View source for Ovum

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==Overview==
[[Image:Gray3.png|left|thumb|A human ovum]]
[[Image:Acrosome reaction diagram.svg|left|thumb|400px|Sperm cells attempting to fertilize an ovum]]
An '''ovum''' (plural '''ova''') is a [[haploid]] [[female]] reproductive cell or [[gamete]]. The word is derived from [[Latin language|Latin]], meaning [[egg (biology)|egg]] or [[egg (biology)|egg cell]]. Both [[animal]]s and [[embryophyte]]s have ova. The term '''ovule''' is used for the young ovum of an animal, as well as the plant structure that carries the female [[gametophyte]] and egg cell and develops into a seed after fertilization. In some [[plant]]s, such as [[algae]], it is also called '''oosphere'''.
==Material contribution to offspring==
The egg is the sole provider of such [[endosymbiont|endosymbiotic]] [[organelle]]s, including mitochondria within the cytoplasm. These cannot be produced with nuclear DNA alone and must be manufactured from DNA within existing organelles of their type (such as [[mitochondrial DNA]]) &mdash; this is important in [[Human mitochondrial genetics]] and can be used to trace maternal and paternal [[ancestry]], especially as plants contain chloroplasts as well. Sperm are often too small to contribute anything physical except DNA and its own mitochondria gets destroyed by the egg.

==Ova production==
In higher animals, ova are produced by female [[gonad]]s (sexual glands) called [[ovary|ovaries]] and all of them are present at birth in [[mammal]]s, and mature via [[oogenesis]].

===Human and mammal ova===
[[Image:Sperm-egg.jpg||300px|left|thumb|A sperm cell fusing with an ovum]]
In the [[viviparous]] animals (which include [[human]]s and all other [[Eutheria|placental mammal]]s), the ovum is fertilized inside the female body, and the [[embryo]] then develops inside the [[uterus]], receiving nutrition directly from the mother. The ovum is the largest [[biological cell|cell]] in the human body, typically visible to the naked eye without the aid of a [[microscope]] or other magnification device. The human ovum measures on average, 145&nbsp;[[micrometre|µm]] in diameter.

===Protist and plant ova===
In [[protist]]s, [[Fungus|fungi]] and many plants, such as [[bryophyte]]s, [[fern]]s, and [[gymnosperm]]s, ova are produced inside [[Archegonium|archegonia]]. Since the archegonium is a [[haploid]] structure, egg cells are produced via [[mitosis]]. The typical bryophyte archegonium consists of a long neck with a wider base containing the egg cell. Upon maturation, the neck opens to allow sperm cells to swim into the archegonium and fertilize the egg. The resulting zygote then gives rise to an embryo, which will grow out of the archegonium as a [[sporeling]] (young [[sporophyte]]).

In the [[flowering plant]]s, the female [[gametophyte]], which usually gives rise to the archegonium, has been reduced to just eight cells referred to as the [[embryo sac]] inside the [[ovule]]. The gametophyte cell closest to the [[micropyle]] opening of the embryo sac develops into the egg cell. Upon [[pollination]], a pollen tube delivers [[sperm]] into the embryo sac 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]] and in many cases the [[Ovary (plants)|plant ovary]] develops into a [[fruit]] to facilitate the [[Biological dispersal|dispersal]] of the seeds. Upon [[germination]], the embryo grows into a [[seedling]].

==Ova development in oviparous animals==
In the [[oviparous]] animals (all [[bird]]s, most [[fish]]es, [[amphibia]]ns and [[reptile]]s) the ova develop protective layers and pass through the [[oviduct]] to the outside of the body. They are [[fertilization|fertilized]] by male [[sperm]] either inside the female body (as in birds), or outside (as in many fishes). After fertilization, an [[embryo]] develops, nourished by nutrients contained in the egg. It then hatches from the egg, outside the mother's body. See [[egg (biology)]] for a discussion of eggs of oviparous animals.

The egg cell's [[cytoplasm]] and [[mitochondria]] (and [[chloroplast]]s in plants) are the sole means of the egg being able to reproduce by [[mitosis]] and eventually form a [[blastocyst]] after fertilization.

==Ovoviviparity==
There is an intermediate form, the [[Ovoviviparity|ovoviviparous]] animals: the embryo develops within and is nourished by an egg as in the oviparous case, but then it hatches inside the mother's body shortly before birth, or just after the egg leaves the mother's body. Some fish, reptiles and many [[invertebrate]]s use this technique.

==See also==
* [[Egg (biology)]]
* [[Fertilisation]]
* [[Insemination]]
* [[Menstrual cycle]]
* [[Ovulation]]
* [[Pollination]]
* [[Mammalian pregnancy|Pregnancy]]
* [[Respiratory burst]]
* [[Sperm]]
* [[Zygote]]
* [[Male egg]]

==External links==
* [http://ovary.stanford.edu The Ovarian Kaleidoscope Database] description of 1800 genes involved in ovarian functions



[[Category:Reproductive system]]
[[Category:Germ cells]]
[[Category:Gynecology]]
[[Category:Female reproductive system]]

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