Polar body

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. 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 (apoptose) and disappear, but in some cases they remain and can be important in the life cycle of the organism.

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.

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.

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.

The process of fertilization in the ovum of a mouse.

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.

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