Z is for Zygote


A zygote is the union of the sperm cell and the egg cell. Also known as a fertilized ovum, the zygote begins as a single cell but divides rapidly in the days following fertilization. After this two-week period of cell division, the zygote eventually becomes an embryo. If this goes well, the embryo becomes a fetus.

The Formation of Zygotes

In order for reproduction to take place, a single sperm cell must penetrate the outer surface of an egg. In most cases, a single egg cell is released during the ovulation phase of a woman's monthly reproductive cycle. Normally, thousands of sperm attempt to penetrate this single egg cell. Once a single sperm has broken through the outer surface, chemical changes in the surface of the egg prevent other sperm from entering.

This process usually happens during sexual intercourse, although medically assisted fertilization is also possible. Intrauterine insemination (IUI) and in vitro fertilization (IVF) are two commonly used assisted reproductive techniques.


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During an IUI, semen is inserted into the uterus using a catheter so that fertilization takes place inside the woman's body. In IVF, eggs are removed from the ovaries and fertilized in a lab. The zygote is then implanted in the uterus.

What Happens After Fertilization?

The zygote divides through a process known as mitosis, in which each cell doubles by dividing into two cells. This two-week stage is known as the germinal period of development and covers the time of conception to the implantation of the embryo in the uterus.

In most cases, each male and female sex cell contains 23 chromosomes. The sperm cell contains the genetic information from the father while the egg cell contains the genetic information from the mother. Because each cell contains half of the genetic material, each cell is known as a haploid cell.

When these two haploid cells join, they form a single diploid cell that contains a total of 46 chromosomes. The zygote then travels down the fallopian tube to the uterus where it must implant in the lining in order to obtain the nourishment it needs to grow and survive.

If this process goes well, the zygote will continue to grow until it reaches the next phase of prenatal development.

How Long Does the Zygote Phase Last?

The period of the zygote is quite brief, lasting for about four days. Around the fifth day, the mass of cells becomes known as a blastocyst. The germinal period will last approximately fourteen days, after which the embryonic period will begin.

The second period of development lasts from two weeks after conception through the eighth week, during which time the organism is known as an embryo. At the ninth week post-conception, the fetal period begins. From this point until birth, the organism is known as a fetus.

Not all zygotes make it to the next stage of prenatal development, however. Researchers estimate that 30 to 70 percent of all naturally occurring conceptions fail either before or at the time of implantation.


Kim Sabin