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 occurred, 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 reported in 1824 by Carus in
gastropods, but their role was not clarified until the work of Butschli in 1875, Giard in 1876, and finally Hertwig in 1877. These structures were often confused with egg fragments or expelled yolk masses, but were eventually referred to as directional bodies (or Richtungskörper), a term implying the place where the maturation divisions start. The common names "polocytes" and "polar bodies" derive from their polar position in the eggs. 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 chromosomal makeup of 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 (see also
uniparental disomy). If the extra chromosome is absorbed into a polar body rather than being passed into the oocyte,
trisomy can be averted. 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. ==Medical applications==