fertilization

Genetics Encyclopedia: Fertilization

Fertilization is the fusion of a female's egg cell (oocyte) and a male's sperm cell (spermatozoa) to form the first cell of a new and unique being. While on the surface this sounds like a simple process, there are many factors that make this possible.

Gametes

Gametes are unique from all other cells. Typically, each cell in the human body contains twenty-three pairs of chromosomes (for a total of forty-six). Mature egg and sperm cells contain only one copy of each chromosome (for a total of twenty-three). At fertilization the fusion of the two gametes will create a cell with the appropriate twenty-three pairs of chromosomes (forty-six individual chromosomes) necessary for human development. In this way, one chromosome of each pair will originate from each parent, making the new individual unique from any other person that came before. The specialized process by which the genetic material is shuffled and the chromosome number is cut in half, from forty-six to twenty-three chromosomes, is called meiosis.

Ovulation and Ejaculation

Sperm and egg cells are not only different from other cells, but are different from each other. A female is born with all the eggs she will ever have. At birth, the chromosomes of these eggs have only completed the beginning of meiosis (meiosis I) and will remain dormant (inactive) until the onset of menstrual cycles in puberty. Specific hormones produced during the menstrual cycle around day thirteen or fourteen trigger the continuation of meiosis in one egg each month. Meiosis is suspended for the second time in the middle of meiosis II around three hours prior to ovulation, and does not resume unless fertilization occurs.

During ovulation this egg, enclosed in two layers of protective material, is released from the ovary. The outer layer, the cumulus oophorus, is comprised of cells called cumulus cells, and the inner layer, the zona pellucida, is comprised of a jellylike coating made of protein and sugar. Once released from the ovary, the egg is swept into the fallopian tube. It is receptive to fertilization for only about one day. The sperm must reach the egg during this time, usually in the fallopian tubes, or fertilization will be impossible.

During sexual intercourse, millions of sperm are deposited into the vagina. They travel through the cervix and uterus to the fallopian tubes. Sperm can live within a woman's body for up to three days. Each sperm contain three distinct parts, the head, mid-piece, and tail. Each of these parts has a distinct purpose. The head of the sperm is composed of the nucleus (containing the chromosomes), an acrosome cap (containing enzymes crucial in fertilization), and an outer membrane. The mid-piece contains energy-producing mitochondria, and the tail is the mechanism for movement. Despite the ingenious design, fewer than 1 percent of the sperm released in an ejaculation ever make it to the egg. Factors inhibiting the success of sperm include abnormal formation and premature death from exposure to acidic vaginal secretions. Sperm can also be blocked by excess mucus covering the cervix, or they may travel to the fallopian tube that does not contain the egg. Fortunately, only one sperm is required to fertilize the egg.

In order to fertilize an egg, sperm must undergo the poorly understood process of capacitation. Capacitation involves changes to the acrosome, triggered by the cervical mucus, to prepare it to release the enzymes necessary to break through the zona pellucida. Upon reaching the surface of the zona pellucida, the sperm releases enzymes to break through. Once through the zona pellucida, the head of the sperm fuses to the egg's membrane, the tail of the sperm stops moving, and the egg engulfs the contents of the sperm.

It is crucial that only one sperm enters the egg. If an extra sperm passes through, a lethal condition known as polyspermy (many sperm) will occur. On the rare occasion this occurs, the fetus will be miscarried as a result of the extra set of chromosomes. To prevent this in most instances, a substance is released from the egg that changes the zona pellucida once it has been penetrated, blocking entry of any other sperm. Sperm penetration triggers the completion of the second meiotic division in the egg. With this division, the chromosomes of the sperm and egg come together in their own nucleus. The cell now officially becomes a zygote, the first cell of a new individual.

Variations

In some instances, an ovary releases more than one egg at one time, or both ovaries release an egg simultaneously. Each egg has the potential to be fertilized, resulting in multiple pregnancies. Since each conception originates from a separate egg and sperm, individuals created in this way are as different as those conceived as separate births. These are referred to as fraternal twins. In rare instances, cells from the same embryo separate to create distinct embryos that are genetically identical and are referred to as monozygotic, or identical, twins.

The exact details of fertilization vary from animal to animal. Fertilization does not always take place inside an animal. For example, sea urchins, spiny animals attached to rocks on the ocean floor, release their eggs and sperm directly into the water. Large numbers of each (millions of eggs and billions of sperm) are necessary to ensure that enough eggs will be fertilized to maintain the population. Many other ocean creatures also release egg and sperm cells into the water. However, the eggs are fertilized only by sperm of the same species because of unique proteins on the surface of the egg. As in humans, fertilization immediately triggers a change in the surface of the egg, protecting it from penetration by other sperm, even sperm of the same species.

Reproductive technology has introduced further variations in how eggs may become fertilized, permitting the process to occur outside the fallopian tubes. One of the most common is in vitro fertilization, in which eggs and artificially capacitated sperm are combined in a glass dish and the dividing embryos are later transplanted into the uterus.

Bibliography

Primakoff, Paul, and Diana G. Myles. "Penetration, Adhesion, and Fusion in Mammalian Sperm-Egg Interaction." Science (Jun. 21, 2002): 2183-2185.

Tobin, Allan, and Jennie Dusheck. Asking about Life, 2nd ed. Orlando, FL: Harcourt,2001.

Wasserman, Paul. "The Biology and Chemistry of Fertilization." Science 235 no. 4788 (1987): 553-560.

—Susan E. Estabrooks