Fertilization
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: November 23, 2014
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
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Chapter 1. Fertilization
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Fertilization
Describe the process of fertilization.
Outline the events that occur between fertilization and the embryonic stage.
Distinguish a morula from a blastocyst.
Define implantation.
How far does a sperm have to swim?
Sperm swimming to an egg. If fertilization occurs, the egg will have all the "instructions" to grow into a new
organism. That one cell will become two, then four, then eight, then sixteen, and on and on and on. And after about
9 months, that one cell will have become a new baby. But it all starts with the sperm swimming to the egg. A sperm
cell is about two thousandths of an inch long. And although they are small, they can swim roughly 8 inches in an
hour. To reach an egg, they will ultimately they have to swim around 192,000 times their own length.
Cleavage and Implantation
A day or two after an ovary releases an egg, the egg may unite with a sperm. Sperm are deposited in the vagina
during sexual intercourse. They propel themselves through the uterus and enter a fallopian tube. This is where
fertilization usually takes place.
When a sperm penetrates the egg, it triggers the egg to complete meiosis. The sperm also undergoes changes. Its
tail falls off, and its nucleus fuses with the nucleus of the egg. The resulting cell, called a zygote, contains all the
chromosomes needed for a new human organism. Half the chromosomes come from the egg and half from the
sperm.
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Morula and Blastocyst Stages
The zygote spends the next few days traveling down the fallopian tube toward the uterus, where it will take up
residence. As it travels, it divides by mitosis several times to form a ball of cells called a morula. The cell divisions
are called cleavage. They increase the number of cells but not the overall size of the new organism. As more cell
divisions occur, a fluid-filled cavity forms inside the ball of cells. At this stage, the ball of cells is called a blastocyst.
The cells of the blastocyst form an inner cell mass and an outer cell layer, as shown in Figure 1.1. The inner cell
mass is called the embryoblast. These cells will soon develop into an embryo. The outer cell layer is called the
trophoblast. These cells will develop into other structures needed to support and nourish the embryo.
FIGURE 1.1
Blastocyst. The blastocyst consists of an
outer layer of cells called the trophoblast
and an inner cell mass called the embryoblast. The blastocyst fluid-filled cavity is
also known as the blastocoel or blastocoele.
Implantation
The blastocyst continues down the fallopian tube and reaches the uterus about 4 or 5 days after fertilization. When
the outer cells of the blastocyst contact cells of the endometrium lining the uterus, the blastocyst embeds in the
endometrium. The process of embedding is called implantation. It generally occurs about a week after fertilization.
For a detailed look at fetal development, see http://php.med.unsw.edu.au/embryology/index.php?title=Main_Page .
Summary
• Fertilization is the union of a sperm and egg, resulting in the formation of a zygote.
• The zygote undergoes many cell divisions before it implants in the lining of the uterus.
Explore More
Review
1. What happens during fertilization? Where does it usually take place?
2. What is implantation? When does it occur?
3. Describe a morula and blastocyst.
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Chapter 1. Fertilization
References
1. Zachary Wilson. Blastocyst diagram. CC BY-NC 3.0
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