Polar body

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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 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 either one or two inside the ovum, usually two. During meiosis, a diploid oocyte replicates its DNA once and divides twice. One product of these divisions will continue to develop into an ovum; the others become polar bodies. 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-identical twins, when the second polar body does not disintegrate and is fertilized by a 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 seeds. Cereal grain is an example of this, and is consumed for its nutritional value by most 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

Diagram showing the reduction in number of the chromosomes in the process of maturation of the ovum.

Scheme showing analogies in the process of maturation of the ovum and the development of the spermatids.

Gray8.png The process of fertilization in the ovum of a mouse.