11.71: Fertilization

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Where do sperm meet the egg?

You might guess that sperm meet the egg in the uterus, the organ where the baby develops. But that is incorrect. The sperm meet the egg in the fallopian tubes that carry the egg from the ovary to the uterus. Then the developing embryo travels to the uterus.

Fertilization and Implantation

The sperm and egg don’t look anything like a human baby (Figure below). After they come together, they will develop into a human being. How does a single cell become a complex organism made up of billions of cells? Keep reading to find out.

Sexual reproduction happens when a sperm and an egg cell combine together. This is called fertilization. Sperm are released into the vagina during sexual intercourse. They “swim” through the uterus and enter a fallopian tube. This is where fertilization normally takes place.

A sperm that is about to enter an egg is pictured below (Figure below). If the sperm breaks through the egg’s membrane, it will immediately cause changes in the egg that keep other sperm out. This ensures that only a single sperm can penetrate an egg. It will also cause the egg to go through meiosis. Recall that meiosis, cell division that creates the egg, begins long before an egg is released from an ovary. In fact, it begins prior to birth.

The sperm and egg each have only half the number of chromosomes as other cells in the body. These cells are haploid, with a single set of chromosomes. This is because when they combine together, they form a cell with the full number of chromosomes. The cell they form is called a zygote. The zygote is diploid, with two sets of chromosomes, one from each parent. A human zygote has two sets of 23 chromosomes, for a total of 46 chromosomes (23 pairs). The zygote slowly travels down the fallopian tube to the uterus. As it travels, it divides by mitosis many times. It forms a hollow ball of cells.

After the ball of cells reaches the uterus, it fixes itself to the side of the uterus. This is called implantation. It usually happens about a week after fertilization. Now the implanted ball of cells is ready to continue its development into a baby.

Summary

Fertilization occurs when an egg and sperm come together to form a zygote.
Implantation occurs when the developing embryo fixes itself to the side of the uterus.

Explore More

How long after fertilization does it take the zygote to reach the uterus?
About how many cells is the embryo at the time of implantation?
What prevents the uterine lining from being shed if an egg is fertilized?
How does the developing embryo receive nutrition?

Review

What is fertilization?
Where does fertilization take place?
What is a zygote? How many chromosomes does a human zygote normally have?
If the egg is fertilized, what immediately happens next?
Where does implantation take place?