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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 fertilization, or outside in the case of external fertilization.
The entire process of development of new individuals is called
procreation, the act of species reproduction.
Fertilization in plants
Flowering plants
After the
pistil is pollination, the pollen grain germinates in a response to a sugary fluid secreted by the mature stigma called semen. 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 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 fertilize it. This is the point when fertilization actually occurs. Note the pollination and fertilisation are two separate processes. After being fertilized, 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 visualize 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 fertilized 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 fertilization
Double fertilization refers to the process in
angiosperms (flowering plants) during reproduction, in which two sperm nuclei from each pollen tube fertilize two cell (biology) in an
plant ovary. The pollen grain adheres to the
stigma (flower) 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
fertilities 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) 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.
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 fertilization.
Fertilization in animals
The mechanics behind fertilisation has been studied extensively in sea urchins and mice. This research addresses the question of how the Spermatozoon 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 fertilization 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
mammals 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. Once the male male ejaculation, a large number of sperm cells swim from the upper vagina through the cervix and across the length of the
uterus toward the ovum—a considerable distance compared to the size of the sperm cell. The capacitation 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 membranes follows, allowing the entry of the sperm
cell nucleus, mitochondrion, 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 performs implantation in the endometrium. At this point the female is said to be pregnancy. If the embryo emplants in any tissue other than the
uterus 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
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 Beginning of pregnancy controversy, within the
abortion debate.Gastrulation is the point in development when the implanted blastocyst 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. 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
chromosomes donated by the
parents are combined, and, in
humans, this means that (2²²)² (17,592,186,044,416 possible zygotes), chromosomally different
zygotes 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.
Parthenogenesis
Another method of fertilisation occurs among animals that normally reproduce sexually, through
parthenogenesis: when the gamete of a female is not fertilized by a male, yet produces viable and unique offspring that are not clones. Only DNA from the mother is inherited, but it is not identical to her. Normal eggs of the mother become fertilised, without sperm, and development proceeds normally. This occurs naturally in several species and may be induced in others through a chemical or electrical stimulus.
See also
- Female sperm
- In vitro fertilization
- Fetal development
- Superfetation
- Superfecundation
Notes and references
http://www.tldm.org/directives/d196.htm
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 fertilization, or outside in the case of external fertilization.
The entire process of development of new individuals is called
procreation, the act of species
reproduction.
Fertilization in plants
Flowering plants
After the
pistil is pollination, the pollen grain germinates in a response to a sugary fluid secreted by the mature stigma called semen. 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 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 fertilize it. This is the point when fertilization actually occurs. Note the pollination and fertilisation are two separate processes. After being fertilized, 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 visualize 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 fertilized 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 fertilization
Double fertilization refers to the process in
angiosperms (flowering plants) during reproduction, in which two sperm nuclei from each pollen tube fertilize two cell (biology) in an plant ovary. The pollen grain adheres to the stigma (flower) 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
fertilities 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) 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.
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 fertilization.
Fertilization in animals
The mechanics behind fertilisation has been studied extensively in sea urchins and mice. This research addresses the question of how the
Spermatozoon 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 fertilization 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
mammals 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. Once the male
male ejaculation, a large number of sperm cells swim from the upper vagina through the cervix and across the length of the
uterus toward the ovum—a considerable distance compared to the size of the sperm cell. The
capacitation 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 membranes follows, allowing the entry of the sperm cell nucleus, mitochondrion, 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 performs
implantation in the endometrium. At this point the female is said to be
pregnancy. If the embryo emplants in any tissue other than the
uterus 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
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 Beginning of pregnancy controversy, within the
abortion debate.Gastrulation is the point in development when the implanted blastocyst 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. 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
chromosomes donated by the
parents are combined, and, in humans, this means that (2²²)² (17,592,186,044,416 possible zygotes), chromosomally different zygotes 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.
Parthenogenesis
Another method of fertilisation occurs among animals that normally reproduce sexually, through parthenogenesis: when the gamete of a female is not fertilized by a male, yet produces viable and unique offspring that are not clones. Only DNA from the mother is inherited, but it is not identical to her. Normal eggs of the mother become fertilised, without sperm, and development proceeds normally. This occurs naturally in several species and may be induced in others through a chemical or electrical stimulus.
See also
Notes and references
http://www.tldm.org/directives/d196.htm
Fertilisation - Wikipedia, the free encyclopedia
Fertilisation [1] (also known as conception, fecundation and syngamy), is fusion of gametes to produce a new organism of the same species. In animals, the process involves a sperm ...
Human Fertilisation and Embryology Authority (HFEA) - Regulator of UK ...
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Bills and Legislation - Human Fertilisation and Embryology Bill [HL]
Human Fertilisation and Embryology Bill [HL] 2007-08 Government Bill introduced by Lord Darzi of Denham, Department of Health and Alan Johnson, Department of Health
Fertilisation
P rices : Biology (450 slides) : £179, Chemistry (280 slides) : £129, Physics (370 slides) : £159, All 3 CD-ROMS : £399 Order form
Human Fertilisation and Embryology Act 1990 (c. 37)
An Act to make provision in connection with human embryos and any subsequent development of such embryos; to prohibit certain practices in connection with embryos and gametes; to ...
In vitro fertilisation (IVF)
IVF - How does it work? We explain the process, the advantages and the disadvantages ... Fertility treatment: In vitro fertilisation (IVF) Approved by the BabyCentre Medical ...
BBC - Schools - KS3 Bitesize - Science - Biology - Reproduction
KS3 revision on reproduction ... Fertilisation Fertilisation happens when an egg cell meets with a sperm cell and joins with it.
Abortion Rights
The Report stage of the Human Fertilisation and Embryology Bill will take place on 22nd October. Anti-abortionists are set to attack women's abortion rights again, whilst pro ...
Human Fertilisation and Embryology Bill : Department of Health ...
The Bill is intended to update the regulation of assisted reproduction and embryo research (primarily by amending the Human Fertilisation and Embryology Act 1990), to ensure that ...
fertilisation - Hutchinson encyclopedia article about fertilisation
In sexual reproduction, the union of two gametes (sex cells, often called egg or ovum, and sperm) to produce a zygote, which combines the genetic material contributed by each ...
