Fertilisation
From Freepedia
Fertilisation (also known as conception, fecundation and syngamy) is fusion of gametes to form a new organism. 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, pollen grains attempt to travel into the ovary by creating a path called "pollen tube." 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 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 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, 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, so one can carefully remove the husk to show the floral structures. 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.
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 fertilisation
There have been some discrepancies regarding the terminology of "conception" (fertilisation) 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 rationalize that the destruction of the human embryo between fertilisation and implantation should be labeled "contraception", rather than "early abortion".
See also
References
- Evans JP, Florman HM. 2002. The state of the union: the cell biology of fertilization. Nature Medicine. 8 Suppl S57-63.
