The zygote is the single cell that is formed when the sperm cell fertilizes the egg cell. The zygote divides multiple times, producing identical copies of itself. The cells produced by the division of the zygote form the developing embryo, fetus, and baby. The zygote is the first step in the formation of a new person.
Description
When the sperm fuses with the egg, a cascade of events begins. Additional sperm are prevented from fertilizing
the egg. The membranes of the egg and sperm combine, producing one single cell. The egg and sperm prepare to fuse their genetic material (DNA/chromosomes). Finally, the genetic material combines to produce the zygote with one complete set of chromosomes.
Most cells in the human body have two pairs of 23 chromosomes, i.e. 46 chromosomes total. One set of 23 chromosomes is inherited from the mother, and the complementary set is inherited from the father. When the egg and sperm are formed, the two sets of chromosomes divide evenly, from 46 to 23 chromsomes to produce eggs and sperm with 23 chromosomes each. This ensures that when the egg and sperm fuse during conception, the original number of chromosomes (46) is restored.
The reduction of each parent cell from 46 to 23 chromosomes ensures that each parent contributes half of his or her genetic material to form the zygote and the offspring shares 50% of his or her genes with each parent. Duplication of the single zygote occurs through a complete division of the single ball of cells. This begins the process of forming the fetus and eventually the baby. The first division produces two identical cells, the second produces four cells, the third produces eight cells, etc. After many cell divisions, the cells begin to specialize and differentiate (form particular tissues and organs).
Fertilization usually occurs in the fallopian tube, and the first few cell divisions occur as the developing embryo moves to the uterus. The first division occurs about 30 hours after fertilization. As the zygote divides, some of the cells formed will develop into the placenta. Approximately six days after fertilization, the ball of cells attaches to the uterine wall.
Sex determination
Men and women each have 22 pairs of non-sex chromosomes and two sex chromosomes. Men's sex chromosomes are X and Y. A mature sperm cell that has undergone the chromosome division process from 46 to 23 chromosomes produces a cell that is either X or Y. Women's sex chromosomes are X and X. The eggs that women produce have only X chromosomes. Therefore, the sperm determines whether the zygote is XY or XX, which is the initial step on the biological path to becoming a male or female.
Developmental periods
The term embryo refers to the developing baby between the second week after conception and the eighth week after conception. Doctors use the term fetus from the ninth week after conception to birth. A pregnancy is broken down into three trimesters. The first trimester begins with the first day of the woman's last menstrual period and each trimester is three calendar months.
Twins
Twins may arise in two ways. Identical twins are called "monozygotic" because both individuals are formed from the same zygote. As the zygote divides to form the baby, two separate individuals form instead of one. Fraternal twins are called "dizygotic" because each individual develops from a different zygote. Two eggs are ovulated, and a separate sperm fertilizes each egg. Therefore, identical twins have exactly the same DNA in each cell and fraternal twins share the same amount of DNA as brothers and sisters. Sometimes it is impossible to tell monozygotic twins from dizygotic twins based on the placenta and the fetal membranes. If a person wants to determine whether twins are monozygotic or dizygotic, DNA studies of blood cells will provide a definitive answer.
Abnormalities
The zygote normally contains two complete sets of 23 chromosomes, and two copies of every gene. If the egg or sperm that fuse to form the zygote is abnormal, the zygote will also be abnormal. For example,Down syndrome is caused by an extra chromosome number 21 from the egg or sperm cell. Since the cells formed by division of the zygote are identical to the zygote, any abnormality in the zygote will be in every cell of the baby.
Abnormalities can also arise when the zygote begins to divide. This type of abnormality is usually severe, eventually leading to a miscarriage. If an abnormality occurs after the zygote has divided one or more times, the baby will have some normal cells and some abnormal cells. This situation is referred to as "mosaicism" and "mosaic" may be used to describe the person's condition.
Molar pregnancies
Molar pregnancies can occur in one of two ways. Sometimes the original cell that duplicates and divides to form the fetus is completely of paternal origin. The chromosomes in a sperm duplicate themselves, then proceed to divide as if they were a normal zygote. These pregnancies are completely abnormal and miscarry. Another type of molar pregnancy occurs when two sperm fertilize one egg. The zygote is triploidy and has 69 chromosomes instead of 46. Although some fetal parts can be seen, these pregnancies normally miscarry in the first or second trimester.
Birth defects
The term birth defect describes many different types of abnormalities, including physical malformations. Abnormalities of anatomical structures may be significant or insignificant; minor variations in structure are common. Approximately 3% of newborns have major malformations. The causes are: chromosome abnormalities (6–7%), inherited genetic conditions (7-8%), environmental factors (7–10%), and multifactorial causes (20–25%). The cause of the remaining 50–60% of malformations is unknown. Multifactorial refers to causes with both genetic and environmental components. Environmental factors include exposures to drugs, chemicals, or other substances that affect the development of the fetus while he/she is in the uterus. Substances that cause birth defects are referred to as teratogens.
Artificial reproductive technology
Couples may pursue assisted reproductive technologies for a number of reasons. If a couple has artificial insemination, the sperm is inserted into the uterus when the woman in ovulating. Fertilization then occurs as it would normally. If a couple has in vitro fertilization(IVF), the egg and sperm are mixed outside the body in the laboratory. The zygote forms in a petri dish if fertilization occurs. After a number of cell divisions, the developing embryo is placed in the woman's uterus. If the sperm are incapable of fusing with the egg themselves, the sperm may be injected into the egg. This additional step to the IVF procedure is called intracytoplasmic sperm injection (ICSI).
In the year 2001, preimplantation diagnosis is possible for a number of genetic diseases. Couples may pursue this if they are at a significant risk for having a child with a disease that could be diagnosed prior to becoming pregnant through preimplantation diagnosis. The procedure is like that of in vitro fertilization, with an additional step. After fertilization occurs and the zygote has begun to divide, a single cell is removed. Removing the cell does not harm the other cells. The cell that is removed is tested for the genetic disease for which the couple is at risk. Multiple developing embryos are tested. Only the embryos that do not have the condition are placed in the woman's uterus to complete development.
The development of a person from the zygote is a fascinating and amazing process. It is a difficult area to study because scientists cannot manipulate human embryos to observe the effects, and the development of the fetus cannot be directly observed. Researchers still have many unanswered questions. Following a doctor's recommendations from prior to the pregnancy throughout pregnancy (such as folic acid intake and avoidance of alcohol and other drugs) increases the chances that the development of a zygote into a full-term infant will be normal. However, there are many babies born with severe birth defects or genetic diseases despite the parents' efforts at doing everything in their power to prevent a problem. Most birth defects and genetic disorders occur because of an event out of control of the parents.
BOOKS
Agnew, Connie L. Twins!: Expert Advice From Two Practicing Physicians on Pregnancy, Birth, and the First Year. New York: HarperCollins, 1997.
Brasner, Shari E. Advice From a Pregnant Obstetrician. New York: Hyperion, 1998.
Nathanielsz, P.W. Life in the Womb: The Origin of Health and Disease. Ithaca, NY: Promethean Press, 1999.
Vaughn, Christopher C. How Life Begins: The Science of Life in the Womb. New York: Times Books, 1996.
PERIODICALS
Check, Erika. "What Moms Can Do Now." Newsweek (27 September 1999): 57–58.
Christensen, Damaris. "Sobering Work." Science News (8 July 2000): 28–29.
Kowalski, Kathiann. "High-tech Conception in the 21st Century." Current Health (January 2000): 1–4.
Miller, Annetta, and Joan Raymond. "The Infertility Challenge." Newsweek (Spring/Summer 1999 Special Edition): 26–28.
ORGANIZATIONS
American College of Obstetricians and Gynecologists. PO Box 96920, 409 12th St. SW, Washington, DC 20090-6920. <http://www.acog.org>.
American Society for Reproductive Medicine. 1209 Montgomery Highway, Birmingham, AL 35216-2809. (205) 978-5000. asrm@asrm.org. <http://www.asrm.org>.
RESOLVE, The National Infertility Association. 1310 Broadway, Somerville, MA 02144-1779. (617) 623-0744. resolveinc@aol.com. <http://www.resolve.org>.
WEBSITES
The InterNational Council on Infertility Information Dissemination, Inc. <http://www.inciid.org>.
