Fertilisation
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Fertilisation or fertilizationTemplate:Fn (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.
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Fertilisation in plants
After the female part of the flower is 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 attempt to travel into the ovary by creating a path. The vegetative and generative nuclei of the pollen grain pass into its repective pollen tube. The growth of the pollen tube is controlled by the vegetative nucleus. Enzymes are secreted to digest the tissue of the stigma sn style as the polen tube grows.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 microphyle (an opening in the ovule wal) and reaches the ovum to fertilise it. This is the point when fertilisation actually occurs. After being fertilised, the ovary starts to swell and becomes 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 shown. 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 a process in angiosperms (flowering plants) during reproduction, in which two sperm cells fertilise two cells in the 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 are released into the ovary through this tube. One of the two sperm cells fertilises the egg cell, forming a diploid zygote. The other sperm cell fuses with two haploid polar nuclei in the center of the embryo sac. The resulting cell is triploid (3n). This triploid cell divides through mitosis and forms the endosperm, a nutrient-rich tissue inside the fruit. Humans and other animals ingest this tissue for nutrition.
Fertilisation in 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. (This process is a part of copulation.) Once the male ejaculates, a large number of sperm cells swim toward the ovum.
The capacitated spermatozoon and the oocyte meet and interact in the ampulla of the fallopian tube. In mammals, binding of the spermatozoon to the zona pellucida, an extracellular layer surrounding the oocyte, 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. Once the ovum fuses with a single sperm cell, its cell membrane changes, preventing fusion with other sperm.
This process ultimately leads to the formation of a diploid cell called a zygote. When the zygote reaches the uterus and implants in the endometrium, the female is said to be pregnant.
If fertilisation takes place, the sperm usually meet the ovum in the fallopian tube, requiring the sperm cells to swim from the upper vagina through the cervix and across the length of the uterus before reaching the fallopian tube—a considerable distance compared to the size of the sperm cell.
Human fertilization
Template:Main 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. In a statement by the American Association of Pro-Life Obstetricians & Gynecologists (AAPLOG), regarding the controversial morning-after pill, AAPLOG claims:
- "[Again,] one must be careful of the terminology. Many now speak of "conception" as that moment when the human blastocyst, the early ball of approximately 100 cells, implants in the mother’s uterus (womb). The time from actual fertilisation (sperm and egg unite in the Fallopian Tube) until implantation, a period of about 7-10 days, is ignored, even though no genetic change occurs in the cells during this time period. Many family planning specialists who have supported the terminology change can thus rationalise that the destruction of the human embryo between fertilisation and implantation should be labeled "contraception", rather than "early abortion".
Gastrulation is the point in development when the implanted blastocyst developes 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.Template:Ref The term "conception" is not usually used in scientific literature because of its variable definition and connotation.
Fertilisation and Genetic Recombination
The variations that result from meiosis is enhanced by fertilisation. Each person has genes for the same traits, but again, each gene's specific instructions can vary. Therefore, the gametes produced by one person are expected to be genetically different 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 (2²³)², or 70,368,744,000,000 chromosomally different zygotes are possible, even assuming no chromosomal crossover. If crossover occurs once, then (4²³)², or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possible for every couple.
Notes
Template:Fnb The spelling fertilisation is the British variant variant of the term. The spelling fertilization is used in American and Candian English, and often in academic British English. See English Dialects for more information.
See also
References
- Evans JP, Florman HM. 2002. The state of the union: the cell biology of fertilisation. Nature Medicine. 8 Suppl S57-63.
- Template:NoteMoore KL, Persaud TVM. 2003. The Developing Human: Clinically Oriented Embryology.bg:Зачеване
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