# Oocyte

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Immature ovum or egg cell

Oocyte Identifiers MeSH D009865 FMA 18644 Anatomical terminology [edit on Wikidata]

An **oocyte** ([/ˈoʊəsaɪt/](https://en.wikipedia.org/wiki/Help:IPA/English), **oöcyte**, or **ovocyte**) is a female [germ cell](/source/Germ_cell) involved in [sexual reproduction](/source/Sexual_reproduction). An oocyte is an [immature ovum](/source/Immature_ovum), an immature [egg cell](/source/Egg_cell) produced in a female fetus in the [ovary](/source/Ovary) during [gametogenesis](/source/Gametogenesis). During [oogenesis](/source/Oogenesis), the oogonia become primary oocytes. An oocyte is a form of genetic material that can be collected for [cryopreservation](/source/Cryopreservation).

## Formation

Diagram showing the reduction in number of the [chromosomes](/source/Chromosomes) in the process of maturation of the [ovum](/source/Ovum); the process is known as [meiosis](/source/Meiosis).

Main article: [Oogenesis](/source/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** during fetal period, and of **secondary** oocytes after it as part of [ovulation](/source/Ovulation).

Cell type ploidy/chromosomes chromatids Process Time of completion Oogonium diploid/46(2N) 2C Oocytogenesis (mitosis) third trimester primary Oocyte diploid/46(2N) 4C Ootidogenesis (meiosis I) (Folliculogenesis) Dictyate in prophase I for up to 50 years secondary Oocyte haploid/23(1N) 2C Ootidogenesis (meiosis II) Halted in metaphase II until fertilization Ootid haploid/23(1N) 1C Ootidogenesis (meiosis II) Minutes after fertilization Ovum haploid/23(1N) 1C

## Characteristics

### Cytoplasm

Oocytes are rich in [cytoplasm](/source/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 human type of secondary oocyte has the 23rd (sex) chromosome as 23,X (female-determining), whereas sperm can have 23,X (female-determining) or 23,Y (male-determining).

### Nest

The space within an ovum or immature ovum is located is the [cell-nest](/source/Germ_cell_nest).[3]

### Cumulus-oocyte complex

The cumulus-oocyte complex contains layers of tightly packed cumulus cells surrounding the oocyte in the Graafian follicle. The oocyte is arrested in Meiosis II at the stage of metaphase II at the diplotene stage and is considered a secondary oocyte. Before ovulation, the [cumulus complex](/source/Cumulus_oophorus) goes through a structural change known as cumulus expansion. The granulosa cells transform from tightly compacted to an expanded mucoid matrix. Many studies show that cumulus expansion is critical for the maturation of the oocyte because the cumulus complex is the oocyte's direct communication with the developing follicle environment. It also plays a significant role in fertilization, though the mechanisms are not entirely known and are species specific.[4][5][6]

## Maternal contributions

Oocyte poles

In order for an oocyte to become fertilized and ultimately grow into a fully functioning organism, it must be able to regulate multiple cellular and developmental processes. The oocyte, a large and complex cell, must be able to direct the growth of the embryo and control cellular activities. As the oocyte is a product of female [gametogenesis](/source/Gametogenesis), the maternal contribution to the oocyte and consequently the newly fertilized egg, is enormous. There are many types of molecules that are maternally supplied to the oocyte, which will direct various activities within the growing [zygote](/source/Zygote).

### Avoidance of damage to germ-line DNA

The DNA of a cell is vulnerable to the damaging effect of oxidative free radicals produced as byproducts of cellular metabolism. DNA damage occurring in oocytes, if not repaired, can be lethal and result in reduced fecundity and loss of potential progeny. Oocytes are substantially larger than the average somatic cell, and thus considerable metabolic activity is necessary for their provisioning. If this metabolic activity were carried out by the oocyte's metabolic machinery, the oocyte genome would be exposed to the reactive oxidative by-products generated. Thus it appears that a process evolved to avoid this vulnerability of germline DNA. It was proposed that, in order to avoid damage to the DNA genome of the oocytes, the metabolism contributing to the synthesis of much of the oocyte's constituents was shifted to other maternal cells that then transferred these constituents to oocytes.[7][8] Thus, oocytes of many organisms are protected from oxidative DNA damage while storing up a large mass of substances to nurture the zygote in its initial embryonic growth.

### mRNAs and proteins

During the growth of the oocyte, a variety of maternally transcribed messenger RNAs, or [mRNAs](/source/Messenger_RNA), are supplied by maternal cells. These mRNAs can be stored in mRNP (message ribonucleoprotein) complexes and be translated at specific time points, they can be localized within a specific region of the cytoplasm, or they can be homogeneously dispersed within the cytoplasm of the entire oocyte.[9] Maternally loaded [proteins](/source/Protein) can also be localized or ubiquitous throughout the cytoplasm. The translated products of the mRNAs and the loaded proteins have multiple functions; from regulation of cellular "house-keeping" such as cell cycle progression and cellular metabolism, to regulation of developmental processes such as [fertilization](/source/Fertilisation), activation of zygotic transcription, and formation of body axes.[9] Below are some examples of maternally inherited mRNAs and proteins found in the oocytes of the [African clawed frog](/source/African_clawed_frog).

Name Type of maternal molecule Localization Function VegT[10] mRNA Vegetal hemisphere Transcription factor Vg1[11] mRNA Vegetal hemisphere Transcription factor XXBP-1[12] mRNA Not known Transcription factor CREB[13] Protein Ubiquitous Transcription factor FoxH1[14] mRNA Ubiquitous Transcription factor p53[15] Protein Ubiquitous Transcription Factor Lef/Tcf[16] mRNA Ubiquitous Transcription factor FGF2[17] Protein Nucleus Not known FGF2, 4, 9 FGFR1[16] mRNA Not known FGF signaling Ectodermin[18] Protein Animal hemisphere Ubiquitin ligase PACE4[19] mRNA Vegetal hemisphere Proprotein convertase Coco[20] Protein Not known BMP inhibitor Twisted gastrulation[16] Protein Not known BMP/Chordin binding protein fatvg[21] mRNA Vegetal hemisphere Germ cell formation and cortical rotation

 Maternal determinants in [*Xenopus laevis*](/source/African_clawed_frog) oocyte

### Mitochondria

The oocyte receives [mitochondria](/source/Mitochondrion) from maternal cells, which will go on to control embryonic metabolism and apoptotic events.[9] The partitioning of mitochondria is carried out by a system of [microtubules](/source/Microtubule) that will localize mitochondria throughout the oocyte. In certain organisms, such as mammals, paternal mitochondria brought to the oocyte by the spermatozoon are degraded through the attachment of ubiquitinated proteins. The destruction of paternal mitochondria ensures the strictly maternal inheritance of mitochondria and mitochondrial DNA (mtDNA).[9]

### Nucleolus

In mammals, the [nucleolus](/source/Nucleolus) of the oocyte is derived solely from maternal cells.[22] The nucleolus, a structure found within the nucleus, is the location where rRNA is transcribed and assembled into ribosomes. While the nucleolus is dense and inactive in a mature oocyte, it is required for proper development of the embryo.[22]

### Ribosomes

Maternal cells also synthesize and contribute a store of [ribosomes](/source/Ribosome) that are required for the translation of proteins before the [zygotic genome is activated](/source/Zygotic_genome_activation). In mammalian oocytes, maternally derived ribosomes and some mRNAs are stored in a structure called cytoplasmic lattices. These cytoplasmic lattices, a network of fibrils, protein, and RNAs, have been observed to increase in density as the number of ribosomes decrease within a growing oocyte[23] and mutation in them have been linked to infertility.[24][25]

## Prophase I arrest

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of [meiosis](/source/Meiosis).[26] In humans, as an example, oocytes are formed between three and four months of [gestation](/source/Gestation) within the fetus and are therefore present at birth. During this prophase I arrested stage ([dictyate](/source/Dictyate)), which may last for many years, four copies of the [genome](/source/Genome) are present in the oocytes. The arrest of ooctyes at the four genome copy stage appears to provide the informational redundancy needed to [repair damage in the DNA](/source/DNA_repair) of the [germline](/source/Germline).[26] The repair process used likely involves [homologous recombinational](/source/Homologous_recombination) repair.[26][27][28] Prophase arrested oocytes have a high capability for efficient repair of [DNA damages](/source/DNA_damage_(naturally_occurring)).[27] In particular, [DNA double-strand breaks](/source/DNA_damage_(naturally_occurring)) can be repaired during the period of prophase arrest by [homologous recombinational repair](/source/Homologous_recombination) and by [non-homologous end joining](/source/Non-homologous_end_joining).[29] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of [fertility](/source/Fertility).[27]

## Paternal contributions

The [spermatozoon](/source/Spermatozoon) that fertilizes an oocyte will contribute its [pronucleus](/source/Pronucleus), the other half of the zygotic [genome](/source/Genome). In some species, the spermatozoon will also contribute a [centriole](/source/Centriole), which will help make up the zygotic [centrosome](/source/Centrosome) required for the first division. However, in some species, such as in the mouse, the entire centrosome is acquired maternally.[30] Currently under investigation is the possibility of other cytoplasmic contributions made to the embryo by the spermatozoon.

During fertilization, the sperm provides three essential parts to the oocyte: (1) a signalling or activating factor, which causes the metabolically dormant oocyte to activate; (2) the haploid paternal genome; (3) the centrosome, which is responsible for maintaining the microtubule system. See anatomy of [sperm](/source/Sperm)

## Abnormalities

- [Nondisjunction](/source/Nondisjunction)—a failure of proper homolog separation in meiosis I, or sister [chromatid](/source/Chromatid) separation in meiosis II can lead to [aneuploidy](/source/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](/source/Down_syndrome) and [Edwards syndrome](/source/Edwards_syndrome) in humans. It is more likely with [advanced maternal age](/source/Advanced_maternal_age).

- Some oocytes have multiple [nuclei](/source/Cell_nucleus), although it is thought they never mature.

## See also

- [Cortical granule](/source/Cortical_granule)

- [Cryoconservation of animal genetic resources](/source/Cryoconservation_of_animal_genetic_resources)

- [Folliculogenesis](/source/Folliculogenesis)

- [Oocyte maturation inhibitor](/source/Oocyte_maturation_inhibitor)

- [Polar body](/source/Polar_body)

- [Symmetry breaking and cortical rotation](/source/Symmetry_breaking_and_cortical_rotation)

- [Oocyte abnormalities](/source/Oocyte_abnormalities)

- [List of distinct cell types in the adult human body](/source/List_of_distinct_cell_types_in_the_adult_human_body)

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1. **[^](#cite_ref-28)** He, Da-Jian; Wang, Lin; Zhang, Zhi-Bi; Guo, Kun; Li, Jing-Zheng; He, Xie-Chao; Cui, Qing-Hua; Zheng, Ping (2018-11-18). ["Maternal gene Ooep may participate in homologous recombination-mediated DNA double-strand break repair in mouse oocytes"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6085769). *Zoological Research*. **39** (6): 387–395. [doi](/source/Doi_(identifier)):[10.24272/j.issn.2095-8137.2018.067](https://doi.org/10.24272%2Fj.issn.2095-8137.2018.067). [PMC](/source/PMC_(identifier)) [6085769](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6085769). [PMID](/source/PMID_(identifier)) [29955025](https://pubmed.ncbi.nlm.nih.gov/29955025).

1. **[^](#cite_ref-29)** Leem J, Lee C, Choi DY, Oh JS. Distinct characteristics of the DNA damage response in mammalian oocytes. Exp Mol Med. 2024 Feb 14. [doi](/source/Doi_(identifier)):[10.1038/s12276-024-01178-2](https://doi.org/10.1038%2Fs12276-024-01178-2). Epub ahead of print. PMID 38355825

1. **[^](#cite_ref-30)** Sutovsky P, Schatten G (2000). *Paternal contributions to the mammalian zygote: fertilization after sperm-egg fusion*. International Review of Cytology. Vol. 195. pp. 1–65. [doi](/source/Doi_(identifier)):[10.1016/s0074-7696(08)62703-5](https://doi.org/10.1016%2Fs0074-7696%2808%2962703-5). [ISBN](/source/ISBN_(identifier)) [978-0-12-364599-9](https://en.wikipedia.org/wiki/Special:BookSources/978-0-12-364599-9). [PMID](/source/PMID_(identifier)) [10603574](https://pubmed.ncbi.nlm.nih.gov/10603574).

## Sources

- Purves WK, Orians GH, Sadava D, Heller HC (2004). *Life: The Science of Biology* (7th ed.). Freeman, W. H. & Company. pp. 823–824. [ISBN](/source/ISBN_(identifier)) [978-0-7167-9856-9](https://en.wikipedia.org/wiki/Special:BookSources/978-0-7167-9856-9).

## External links

- [Micrograph of a primary oocyte and follicle of a monkey](http://it.stlawu.edu/~mtem/devbiol/atlas/ANTRAL1.JPG) [Archived](https://web.archive.org/web/20181005005737/http://it.stlawu.edu/~mtem/devbiol/atlas/ANTRAL1.JPG) 2018-10-05 at the [Wayback Machine](/source/Wayback_Machine)

Preceded by None Stages of human development Sperm + Oocyte Succeeded by Zygote

v t e Female reproductive system Internal Adnexa Ovaries Follicles Corpus Hemorrhagicum Luteum Albicans Thecae Externa Interna Follicular antrum Follicular fluid Corona radiata Zona pellucida Membrana granulosa Perivitelline space Other Germinal epithelium Tunica albuginea Cortex Cumulus oophorus Stroma Medulla Retia ovarii Oogenesis Oogonium Oocytogenesis Oocyte Ootidogenesis Ootid Ovum Fallopian tubes Isthmi Ampullae Infundibula Fimbriae Ostia Intramural segments Ligaments Ovarian ligaments Suspensory ligaments Pampiniform plexus Wolffian vestiges Gartner's ducts Epoophora Vesicular appendages Paroophora Uterus Regions Body Cavity Fundus Isthmus Cervix Canal Internal os External os Supravaginal portion Horns Layers Wall Endometrium Myometrium Perimetrium Parametrium Epithelium Ligaments Round ligaments Broad ligaments Cardinal ligaments Uterosacral ligaments Pubocervical ligaments General Glands Urogenital diaphragm Vestibular glands Bartholin's glands Skene's glands Vagina Canal Rugae Walls Fornices Support structures Epithelium External Vulva Labia Mons pubis Labia majora Pudendal cleft Labiocrural folds Anterior commissure Posterior commissure Dartos muliebris Posterior labial nerves Anterior labial nerves Perineal nerve Labia minora Frenulum Posterior labial arteries Posterior labial veins Interlabial sulci Clitoris Root Crura Bulbs Suspensory ligament Fundiform ligament Body Corpora cavernosa Trabeculae of corpora cavernosa Infra-corporeal residual spongy part Angle Commissure of bulbs Pars intermedia Glans Corona Frenulum Arteries Dorsal arteries Deep arteries Arteries of bulbs Internal pudendal artery Veins Superficial dorsal veins Deep dorsal veins Veins of bulbs Internal pudendal veins Nerves Dorsal nerves Pudendal nerve Clitoral hood Fascia Tunica albuginea Septum Vestibule Fossa Vaginal orifice Hymen Vestibular gland openings Urethra External urethral orifice Blood supply Arteries Ovarian arteries Uterine artery Arcuate artery Vaginal artery Spiral arteries Veins Ovarian veins Uterine vein Uterine venous plexuses Vaginal venous plexus Other G-spot Urethral sponge Perineal sponge Rectouterine pouch Vesicouterine pouch Uterotubal junctions

Authority control databases: National Czech Republic

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Adapted from the Wikipedia article [Oocyte](https://en.wikipedia.org/wiki/Oocyte) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Oocyte?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.