# Egg cell
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{{Short description|Female reproductive cell}}
{{Redirect|Ova|anime films released direct-to-video|Original video animation|other uses}}
{{Confused|Egg}}
{{Infobox cell
| Name = Egg cell
| Latin = ovum
| Greek = ωάριον (ōárion)
| Image = Gray3.png
| Caption = A human egg cell with surrounding [corona radiata](/source/Corona_radiata_(embryology))
| Width =
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| System =
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thumb|372x372px|Human egg cell
The '''egg cell''' or '''ovum''' ({{plural form}}: '''ova''') is the [female](/source/female) [reproductive](/source/Reproduction) cell, or [gamete](/source/gamete),[{{cite web |author= |url=https://www.biologyonline.com/dictionary/ovum |title=Ovum |date=7 October 2019 |website=Biology Dictionary |publisher=BiologyOnline |access-date=21 January 2023 }}] in most [anisogamous](/source/anisogamous) organisms (organisms that [reproduce sexually](/source/reproduce_sexually) with a larger, female gamete and a smaller, male one). The term is used when the female gamete is not capable of movement (non-[motile](/source/motile)). If the male gamete ([sperm](/source/sperm)) is capable of movement, the type of sexual reproduction is also classified as [oogamous](/source/oogamous). A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is an '''oosphere'''.[{{Cite web|title=Oosphere Meaning |url=https://www.yourdictionary.com/oosphere |website=YourDictionary |access-date=12 April 2021}}] When fertilized, the oosphere becomes the [oospore](/source/oospore).{{clarification needed|reason=the next sentence calls the product of fertilisation the zygote|date=May 2021}}
When egg and sperm fuse together during [fertilisation](/source/fertilisation), a [diploid](/source/diploid) cell (the [zygote](/source/zygote)) is formed, which rapidly grows into a new organism.
== History ==
While the non-mammalian animal egg was obvious, the doctrine ''ex ovo omne vivum'' ("every living [animal comes from] an egg"), associated with [William Harvey](/source/William_Harvey) (1578–1657), was a rejection of [spontaneous generation](/source/spontaneous_generation) and [preformationism](/source/preformationism) as well as a bold assumption that mammals also reproduced via eggs. [Karl Ernst von Baer](/source/Karl_Ernst_von_Baer) discovered the mammalian ovum in 1827.[{{cite journal |last1=Cobb |first1=M. |title=An amazing 10 years: the discovery of egg and sperm in the 17th century |journal=Reprod Domest Anim |date=August 2012 |volume=47 |issue=Suppl 4 |pages=2–6 |doi=10.1111/j.1439-0531.2012.02105.x |pmid=22827343 |doi-access=free }}][{{cite web|url=http://commons.wikimedia.org/wiki/File:%22Conclusio%22_from_Carl_Ernst_von_Baer%27s_De_Ovi_Mammalium_et..._Wellcome_L0013369.jpg |type=jpeg |title="Conclusio" from Carl Ernst von Baer's De Ovi Mammalium et...}}] The fusion of spermatozoa with ova (of a starfish) was observed by [Oskar Hertwig](/source/Oskar_Hertwig) in 1876.[{{cite book|first=Joseph |last=Needham |author-link=Joseph Needham |year=1959 |title=A History of Embryology |edition=2nd, revised |location=Cambridge, England, UK |publisher=Cambridge University Press}}][{{cite journal|last=Lopata |first=Alex |title=History of the Egg in Embryology |date=April 2009 |journal=Journal of Mammalian Ova Research |volume=26 |issue=1 |pages=2–9 |doi=10.1274/jmor.26.2 |s2cid=86828337}}]
== Animals ==
In animals, egg cells are also known as ''ova'' (singular ''ovum'', from the [Latin](/source/Latin_language) word {{lang|la|ovum}} meaning '[egg](/source/egg_(biology))').[{{cite dictionary |url= https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dovum |title=ōvum |last1=Lewis |first1= Charlton T. |last2=Short |first2=Charles |dictionary= A Latin Dictionary |publisher= Perseus Digital Library |date = 1879 }}] The term '''ovule''' in animals is used for the young ovum of an animal. In vertebrates, ova are produced by female [gonad](/source/gonad)s (sex glands) called [ovaries](/source/ovary). A number of ova are present at birth in [mammal](/source/mammal)s and mature via [oogenesis](/source/oogenesis).
Studies performed on humans, dogs, and cats in the 1870s suggested that the production of [oocyte](/source/oocyte)s (immature egg cells) stops at or shortly after birth. A review of reports from 1900 to 1950 by zoologist [Solomon Zuckerman](/source/Solly_Zuckerman%2C_Baron_Zuckerman) cemented the belief that females have a finite number of oocytes that are formed before they are born. This dogma has been challenged by a number of studies since 2004. Several studies suggest that [ovarian stem cells](/source/ovarian_stem_cells) exist within the mammalian ovary. Whether or not mature mammals can actually create new egg cells remains uncertain and is an ongoing research question.[{{cite journal | vauthors = Horan CJ, Williams SA | title = Oocyte stem cells: fact or fantasy? | language = en-US | journal = Reproduction | volume = 154 | issue = 1 | pages = R23–R35 | date = July 2017 | pmid = 28389520 | doi = 10.1530/REP-17-0008 | s2cid = 207156647 | doi-access = free }}][{{cite journal | vauthors = Telfer EE, Anderson RA | title = The existence and potential of germline stem cells in the adult mammalian ovary | journal = Climacteric | volume = 22 | issue = 1 | pages = 22–26 | date = February 2019 | pmid = 30601039 | pmc = 6364305 | doi = 10.1080/13697137.2018.1543264 }}]
=== Mammals including humans ===
thumb|Diagram of a human egg cell
right|thumb|Ovum and sperm fusing together
thumb|The process of fertilizing an ovum (top to bottom).
In all [mammals](/source/mammals), the ovum is fertilized inside the female body. Human ova grow from primitive germ cells that are embedded in the substance of the [ovaries](/source/ovary).[{{cite book|last=Regan |first=Carmen L. |editor=Worell, Judith |year=2001 |chapter=Pregnancy |title=Encyclopedia of Women and Gender: Sex Similarities and Differences and the Impact of Society on Gender |url=https://archive.org/details/encyclopediaofwo0000unse_g9b9/page/859 |url-access=registration |access-date=3 November 2013 |volume=1 |publisher=Academic Press |page=859 |isbn=9780122272455 |chapter-url=https://books.google.com/books?id=7SXhBdqejgYC&pg=PA859-11-039}}]
The ovum is one of the largest [cells](/source/biological_cell) in the human body, typically visible to the naked eye without the aid of a [microscope](/source/microscope) or other magnification device.[{{cite book |last1=Alexander |first1=Rachel |last2=Davies |first2=Mary-Ann |last3=Major |first3=Vicky |last4=Singaram |first4=S. Veena |last5=Dale-Jones |first5=Barbara |title=X-kit Anatomy |date=2006 |publisher=Pearson South Africa |isbn=978-1-86891-380-0 |page=3 |url=https://books.google.com/books?id=ycuCJGCL75EC&pg=PA3 |language=en}}] The human ovum measures approximately {{cvt|120|μm}} in diameter.[{{Cite book|last1=Alberts |first1=Bruce |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter |year=2002 |chapter=Eggs |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK26842/ |title=Molecular Biology of the Cell |edition=4th |location=New York, US |publisher=Garland Science |isbn=0-8153-3218-1}}]
In humans, recombination rates differ between maternal and paternal DNA:
* '''Maternal DNA:''' Recombines approximately '''42 times''' on average.
* '''Paternal DNA:''' Recombines approximately '''27 times''' on average.
=== Ooplasm ===
Ooplasm is like the yolk of the ovum, a cell substance at its center, which contains its [nucleus](/source/cell_nucleus), named the [germinal vesicle](/source/germinal_vesicle), and the [nucleolus](/source/nucleolus), called the [germinal disc](/source/germinal_disc).[{{cite web |url=http://www.theodora.com/anatomy/the_ovum.html |title=The Ovum
|work=Gray's Anatomy |access-date=18 October 2010}}]
The ooplasm consists of the [cytoplasm](/source/cytoplasm) of the ordinary animal cell with its [spongioplasm](/source/spongioplasm) and [hyaloplasm](/source/hyaloplasm), often called the ''formative yolk''; and the ''nutritive yolk'' or ''[deutoplasm](/source/deutoplasm)'', made of rounded granules of fatty and [albumin](/source/albumin)oid substances imbedded in the cytoplasm.[
[Mammal](/source/Mammal)ian ova contain only a tiny amount of the nutritive yolk, for nourishing the [embryo](/source/embryo) in the early stages of its development only. In contrast, bird eggs contain enough to supply the chick with nutriment throughout the whole period of incubation.][{{cite book |author=Esau, K. |year=1977 |title=Anatomy of seed plants |url=https://archive.org/details/anatomyofseedpla00esau |url-access=registration |publisher=John Wiley and Sons |location=New York |isbn=978-0-471-24520-9 |edition=second}}]
In [flowering plant](/source/flowering_plant)s, the female gametophyte (sometimes referred to as the embryo sac) has been reduced to just eight cells inside the [ovule](/source/ovule). The gametophyte cell closest to the [micropyle](/source/Ovule) opening of the ovule develops into the egg cell. Upon [pollination](/source/pollination), a pollen tube delivers sperm into the gametophyte and one sperm nucleus fuses with the egg nucleus. The resulting zygote develops into an embryo inside the ovule. The ovule, in turn, develops into a [seed](/source/seed) and in many cases, the [plant ovary](/source/Ovary_(plants)) develops into a [fruit](/source/fruit) to facilitate the [dispersal](/source/Biological_dispersal) of the seeds. Upon [germination](/source/germination), the embryo grows into a [seedling](/source/seedling).[
[[File:DEV035048A.jpg|thumb|Gene expression pattern determined by histochemical GUS assays in ''[Physcomitrella patens](/source/Physcomitrella_patens)''. The Polycomb gene FIE is expressed (blue) in unfertilized egg cells of the moss ''Physcomitrella patens'' (right) and expression ceases after fertilization in the developing diploid sporophyte (left). In situ GUS staining of two female sex organs ([archegonia](/source/archegonium)) of a transgenic plant expressing a translational fusion of FIE-uidA under control of the native FIE promoter]]
In the [moss](/source/moss) ''Physcomitrella patens'', the [Polycomb](/source/Polycomb) protein FIE is expressed in the unfertilised egg cell (Figure, right) as the blue colour after [GUS staining](/source/GUS_staining) reveals. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo.][{{cite journal | last1 = Mosquna | first1 = Assaf | author-link5 = Ralf Reski | last2 = Katz | first2 = Aviva | last3 = Decker | first3 = Eva L. | last4 = Rensing | first4 = Stefan A. | last5 = Reski | first5 = Ralf | last6 = Ohad | first6 = Nir | year = 2009 | title = Regulation of stem cell maintenance by the Polycomb protein FIE has been conserved during land plant evolution | journal = Development | volume = 136 | issue = 14| pages = 2433–2444 | doi = 10.1242/dev.035048 | pmid = 19542356 | doi-access = free }}]
==Other organisms==
In [algae](/source/algae), the egg cell is often called oosphere.{{Citation needed|date=October 2014}} ''Drosophila'' oocytes develop in individual egg chambers that are supported by nurse cells and surrounded by somatic follicle cells. The nurse cells are large polyploid cells that synthesize and transfer RNA, proteins, and organelles to the oocytes. This transfer is followed by the [programmed cell death](/source/programmed_cell_death) (apoptosis) of the nurse cells. During oogenesis, 15 nurse cells die for every oocyte that is produced.[{{cite journal |author=McCall K |title=Eggs over easy: cell death in the ''Drosophila'' ovary |journal=Dev. Biol. |volume=274 |issue=1 |pages=3–14 | date=October 2004 |pmid=15355784 |doi=10.1016/j.ydbio.2004.07.017 |doi-access= }}] In addition to this developmentally regulated cell death, egg cells may also undergo apoptosis in response to starvation and other insults.