Oocyte
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 oocyte divides in
meiosis II into one
ootid and the second polar body. The ootid then
differentiates into an
ovum.
* 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).
* 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 syndrome and
Edward's syndrome.
* Some oocytes have multiple
nuclei, although it is thought they never mature.
William K. Purves, Gordon H. Orians, David Sadava, H. Craig Heller, Craig Heller (2003).
Life: The Science of Biology(7th ed.), pp. 823–824
*
polar body*
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