Fertilization



Fertilization (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.

Fertilization 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 fertilization 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&mdash;a considerable distance compared to the size of the sperm cell.

Human fertilization
The term "conception" commonly refers to fertilization, 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 rationalize that the destruction of the human embryo between fertilisation and implantation should be labeled "contraception", rather than "early abortion".

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.

Fertilization 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.