polar body

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First stages of segmentation of a mammalian embryo. 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. It is also known as a polar cell.

Asymmetrical cell division (cytokinesis) leads to the production of polar bodies during oogenesis. To conserve nutrients, the majority of cytoplasm is segregated into the secondary oocyte during meiosis I, and the ovum during meiosis II. The remaining daughter cells generated from the meiotic events are small and 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. The first polar body is one of the two products in the first stage of meiosis and is considered haploid, with 23 chromosomes and 46 chromatids. The second polar body is also haploid, with 23 chromosomes and 23 chromatids. Sometimes the first polar body undergoes the second meiotic cell division.

In flowering 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 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 three haploid polar bodies are the by-products of this division, and are essentially 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.

Polar body twinning

Polar body twinning is a hypothesized form of twinning in the female, where a polar body does not disintegrate and is fertilized by a sperm.^[1]

However, such development would usually be impossible because it does not have enough cytoplasm (or yolk, if the ovum has yolk) to feed the developing embryo.

Twinning would theoretically occur if two sperm fertilize both the egg and a polar body. Additional fertilization of the first polar body would result in only 25% genetic identity, since the twins would share none of the maternal genome. On the other hand, additional fertilization of the second polar body would result in half twinning with 75% genetic identity, because the twins would share all the maternal genome.

Additional images

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

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

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  The process of fertilization in the ovum of a mouse.

References