Polar body

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First stages of segmentation of a mammalian embryo. Semidiagrammatic. z.p. Zona pellucida. p.gl. Polar bodies. a. Two-cell stage. b. Four-cell stage. c. Eight-cell stage. d, e. Morula stage. Gray9.png
First stages of segmentation of a mammalian embryo. Semidiagrammatic. z.p. Zona pellucida. p.gl. Polar bodies. a. Two-cell stage. b. Four-cell stage. c. Eight-cell stage. d, e. Morula stage.

A polar body is a small haploid cell that is formed at the same time as an egg cell during oogenesis, but generally does not have the ability to be fertilized. It is named from its polar position in the egg.

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When certain diploid cells in animals undergo cytokinesis after meiosis to produce egg cells, they sometimes divide unevenly. Most of the cytoplasm is segregated into one daughter cell, which becomes the egg or ovum, while the smaller polar bodies only get a small amount of cytoplasm. They frequently die and disintegrate by apoptosis, but in some cases remain and can be important in the life cycle of the organism. [1]

Twinning

Polar body twinning is a hypothesized form of twinning in meiosis, where one or more polar bodies do not disintegrate and are fertilized by sperm. [2]

Twinning would occur, in principle, if the egg cell and a polar body were both fertilized by separate sperms. However, even if fertilization occurs, further development would usually not occur because the zygote formed by the fusion of the sperm and polar body would not have enough cytoplasm or stored nutrients to feed the developing embryo.

Polar bodies were first reported in 1824 by Carus in gastropods, but their role was not clarified until the work of Butschli in 1875, Giard in 1876, and finally Hertwig in 1877. These structures were often confused with egg fragments or expelled yolk masses, but were eventually referred to as directional bodies (or Richtungskörper), a term implying the place where the maturation divisions start. The common names "polocytes" and "polar bodies" derive from their polar position in the eggs. [1] Polar bodies were characterized in the early 20th century, by O. Hertwig, T. Boveri, and E.L. Mark, as non-functioning egg cells which disintegrated because the spermatozoon, with rare exceptions, could not fertilize them and instead chemically triggered their dissolution. [3]

Polar bodies serve to eliminate one half of the diploid chromosome set produced by meiotic division in the egg, leaving behind a haploid cell. To produce the polar bodies, the cell must divide asymmetrically, which is fueled by furrowing (formation of a trench) near a particular point on the cell membrane. The presence of chromosomes induces the formation of an actomyosin cortical cap, a myosin II ring structure and a set of spindle fibers, the rotation of which promotes invagination at the edge of the cell membrane and splits the polar body away from the oocyte. [4]

Meiotic errors can lead to aneuploidy in the polar bodies, which, in the majority of cases, produces an aneuploid zygote. Errors can occur during either of the two meiotic divisions that produce each polar body, but are more pronounced if they occur during the formation of the first polar body, because the formation of the first polar body influences the chromosomal makeup of the second. For example, predivision (the separation of chromatids before anaphase) in the first polar body can induce the formation of an aneuploid polar body. Therefore, the formation of the first polar body is an especially important factor in forming a healthy zygote. [5]

However, chromosomally abnormal polar bodies are not guaranteed to induce the development of an abnormal zygote. A euploid zygote can be produced if the aneuploidy is reciprocal: one polar body has an extra chromosome and the other lacks the same chromosome (see also uniparental disomy). If the extra chromosome is absorbed into a polar body rather than being passed into the oocyte, trisomy can be avoided. Whether this is a chance event or is some way influenced by the microenvironment is unclear. In at least one case, this euploid zygote has been traced through development to birth as a healthy child with a normal chromosome count. [6]

Medical applications

A polar body biopsy is the sampling of a polar body of an oocyte. After sampling of a polar body, subsequent analysis can be used to predict viability and pregnancy chance of the oocyte, as well as the future health of a person resulting such pregnancy. The latter use makes it a form of preimplantation genetic screening (PGS). Compared to a blastocyst biopsy, a polar body biopsy can potentially be of lower costs, less harmful side-effects, and more sensitive in detecting abnormalities. [6] The main advantage of the use of polar bodies in PGD is that they are not necessary for successful fertilisation or normal embryonic development, thus ensuring no deleterious effect for the embryo.

One of the disadvantages of PB biopsy is that it only provides information about the maternal contribution to the embryo, which is why cases of autosomal dominant and X-linked disorders that are maternally transmitted can be diagnosed, and autosomal recessive disorders can only partially be diagnosed. Another drawback is the increased risk of diagnostic error, for instance due to the degradation of the genetic material or events of recombination that lead to heterozygous first polar bodies.

Parthenogenesis

In some species, the polar body may re-merge with the egg cell. This can result in a viable embryo that has only one parent, a process called parthenogenesis. [7]

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Related Research Articles

<span class="mw-page-title-main">Gamete</span> Cell that fuses during fertilisation, such as a sperm or egg cell

A gamete is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual that produces the larger type of gamete—called an ovum—and a male produces the smaller type, called a sperm. Sperm cells or spermatozoa are small and motile due to the flagellum, a tail-shaped structure that allows the cell to propel and move. In contrast, each egg cell or ovum is relatively large and non-motile. In short a gamete is an egg cell or a sperm. In animals, ova mature in the ovaries of females and sperm develop in the testes of males. During fertilization, a spermatozoon and ovum unite to form a new diploid organism. Gametes carry half the genetic information of an individual, one ploidy of each type, and are created through meiosis, in which a germ cell undergoes two fissions, resulting in the production of four gametes. In biology, the type of gamete an organism produces determines the classification of its sex.

<span class="mw-page-title-main">Meiosis</span> Cell division producing haploid gametes

Meiosis is a special type of cell division of germ cells and apicomplexans in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome (haploid). Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a cell with two copies of each chromosome again, the zygote.

<span class="mw-page-title-main">Spermatozoon</span> Motile sperm cell

A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote.

<span class="mw-page-title-main">Zygote</span> Diploid eukaryotic cell formed by fertilization between two gametes

A zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism.

<span class="mw-page-title-main">Fertilisation</span> Union of gametes of opposite sexes during the process of sexual reproduction to form a zygote

Fertilisation or fertilization, also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. While processes such as insemination or pollination which happen before the fusion of gametes are also sometimes informally referred to as fertilisation, these are technically separate processes. The cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation.

<span class="mw-page-title-main">Intracytoplasmic sperm injection</span> In vitro fertilization procedure

Intracytoplasmic sperm injection is an in vitro fertilization (IVF) procedure in which a single sperm cell is injected directly into the cytoplasm of an egg. This technique is used in order to prepare the gametes for the obtention of embryos that may be transferred to a maternal uterus. With this method, the acrosome reaction is skipped.

<span class="mw-page-title-main">Egg cell</span> Female reproductive cell in most anisogamous organisms

The egg cell, or ovum, is the female reproductive cell, or gamete, in most anisogamous organisms. The term is used when the female gamete is not capable of movement (non-motile). If the male gamete (sperm) is capable of movement, the type of sexual reproduction is also classified as oogamous. A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is an oosphere. When fertilized, the oosphere becomes the oospore.

<span class="mw-page-title-main">Germ cell</span> Gamete-producing cell

A germ cell is any cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have germ cells designated in early development. Instead, germ cells can arise from somatic cells in the adult, such as the floral meristem of flowering plants.

An oocyte, oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell (PGC), which then undergoes mitosis, forming oogonia. During oogenesis, the oogonia become primary oocytes. An oocyte is a form of genetic material that can be collected for cryoconservation.

<span class="mw-page-title-main">Oogenesis</span> Egg cell production process

Oogenesis, ovogenesis, or oögenesis is the differentiation of the ovum into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage.

<span class="mw-page-title-main">Pronucleus</span> Nucleus of a sperm or an egg cell during fertilization

A pronucleus denotes the nucleus found in either a sperm or egg cell during the process of fertilization. The sperm cell undergoes a transformation into a pronucleus after entering the egg cell but prior to the fusion of the genetic material of both the sperm and egg. In contrast, the egg cell possesses a pronucleus once it becomes haploid, not upon the arrival of the sperm cell. Haploid cells, such as sperm and egg cells in humans, carry half the number of chromosomes present in somatic cells, with 23 chromosomes compared to the 46 found in somatic cells. It is noteworthy that the male and female pronuclei do not physically merge, although their genetic material does. Instead, their membranes dissolve, eliminating any barriers between the male and female chromosomes, facilitating the combination of their chromosomes into a single diploid nucleus in the resulting embryo, which contains a complete set of 46 chromosomes.

Reproductive biology includes both sexual and asexual reproduction.

<span class="mw-page-title-main">Human fertilization</span> Union of a human egg and sperm

Human fertilization is the union of a human egg and sperm, occurring primarily in the ampulla of the fallopian tube. The result of this union leads to the production of a fertilized egg called a zygote, initiating embryonic development. Scientists discovered the dynamics of human fertilization in the 19th century.

A chromosomal abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities, where there is an atypical number of chromosomes, or as structural abnormalities, where one or more individual chromosomes are altered. Chromosome mutation was formerly used in a strict sense to mean a change in a chromosomal segment, involving more than one gene. Chromosome anomalies usually occur when there is an error in cell division following meiosis or mitosis. Chromosome abnormalities may be detected or confirmed by comparing an individual's karyotype, or full set of chromosomes, to a typical karyotype for the species via genetic testing.

Oocyte selection is a procedure that is performed prior to in vitro fertilization, in order to use oocytes with maximal chances of resulting in pregnancy. In contrast, embryo selection takes place after fertilization.

Oocyteactivation is a series of processes that occur in the oocyte during fertilization.

Embryo quality is the ability of an embryo to perform successfully in terms of conferring a high pregnancy rate and/or resulting in a healthy person. Embryo profiling is the estimation of embryo quality by qualification and/or quantification of various parameters. Estimations of embryo quality guides the choice in embryo selection in in vitro fertilization.

Polar body biopsy is the sampling of a polar body of an oocyte. It was first applied clinically in humans in 1987 after extensive animal studies. A polar body is a small haploid cell that is formed concomitantly as an egg cell during oogenesis, but which generally does not have the ability to be fertilized.

Ovum quality is the measure of the ability of an oocyte to achieve successful fertilisation. The quality is determined by the maturity of the oocyte and the cells that it comprises, which are susceptible to various factors which impact quality and thus reproductive success. This is of significance as an embryo's development is more heavily reliant on the oocyte in comparison to the sperm.

Oocytes are immature egg cells that develop to maturity within a follicle in the ovary. Oocyte abnormalities can occur due to several factors, including premature ovarian insufficiency (POI), other maturation abnormalities, maternal ageing, and mitochondrial abnormalities.

References

  1. 1 2 Schmerler, S.; G.M. Wessel (2011). "Polar bodies—more a lack of understanding than a lack of respect". Mol. Reprod. Dev. 78 (1): 3–8. doi:10.1002/mrd.21266. PMC   3164815 . PMID   21268179.
  2. ↑ Bieber, F.R.; Nance, W.E.; Morton, C.C.; Brown, J.A.; Redwine, F.O.; Jordan, R.L.; Mohanakumar, T. (1981). "Genetic studies of an acardiac monster: evidence of polar body twinning in man". Science. 213 (4509): 775–777. Bibcode:1981Sci...213..775B. doi:10.1126/science.7196086. PMID   7196086.
  3. ↑ Conklin, E. G. (1915). "Why Polar Bodies do Not Develop". Proceedings of the National Academy of Sciences of the United States of America. 1 (9): 491–6. Bibcode:1915PNAS....1..491C. doi: 10.1073/pnas.1.9.491 . PMC   1090872 . PMID   16576055.
  4. ↑ Wang, Q.; Racowsky, C.; Deng, M. (2011). "Mechanism of the chromosome-induced polar body extrusion in mouse eggs". Cell Division. 6: 17. doi: 10.1186/1747-1028-6-17 . PMC   3179692 . PMID   21867530.
  5. ↑ Geraedts, J.; Collins, J.; Gianaroli, L.; Goossens, V.; Handyside, A.; Harper, J.; Montag, M.; Repping, S.; Schmutzler, A. (2010). "What next for preimplantation genetic screening? A polar body approach!". Human Reproduction (Oxford, England). 25 (3): 575–7. doi:10.1093/humrep/dep446. PMC   2817568 . PMID   20031957.
  6. 1 2 Scott Jr, R. T.; Treff, N. R.; Stevens, J.; Forman, E. J.; Hong, K. H.; Katz-Jaffe, M. G.; Schoolcraft, W. B. (2012). "Delivery of a chromosomally normal child from an oocyte with reciprocal aneuploid polar bodies". Journal of Assisted Reproduction and Genetics. 29 (6): 533–7. doi:10.1007/s10815-012-9746-6. PMC   3370038 . PMID   22460080.
  7. ↑ Zhang, Sarah (2021-10-28). "After 30 Years of Breeding Condors, a Secret Comes Out". The Atlantic. Retrieved 2021-10-30.
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