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Fertilization is the process of an egg and sperm uniting to form a zygote, which can lead to pregnancy. Mammals experience fertilization through sexual intercourse, while other species have different methods. Ovulation is when the ovary releases an egg, and sperm cells are produced in the male testicles. Successful fertilization depends on various factors, and reproductive assistance techniques like intrauterine insemination and IVF are available for those unable to conceive through sexual intercourse.
Fertilization is the process in which an egg and sperm unite to form a zygote, initiating the series of events that can lead to pregnancy. In mammals, including humans, eggs are released from the female ovary and sperm from the male penis. Fertilization typically occurs through sexual intercourse, but scientific advances have paved the way for in vitro techniques, which are performed in the laboratory.
Mammalian species versus non-mammalian species
Many animal species experience fertilization in a similar way, but the events leading up to the event can differ greatly. In mammals it is preceded by sexual intercourse, but in other species this is not always the case. Some non-mammalian species, such as birds and some reptiles, engage in some form of intercourse. Males of these species do not have a penis, but instead have an opening called the cloaca, which is pressed against the female cloaca to allow sperm to be transferred. The eggs are fertilized inside the female, so that protective shells can form around each zygote before it is laid.
Other reptile species, including amphibians, have different ways of ensuring that the egg and sperm make contact with each other. In frogs, for example, the male typically mounts the female and releases sperm onto the eggs as they are laid. Many fish species reproduce in a similar way. For others, the process can occur without any physical contact, with males swimming around and fertilizing eggs previously deposited by a female.
Ovulation
In mammals, the process by which an ovary releases an egg is called ovulation. This event occurs approximately 14 days after the start of a woman’s menstrual cycle, which is also considered the first day of the menstrual cycle. Ovulation, therefore, occurs approximately in the middle of the cycle. After an egg is released, it travels through the fallopian tube, where it stays for about a day. If fertilization does not occur during this time, the egg ruptures.
Sperm production
Sperm cells are produced in large numbers in the male testicles and approximately 300 million are expelled into the vagina during ejaculation. Of these, about 200 reach the egg and typically only one will fertilize it. This ensures that the resulting zygote has the correct combination of genetic material. If two sperm fertilize a single egg, the zygote fails to divide properly and does not develop further.
Events in Fertilization
Once they penetrate the cervix and enter the uterus, the sperms are able to stay for up to five days before dying. These sperm cells use a phenomenon called chemotaxis to locate the egg, if present. This refers to how the concentration of a given chemical in the environment can change based on location. In this case, sperm that approach the fallopian tubes are able to locate the egg by following a trail of hormones that the egg releases.
The first sperm to reach the egg binds with molecules on its surface, and this interaction causes chemical changes in the egg’s membrane. These changes help the sperm move through the egg membrane and also prevent any other sperm from binding. Once the sperm is inside the egg, its own membrane dissolves, allowing the contents of the two to mix. When the genetic material from the egg and sperm come together, the new zygote begins dividing. Fertilization is considered complete once the genetic material has combined.
Successful fertilization depends on several factors, including the timing of intercourse relative to the woman’s menstrual cycle and the reproductive health of both the woman and the man involved in the process. A variety of different conditions can reduce the fertility of both parties. In men, a low sperm count, or sperm with poor mobility, can reduce the chance of a sperm reaching an egg. In women, a condition such as endometriosis, a disease of the uterine lining, can prevent implantation from occurring.
Implantation in the uterus
Fertilization typically occurs in one of the fallopian tubes, after which the egg travels to the uterus over a period of about three days. Once in the uterus, the zygote implants itself in the endometrial lining to continue development. On rare occasions, a zygote fails to move into the uterus and instead implants itself in the wall of the fallopian tube. This results in an ectopic pregnancy, an event that ultimately impedes fetal development and can endanger the health of the pregnant woman.
Reproductive assistance
For people who are unable to conceive through sexual intercourse, modern technology provides ways around some of the biological requirements of fertilization. One such method is intrauterine insemination, which ensures that the egg and sperm are able to come into contact. In this process, a woman is given medications that promote the release of more eggs during ovulation. When ovulation occurs, the prepared sperm cells supplied by your male partner are introduced directly into your uterus.
IVF involves similar techniques for increasing a woman’s egg production with medication. The sperm is collected in the same way as before, but instead of introducing the sperm into the uterus, the woman’s eggs are retrieved from her fallopian tubes and mixed with the sperm in a culture dish. After several days, the fertilized eggs are selected and implanted in the woman’s uterus. Since more embryos are usually implanted to increase the chances of pregnancy, this method often results in multiple births.
