In Vitro and in Vivo Fertilization
In animals, fertilization is the fusion of a sperm cell with an egg cell. The penetration of the egg cell by the chromosome-containing part of the sperm cell causes a reaction that prevents additional sperm cells from entering the egg. The egg and sperm each contribute half of the new organism's genetic material. A fertilized egg cell is known as a zygote. Following fertilization, the zygote undergoes continuous cell division that eventually produces a new multicellular organism.
Human fertilization in vivo (in the living body) occurs in oviducts (fallopian tubes) of the female reproductive tract, and takes place within hours following sexual intercourse. Only one of the approximately 300 million sperm released into a female's vagina during intercourse can fertilize the single female egg cell (ovum). The successful sperm cell must enter the uterus and swim up the fallopian tube to meet the ovum cell, where it passes through the thick coating surrounding the egg. This coating, consisting of sugars and proteins, is known as the zona pellucida. The tip of the head of the sperm cell contains enzymes which break through the zona pellucida and aid the penetration of the sperm into the egg. Once the head of the sperm is inside the egg, the tail of the sperm falls off, and the perimeter of the egg thickens to prevent another sperm from entering.
The sperm and the egg each contain only half the normal number of chromosomes, a condition known as the haploid chromosome state. When the genetic material of the two cells fuses, fertilization is complete.
In humans, a number of variables affect whether or not fertilization occurs following intercourse. One factor is a woman's ovulatory cycle. Human eggs can only be fertilized a few days after ovulation, which usually occurs only once every 28 days.
In vitro fertilization (IVF) is a procedure in which eggs (ova) from a woman's ovary are removed, fertilized with sperm in a laboratory procedure, and then the resulting fertilized egg (embryo) is returned to the woman's uterus. IVF is one of several assisted reproductive techniques (ART) used to help infertile couples to conceive a child. If after one year of having sexual intercourse without the use of birth control, a woman is unable to become pregnant, infertility is suspected. Some of the reasons for infertility are damaged or blocked fallopian tubes, hormonal imbalance, or endometriosis in the woman. In the man, low sperm count or poor quality sperm can cause infertility.
IVF is one of several possible methods to increase the chance for an infertile couple to become pregnant. Its use depends on the reason for infertility. IVF may be an option if there is a blockage in the fallopian tube or endometriosis in the woman, or low sperm count or poor quality sperm in the man. There are other possible treatments for these conditions often attempted prior to IVF, such as surgery for blocked tubes or endometriosis. IVF will not be successful for a woman who is not capable of ovulating, or a man who is not able to produce a certain number of healthy sperm.
During in vitro fertilization, the joining of egg and sperm takes place outside of the woman's body. A woman may be given fertility drugs before this procedure so that several eggs mature in the ovaries at the same time. Mature eggs (ova) are removed from a woman's ovaries, and are mixed with sperm in a laboratory dish or test tube, hence, the early term "test tube baby." The eggs are monitored for several days. Once there is evidence that fertilization has occurred and the cells begin to divide, they are then returned to the woman's uterus.
In the procedure to remove eggs, enough may be gathered to be frozen and saved (either fertilized or unfertilized) for additional IVF attempts.
IVF has been used successfully since 1978, when the first child to be conceived by this method was born in England. Over the past 20 years, thousands of couples have used IVF or other assisted reproductive technologies to conceive. The risks associated with in vitro fertilization include the possibility of multiple pregnancy (since several embryos may be implanted) and ectopic pregnancy (an embryo that implants in the fallopian tube or in the abdominal cavity outside the uterus). There is a slight risk of ovarian rupture, bleeding, infections, and complications of anesthesia. If the procedure is successful and pregnancy is achieved, the pregnancy would carry the same risks as any pregnancy achieved without assisted technology.
Success rates vary widely between clinics and between physicians performing the procedure. A couple has about a 10% chance of becoming pregnant each time the procedure is performed. Therefore, the procedure may have to be repeated more than once to achieve pregnancy.
Other similar types of assisted reproductive technologies are also used to achieve pregnancy. A procedure called intracytoplasmic sperm injection (ICSI) uses a manipulation technique that uses a microscope to inject a single sperm into each egg. The fertilized eggs can then be returned to the uterus, as in IVF. In gamete intrafallopian tube transfer (GIFT) the eggs and sperm are mixed in a narrow tube and then deposited in the fallopian tube, where fertilization normally takes place. Another variation on IVF is zygote intrafallopian tube transfer (ZIFT). As in IVF, the fertilization of the eggs occurs in a laboratory dish. And, similar to GIFT, the embryos are placed in the fallopian tube (rather than the uterus as with IVF).
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