Mammalian gametes and fertilisation

Specialised gametes

Sperm: a haploid nucleus; an acrosome (a special lysosome) containing digestive enzymes; a midpiece packed with mitochondria to release energy for movement; a flagellum (tail).

Egg: much larger, with cytoplasm containing food stores; a haploid nucleus; cortical granules just under the membrane; surrounded by a glycoprotein layer, the zona pellucida, and follicle cells.

Fertilisation

1. Sperm swim through the oviduct and reach the egg.

2. Acrosome reaction: on contact with the zona pellucida, the acrosome releases its digestive enzymes, which break down the zona pellucida so the sperm can get through.

3. The sperm cell membrane fuses with the egg cell membrane and the sperm nucleus enters.

4. Cortical reaction: cortical granules release their contents by exocytosis, making the zona pellucida thicken and harden so no more sperm can enter (preventing polyspermy).

5. The sperm and egg nuclei fuse, forming a diploid zygote.

Why the order matters

The acrosome reaction lets a sperm in; the cortical reaction keeps the rest out. If two sperm fertilised one egg, the zygote would have too many chromosomes and would not develop normally.

Worked example

Explain why the cortical reaction is important.

1. After one sperm enters, cortical granules release their contents.
2. The zona pellucida hardens.
3. Other sperm cannot get through, so the zygote has the correct diploid number.

Answer: It prevents more than one sperm fertilising the egg.

Check yourself

1. What does the acrosome contain?
   • Mitochondria
   • Cortical granules
   • Digestive enzymes
   • Food stores
   Show answer

   Digestive enzymes. Acrosome enzymes digest the zona pellucida.

2. Why does the sperm midpiece contain many mitochondria?
   • To release energy (ATP) for the flagellum to move
   • To digest the zona pellucida
   • To store food
   • To make the zona pellucida
   Show answer

   To release energy (ATP) for the flagellum to move. Swimming needs a lot of ATP.

3. What is the zona pellucida?
   • The tail of the sperm
   • The nucleus of the egg
   • A glycoprotein layer around the egg
   • An enzyme in the acrosome
   Show answer

   A glycoprotein layer around the egg. It surrounds the egg cell membrane.

4. What is the result of the cortical reaction?
   • The acrosome releases enzymes
   • The zona pellucida hardens, preventing other sperm entering
   • The sperm grows a tail
   • The egg divides by meiosis
   Show answer

   The zona pellucida hardens, preventing other sperm entering. Hardening blocks polyspermy.

5. Which happens first?
   • The cortical reaction
   • Fusion of the nuclei
   • Formation of the zygote
   • The acrosome reaction
   Show answer

   The acrosome reaction. The sperm must first get through the zona pellucida.

6. A human egg and sperm fuse. How many chromosomes does the zygote have?
   Show answer

   46. 23 + 23 = 46: diploid.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.