Talk:HMS Waterwitch (1892) and Fertilisation: Difference between pages

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[[Image:Sperm-egg.jpg|right|300px|thumb|A sperm cell fertilizing an ovum]]

{{otheruses4|reproduction in organisms|soil improvement|Fertilization (soil)}}

'''Fertilisation''' or '''fertilization''' (also known as '''conception''', '''fecundation''' and '''syngamy'''), is fusion of gametes to form a new organism of the same species. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation.

The entire process of development of new individuals is called [[procreation]], the act of species [[reproduction]].

== Fertilisation in plants ==

=== Flowering plants ===

After the [[pistil]] is [[pollination|pollinated]], the pollen grain germinates in a response to a sugary fluid secreted by the mature stigma. From each pollen grain, a [[pollen tube]] grows out attempting to travel into the ovary by creating a path through the female tissue. The vegetative (or tube) and generative nuclei of the pollen grain pass into its respective pollen tube. The growth of the pollen tube is controlled by the vegetative (or tube) nucleus. Hydrolytic [[enzymes]] are secreted by the pollen tube to digest the female tissue (stigma and style) as the pollen tube grows. During pollen tube growth toward the ovary, the generative nucleus divides to produce two separate sperm nuclei - a growing pollen tube therefore contains 3 separate nuclei. The pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the [[Carpel|style]] and curls to the bottom of the ovary, then near the [[receptacle]], it breaks through the [[ovule]] through the micropyle (an opening in the ovule wall) and reaches the ovum (or egg cell) to fertilise it. This is the point when fertilisation actually occurs. Note the pollination and fertilisation are two separate processes. After being fertilised, the ovary starts to swell and will develop a [[fruit]]. With multi-seeded fruits, multiple grains of pollen are necessary for [[syngamy]] with each ovule.

The process is easy to visualise if one looks at [[maize]] silk, which is the female flower of corn. Pollen from the [[tassel]] (the male flower) falls on the sticky external portion of the silk, and then pollen tubes grow down the silk to the attached ovule. The dried silk remains inside the husk of the ear as the seeds mature; if one carefully removes the husk, the floral structures may be seen. In many plants, the development of the flesh of the fruit is proportional to the percentage of fertilised ovules. For example, with [[watermelon]], about a thousand grains of pollen must be delivered and spread evenly on the three lobes of the stigma to make a normal sized and shaped fruit.

=== Double fertilisation ===

''Double fertilisation'' refers to the process in [[angiosperms]] (flowering plants) during [[reproduction]], in which two [[sperm]] nuclei from each pollen tube fertilise two [[cell (biology)|cells]] in an [[plant ovary|ovary]]. The [[pollen]] grain adheres to the [[stigma (flower)|stigma]] of the [[carpel]] (female reproductive structure) and grows a [[pollen tube]] that penetrates the [[ovum]] through a tiny pore called a [[micropyle]]. Two sperm cells (derived from the generative nucleus) are released into the ovary through this tube. One of the two sperm cells ''fertilises'' the egg cell (at the end of the ovary), forming a [[diploid]] (2n) [[zygote]]. The other sperm cell fuses with two haploid polar nuclei (contained in the central cell) in the centre of the [[embryo]] sac (or ovule). The resulting cell is [[triploid]] (3n). This triploid cell divides through [[mitosis]] and forms the [[endosperm]], a [[nutrient]]-rich [[tissue (biology)|tissue]] inside the [[fruit]].

The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm arise by mitosis from a single meiotic product. Therefore, maternal contribution to the genetic constitution of the triploid endosperm is different from that of the embryo.

Recently research has shown that in one primitive group of flowering plants, the water lilies, ''[[Nuphar]]'', the endosperm is diploid, resulting from the fusion of a pollen nucleus with one, rather than two, maternal nuclei.<ref>{{cite journal |quotes=no |author=Friedman, W. E. & J. H. Williams |title=Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants |journal=[[Evolution (journal)|Evolution]] |year=2003 |volume=57 |issue=2 |pages=216-30}}</ref>

In [[gymnosperms]], such as conifers, the food storage tissue is part of the female [[gametophyte]] only, a [[haploid]] (1n) tissue, so there is no double fertilisation. Poop

== Fertilisation in animals ==

The mechanics behind fertilisation has been studied extensively in sea urchins and mice. This research addresses the question of how the [[Spermatozoon|sperm]] and the appropriate egg find each other and the question of how only one sperm gets into the egg and delivers its contents. There are three steps to fertilisation that insure species-specificity: 1. Chemotaxis 2. Sperm activation/acrosomal reaction 3. Sperm/egg adhesion.

=== Sea Urchins ===

[[Chemotaxis]] was discovered as the method for which sperm find the eggs. This chemotaxis is an example of a ligand/receptor interaction. [[Resact]] is a 14 amino acid peptide purified from the jelly coat of A. punctulata that attracts the migration of sperm.

After finding the egg, the sperm gets through the jelly coat through a process called [[sperm activation]]. In another ligand/receptor interaction, an oligosaccharide component of the egg binds and activates a receptor on the sperm and causes the [[acrosomal reaction]]. The acrosomal vesicles of the sperm fuse with the plasma membrane and are released. In this process, molecules bound to the acrosomal vesicle membrane, such as bindin, are exposed on the surface of the sperm. These contents digest the jelly coat and eventually the vitelline membrane. In addition to the release of acrosomal vesicles, there is explosive polymerization of actin to form a thin spike at the head of the sperm called the [[acrosomal process]].

The sperm binds to the egg through another ligand reaction between receptors on the [[vitelline membrane]]. The sperm surface protein [[bindin]], binds to a receptor on the vitelline membrane identified as [[ERB1]].

Fusion of the plasma membranes of the sperm and egg are likely mediated by bindin. At the site of contact, fusion causes the formation of a [[fertilisation cone]].

=== Mammals ===

All [[mammal]]s rely on internal fertilisation through [[copulation]]. To deliver the sperm to the [[female]], the [[male]] inserts his [[sexual organ]], the [[penis]], into the opening of the [[vagina]], the passage into the female's other sexual organs. (This process is a part of copulation.) Once the male [[male ejaculation|ejaculates]], a large number of sperm cells swim from the upper vagina through the [[cervix]] and across the length of the [[uterus]] toward the ovum&mdash;a considerable distance compared to the size of the sperm cell. The [[capacitation|capacitated]] spermatozoon and the oocyte meet and interact in the ''ampulla'' of the [[fallopian tube]]. It is probable that chemotaxis is involved in directing the sperm to the egg, but the mechanism has yet to be worked out.

After finding the egg, the sperm binds to the [[zona pellucida]]. In contrast to sea urchins, the sperm binds to the egg before the acrosmal reaction. The zona pellucida is a thick layer of extracellular matrix that surrounds the egg and is similar to the role of the vitelline membrane in sea urchins. A glycoprotein in the zona pellucida, [[ZP3]] was discovered to be responsible for egg/sperm adhesion ''in mice''. The receptor [[galactosyltransferase]] (GalT) binds to the N-acetylglucosamine residues on the ZP3 and is important for binding to sperm and activating the acrosome reaction. ZP3 is sufficient for sperm/egg binding but not necessary. There are two additional sperm receptors: a 250kD protein that binds to an oviduct secreted protein and SED1 which binds independently to the zona. After the acrosome reaction, it is believed that the sperm remains bound to the zona pellucida through exposed ZP2 receptors. These receptors are unknown in mice but have been identified in guinea pigs.

In mammals, binding of the spermatozoon to the GalT initiates the [[acrosome reaction]]. This process releases the [[enzyme]] [[hyaluronidase]], which digests the matrix of [[hyaluronic acid]] in the vestments surrounding the oocyte. Fusion between the sperm and oocyte [[plasma membrane]]s follows, allowing the entry of the sperm [[cell nucleus|nucleus]], [[mitochondrion|mitochondria]], [[centriole]] and [[flagellum]] into the oocyte. The fusion is likely mediated by the protein CD9 in mice (the bindin homolog). Once the ovum fuses with a single sperm cell, its cell membrane changes, preventing fusion with other sperm (see [[Egg activation]]).

This process ultimately leads to the formation of a [[diploid]] cell called a [[zygote]]. The zygote begins to divide and form a [[blastocyst]] and when it reaches the uterus, it implants in the [[endometrium]]. At this point the female is said to be [[pregnancy|pregnant]]. If the embryo emplants in any tissue other than the [[uterus|uterine]] wall, an [[ectopic pregnancy]] results, which can be fatal to the mother.

In some animals (e.g. rabbit) the act of coitus induces ovulation by stimulating release of the pituitary hormone gonadotropin. This greatly increases the probability that coitus will result in pregnancy.

=== Humans ===

{{main|Human fertilization}}

The term "conception" commonly refers to fertilisation, but is sometimes defined as implantation or even "the point at which human life begins" and is thus a subject of semantic arguments within the [[abortion]] debate.

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Gastrulation is the point in development when the implanted blastoplyst develops three germ layers, the endoderm, the exoderm and the mesoderm. It is at this point that the genetic code of the father becomes fully involved in the development of the embryo. Until this point in development, twinning is possible. Additionally, interspecies hybrids which have no chance of development survive until gastrulation.

However this stance is not entirely warranted since human developmental biology literature refers to the "conceptus" and the medical literature refers to the "products of conception" as the post-implantation embryo and its surrounding membranes.<ref>{{cite book |author=Moore, K. L. & T. V. M. Persaud |year=2003 |title=The Developing Human: Clinically Oriented Embryology |publisher=W. B. Saunders Company |id=ISBN 0-7216-6974-3}}</ref> The term "conception" is not usually used in scientific literature because of its variable definition and connotation.

== Fertilisation and genetic recombination ==

[[Meiosis]] results in a random segregation of the genes contributed from each parent. Each parent organism generally has the same genetic make-up, but differs for a fraction of their genes. Therefore, each [[gamete]] produced by a person will be genetically different from the others from that person, as well as from the gametes produced by another person. When gametes first fuse at fertilisation, the [[chromosome]]s donated by the [[parent]]s are combined, and, in [[human]]s, this means that (2²²)² (17,592,186,044,416 possible zygotes), chromosomally different [[zygote]]s are possible for the non-sex chromosomes, even assuming no [[chromosomal crossover]]. If crossover occurs once, then on average (4²²)² (309,485,009,821,274,699,980,603,392) genetically different zygotes are possible for every couple, not considering that crossover events can take place at most points along each chromosome. The X and Y chromosomes do not undergo crossover events, so are excluded from the calculation. Note that the [[mitochondrial DNA]] is only inherited from the maternal parent.

==See also==

* [[Female sperm]]

* [[In vitro fertilization]]

* [[Fetal development]]

* [[Superfetation]]

* [[Superfecundation]]

== Notes and references ==

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* {{cite journal |quotes=no |author=Evans, J. P. & H. M. Florman |year=2002 |title=The state of the union: the cell biology of fertilization |journal=Nature Medicine |volume=8 |issue=Suppl |pages=S57-63}}-->

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{{Developmental biology}}

{{Reproductive physiology}}

[[Category:Biological reproduction]]

[[Category:Fertility]]

[[Category:Pollination]]

[[Category:Pregnancy]]

[[bg:Зачеване]]

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[[es:Fecundación]]

[[fr:Fécondation]]

[[id:Pembuahan]]

[[it:Fecondazione]]

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[[nl:Bevruchting]]

[[ja:受精]]

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[[pl:Zapłodnienie]]

[[pt:Fecundação]]

[[ru:Половой процесс]]

[[simple:Fertilization]]

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