Encyclopedia
Fertilisation, also spelt
fertilization , 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.
Fertilisation in plants
After the
pistil 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 attempting to travel into the ovary by creating a path. The vegetative and generative nuclei of the pollen grain pass into its respective pollen tube. The growth of the pollen tube is controlled by the vegetative nucleus.
Enzymes are secreted to digest the tissue of the stigma as the pollen 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 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 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 the process in
angiosperms 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 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 centre of the
embryo sac. The resulting cell is
triploid . This triploid cell divides through
mitosis and forms the endosperm, a
nutrient-rich tissue inside the
fruit.
The two central maternal 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 one primitive group of flowering plants, the water lilly,
Nuphar is genus of aquatic plants in the family Nymphaeaceae [i], with a temperate Northern Hemisphere [i] ...
, 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, a haploid tissue, so there is no double fertilisation.
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. 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 embryo reaches the uterus and implants in the endometrium, it begins to divide and form an embryo. At this point the female is said to be
pregnant. If the embryo emplants in the fallopian tubes, rather than in the uterus, an ectopic pregnancy results, which can be fatal to the mother.
In some animals 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.
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 fertilisation
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.
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 segragation of the genes contributed from each parent. Each parent organism generally has the same genetic makeup, but differs for a fraction of their genes. Therefore, each gamete produced 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 ˛, chromosomally different zygotes are possible for the non-sex chromosomes, even assuming no
chromosomal crossover. If crossover occurs once, then on average ˛ 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.
See also
Notes and references
