Oocyte
An oocyte, ovocyte, or rarely oöcyte, is a female gametocyte or germ cell involved in reproduction.
Formation
The formation of an oocyte is called oogenesis. Thus, oogenesis is a form of gametogenesis whose male counterpart is spermatocytogenesis. Oogenesis 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, known as the Dictyate stage, ready to continue meiosis when puberty begins.
- 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 oocyte divides in meiosis II into one ootid and the second polar body. The ootid then differentiates into an ovum.
Characteristics
- 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
- nondisjunction -- a failure of proper homolog separation in meiosis I, or sister chromatid separation in meisosis II can lead to aneuploidy, in which the oocyte has the wrong number of chromosomes, for example 22,X or 24,X. This is the cause of conditions like Down syndrome and Edwards syndrome. It is more likely with advance maternal age.
- Some oocytes have multiple nuclei, although it is thought they never mature.
Additional Images
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Image:Gray7.png|Scheme showing analogies in the process of maturation of the ovum and the development of the spermatids.
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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
External links
