Diagram showing the reduction in number of the chromosomes in the process of maturation of the ovum.
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|MeSH
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An oocyte or ovocyte is a female gametocyte or germ cell involved in reproduction.
The formation of an oocyte is called oocytogenesis. Thus, oocytogenesis is a form of gametocytogenesis whose male counterpart is spermatocytogenesis. Oocytogenesis results in the formation of both primary oocytes before birth, and of secondary oocytes after it as part of ovulation.
The division of an oogonium by mitosis creates two diploid primary oocytes which are kept in a prolonged prophase I ready for meiosis.
As part of ovulation, primary oocytes undergo the first meiotic division, which sees homologous chromosomes pair during prophase and split from one another during anaphase. This forms one haploid secondary oocyte and the first polar body.
A second period of arrested development occurs after the first meiotic division forms the secondary oocyte. The egg may be expelled from the ovary in this condition, and in many species, including humans, the second meiotic division is not completed until the egg is fertilized by a sperm.
The secondary oocyte is the largest cell in the body, and in humans is just visible to the naked eye.
Oocytes are rich in cytoplasm which contains yolk granules to nourish the cell early in development.
The only normal type of secondary oocyte has sex chromosomes 23,X (where sperm can be 23,X or 23,Y).
Abnormalities
If meiosis goes wrong, a secondary oocyte may have the wrong number of sex chromosomes, being 22,X or 24,X. This is the cause of conditions like Down and Edward's syndrome.
Some oocytes have multiple nuclei, although it is thought they never mature.
Resources
William K. Purves, Gordon H. Orians, David Sadava, H. Craig Heller, Craig Heller (2003). Life: The Science of Biology(7th ed.), pp. 823–824
