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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. The other, smaller cells are called polar bodies. They frequently die (apoptose) and disappear, but in some cases they remain and can be important in the life cycle of the organism.
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 identified and characterized in the early 20th century, primarily by O. Hertwig, T. Boveri, and E.L. Mark. They were described as non-functioning egg cells which disintegrated because the spermatozoon, with rare exceptions, could not fertilize them and instead chemically triggered their dissolution.
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 
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 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. 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. 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.
Because non-disjunction in the polar bodies directly influences the viability and health of the zygote, it has been proposed that screening of polar bodies be used as an alternative form of preimplantation genetic screening (PGS), due to the costs, harmful side-effects and overall ineffective nature of blastocyst biopsy. By screening the first polar body for chromosomal anomalies, non-viable eggs can be reliably identified, though eggs with normal first polar bodies can still be affected by errors. This method was initially performed with fluorescence in situ hybridization (FISH), then by hybridizing a sample into lymphocytes to observe it in metaphase, and more recently by microarrays, which are fully automated and make it easier to distinguish between chromosome vs. chromatid abnormalities.
Several studies have suggested that polar body screening for aneuploidy may not be optimal. When the majority of errors occur in chromatids rather than entire chromosomes (a condition correlated with the age of the mother), screening only the first polar body will fail to detect a large percentage of defective eggs. As mentioned earlier, chromosomal abnormality in the first polar body can result in a healthy embryo, meaning that eggs may in fact be wasted as a result of the screening. Polar body testing will also be unable to detect post-zygotic errors in an oocyte.
Because euploid polar bodies contain the same chromosomes as the oocyte, they can be used as a source of genetic material that is easily accessed without destroying the egg. This presents a research advantage by minimizing damage to the oocytes under investigation.
- Schmerler, S.; G.M. Wessel (2011). "Polar bodies—more a lack of understanding than a lack of respect.". Mol. Reprod. Dev 78: 3–8. doi:10.1002/mrd.21266. Retrieved May 24, 2012.
- Kris Bigalk. "Rare Forms of Twinning". bellaonline.com. Retrieved 2007-03-22.
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