The structure of the sperm and the ovum

Two cells with different jobs

A sperm and an ovum each carry 23 chromosomes. Yet the two cells look nothing alike. The sperm is tiny and must swim a long way. The ovum is among the largest cells of the human body and does not move by itself. Its size comes from the stored food that the early embryo uses.

The sperm

A sperm has four parts.

• The head holds the haploid nucleus. Over the front of the nucleus is a cap shaped acrosome. It is filled with enzymes that help the sperm enter the ovum.
• The neck joins the head to the middle piece.
• The middle piece holds many mitochondria. They give the energy for the movement of the tail.
• The tail is a long whip. Its movement makes the sperm swim.

A sperm is made for one thing: to reach the ovum and deliver its nucleus. It has almost no cytoplasm and carries no food.

The semen

At each ejaculation, a man releases about 200 to 300 million sperms. They are mixed with the seminal plasma from the glands, and the mixture is called semen.

For a man to be fertile, the textbook (NCERT) says two things must be true.

• At least 60 per cent of the sperms must have normal shape and size.
• At least 40 per cent of them must show vigorous motility, which means strong swimming.

A note on the numbers. The World Health Organization (2021) gives other lower limits for a fertile sample: 16 million sperms in each mL, 39 million in the whole sample, 4 per cent normal shapes and 30 per cent forward swimming. The textbook values are used in many exams, so learn them, but know that laboratories use the WHO figures.

The ovum

An ovum is a large round cell. The human ovum is about 0.1 mm across, which you can just see without a lens. It has these parts.

• The nucleus is haploid. It lies in the cytoplasm.
• The cytoplasm holds food, so the early embryo can grow before it is fed by the mother.
• The plasma membrane encloses the cell.
• The zona pellucida is a thick clear membrane outside the plasma membrane. A sperm must pass through it.
• The corona radiata is a layer of granulosa cells that stays around the ovum after ovulation.
• A small polar body lies in the space between the plasma membrane and the zona pellucida.

The word "ovum" is used loosely. At ovulation the cell released is really a secondary oocyte. It becomes a true ovum only if a sperm enters. The next course explains this.

Comparing the two gametes

You should find that a sample can have a large total count and still fail the test, if too few sperms are normal or motile. The test looks at shares, not only at the total.

Try it in Python

The program counts the sperms in a sample and checks the two tests of the textbook.

Practise with new numbers

Common mistakes

• Saying that the acrosome is in the tail. It is a cap over the front of the head.
• Saying that the mitochondria of a sperm are in the head. They are in the middle piece.
• Saying that the ovum is released from the ovary as a true ovum. It is a secondary oocyte.
• Saying that the sperm brings the food for the embryo. The ovum stores the food.
• Learning only the total number of sperms. The tests also use the shares of normal and motile sperms.

Remember these

• Sperm: head (nucleus and acrosome), neck, middle piece (mitochondria), tail.
• Semen: 200 to 300 million sperms in one ejaculation. NCERT: at least 60 per cent normal and 40 per cent vigorously motile.
• Ovum: nucleus, cytoplasm with food, plasma membrane, zona pellucida, corona radiata, polar body.
• Both gametes are haploid, with 23 chromosomes.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does the acrosome hold, and where is it?

2. Why does the middle piece of a sperm have many mitochondria?

3. What two conditions does the textbook give for a fertile sample of semen?

4. Name the layers around the cytoplasm of an ovum, from the inside to the outside.

5. Give two differences between a sperm and an ovum.

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 2, Human Reproduction.
• World Health Organization (2021). WHO laboratory manual for the examination and processing of human semen, 6th edition, Geneva.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 27, The Reproductive System.