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|Diagram showing the reduction in number of the chromosomes in the process of maturation of the ovum.|
|Gray's||subject #3 38|
An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is part of the ovary development. The germ cells produce a primordial germ cell (PGC) which becomes an oogonium 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-site).
- Main article: Oogenesis
The formation of an oocyte is called oocytogenesis, which is a part of oogenesis. 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|
Oocytes are rich in cytoplasm which contains yolk granules to nourish the cell early in development.
During the primary oocyte stage of oogenesis, the nucleus is called a germinal vesicle
The only normal human type of secondary oocyte has sex chromosomes 23,X (where sperm can be 23,X or 23,Y).
The space wherein an ovum or immature ovum is located is the cell-nest.
- 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 advanced maternal age.
- 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
|Stages of human development
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