The egg cell, or ovum (PL: ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that 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). If the male gamete (sperm) is capable of movement, the type of sexual reproduction is also classified as oogamous. A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is an oosphere. When fertilized the oosphere becomes the oospore.[clarification needed]
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 (1578–1657), was a rejection of spontaneous generation and preformationism as well as a bold assumption that mammals also reproduced via eggs. Karl Ernst von Baer discovered the mammalian ovum in 1827. The fusion of spermatozoa with ova (of a starfish) was observed by Oskar Hertwig in 1876.
In animals, egg cells are also known as ova (singular ovum, from the Latin word ovum meaning 'egg'). The term ovule in animals is used for the young ovum of an animal. In vertebrates, ova are produced by female gonads (sex glands) called ovaries. A number of ova are present at birth in mammals and mature via oogenesis.
Studies performed on humans, dogs, and cats in the 1870s suggested that the production of oocytes (immature egg cells) stops at or shortly after birth. A review of reports from 1900 to 1950 by zoologist Solomon 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 exist within the mammalian ovary. Whether or not mature mammals can actually create new egg cells remains uncertain and is an ongoing research question.
Mammals including humans Edit
The human ova grow from primitive germ cells that are embedded in the substance of the ovaries. Each of them divides repeatedly to give secretions of the uterine glands, ultimately forming a blastocyst.
The ovum is one of the largest cells in the human body, typically visible to the naked eye without the aid of a microscope or other magnification device. The human ovum measures approximately 120 μm (0.0047 in) in diameter.
The ooplasm consists of the cytoplasm of the ordinary animal cell with its spongioplasm and hyaloplasm, often called the formative yolk; and the nutritive yolk or deutoplasm, made of rounded granules of fatty and albuminoid substances imbedded in the cytoplasm.
Mammalian ova contain only a tiny amount of the nutritive yolk, for nourishing the 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.
Ova development in oviparous animals Edit
In the oviparous animals (all birds, most fish, amphibians and reptiles) the ova develop protective layers and pass through the oviduct 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. See egg for a discussion of eggs of oviparous animals.
There is an intermediate form, the 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 invertebrates use this technique.
Nearly all land plants have alternating diploid and haploid generations. Gametes are produced by the gametophyte, which is the haploid generation. The female gametophyte produces structures called archegonia, and the egg cells form within them 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 into a new diploid individual (sporophyte). In seed plants, a structure called the ovule contains the female gametophyte. The gametophyte produces an egg cell. After fertilization, the ovule develops into a seed containing the embryo.
In flowering plants, the female gametophyte (sometimes referred to as the embryo sac) has been reduced to just eight cells inside the ovule. The gametophyte cell closest to the micropyle opening of the ovule develops into the egg cell. Upon 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 and in many cases, the plant ovary develops into a fruit to facilitate the dispersal of the seeds. Upon germination, the embryo grows into a seedling.
In the moss Physcomitrella patens, the Polycomb protein FIE is expressed in the unfertilised egg cell (Figure, right) as the blue colour after GUS staining reveals. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo.
Other organisms Edit
In algae, the egg cell is often called oosphere. 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 (apoptosis) of the nurse cells. During oogenesis, 15 nurse cells die for every oocyte that is produced. In addition to this developmentally regulated cell death, egg cells may also undergo apoptosis in response to starvation and other insults.
See also Edit
- "Ovum". Biology Dictionary. BiologyOnline. 7 October 2019. Retrieved 21 January 2023.
- "Oosphere Meaning". YourDictionary. Retrieved 12 April 2021.
- Cobb, M. (August 2012). "An amazing 10 years: the discovery of egg and sperm in the 17th century". Reprod Domest Anim. 47 (Suppl 4): 2–6. doi:10.1111/j.1439-0531.2012.02105.x. PMID 22827343.
- ""Conclusio" from Carl Ernst von Baer's De Ovi Mammalium et..." (jpeg).
- Needham, Joseph (1959). A History of Embryology (2nd, revised ed.). Cambridge, England, UK: Cambridge University Press.
- Lopata, Alex (April 2009). "History of the Egg in Embryology". Journal of Mammalian Ova Research. 26 (1): 2–9. doi:10.1274/jmor.26.2. S2CID 86828337.
- Lewis, Charlton T.; Short, Charles (1879). "ōvum". A Latin Dictionary. Perseus Digital Library.
- Horan CJ, Williams SA (July 2017). "Oocyte stem cells: fact or fantasy?". Reproduction. 154 (1): R23–R35. doi:10.1530/REP-17-0008. PMID 28389520. S2CID 207156647.
- Telfer EE, Anderson RA (February 2019). "The existence and potential of germline stem cells in the adult mammalian ovary". Climacteric. 22 (1): 22–26. doi:10.1080/13697137.2018.1543264. PMC 6364305. PMID 30601039.
- Regan, Carmen L. (2001). "Pregnancy". In Worell, Judith (ed.). Encyclopedia of Women and Gender: Sex Similarities and Differences and the Impact of Society on Gender. Vol. 1. Academic Press. p. 859. ISBN 9780122272455. Retrieved 3 November 2013.
- Alexander, Rachel; Davies, Mary-Ann; Major, Vicky; Singaram, S. Veena; Dale-Jones, Barbara (2006). X-kit Anatomy. Pearson South Africa. p. 3. ISBN 978-1-86891-380-0.
- Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002). "Eggs". Molecular Biology of the Cell (4th ed.). New York, USA: Garland Science. ISBN 0-8153-3218-1.
- "The Ovum". Gray's Anatomy. Retrieved 18 October 2010.
- Esau, K. (1977). Anatomy of seed plants (second ed.). New York: John Wiley and Sons. ISBN 978-0-471-24520-9.
- Mosquna, Assaf; Katz, Aviva; Decker, Eva L.; Rensing, Stefan A.; Reski, Ralf; Ohad, Nir (2009). "Regulation of stem cell maintenance by the Polycomb protein FIE has been conserved during land plant evolution". Development. 136 (14): 2433–2444. doi:10.1242/dev.035048. PMID 19542356.
- McCall K (October 2004). "Eggs over easy: cell death in the Drosophila ovary". Dev. Biol. 274 (1): 3–14. doi:10.1016/j.ydbio.2004.07.017. PMID 15355784.
- The Ovarian Kaleidoscope Database description of 1800 genes involved in ovarian functions