After fertilization is initiated, the secondary oocyte begins its second meiotic division, resulting in the formation of a mature ovum and another polar body. At this point, the ovum is ready to fuse with the spermatozoan.
Fertilization typically occurs in the fallopian tube. After fertilization, the diploid zygote will begin the process of implantation into the uterus, where it undergoes mitotic divisions. The resulting multicellular organism is called the embryo, which further develops in the uterus into a fetus. Normally, one ovum is released every 28 days in human females.
These 28 days constitute the menstrual cycle, which may vary in length, but on average, the cycle takes 28 days. A female is born with all the eggs she will ever have, and after the initiation of menstruation, one egg is released every 28 days until menopause. At the beginning of the menstrual cycle, elevated follicle-stimulating hormone FSH stimulates primary oocytes to resume meiosis. A spike in luteinizing hormone LH triggers ovulation.
Thyroid-stimulating hormone TSH and human chorionic gonadotropin hCG are involved in thyroid hormone secretion, and in maintenance of the corpus luteum during pregnancy, respectively.
Estrogen is a female sex hormone, and has an indirect regulatory role in meiotic divisions of ova. An oocyte develops in the ovaries during female gametogenesis.
If the oocyte eventually becomes an ovum and is fertilized in the fallopian tubes, the resulting gamete will be implanted in the uterus. The placenta is an organ that connects the developing fetus to the uterine wall, and the cervix is the lower portion of the uterus that separates the vagina from the uterus. When gametes join they form a cell called a zygote.
Human sperm and eggs contain 23 chromosomes. Human zygotes contain 46 chromosomes. The type of cell division that produces gametes with half the normal chromosomes is called meiosis. During oogenesis—at the end of meiosis and cytokinesis—an oogonium divides into one mature ovum capable of being fertilized and three polar bodies that are reabsorbed, while a spermatogonium divides into four viable spermatozoa capable of fertilization.
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Correct answer: more than one follicle maturing in a single menstrual cycle. Explanation : During the menstrual cycle, typically only one follicle matures and is fertilized. Report an Error. Example Question 12 : Understanding Oogenesis. Possible Answers: Zygotes. Correct answer: Polar bodies. Chapter 4: Cell Structure and Function. Chapter 5: Membranes and Cellular Transport. Chapter 6: Cell Signaling. Chapter 7: Metabolism. Chapter 8: Cellular Respiration.
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This is a sample clip. Sign in or start your free trial. Even the evolutionary distant but important model organism Drosophila melanogaster can display increased incidence of age-related aneuploidy on a background where sister chromatid cohesion is perturbed Jeffreys et al. It is true that many strains of mice have low rates of aneuploidy and the above age-related aneuploidy in Drosophila requires disruption in the expression of the ORD gene product involved in sister chromatid cohesion.
However, high, human-like levels of aneuploidy can exist without perturbation in some mice. This increase in aneuploidy was associated with a decrease in recombination frequency between homologs and is likely due to sequence divergence in the homologs of these two strains given the same phenomenon can be observed in close strains of yeasts Hunter et al. Also some strains of mice such as CBA display higher rates of aneuploidy than other strains Eichenlaub-Ritter et al.
As an aside it is important to note that achiasmate homologs are a feature of meiosis in a number of organisms, especially in Drosophila Wolf, ; Thomas et al. One mechanism to account for proper segregation of achiasmate homologs in Drosophila is heterochromatin pairing Karpen et al.
It would be interesting to determine if pairing of homologs in mammalian oocytes, independent of chiasmata, play any part in MI as it can in other organisms Gerton and Hawley, This review has had two main purposes: first, to give a broad review of the cell biology responsible for the meiotic cell cycle transitions which define the remarkable life of an oocyte; second, to put our knowledge of cell biology into a clinical context by using it to discuss our current understanding of the unique susceptibility of the oocyte to aneuploidy.
It is hoped that despite the fact that the etiology of aneuploidy is unlikely to be found in one particular meiotic defect or even pathway, our understanding of its causes will likely come from basic cell biology done on model organisms which are more tractable than human, and such knowledge will eventually feed into a clinical setting by collaborations of basic and applied researchers who together may find newer and better methods for prevention, screening and possible treatment.
Major themes basic cell biologist are likely to make substantial progress in the next decade are i how germ cells commit to entry into meiosis; ii how homologs are held together for the maintenance of cohesion and how they may deteriorate with age; iii how the oocyte remains viable during a protracted period of GV arrest; iv how the oocyte controls the segregation of homologs during the first meiotic division and v how the oocyte maintains MetII arrested and how this is detrimentally affected by post-ovulatory ageing.
Pursuit of answers to these questions will likely lead to a better understanding of aneuploidy in oocytes. The author would like to acknowledge continued funding from the Wellcome Trust. Google Scholar. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide.
Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract.
Passage from mitosis to GV stage arrest. Passage through meiosis I. Completion of meiosis II at fertilization. Aneuploidy in oocytes: is it a human condition? Concluding remarks. Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age. Jones Keith T.
Jones 1. Oxford Academic. Revision received:. Cite Cite Keith T. Select Format Select format. Permissions Icon Permissions. Abstract Mammalian oocytes begin meiosis in the fetal ovary, but only complete it when fertilized in the adult reproductive tract. Figure Open in new tab Download slide. The life of a mammalian oocyte The figures depicts the life of an oocyte beginning with its inception from oogonia following PGC colonization of the ovary left to pronucleus formation following fertilization right , which marks the completion of meiosis and entry into the first embryonic cell cycle.
Aneuploidy in oocytes during meiosis I A homolog pair, aligned and under tension on a metaphase I spindle: A , C , D and E these pairs have a single crossover event, i. Loss of cohesin with age resolves chiasmata A single crossover event has happened in fetal life for this homolog pair A. The cohesin complex Sister chromatids are held together by the cohesin complex, which embraces them in a ring-like structure.
Incompetence of preovulatory mouse oocytes to undergo cortical granule exocytosis following induced calcium oscillations. Google Scholar Crossref. Search ADS. Predivision in human oocytes at meiosis I: a mechanism for trisomy formation in man.
In germ cells of mouse embryonic ovaries, the decision to enter meiosis precedes premeiotic DNA replication. Chromosome synapsis defects and sexually dimorphic meiotic progression in mice lacking Spo Number of germ cells and somatic cells in human fetal ovaries during the first weeks after sex differentiation.
Functionality of the spindle checkpoint during the first meiotic division of mammalian oocytes. Germ cells enter meiosis in a rostro-caudal wave during development of the mouse ovary. Mad3p, a pseudosubstrate inhibitor of APC Cdc20 in the spindle assembly checkpoint. Bisphenol-A induces cell cycle delay and alters centrosome and spindle microtubular organization in oocytes during meiosis. Activation of p34cdc2 protein kinase activity in meiotic and mitotic cell cycles in mouse oocytes and embryos.
Google Scholar PubMed. Disruption of c-mos causes parthenogenetic development of unfertilized mouse eggs. Fission yeast Taz1 protein is required for meiotic telomere clustering and recombination. Antenatal care implications of population-based trends in Down syndrome birth rates by rurality and antenatal care provider, Queensland, — Influence of follicular fluid meiosis-activating sterol on aneuploidy rate and precocious chromatid segregation in aged mouse oocytes.
An accumulation of p34cdc2 at the end of mouse oocyte growth correlates with the acquisition of meiotic competence. Acquisition of meiotic competence in growing mouse oocytes is controlled at both translational and posttranslational levels. Cellular, biochemical and molecular mechanisms regulating oocyte maturation. Mechanisms of aneuploidy induction in human oogenesis and early embryogenesis. The Ran GTPase mediates chromatin signaling to control cortical polarity during polar body extrusion in mouse oocytes.
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The development of mouse oocyte cortical reaction competence is accompanied by major changes in cortical vesicles and not cortical granule depth. G2 arrest in Xenopus oocytes depends on phosphorylation of cdc25 by protein kinase A. Formin-2 is required for spindle migration and for the late steps of cytokinesis in mouse oocytes.
The CBA mouse as a model for age-related aneuploidy in man: studies of oocyte maturation, spindle formation and chromosome alignment during meiosis. The participation of cyclic adenosine monophosphate cAMP in the regulation of meiotic maturation of oocytes in the laboratory mouse. In vitro maturation of mouse oocytes isolated from late, middle, and pre-antral graafian follicles.
D-box is required for the degradation of human Shugoshin and chromosome alignment. Development of the first meiotic prophase stages in human fetal oocytes observed by light and electron microscopy. Histone H1 kinase activity, germinal vesicle breakdown and M phase entry in mouse oocytes. Homologous chromosome interactions in meiosis: diversity amidst conservation. Molecular basis of oocyte-paracrine signalling that promotes granulosa cell proliferation. Intracellular calcium oscillations signal apoptosis rather than activation in in vitro aged mouse eggs.
Essential CDK1-inhibitory role for separase during meiosis I in vertebrate oocytes. The origin and evolution of the pseudoautosomal regions of human sex chromosomes. Rac activity is polarized and regulates meiotic spindle stability and anchoring in mammalian oocytes. Wee1B Is an oocyte-specific kinase involved in the control of meiotic arrest in the mouse. Cohesin cleavage by separase required for anaphase and cytokinesis in human cells. Dissociation of cohesin from chromosome arms and loss of arm cohesion during early mitosis depends on phosphorylation of SA2.
Heterogeneity of cell populations that contribute to the formation of primordial follicles in rats. SMC1beta-deficient female mice provide evidence that cohesins are a missing link in age-related nondisjunction. Cyclin-B1-mediated inhibition of excess separase is required for timely chromosome disjunction. Mad2 is required for inhibiting securin and cyclin B degradation following spindle depolymerisation in meiosis I mouse oocytes. Mad2 prevents aneuploidy and premature proteolysis of cyclin B and securin during meiosis I in mouse oocytes.
Changes in histone acetylation during postovulatory aging of mouse oocyte. The mismatch repair system contributes to meiotic sterility in an interspecific yeast hybrid. Degradation of securin in mouse and pig oocytes is dependent on ubiquitin-proteasome pathway and is required for proteolysis of the cohesion subunit, Rec8, at the metaphase-to-anaphase transition.
Oocytes prevent cumulus cell apoptosis by maintaining a morphogenic paracrine gradient of bone morphogenetic proteins. Aged mouse oocytes fail to readjust intracellular adenosine triphosphates at fertilization. Germline stem cells and follicular renewal in the postnatal mammalian ovary. Oocyte generation in adult mammalian ovaries by putative germ cells in bone marrow and peripheral blood.
Turning it on and off: M-phase promoting factor during meiotic maturation and fertilization. The evolving national birth prevalence of Down syndrome in Taiwan.
A study on the impact of second-trimester maternal serum screening. Acquisition of meiotic competence in mouse oocytes: absolute amounts of p34 cdc2 , cyclin B1, cdc25C, and wee1 in meiotically incompetent and competent oocytes.
Centric heterochromatin and the efficiency of achiasmate disjunction in Drosophila female meiosis. Initiation of meiotic recombination by formation of DNA double-strand breaks: mechanism and regulation. Maternal education modifies the age-related increase in the birth prevalence of Down syndrome.
A central role for cohesins in sister chromatid cohesion, formation of axial elements, and recombination during yeast meiosis. Recruitment and selection of ovarian follicles for determination of ovulation rate in the pig.
Sex-specific differences in meiotic chromosome segregation revealed by dicentric bridge resolution in mice. Retinoic acid regulates sex-specific timing of meiotic initiation in mice. Bi-orientation of achiasmatic chromosomes in meiosis I oocytes contributes to aneuploidy in mice. Resolution of chiasmata in oocytes requires separase-mediated proteolysis.
The spindle checkpoint rescues the meiotic segregation of chromosomes whose crossovers are far from the centromere. Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and meiosis II. Characterization of susceptible chiasma configurations that increase the risk for maternal nondisjunction of chromosome Formin-2, polyploidy, hypofertility and positioning of the meiotic spindle in mouse oocytes. Meiotic maturation of the mouse oocyte requires an equilibrium between cyclin B synthesis and degradation.
Temporally and spatially selective loss of Rec8 protein from meiotic chromosomes during mammalian meiosis. Loss of Rec8 from chromosome arm and centromere region is required for homologous chromosome separation and sister chromatid separation, respectively, in mammalian meiosis.
The mitotic feedback control gene MAD2 encodes the alpha-subunit of a prenyltransferase. Mouse pachytene checkpoint 2 trip13 is required for completing meiotic recombination but not synapsis. Cdc25b phosphatase is required for resumption of meiosis during oocyte maturation. Cohesin release is required for sister chromatid resolution, but not for condensin-mediated compaction, at the onset of mitosis. Reduced expression of MAD2, BCL2, and MAP kinase activity in pig oocytes after in vitro aging are associated with defects in sister chromatid segregation during meiosis II and embryo fragmentation after activation.
Maintenance of sister chromatid attachment in mouse eggs through maturation-promoting factor activity.
Mouse Emi2 establishes a metaphase II spindle by re-stabilizing cyclin B1 during interkinesis. Cytoplasmic control of nuclear behavior during meiotic maturation of frog oocytes. Shugoshin prevents dissociation of cohesin from centromeres during mitosis in vertebrate cells.
Stops and starts in mammalian oocytes: recent advances in understanding the regulation of meiotic arrest and oocyte maturation. Meiotic arrest in the mouse follicle maintained by a Gs protein in the oocyte. Meiotic resumption in response to luteinizing hormone is independent of a Gi family G protein or calcium in the mouse oocyte. The Gs-linked receptor GPR3 maintains meiotic arrest in mammalian oocytes. Sexual differentiation of germ cells in XX mouse gonads occurs in an anterior-to-posterior wave.
Timing of activation of primordial follicles in mature rats is only slightly affected by fetal stage at meiotic arrest. Regulation of the acquisition of meiotic competence in the mouse: changes in the subcellular localization of cdc2, cyclin B1, cdc25C and wee1, and in the concentration of these proteins and their transcripts. Kinetochore orientation during meiosis is controlled by aurora b and the monopolin complex.
Not all germ cells are created equal: Aspects of sexual dimorphism in mammalian meiosis. Asymmetric positioning and organization of the meiotic spindle of mouse oocytes requires CDC42 function.
Defective meiosis in telomere-silencing mutants of Schizosaccharomyces pombe. Phosphorylation of Erp1 by p90rsk is required for cytostatic factor arrest in Xenopus laevis eggs. Microinjection of antisense c-mos oligonucleotides prevents meiosis II in the maturing mouse egg.
First-trimester combined screening for Down syndrome and other fetal anomalies. Maternal primary imprinting is established at a specific time for each gene throughout oocyte growth.
Rec8p, a meiotic recombination and sister chromatid cohesion phosphoprotein of the Rad21p family conserved from fission yeast to humans. Maternal aging and chromosomal abnormalities: new data drawn from in vitro unfertilized human oocytes. Effect of maternal age on the frequency of cytogenetic abnormalities in human oocytes.
The occurrence of aneuploidy in human: lessons from the cytogenetic studies of human oocytes. From primordial germ cell to primordial follicle: mammalian female germ cell development.
Mouse ovarian germ cell cysts undergo programmed breakdown to form primordial follicles. Migration of gonocytes into the mammalian gonad and their differentiation.
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