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Fertilization (also known as conception, fecundation and syngamy) is the fusion of gametes to initiate the development of a new individual organism. In animals, the process involves the fusion of an ovum with a sperm, 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 (external fertilisation). The cycle of fertilisation and development of new individuals is called sexual reproduction.
After the carpel is pollinated, the pollen grain germinates. From each pollen grain, a pollen tube grows out that attempts to travel to the ovary by creating a path through the female tissue. The vegetative (or tube) and generative nuclei of the pollen grain pass into the pollen tube. After the pollen grain adheres to the stigma of the carpel (female reproductive structure) a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle.
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 the pollen tube "bursts" into the embryo sac.
After being fertilised, the ovary starts to swell and will develop into the fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule. The growth of the pollen tube is controlled by the vegetative (or tube) cytoplasm. Hydrolytic enzymes are secreted by the pollen tube that digest the female tissue as the tube grows down the stigma and style; the digested tissue is used as a nutrient source for the pollen tube as it grows. During pollen tube growth toward the ovary, the generative nucleus divides to produce two separate sperm nuclei (haploid number of chromosomes) – a growing pollen tube therefore contains three separate nuclei, two sperm and one tube. The sperms are interconnected and dimorphic, the large one, in a number of plants, is also linked to the tube nucleus and the interconnected sperm and the tube nucleus form the "male germ unit".
Double fertilisation is the process in angiosperms (flowering plants) in which two sperm from each pollen tube fertilise two cells in a female gametophyte (sometimes called an embryo sac) that is inside an ovule. After the pollen tube enters the gametophyte, the pollen tube nucleus disintegrates and the two sperm cells are released; one of the two sperm cells fertilises the egg cell (at the bottom of the gametophyte near the micropyle), forming a diploid (2n) zygote. This is the point when fertilisation actually occurs; pollination and fertilisation are two separate processes. The nucleus of the other sperm cell fuses with two haploid polar nuclei (contained in the central cell) in the centre of the gametophyte. 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 the single meiotic product that also gave rise to the egg. Therefore, maternal contribution to the genetic constitution of the triploid endosperm is double that of the embryo.
One primitive species of flowering plant, Nuphar polysepala, has endosperm that is diploid, resulting from the fusion of a sperm with one, rather than two, maternal nuclei. It is believed that early in the development of angiosperm linages, there was a duplication in this mode of reproduction, producing seven-celled/eight-nucleate female gametophytes, and triploid endosperms with a 2:1 maternal to paternal genome ratio.
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.
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 ensure species-specificity:
Consideration as to whether an animal (more specifically a vertebrate) uses internal or external fertilization is often dependent on the method of birth. Oviparous animals laying eggs with thick calcium shells, such as chickens, or thick leathery shells generally reproduce via internal fertilisation so that the sperm fertilises the egg without having to pass through the thick, protective, tertiary layer of the egg. Ovoviviparous and viviparous animals also use internal fertilisation. It is important to note that although some organisms reproduce via amplexus, they may still use internal fertilisation, as with some salamanders. Advantages to internal fertilisation include: minimal waste of gametes; greater chance of individual egg fertilisation, relatively "longer" time period of egg protection, and selective fertilisation; many females have the ability to store sperm for extended periods of time and can fertilise their eggs at their own desire.
Oviparous animals producing eggs with thin tertiary membranes or no membranes at all, on the other hand, use external fertilisation methods. Advantages to external fertilisation include: minimal contact and transmission of bodily fluids; decreasing the risk of disease transmission, and greater genetic variation (especially during broadcast spawning external fertilisation methods).
Sperm find the eggs via chemotaxis, a type of 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 penetrates 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 polymerisation 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 EBR1.
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 internally fertilise through copulation. After a male ejaculates, many sperm move to the upper vagina (via contractions from the vagina) through the cervix and across the length of the uterus to meet the ovum. In cases where fertilisation occurs, the female usually ovulates during a period that extends from hours before copulation to a few days after; therefore, in most mammals it is more common for ejaculation to precede ovulation than vice versa.
The capacitated spermatozoon and the oocyte meet and interact in the ampulla of the fallopian tube. Thermotactic and chemotactic gradients are involved in guiding sperm towards the egg during the final stage of sperm migration. Spermatozoa respond to the temperature gradient of ~2°C between the oviduct and the ampulla, and chemotactic gradients of progesterone have been confirmed as the signal emanating from the cumulus oophorus cells surrounding rabbit and human oocytes. Capacitated and hyperactivated sperm respond to these gradients by changing their behaviour and moving towards the cumulus-oocyte complex. Other chemotactic signals such as formyl Met-Leu-Phe (fMLF) may also guide spermatozoa.
The zona pellucida, a thick layer of extracellular matrix that surrounds the egg and is similar to the role of the vitelline membrane in sea urchins, binds with the sperm. Unlike sea urchins, the sperm binds to the egg before the acrosomal reaction. ZP3, a glycoprotein in the zona pellucida, is 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 with the sperm and activating the acrosome reaction. ZP3 is sufficient though unnecessary for sperm/egg binding. Two additional sperm receptors exist: a 250kD protein that binds to an oviduct secreted protein, and SED1, which independently binds to the zona. After the acrosome reaction, the sperm is believed to remain bound to the zona pellucida through exposed ZP2 receptors. These receptors are unknown in mice but have been identified in guinea pigs.
In mammals, the binding of the spermatozoon to the GalT initiates the acrosome reaction. This process releases the hyaluronidase that digests the matrix of hyaluronic acid in the vestments around the oocyte. Fusion between the oocyte plasma membranes and sperm follows and allows the sperm nucleus, centriole and flagellum, but not the mitochondria, to enter the oocyte. The protein CD9 likely mediates this fusion in mice (the binding homolog). The egg "activates" itself upon fusing with a single sperm cell and thereby changes its cell membrane to prevent fusion with other sperm.
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This process ultimately leads to the formation of a diploid cell called a zygote. The zygote divides to form a blastocyst and, upon entering the uterus, implants in the endometrium, beginning pregnancy. Embryonic implantation not in the uterine wall results in an ectopic pregnancy that can kill the mother.
In such animals as rabbits, coitus induces ovulation by stimulating the release of the pituitary hormone gonadotropin; this release greatly increases the likelihood of pregnancy.
The term conception commonly refers to fertilisation, which is the successful fusion of gametes to form a new organism. Its use 'conception' by some to refer to implantation makes it a subject of semantic arguments about the beginning of pregnancy, typically in the context of the abortion debate. Upon gastrulation, which occurs around 16 days after fertilisation, the implanted blastocyst develops three germ layers, the endoderm, the ectoderm and the mesoderm, and the genetic code of the father becomes fully involved in the development of the embryo; later twinning is impossible. Additionally, interspecies hybrids survive only until gastrulation and cannot further develop. However, some human developmental biology literature refers to the conceptus and such 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.
There are three types of fertilisation processes in protozoa:
Meiosis results in a random segregation of the genes that each parent contributes. Each parent organism is usually identical save for a fraction of their genes; each gamete is therefore genetically unique. At fertilisation, parental chromosomes combine. In humans, (2²²)² = 17.6x1012 chromosomally different zygotes are possible for the non-sex chromosomes, even assuming no chromosomal crossover. If crossover occurs once, then on average (4²²)² = 309x1024 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 undergo no crossover events and are therefore excluded from the calculation. The mitochondrial DNA is only inherited from the maternal parent.
Organisms that normally reproduce sexually can also reproduce via parthenogenesis, wherein an unfertilized female gamete produces viable offspring. These offspring may be clones of the mother, or in some cases genetically differ from her but inherit only part of her DNA. Parthenogenesis occurs in many plants and animals and may be induced in others through a chemical or electrical stimulus to the egg cell. In 2004, Japanese researchers led by Tomohiro Kono succeeded after 457 attempts to merge the ova of two mice by blocking certain proteins that would normally prevent the possibility; the resulting embryo normally developed into a mouse.
Allogamy, which is also known as cross-fertilization, refers to the fertilization of an egg cell from one individual with the male gamete of another.
Autogamy, which is also known as self-fertilization, occurs in such hermaphroditic organisms as plants and flatworms; therein, two gametes from one individual fuse.
Gynogenesis: A sperm stimulates the egg to develop without fertilization or syngamy. The sperm may enter the egg.
Hybridogenesis: One genome is eliminated to produce haploid eggs.
Canina meiosis: (sometimes called "permanent odd polyploidy") one genome is transmitted in the Mendelian fashion, others are transmitted clonally.
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The human sperm cell has been accredited in scientific literature for actively penetrating the egg and causing the fertilization process to initiate, while the egg cell has been seen to be the unmoving receiving end. However, Feminist scholars, such as Emily Martin and Londa Schiebinger, tell a different story of the textbook accounts of conception and the role of the egg. Schiebinger claims in her book Has Feminism Changed Science? that "the supermatic hero actively pursues the egg, surviving the hostile environment of the vagina and defeating his many rivals. Meanwhile, the egg moves along the fallopian tubes until it makes contact with the sperm." In his article, "The Aggressive Egg," Freedman describes the studies of researcher Emily Martin; this article tells a different view of the process of fertilization. According to this view by her, the egg is a partner and active contributor to fertilization. Freedman also writes that Martin does not suggest that researchers have distorted the imagery. Schiebinger shines light on the 1983 efforts of scientists Gerald and Hieden Schatten to revise fundamental notions of fertilization in their article titled "The Energetic Egg." They portrayed the egg as an active agent, directing the growth of microvilli, to capture and tether the sperm bringing it to the egg. Here Gerald and Hieden Schatten portrayed the egg and sperm as partners, working together toward successful fertilization.