Difference between revisions of "Cell Division - Meiosis"

From Embryology
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* Only small regions of non-paired sex chromosomes interact
* Only small regions of non-paired sex chromosomes interact
* Mutations that compromise meiotic recombination in male spermatocytes result in arrest and apoptosis at this stage.  
* Mutations that compromise meiotic recombination in male spermatocytes result in arrest and apoptosis at this stage.  
[[File:Mouse meiosis pachytene 01.jpg|800px]]
Mouse meiosis pachytene<ref name="PMID25768017"><pubmed>25768017</pubmed></ref>
* diplotene phase, diplonema; Greek, ''diplonema'' = "two threads"
* diplotene phase, diplonema; Greek, ''diplonema'' = "two threads"

Revision as of 19:21, 15 May 2016

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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!


Simple cartoon of Mitosis and Meiosis
Mitosis and Meiosis

Meiosis is the special type of reductive cell division occurring only in the generation of the gametes or germ cells (oocyte and spermatozoa). Meiotic cell division reduces (halves) the chromosomal content. The overall process of germ cell development is called "gametogenesis" and includes not only meiosis but also the cellular changes, that occur differently in male and female gametes.

Cell Division Links: meiosis | mitosis | Lecture - Cell Division and Fertilization | spermatozoa | oocyte | fertilization | zygote | Genetics
Female gametogenesis.jpg Male gametogenesis.jpg
Female gametogenesis Male gametogenesis

Some Recent Findings

Bivalent separation into univalents precedes age-related meiosis I errors in oocytes[1]
Karyotype of parthenogenetic blastocysts[2]
  • Silencing of X-Linked MicroRNAs by Meiotic Sex Chromosome Inactivation [3] "During the pachytene stage of meiosis in male mammals, the X and Y chromosomes are transcriptionally silenced by Meiotic Sex Chromosome Inactivation (MSCI). MSCI is conserved in therian mammals and is essential for normal male fertility. ...Here we use RNA FISH to examine X-miRNA expression in the male germ line. We find that, like protein-coding X-genes, X-miRNAs are expressed prior to prophase I and are thereafter silenced during pachynema. X-miRNA silencing does not occur in mouse models with defective MSCI. Furthermore, X-miRNAs are expressed at pachynema when present as autosomally integrated transgenes. Thus, we conclude that silencing of X-miRNAs during pachynema in wild type males is MSCI-dependent. Importantly, misexpression of X-miRNAs during pachynema causes spermatogenic defects. We propose that MSCI represents a chromosomal mechanism by which X-miRNAs, and other potential X-encoded repressors, can be silenced, thereby regulating genes with critical late spermatogenic functions." Spermatozoa Development
  • Bivalent separation into univalents precedes age-related meiosis I errors in oocytes[1] "The frequency of chromosome segregation errors during meiosis I (MI) in oocytes increases with age. The two-hit model suggests that errors are caused by the combination of a first hit that creates susceptible crossover configurations and a second hit comprising an age-related reduction in chromosome cohesion. This model predicts an age-related increase in univalents, but direct evidence of this phenomenon as a major cause of segregation errors has been lacking. Here, we provide the first live analysis of single chromosomes undergoing segregation errors during MI in the oocytes of naturally aged mice. Chromosome tracking reveals that 80% of the errors are preceded by bivalent separation into univalents. The set of the univalents is biased towards balanced and unbalanced predivision of sister chromatids during MI. Moreover, we find univalents predisposed to predivision in human oocytes. This study defines premature bivalent separation into univalents as the primary defect responsible for age-related aneuploidy."
  • Premature dyad separation in meiosis II is the major segregation error with maternal age in mouse oocytes[4] "Changes consistent with chromosome cohesion deterioration were found with age, including increased interkinetochore distance and loss of the centromeric protector of cohesion SGO2 in metaphase II arrested (metII) eggs, as well as a rise in the number of weakly attached bivalents in meiosis I (MI) and lagging chromosomes at anaphase I. However, there were no MI errors in congression or biorientation. Instead, premature separation of dyads in meiosis II was the major segregation defect in aged eggs and these were associated with very low levels of SGO2. These data show that although considerable cohesion loss occurs during MI, its consequences are observed during meiosis II, when centromeric cohesion is needed to maintain dyad integrity."
  • Chromosomes in the Porcine First Polar Body Possess Competence of Second Meiotic Division within Enucleated MII Stage Oocytes[2] "These results demonstrate that chromosomes in PB1 can participate in normal pre-implantation embryonic development when injected into enucleated MII stage oocytes, and that tetraploid PA blastocysts are produced (although at a low proportion) when PB1 chromosomes are injected into intact MII stage oocytes."
More recent papers
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<pubmed limit=5>Meiosis</pubmed>

Older recent papers
  • Bora regulates meiotic spindle assembly and cell cycle during mouse oocyte meiosis[5] "Bora is the binding partner of Aurora A, which is required for its activation and phosphorylation of Polo like kinase 1 (Plk1), and is involved in the spindle assembly and progress of the cell cycle during mitosis. In this study, we examined the expression, localization, and function of Bora during mouse oocyte meiosis. The expression level of Bora was increased during oocyte meiotic maturation, with an elevated level at metaphase. Immunofluorescence analysis showed that Bora was concentrated as a dot shortly after germinal vesicle breakdown (GVBD), associating first with the surrounding chromosomes and then with the spindle throughout the oocyte meiotic maturation. Further experiments confirmed that Bora co-localized with α-tubulin at prometaphase/metaphase, but dissociated from α-tubulin at anaphase/telophase."


Oocyte Meiosis 01 icon.jpg
 ‎‎Oocyte Meiosis
Page | Play
A mouse oocyte undergoing meiosis spindle migration followed by first polar body extrusion and MII spindle positioning.[6]
  • blue - Hoechst staining of chromosomes.
  • green - UtrCH-GFP was used to label cortical changes during spindle migration.

The video shows that cytoplasmic streaming continues to the MII arrest stage to maintain the oocyte set of chromosomes/MII spindle in place close to the cortex. Frames are 11 min apart, and video length is 840 min. Bar, 20 µm.

Comparison of Meiosis/Mitosis

Mitosis and meiosis.jpg

  • After DNA replication 2 nuclear (and cell) divisions required to produce haploid gametes
  • Each diploid cell in meiosis produces 4 haploid cells (sperm) 1 haploid cell (egg)
  • Each diploid cell mitosis produces 2 diploid cells

Meiosis Germ cell division (haploid)

  • Reductive division
  • Generates haploid gametes (egg, sperm)
  • Each genetically distinct from parent
  • Genetic recombination (prophase 1)
    • Exchanges portions of chromosomes maternal/paternal homologous pairs
  • Independent assortment of paternal chromosomes (meiosis 1)

Homologous chromosomes pairing unique to meiosis

  • Each chromosome duplicated and exists as attached sister chromatids before pairing occurs
  • Genetic Recombination shown by chromosomes part red and part black
    • chromosome pairing in meiosis involves crossing-over between homologous chromosomes

Meiosis I and II

  • Meiosis I separates the pairs of homologous chromosomes, reduces the cell from diploid to haploid.
  • Meiosis II separates each chromosome into two chromatids (chromosome behavior in meiosis II is like that of mitosis).

Figure 14.32. Comparison of meiosis and mitosis

Prophase I

  • The homologous chromosomes pair and exchange DNA to form recombinant chromosomes.
  • Note - in oocyte development, from birth until puberty oocytes are in "prophase I arrest" at diplotene stage. This is important for sustaining the ovarian oocyte pool and lutenizing hormone (LH) induces resumption of meiosis I.
Meiotic prophase I stages
Mouse early meiotic prophase I stages[7]

Prophase I is further divided into five stages (phases):


  • leptotene phase, leptonema; Greek, leptotene = "thin threads"
  • the duplicated paired chromosome homologs condense.


  • zygotene phase, zygonema, Greek, zygotene = "paired threads"
  • homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes consisting of four chromatids (tetrads).
  • the synaptonemal complex begins to form between the two sets of sister chromatids in each bivalent (the duplicated chromosome paired with its homologous duplicated chromosome).


  • pachytene phase, pachynema; Greek, pachytene = "thick threads"
  • crossing over between pairs of homologous chromosomes (meiotic recombination or synapsis) to form chiasmata (form between two nonsister chromatids at points where they have crossed over)
  • synaptonemal complex is complete and can be stable for some time.
  • Autosomal non-sister chromatids of homologous chromosomes can now extensively exchange segments in regions of homology.
  • Only small regions of non-paired sex chromosomes interact
  • Mutations that compromise meiotic recombination in male spermatocytes result in arrest and apoptosis at this stage.

Mouse meiosis pachytene 01.jpg

Mouse meiosis pachytene[8]


  • diplotene phase, diplonema; Greek, diplonema = "two threads"
  • homologous chromosomes begin to separate but remain attached by chiasmata.
  • synaptonemal complex degrades and the chromosomes separate from one another a small amount giving this appearance.
  • It is possible that some chromosome uncoiling may also occur allowing some gene transcription.
    • In the developing human ovary, oocytes remain at the diplotene stage from fetal life through postnatal childhood, until puberty when the lutenizing hormone (LH) surges stimulate the resumption of meiosis.


  • diakinesis phase; Greek, diakinesis = "moving through"
  • homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.
  • prophase I ends and chromosomes now recondense, transcription stops and the transition to metaphase occurs.

Prometaphase I

  • Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.
Mouse oocyte meiosis[9]

Metaphase I

  • Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.)

Anaphase I

  • The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell.

Telophase I

  • The chromosomes become diffuse and the nuclear membrane reforms.

Cytokinesis I

  • Cellular cytoplasmic division to form two new cells, followed by Meiosis II.
  • Note - in oocyte meiosis, the extrusion of the first polar body (1 PB) indicates completion of the first meiotic division.

Prophase II

  • Chromosomes begin to condense, nuclear membrane breaks down and spindle forms.

Metaphase II

  • Spindle fibres attach to chromosomes, chromosomes align in cell centre.

Anaphase II

  • Chromosomes separate and move to the opposite poles of the cell.

Telophase II

  • Chromosomes reach spindle pole ends and the nuclear membrane reforms.


Cellular cytoplasmic division to form new cells.

Meiosis Sex Differences

Female (oogenesis)

  • Meiosis initiated once in a finite population of cells
  • 1 gamete produced / meiosis
  • Completion of meiosis delayed for months or years
  • Meiosis arrested at 1st meiotic prophase and reinitiated in a smaller population of cells
  • Differentiation of gamete occurs while diploid in first meiotic prophase
  • All chromosomes exhibit equivalent transcription and recombination during meiotic prophase

Male (spermatogenesis)

  • Meiosis initiated continuously in a mitotically dividing stem cell population
  • 4 gametes produced / meiosis
  • Meiosis completed in days or weeks
  • Meiosis and differentiation proceed continuously without cell cycle arrest
  • Differentiation of gamete occurs while haploid after meiosis ends

Sex chromosomes excluded from recombination and transcription during first meiotic prophase

Female Gametogenesis

In females, the total number of eggs ever to be produced are present in the newborn female.

  1. All eggs are arrested at an early stage of the first meiotic division as a primary oocyte (primordial follicle). Following purberty, during each menstrual cycle, pituitary gonadotrophin stimulates completion of meiosis 1 the day before ovulation.
  2. In meiosis 1, a diploid cell becomes 2 haploid (23 chromosomes) daughter cells, each chromosome has two chromatids. One cell becomes the secondary oocyte the other cell forms the first polar body.
  3. The secondary oocyte then commences meiosis 2 which arrests at metaphase and will not continue without fertilization.
  4. At fertilization meiosis 2 completes, forming a second polar body. Note that the first polar body may also undergo this process forming a third polar body.

Female gametogenesis

Oogenesis and meiosis cartoon.jpg

Meiosis and Oogenesis[10]

Meiosis - divided into 3 temporally distinct phases.
  1. Prophase - after DNA replication, homologous chromosomes (shown in red and blue) undergo pairing, synapsis and recombination, and arrest at the diplotene (dictyate) stage.
  2. Dictyate arrest - oocytes remain in meiotic arrest until the female reaches maturity and the oocyte has completed an extensive period of growth following follicle formation.
  3. Divisions - luteinizing hormone (LH) surge that triggers ovulation also causes resumption and completion of the first meiotic division in the periovulatory oocyte. The ovulated egg is arrested at second meiotic metaphase, and anaphase onset and completion of meiosis II only occur if the egg is fertilized.

Oogenesis - complex involving 4 distinct phases.

  1. Commitment to meiosis and meiotic initiation - occurs at GA 8–10 weeks in humans.
  2. Follicle formation - occurs during the second trimester in humans.
  3. Oocyte growth - occurs in the sexually mature female under the control of paracrine and endocrine signals. Oocyte growth is thought to take approximately 85 days in humans and typically culminates in the ovulation of a single egg.
  4. Fertilization - of the ovulated egg results in the completion of the second meiotic division.
Oocyte Meiotic Spindles
  • Mammalian oocytes have no centrosomes, but still form spindles using many microtubule-organizing centres lacking centrioles. (cells dividing by mitosis have 2 centrosomes to help form the mitotic spindle) PMID 17693257
  • Meiotic spindle is a microtubule structure but its relocation in the oocyte cytoplasm involves microfilaments, filamentous-actin structures nucleated by Formin-2 (Fmn2). PMID 19062278

Polar Body

Human oocyte at metaphase II showing polar body at 12 o'clock position.

The breakdown of the germinal vesicle indicates a resumption of meiosis and the extrusion of the first polar body (1 PB) indicates completion of the first meiotic division in human oocytes. The polar body is a small cytoplasmic exclusion body formed to enclose the excess DNA formed during the oocyte (egg) meiosis and following sperm fertilization. There are 2-3 polar bodies derived from the oocyte present in the zygote, the number is dependent upon whether polar body 1 (the first polar body formed during meiosis 1) divides during meiosis 2. This exclusion body contains the excess DNA from the reductive division (the second and third polar bodies are formed from meiosis 2 at fertilization). These polar bodies do not contribute to the future genetic complement of the zygote, embryo or fetus.

Recent research in some species suggest that the space formed by the peripheral polar body (between the oocyte and the zona pellucia) can influence the site of spermatozoa fertilization.

Assisted reproductive techniques involving intracytoplasmic sperm injection (ICSI) have looked at the "quality" of the polar body and found that the morphology is related to mature oocyte viability and has the potential to predict oocyte fertilization rates and pregnancy achievement.[11][12]

Links: Category:Polar Body

Female Abnormalities

Trisomy 21 female karyotype

Meiotic non-disjunction resulting in aneuploidy, most are embryonic lethal and not seen. The potential for genetic abnormalities increase with maternal age.

  • Autosomal chromosome aneuploidy
    • trisomy 21 - Down syndrome
    • trisomy 18 - Edwards syndrome
    • trisomy 13 - Patau syndrome
  • Sex chromosome aneuploidy
    • monosomy X - Turner's Syndrome
    • trisomy X - Triple-X syndrome
    • 47 XXY - Klinefelter's Syndrome

Male Gametogenesis

In males, sperm continues to be generated throughout life from a stem cell population in the testis. Spermatozoa maturation involves two processes meiosis and spermiogenesis Male gametogenesis.jpg

The above figure compares meiosis to the female (the polar bodies have been removed and labelling updated).

Human Spermatozoa Development

  • Spermatogenesis process of spermatagonia mature into spermatazoa (sperm).
  • Continuously throughout life occurs in the seminiferous tubules in the male gonad- testis (plural testes).
  • At puberty spermatagonia activate and proliferate (mitosis).
  • about 48 days from entering meiosis until morphologically mature spermatozoa
  • about 64 days to complete spermatogenesis, depending reproduction time of spermatogonia
  • follicle stimulating hormone (FSH) - stimulates the spermatogenic epithelium
  • luteinizing-hormone (LH) - stimulates testosterone production by Leydig cells

Human-spermatozoa EM01.jpg

Spermatozoa animation icon.jpg Mature human spermatozoa
  • 60 µm long, actively motile
  • divided into 3 main regions (head, neck and tail)
  • head - (flattened, 5 µm long by 3 µm wide) the nucleus and acrosome. Posterior part of nuclear membrane forms the basal plate.
  • neck - (1 µm) attached to basal plate, transverse oriented centriole, contains nine segmented columns of fibrous material, continue as outer dense fibres in tail.
  • tail - 3 parts a middle piece, principal piece and end piece
    • middle piece - (5 µm long) axonema and dense fibres surrounded by mitochondria
    • principal piece - (45 µm long) fibrous sheath interconnected by regularly spaced circumferential hoops
    • end piece - (5 µm long) axonema surrounded by small amount of cytoplasm and plasma membrane

Links: Spermatozoa Development


  • In humans at puberty, hormonal and morphological changes occur within the gonad and other systems (secondary sex characteristics).
  • Within the testis the immature Sertoli cells cease to proliferate and differentiate.
  • Spermatogonium proliferate and spermatogenesis begins.
  • It takes about 70 days for cells to mature from the diploid spermatogonium to a primary spermatocyte.
  • This maturation occurs in waves along the seminiferous tubules.

Links: Puberty


Azoospermia - Non-obstructive azoospermia (NOA) and Obstructive azoospermia (OA)
  • release of spermatozoa and accessory gland secretions from the male genital tract (3.5 ml)
  • 200-600 million sperm, by volume less than 10 % spermatozoa
  • Accessory Gland secretions - 60 % seminal vesicle, 30 % prostate and 10 % bulbourethral

Male Abnormalities

  • Oligospermia - (Low Sperm Count) less than 20 million sperm after 72 hour abstinence from sex
  • Azoospermia - (Absent Sperm) blockage of duct network
  • Immotile Cilia Syndrome - lack of sperm motility

Meiosis in Other Species

  • Sea urchin - oocytes complete meiosis before being shed.
  • Starfish - oocytes only complete meiosis upon hormonal stimulation.


Meiotic Nondisjunction

  • Occurs when homologues fail to separate during meiotic division I or II
  • For example trisomy 21 (Down Syndrome) caused by an extra copy of chromosome 21

Links: trisomy 21 | Nondisjunction

Chromosomal Translocations

  • Philadelphia chromosome
  • Chronic myelogenous leukemia
    • Piece of Chr9 exchanged with Chr22 Generates truncated abl

Overstimulates cell production


  1. 1.0 1.1 <pubmed>26130582</pubmed>| Nat Commun.]
  2. 2.0 2.1 <pubmed>24312673</pubmed>
  3. <pubmed>26509798</pubmed>
  4. <pubmed>24346700</pubmed>| Development
  5. <pubmed>23610072</pubmed>
  6. <pubmed>23439682</pubmed>| PMC3587830 | J Cell Biol.
  7. <pubmed>20140225</pubmed>| PLoS One
  8. <pubmed>25768017</pubmed>
  9. <pubmed>23152892</pubmed>| PLoS ONE
  10. <pubmed>22705668</pubmed>| Nat Rev Genet.
  11. <pubmed>10655316</pubmed>
  12. <pubmed>19960239</pubmed>| PMC2799563




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Cite this page: Hill, M.A. (2020, May 25) Embryology Cell Division - Meiosis. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Cell_Division_-_Meiosis

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