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14-September-2008 18:02:49 - Fertilisation Redirected from Fertilization This article is about fertilisation in animals and plants. For fertilisation in humans specifically, see Human fertilization. A sperm cell fertilising an ovum A sperm cell fertilising an ovum For soil improvement see Fertilization soil. Conception redirects here. For other uses, see Conception disambiguation. Conceive redirects here. For the health magazine, see Conceive magazine. Fertilisation1 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. Contents 1 Fertilisation in plants 1.1 Flowering plants 1.2 Double fertilisation 2 Fertilisation in animals 2.1 Sea urchins 2.2 Mammals 2.3 Humans 3 Fertilisation and genetic recombination 4 Parthenogenesis 5 See also 6 Notes and references Fertilisation in plants Flowering plants After the pistil is pollinated, 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 three separate nuclei. The pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the 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 that 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 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 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 is the process in angiosperms flowering plants during reproduction, in which two sperm nuclei from each pollen tube fertilise two cells in an ovary. The pollen grain adheres to the 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, inside the seed. 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.2 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. 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. Sea urchins Acrosome reaction on a Sea Urchin cell Acrosome reaction on a Sea Urchin cell 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. 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 distancevague 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, mitochondria, centriole and flagellum 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. 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.3 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 2²²² 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 4²²² 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 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.456 See also Kaguya mouse Female sperm In vitro fertilisation Fetal development Superfetation Superfecundation 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. ^ Friedman, W. E. J. H. Williams 2003. Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants. Evolution 57 2: 216-30. ^ 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 v d e Developmental biology Human embryogenesis development of embryo and development of fetus some dates are approximate - see Carnegie stages and a timeline Week 1 Fertilization - Egg activation - Zygote - Cleavage - Morula - Blastula Blastomere - Blastocyst - Inner cell mass Week 2 Bilaminar disc Hypoblast, Epiblast Week 3 Trilaminar embryo, germ layers Archenteron/Primitive streak Primitive pit, Primitive knot/Blastopore, Primitive groove - Gastrula/Gastrulation - Regional specification Ectoderm: Surface ectoderm - Neuroectoderm - Somatopleure - Neurulation - Neural crest Endoderm: Splanchnopleure Mesoderm: Chorda- - Paraxial Somite/Somitomere/Sclerotome/Myotome/Dermatome - Intermediate - Lateral plate Intraembryonic coelom, Splanchnopleure/Somatopleure Extraembryonic/uterus Trophoblast Cytotrophoblast, Syncytiotrophoblast Blastocoele - Yolk sack/exocoelomic cavity - Heuser's membrane - Extraembryonic coelom - Vitelline duct Umbilical cord Umbilical artery, Umbilical vein, Wharton's jelly - Allantois Placenta - Decidua Decidual cells - Chorionic villi/Intervillous space - Gestational sac Amnion/Amniotic sac/Amniotic cavity, Chorion Histogenesis Programmed cell death Apoptosis - Stem cells - Germ line development Organogenesis Limb development: Limb bud - Apical ectodermal ridge/AER other structures: Eye development - Cutaneous structure development - Heart development - Development of the urinary and reproductive organs v d e Human physiology and endocrinology of reproduction Anatomy and physiology Reproductive system male, female Menstrual/Estrous cycle Menstruation - Follicular phase - Ovulation - Luteal phase Gametogenesis Spermatogenesis spermatogonium, spermatocyte, spermatid, sperm - Oogenesis oogonium, oocyte, ootid, ovum - Germ cell gonocyte, gamete Sexuality Human sexual behavior - Sexual intercourse - Erection - Ejaculation - Orgasm - Insemination - Fertilisation/Fertility - Masturbation - Pregnancy - Postpartum period Lifespan Prenatal development - Sexual dimorphism - Sexual differentiation - Puberty Menarche, Adrenarche - Maternal age / Paternal age - Climacteric Menopause, Andropause Eggs Ovum - Oviposition - Oviparity - Ovoviviparity - Viviparity Reproductive endocrinology Hypothalamic-pituitary-gonadal axis - Andrology - Hormone Retrieved from http://en..org/wiki/Fertilisation Categories: Reproduction | Fertility | Pollination | PregnancyHidden category: articles needing clarification Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages বাংলা Brezhoneg БългарÑ?ки Català Česky Dansk Deutsch Español Esperanto Français Hrvatski Bahasa Indonesia Italiano עברית Lietuvių МакедонÑ?ки Nederlands 日本語 ‪Norsk bokmÃ¥l‬ Polski Português Română Runa Simi РуÑ?Ñ?кий Simple English SlovenÄ?ina СрпÑ?ки / Srpski Basa Sunda Suomi Svenska ไทย Türkçe УкраїнÑ?ька ä¸æ–‡ This page was last modified on 12 September 2008, at 01:10
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