, SMC1 and SMC3 are consistent with a role in sister chromatid cohesion during meiosis in human oocytes

    Human Reproduction 25:2316–2327.

    https://doi.org/10.1093/humrep/deq180

    • Google Scholar
11. 1. Handyside AH
    2. Montag M
    3. Magli MC
    4. Repping S
    5. Harper J
    6. Schmutzler A
    7. Vesela K
    8. Gianaroli L
    9. Geraedts J

    (2012) Multiple meiotic errors caused by predivision of chromatids in women of advanced maternal age undergoing in vitro fertilisation

    European Journal of Human Genetics 20:742–747.

    https://doi.org/10.1038/ejhg.2011.272

    • Google Scholar
12. 1. Hassold T
    2. Hunt P

    (2001) To err (meiotically) is human: the genesis of human aneuploidy

    Nature Reviews Genetics 2:280–291.

    https://doi.org/10.1038/35066065

    • Google Scholar
13. 1. Holubcova Z
    2. Blayney M
    3. Elder K
    4. Schuh M

    (2015) Error-prone chromosome-mediated spindle assembly favors chromosome segregation defects in human oocytes

    Science 348:1143–1147.

    https://doi.org/10.1126/science.aaa9529

    • Google Scholar
14. 1. Jaffe LA
    2. Terasaki M

    (2004) Quantitative microinjection of oocytes, eggs, and embryos

    Methods Cell Biol 74:219–242.

    https://doi.org/10.1016/S0091-679X(04)74010-8

    • Google Scholar
15. 1. Kim J
    2. Ishiguro Kei-ichiro
    3. Nambu A
    4. Akiyoshi B
    5. Yokobayashi S
    6. Kagami A
    7. Ishiguro T
    8. Pendas AM
    9. Takeda N
    10. Sakakibara Y
    11. Kitajima TS
    12. Tanno Y
    13. Sakuno T
    14. Watanabe Y

    (2015) Meikin is a conserved regulator of meiosis-i-specific kinetochore function

    Nature 517:466–471.

    https://doi.org/10.1038/nature14097

    • Google Scholar
16. 1. Kitajima TS
    2. Ohsugi M
    3. Ellenberg J

    (2011) Complete kinetochore tracking reveals error-prone homologous chromosome biorientation in mammalian oocytes

    Cell 146:568–581.

    https://doi.org/10.1016/j.cell.2011.07.031

    • Google Scholar
17. 1. Knouse KA
    2. Wu J
    3. Whittaker CA
    4. Amon A

    (2014) Single cell sequencing reveals low levels of aneuploidy across mammalian tissues

    Proceedings of the National Academy of Sciences of the United States of America 111:13409–13414.

    https://doi.org/10.1073/pnas.1415287111

    • Google Scholar
18. 1. Kong A
    2. Barnard J
    3. Gudbjartsson DF
    4. Thorleifsson G
    5. Jonsdottir G
    6. Sigurdardottir S
    7. Richardsson B
    8. Jonsdottir J
    9. Thorgeirsson T
    10. Frigge ML
    11. Lamb NE
    12. Sherman S
    13. Gulcher JR
    14. Stefansson K

    (2004) Recombination rate and reproductive success in humans

    Nature Genetics 36:1203–1206.

    https://doi.org/10.1038/ng1445

    • Google Scholar
19. 1. Kuliev A
    2. Zlatopolsky Z
    3. Kirillova I
    4. Spivakova J
    5. Cieslak Janzen J

    (2011) Meiosis errors in over 20,000 oocytes studied in the practice of preimplantation aneuploidy testing

    Reproductive BioMedicine Online 22:2–8.

    https://doi.org/10.1016/j.rbmo.2010.08.014

    • Google Scholar
20. 1. Lane SIR
    2. Yun Y
    3. Jones KT

    (2012) Timing of anaphase-promoting complex activation in mouse oocytes is predicted by microtubule-kinetochore attachment but not by bivalent alignment or tension

    Development 139:1947–1955.

    https://doi.org/10.1242/dev.077040

    • Google Scholar
21. 1. Lee J
    2. Kitajima TS
    3. Tanno Y
    4. Yoshida K
    5. Morita T
    6. Miyano T
    7. Miyake M
    8. Watanabe Y

    (2008) Unified mode of centromeric protection by shugoshin in mammalian oocytes and somatic cells

    Nature Cell Biology 10:42–52.

    https://doi.org/10.1038/ncb1667

    • Google Scholar
22. 1. Lister LM
    2. Kouznetsova A
    3. Hyslop LA
    4. Kalleas D
    5. Pace SL
    6. Barel JC
    7. Nathan A
    8. Floros V
    9. Adelfalk C
    10. Watanabe Y
    11. Jessberger R
    12. Kirkwood TB
    13. Höög C
    14. Herbert M

    (2010) Age-related meiotic segregation errors in mammalian oocytes are preceded by depletion of cohesin and Sgo2

    Current Biology 20:1511–1521.

    https://doi.org/10.1016/j.cub.2010.08.023

    • Google Scholar
23. 1. Musacchio A
    2. Salmon ED

    (2007) The spindle-assembly checkpoint in space and time

    Nature Reviews Molecular Cell Biology 8:379–393.

    https://doi.org/10.1038/nrm2163

    • Google Scholar
24. 1. Nagaoka SI
    2. Hassold TJ
    3. Hunt PA

    (2012) Human aneuploidy: mechanisms and new insights into an age-old problem

    Nature Reviews Genetics 13:493–504.

    https://doi.org/10.1038/nrg3245

    • Google Scholar
25. 1. Obradors A
    2. Rius M
    3. Daina G
    4. Ramos L
    5. Benet J
    6. Navarro J

    (2011) Whole-chromosome aneuploidy analysis in human oocytes: focus on comparative genomic hybridization

    Cytogenetic and Genome Research 133:119–126.

    https://doi.org/10.1159/000324233

    • Google Scholar
26. 1. Ottolini CS
    2. Newnham LJ
    3. Capalbo A
    4. Natesan SA
    5. Joshi HA
    6. Cimadomo D
    7. Griffin DK
    8. Sage K
    9. Summers MC
    10. Thornhill AR
    11. Housworth E
    12. Herbert AD
    13. Rienzi L
    14. Ubaldi FM
    15. Handyside AH
    16. Hoffmann ER

    (2015) Genome-wide maps of recombination and chromosome segregation in human oocytes and embryos show selection for maternal recombination rates

    Nature Genetics 47:727–735.

    https://doi.org/10.1038/ng.3306

    • Google Scholar
27. 1. Pacchierotti F
    2. Adler I-D
    3. Eichenlaub-Ritter U
    4. Mailhes JB

    (2007) Gender effects on the incidence of aneuploidy in mammalian germ cells

    Environmental Research 104:46–69.

    https://doi.org/10.1016/j.envres.2006.12.001

    • Google Scholar
28. 1. Petronczki M
    2. Siomos MF
    3. Nasmyth K

    (2003) Un mnage à quatree

    Cell 112:423–440.

    https://doi.org/10.1016/S0092-8674(03)00083-7

    • Google Scholar
29. 1. Sakakibara Y
    2. Hashimoto S
    3. Nakaoka Y
    4. Kouznetsova A
    5. Höög C
    6. Kitajima TS

    (2015) Bivalent separation into univalents precedes age-related meiosis i errors in oocytes

    Nature Communications 6:7550.

    https://doi.org/10.1038/ncomms8550

    • Google Scholar
30. 1. Schuh M
    2. Ellenberg J

    (2007) Self-organization of MTOCs replaces centrosome function during acentrosomal spindle assembly in live mouse oocytes

    Cell 130:484–498.

    https://doi.org/10.1016/j.cell.2007.06.025

    • Google Scholar
31. 1. Shomper M
    2. Lappa C
    3. FitzHarris G

    (2014) Kinetochore microtubule establishment is defective in oocytes from aged mice

    Cell Cycle 13:1171–1179.

    https://doi.org/10.4161/cc.28046

    • Google Scholar
32. 1. Strickland L
    2. Dassow George von
    3. Ellenberg J
    4. Foe V
    5. Lenart P
    6. Burgess D

    (2004) Light microscopy of echinoderm embryos

    Methods Cell Biol 74:371–409.

    https://doi.org/10.1016/S0091-679X(04)74016-9

    • Google Scholar
33. 1. Tachibana-Konwalski K
    2. Godwin J
    3. van der Weyden L
    4. Champion L
    5. Kudo NR
    6. Adams DJ
    7. Nasmyth K

    (2010) Rec8-containing cohesin maintains bivalents without turnover during the growing phase of mouse oocytes

    Genes & Development 24:2505–2516.

    https://doi.org/10.1101/gad.605910

    • Google Scholar
34. 1. Templado C
    2. Vidal F
    3. Estop A

    (2011) Aneuploidy in human spermatozoa

    Cytogenetic and Genome Research 133:91–99.

    https://doi.org/10.1159/000323795

    • Google Scholar
35. 1. Tsutsumi M
    2. Fujiwara R
    3. Nishizawa H
    4. Ito M
    5. Kogo H
    6. Inagaki H
    7. Ohye T
    8. Kato T
    9. Fujii T
    10. Kurahashi H
    11. Shi Q

    (2014) Age-related decrease of meiotic cohesins in human oocytes

    PLoS ONE 9:e96710.

    https://doi.org/10.1371/journal.pone.0096710

    • Google Scholar
36. 1. Watanabe Y

    (2012) Geometry and force behind kinetochore orientation: lessons from meiosis

    Nature Reviews Molecular Cell Biology 13:370–382.

    https://doi.org/10.1038/nrm3349

    • Google Scholar
37. 1. Yokobayashi S
    2. Watanabe Y

    (2005) The kinetochore protein Moa1 enables cohesion-mediated monopolar attachment at meiosis i

    Cell 123:803–817.

    https://doi.org/10.1016/j.cell.2005.09.013

    • Google Scholar
38. 1. Yun Y
    2. Holt JE
    3. Lane S
    4. McLaughlin E
    5. Merriman J
    6. Jones K

    (2014) Reduced ability to recover from spindle disruption and loss of kinetochore spindle assembly checkpoint proteins in oocytes from aged mice

    Cell Cycle 13:1938–1947.

    https://doi.org/10.4161/cc.28897

    • Google Scholar
39. 1. Zhai Y

    (1995) Kinetochore microtubule dynamics and the metaphase-anaphase transition

    The Journal of Cell Biology 131:721–734.

    https://doi.org/10.1083/jcb.131.3.721

    • Google Scholar

Author details

1. Agata P Zielinska

   Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom

   Contribution

   APZ, generated and analysed all data with the exception of Figure 6 and Figure 6-figure supplement 1, and prepared figures; wrote the manuscript, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   The author declares that no competing interests exist.

2. Zuzana Holubcova

   Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom

   Contribution

   ZH, generated data in Figure 6 and Figure 6-figure supplement 1 and commented on the manuscript, Acquisition of data, Analysis and interpretation of data

   Competing interests

   The author declares that no competing interests exist.

3. Martyn Blayney

   Bourn Hall Clinic, Cambridge, United Kingdom

   Contribution

   MB, supervised the collection and handling of human oocytes in the Bourn Hall Clinic and commented on the manuscript, Drafting or revising the article, Contributed unpublished essential data or reagents

   Competing interests

   The author declares that no competing interests exist.

4. Kay Elder

   Bourn Hall Clinic, Cambridge, United Kingdom

   Contribution

   KE, supervised the collection and handling of human oocytes in the Bourn Hall Clinic and commented on the manuscript, Drafting or revising the article, Contributed unpublished essential data or reagents

   Competing interests

   The author declares that no competing interests exist.

5. Melina Schuh

   1. Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom
   2. Max Planck Institute for Biophysical Chemistry, Goettingen, Germany

   Contribution

   MS, prepared schematic drawings; wrote the manuscript; conceived and supervised the study, Conception and design, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   melina.schuh@mpibpc.mpg.de

   Competing interests

   The author declares that no competing interests exist.

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