Theegg cell orovum (pl.:ova) is thefemalereproductive cell, orgamete,[1] in mostanisogamous organisms (organisms thatreproduce 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). If the male gamete (sperm) is capable of movement, the type of sexual reproduction is also classified asoogamous. A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is anoosphere.[2] When fertilized, the oosphere becomes theoospore.[clarification needed]
When egg and sperm fuse together duringfertilisation, adiploid cell (thezygote) is formed, which rapidly grows into a new organism.
While the non-mammalian animal egg was obvious, the doctrineex ovo omne vivum ("every living [animal comes from] an egg"), associated withWilliam Harvey (1578–1657), was a rejection ofspontaneous generation andpreformationism as well as a bold assumption that mammals also reproduced via eggs.Karl Ernst von Baer discovered the mammalian ovum in 1827.[3][4] The fusion of spermatozoa with ova (of a starfish) was observed byOskar Hertwig in 1876.[5][6]
In animals, egg cells are also known asova (singularovum, from theLatin wordovum meaning 'egg').[7] The termovule in animals is used for the young ovum of an animal. In vertebrates, ova are produced by femalegonads (sex glands) calledovaries. A number of ova are present at birth inmammals and mature viaoogenesis.
Studies performed on humans, dogs, and cats in the 1870s suggested that the production ofoocytes (immature egg cells) stops at or shortly after birth. A review of reports from 1900 to 1950 by zoologistSolomon 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 thatovarian 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.[8][9]
Diagram of a human egg cellOvum and sperm fusing togetherThe process of fertilizing an ovum (top to bottom).
In allmammals, the ovum is fertilized inside the female body. Human ova grow from primitive germ cells that are embedded in the substance of theovaries.[10]
The ovum is one of the largestcells in the human body, typically visible to the naked eye without the aid of amicroscope or other magnification device.[11] The human ovum measures approximately 120 μm (0.0047 in) in diameter.[12]
In humans, recombination rates differ between maternal and paternal DNA:
Maternal DNA: Recombines approximately42 times on average.
Paternal DNA: Recombines approximately27 times on average.
The ooplasm consists of thecytoplasm of the ordinary animal cell with itsspongioplasm andhyaloplasm, often called theformative yolk; and thenutritive yolk ordeutoplasm, made of rounded granules of fatty andalbuminoid substances imbedded in the cytoplasm.[13]
Mammalian ova contain only a tiny amount of the nutritive yolk, for nourishing theembryo 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.[13]
In theoviparous animals (allbirds, mostfish,amphibians andreptiles), the ova develop protective layers and pass through theoviduct to the outside of the body. They are fertilized by male sperm either inside the female body (as in birds), or outside (as in many fish). After fertilization, an embryo develops, nourished by nutrients contained in the egg. It then hatches from the egg, outside the mother's body. Seeegg for a discussion of eggs of oviparous animals.
The egg cell'scytoplasm andmitochondria are the sole means the egg can reproduce by mitosis and eventually form ablastocyst after fertilization.
There is an intermediate form, theovoviviparous 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 manyinvertebrates use this technique.
Nearly all land plants have alternatingdiploid andhaploid generations. Gametes are produced by the haploid generation, which is known as thegametophyte. The female gametophyte produces structures calledarchegonia, and the egg cells form within them viamitosis. The typicalbryophyte 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 into a new diploid individual, known as asporophyte. Inseed plants, a structure called theovule contains the female gametophyte. The gametophyte produces an egg cell. Afterfertilization, the ovule develops into aseed containing the embryo.[14]
Inflowering plants, the female gametophyte (sometimes referred to as the embryo sac) has been reduced to just eight cells inside theovule. The gametophyte cell closest to themicropyle opening of the ovule develops into the egg cell. Uponpollination, 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 aseed and in many cases, theplant ovary develops into afruit to facilitate thedispersal of the seeds. Upongermination, the embryo grows into aseedling.[14]
Gene expression pattern determined by histochemical GUS assays inPhyscomitrella patens. The Polycomb gene FIE is expressed (blue) in unfertilized egg cells of the mossPhyscomitrella patens (right) and expression ceases after fertilization in the developing diploid sporophyte (left). In situ GUS staining of two female sex organs (archegonia) of a transgenic plant expressing a translational fusion of FIE-uidA under control of the native FIE promoter
In themossPhyscomitrella patens, thePolycomb protein FIE is expressed in the unfertilised egg cell (Figure, right) as the blue colour afterGUS staining reveals. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo.[15]
Inalgae, the egg cell is often called oosphere.[citation needed]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 theprogrammed cell death (apoptosis) of the nurse cells. During oogenesis, 15 nurse cells die for every oocyte that is produced.[16] In addition to this developmentally regulated cell death, egg cells may also undergo apoptosis in response to starvation and other insults.[16]
^Alexander, Rachel; Davies, Mary-Ann; Major, Vicky; Singaram, S. Veena; Dale-Jones, Barbara (2006).X-kit Anatomy. Pearson South Africa. p. 3.ISBN978-1-86891-380-0.
^Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002)."Eggs".Molecular Biology of the Cell (4th ed.). New York, US: Garland Science.ISBN0-8153-3218-1.
^abc"The Ovum".Gray's Anatomy. Retrieved18 October 2010.
