Conception
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- This article is about fertilisation in animals. For fertilisation in humans specifically, see Human fertilization.
- "Conceive" redirects here. For the health magazine, see Conceive magazine.
Fertilisation[1] (also known as conception, fecundation and syngamy), is the fusion of gametes to produce 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.
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[edit] Fertilisation in animals
The mechanics behind fertilisation has been studied extensively in sea urchins and mice. This research addresses the question of how the 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:
- Chemotaxis
- Sperm activation/acrosomal reaction
- Sperm/egg adhesion.
[edit] Sea urchins
Chemotaxis was discovered as the method by 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.
[edit] 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 ejaculates, a large number of sperm cells move up to the upper vagina (via contractions from the vagina) through the cervix and across the length of the uterus toward the ovum—a relatively considerable distance compared to the size of the sperm cell. The 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 membranes follows, allowing the entry of the sperm nucleus, centriole and flagellum, but not the mitochondria, into the oocyte. The fusion is likely mediated by the protein CD9 in mice (the binding homolog). The egg "activates" once it fuses with a single sperm cell, i.e., its cell membrane changes to preventing fusion with other sperm.
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 pregnant. If the embryo emplants in any tissue other than the uterine wall, an ectopic pregnancy results, which can be fatal to the mother.
In some animals (e.g. rabbits) 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.
[edit] Humans
- Main article: 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 about the beginning of pregnancy, within the abortion debate. Gastrulation is the point in development when the implanted blastocyst develops three germ layers, the endoderm, the ectoderm 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 survive only until gastrulation, and have no chance of development afterward. 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.[2] The term "conception" is not usually used in scientific literature because of its variable definition and connotation.
[edit] 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. Disruptions to the early embryo can lead to embryo splitting and twinning, according to one biologist and health economist [[1]]. Such monozygotic splitting is far more likely following assisted conception procedures such as in-vitro fertilisation (IVF) [[2]]
[edit] Parthenogenesis
Another method of fertilisation occurs among animals that normally reproduce sexually, through parthenogenesis: when the gamete of a female is not fertilised 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. In 2004, Japanese researchers led by Tomohiro Kono succeeded after 457 attempts to merge the ova of two mice, the result of which developed normally into a mouse. This was achieved by blocking certain proteins that would normally prevent the possibility.[3][4]*[5]
[edit] See also
[edit] Notes and references
- ↑ The spelling fertilisation is a British English variant. The spelling fertilization is also used, and is the official spelling in American and Canadian English.
- ↑ Moore, K. L. & T. V. M. Persaud (2003). The Developing Human: Clinically Oriented Embryology, W. B. Saunders Company. ISBN 0-7216-6974-3.
- ↑ The End of Males? Mouse Made to Reproduce Without Sperm
- ↑ Mice created without fathers
- ↑ http://www.formyodds.com Predicting the take-home baby rate
| This page uses content from the English-language version of Wikipedia. The original article was at Conception. The list of authors can be seen in the page history. As with Psychology Wiki, the text of Wikipedia is available under the GNU Free Documentation License. |
