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Oocyte

Oocyte

Diagram showing the reduction in number of the chromosomes in the process of maturation of the ovum.
Gray's	subject #3 38
MeSH	Oocytes

An oocyte, ovocyte, or rarely oöcyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum. An oocyte is part the ovary development. The germ cells produce a primordial germ cell (PGC) which becomes an oogonia which marks the start of mitosis. After mitosis stops (due to actions of retinoic acid and the mesenephros) meiosis starts. This stage the oogonia is now an Oocyte (pronounced oh'a (like Noah)-site).

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Formation

Main article: Oogenesis

The formation of an oocyte is called oocytogenesis, which is a part of oogenesis^[1]. Oogenesis results in the formation of both primary oocytes before birth, and of secondary oocytes after it as part of ovulation.

Cell type	ploidy/chromosomes	chromatids	Process	Time of completion
Oogonium	diploid/46	2N	Oocytogenesis (mitosis)	third trimester
primary Oocyte	diploid/46	4N	Ootidogenesis (meiosis 1) (Folliculogenesis)	Dictyate in prophase I until ovulation
secondary Oocyte	haploid/23	2N	Ootidogenesis (meiosis 2)	Halted in metaphase II until fertilization
Ootid	haploid/23	1N	?	Minutes after fertilization
Ovum	haploid/23	1N

Characteristics

Cytoplasm

Oocytes are rich in cytoplasm which contains yolk granules to nourish the cell early in development.

Nucleus

During the primary oocyte stage of oogenesis, the nucleus is called a germinal vesicle^[2]

The only normal type of secondary oocyte has sex chromosomes 23,X (where sperm can be 23,X or 23,Y).

Nest

The space wherein an ovum or immature ovum is located is the cell-nest^[3].

Abnormalities

nondisjunction -- a failure of proper homolog separation in meiosis I, or sister chromatid separation in meiosis 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.

References

^ answers.com
^ Biology-online
^ Germinal epithelium, folliculogenesis, and postovulatory follicles in ovaries of rainbow trout, Oncorhynchus mykiss (Walbaum, 1792) (Teleostei, protacanthopterygii, salmoniformes)

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