11.69: Egg Cells

How is egg different from sperm?

Egg and sperm are both gametes, or reproductive cells. Notice how different they are, however. The egg is much larger than the sperm. The egg also does not have a tail. And the female only releases one egg at time, while the male releases millions of sperm at a time.

Eggs and Egg Production

When a baby girl is born, her ovaries contain all of the eggs they will ever produce. But these eggs are not fully developed. They develop only after she starts having menstrual periods at about age 12 or 13. Just one egg develops each month. A woman will release an egg once each month until she is in her 40s. A girl is born with over a million eggs. They die off and by puberty about 40,000 remain.

Eggs

Eggs are very big cells. In fact, they are the biggest cells in the human female body. (How many egg cells are in the human male body?) An egg is about 30 times as wide as a sperm cell! You can even see an egg cell without a microscope. Like a sperm cell, the egg contains a nucleus with half the number of chromosomes as other body cells. Unlike a sperm cell, the egg contains a lot of cytoplasm, the contents of the cell, which is why it is so big. The egg also does not have a tail.

Egg Production

Egg production takes place in the ovaries. It takes several steps to make an egg:

1. Before birth, special cells in the ovaries go through mitosis (cell division), producing identical cells.
2. The daughter cells then start to divide by meiosis. But they only go through the first of the two cell divisions of meiosis at that time. They go through the second stage of cell division after the female goes through puberty.
3. In a mature female, an egg develops in an ovary about once a month. The drawing below shows how this happens (Figure below).

As you can see from the figure, the egg rests in a nest of cells called a follicle. The follicle and egg grow larger and go through other changes. The follicle protects the egg as it matures in the ovary.

After a couple of weeks, the egg bursts out of the follicle and through the wall of the ovary. This is called ovulation, which usually occurs at the midpoint of a monthly cycle. In a 28 day cycle, ovulation would occur around day 14. The moving fingers of the nearby fallopian tube then sweep the egg into the tube. At this time, if sperm are present the egg can be fertilized.

Figure \(\PageIndex{1}\): This diagram shows how an egg and its follicle develop in an ovary. After it develops, the egg leaves the ovary and enters the fallopian tube. (1) Undeveloped eggs, (2) Egg and follicle developing, (3) Egg and follicle developing, (4) Ovulation. After ovulation, what remains of the follicle is known as the corpus luteum, which degenerates (5, 6).

Fertilization occurs if a sperm enters the egg while it is passing through the fallopian tube. When this happens, the egg finally completes meiosis. This results in two daughter cells that are different in size. The smaller cell is called a polar body. It contains very little cytoplasm. It soon breaks down and disappears. The larger cell is the egg. It contains most of the cytoplasm. This will develop into a child.

Summary

• Eggs are female gametes that form in the ovaries and are released into the fallopian tubes.
• The eggs are formed before a baby girl is born, but these eggs are not fully developed.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

• Follicle Development 

1. What happens during ovulation? What happens to an egg after ovulation?
2. At what point is a zygote formed? How many chromosomes does a human zygote normally have?
3. Where does implantation occur?

Explore More II

• Formation of Gametes 

1. Is the primary oocyte diploid or haploid?
2. What structure forms from the primary oocyte? Is this structure diploid or haploid?

Review

1. Describe what happens during ovulation. When does this occur?
2. After ovulation, where does the egg go?
3. Briefly describe egg production.
4. What is a polar body?