Polar body
3607962
204367593
2008-04-09T02:41:52Z
Cewvero
6508231
[[Image:Gray9.png|thumb|300px|First stages of segmentation of a mammalian [[ovum]]. Semidiagrammatic. z.p. [[Zona striata]]. 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 [[cell (biology)|cell]] structure found inside an [[ovum]]. Both [[animal]] and [[plant]] ova possess it.
Asymmetrical cell division ([[cytokinesis]]) leads to the production of polar bodies during [[oogenesis]]. To conserve nutrients, the majority of [[cytoplasm]] is segretated into either the secondary [[oocyte]] or ovum, during [[meiosis]] I or meiosis II, respectively. The remaining daughter cells generated from the meiotic events contain relatively little cytoplasm and are referred to as polar bodies. Eventually, the polar bodies degenerate.
There may be one or two polar bodies in the ovum of plant or possibly animal cells possess it.
The first polar body is one of the two products in the first stage of meiosis and is [[diploid]], whereas the second polar body contains only one [[chromatid]] and is [[haploid]]. Both are relatively small and contain little cytoplasm. Sometimes the first polar body undergoes the second meiotic cell division.
Some say that this second polar body can potentially result in [[half twin|half-identical twins]], when the second polar body does not disintegrate and is fertilized by a [[spermatozoon|sperm]]; but such development would usually be impossible because it does not have enough cytoplasm (and yolk, if the ovum has yolk) to feed the developing embryo.
In plants, the [[pollen]] releases two male [[gametes]] ([[sperm]] nuclei) into the [[ovule]] upon [[fertilization]]. One fertilizes the haploid ovum and develops into a normal embryo, while the other fertilizes the two polar bodies within the center of the ovary, creating a structure called [[endosperm]]. Endosperm stores nutrients for initial growth of plant [[seed]]s. [[Cereal|Cereal grain]] is an example of this, and is consumed for its nutritional value by many animals, including humans.
Polar bodies are the by-products of the egg’s division during meiosis. As an egg matures, it goes through a two-step division process, dividing once at the time when ovulation would occur and again at the time of fertilization. The two polar bodies are the by-products of this division, and are essentially being discarded by the egg. By analyzing the polar bodies, it is possible to infer the genetic status of the egg. Therefore, polar body analysis allows us to test the mother’s genetic contribution to the embryo.
==Additional images==
<gallery>
Image:Gray5.svg|Diagram showing the reduction in number of the chromosomes in the process of maturation of the ovum.
Image:Gray7.png|Scheme showing analogies in the process of maturation of the ovum and the development of the spermatids.
Image:Gray8.png|The process of fertilization in the ovum of a mouse.
</gallery>
{{Cell-biology-stub}}
[[Category:Germ cells]]
[[de:Polkörper]]