Thousands of sperm arrive, and exactly one gets in. The egg does not simply get lucky — the instant the first sperm fuses with it, it chemically slams the door on every other one. That block, and the week-long journey that follows, are pure HL material.
📚 What you need to know
Fertilisation is the fusion of one sperm and one ovum, giving a diploid zygote. The entry of more than one sperm is polyspermy, and it must be prevented.
At this stage the female gamete is still a secondary oocyte, surrounded by follicle cells and a layer of glycoproteins called the zona pellucida.
Sperm release digestive enzymes from the acrosome to digest the zona pellucida — the acrosome reaction.
Complementary receptors on the sperm head bind to proteins on the oocyte membrane, and the two membranes fuse so the sperm nucleus can enter.
Meiosis II then completes, releasing the second polar body and forming the mature ovum.
Membrane fusion triggers cortical granules to release enzymes by exocytosis. These digest the binding proteins and harden the zona pellucida — the cortical reaction — so no more sperm can enter.
Mitosis produces a blastocyst, which reaches the uterus after about seven days with around 125 cells and implants into the endometrium.
Getting through the coats
Many sperm are released and are attracted towards the secondary oocyte by chemical signals. Two layers stand between them and the oocyte membrane: an outer layer of follicle cells and, beneath it, the zona pellucida, a layer of glycoproteins.
To reach the membrane, sperm release digestive enzymes from the acrosome, a structure in the head, which digest the glycoproteins of the zona pellucida. This is the acrosome reaction. The first sperm to digest its way through reaches the oocyte cell surface membrane, where complementary receptors on the head of the sperm bind with proteins on the oocyte membrane. The two cell surface membranes then fuse, and the sperm nucleus enters the oocyte.
At that moment, meiosis II continues in the oocyte, leading to the release of the second polar body and the formation of the mature ovum.
The cortical reaction: shutting the door
The fusion of the two membranes triggers a series of vesicles called cortical granules to move from the outer layer of the ovum cytoplasm to the cell surface membrane. By exocytosis, they release enzymes that do two things: they digest the binding proteins on the cell surface of the ovum, and they cause the zona pellucida to harden.
Follow the logic. With the binding proteins destroyed, no further sperm can attach. With the zona pellucida hardened, no further sperm can digest their way in. Polyspermy is prevented. Inside the ovum, the male and female nuclei then fuse and fertilisation is complete.
Steps one and two are the sperm working; steps three and four are the oocyte responding. Only the first sperm ever gets past step two.
Two reactions, easily confused. The acrosome reaction is the sperm digesting its way in; the cortical reaction is the oocyte keeping everyone else out. Name which cell each belongs to and you will not mix them up.
Why polyspermy has to be prevented
The zygote must be diploid. One haploid sperm nucleus plus one haploid ovum nucleus gives 46 chromosomes. If a second sperm entered, the cell would have 69 chromosomes, the chromosomes could not pair or separate correctly at mitosis, and the embryo would not develop.
Blastocyst development and implantation
Once fertilised, the ovum divides by mitosis to form two diploid nuclei, and the cytoplasm divides equally to give a two-cell embryo. Mitosis continues to a four-cell embryo and onwards until the embryo takes the shape of a hollow ball called a blastocyst, with an internal group of cells called blastomeres that will eventually develop into the foetus.
Up to this point the embryo is still in the oviduct. After about seven days it consists of around 125 cells and has reached the uterus. The zona pellucida, which has protected it throughout, breaks down and is lost at around seven days of age.
By now the blastocyst has used up the nutrient supplies of the egg cell and needs an external supply. It obtains one by implanting into the endometrium. The outer layer of the blastocyst develops finger-like projections that penetrate the lining, and at this stage there is already an exchange of nutrients and oxygen with the mother’s blood. The embryo then grows and develops rapidly.
🧩 From fertilisation to implantation, in order
Fertilisation in the oviduct produces a diploid zygote.
Mitosis gives a two-cell embryo, then four cells, then more.
A hollow ball of cells, the blastocyst, forms while still travelling down the oviduct.
At around 7 days and roughly 125 cells, the blastocyst reaches the uterus and the zona pellucida is lost.
Implantation: finger-like projections penetrate the endometrium and exchange with maternal blood begins.
Worked examples
WORKED EXAMPLE
Explain how the entry of more than one sperm into an oocyte is prevented. [4]
Step 1 — the trigger
When the first sperm fuses with the oocyte cell surface membrane, cortical granules move to the membrane.
Step 2 — the release
The granules release their enzymes by exocytosis.
Step 3 — the two effects
These enzymes digest the binding proteins on the ovum surface, so no further sperm can attach, and they cause the zona pellucida to harden, so no further sperm can digest through it.
Step 4 — the outcome
Polyspermy is prevented, which keeps the zygote diploid.
4 marksBoth effects are separate marking points. Give the two of them explicitly.
WORKED EXAMPLE
Explain why the blastocyst must implant in the endometrium around seven days after fertilisation. [3]
Point 1 — the problem
By this stage the blastocyst has used up the nutrient supplies that were stored in the egg cell cytoplasm.
Point 2 — the solution
It implants into the endometrium, developing finger-like projections that penetrate the uterus lining.
Point 3 — what implantation provides
This allows an exchange of nutrients and oxygen with the mother’s blood, supporting the rapid growth that follows.
3 marksLink the timing to the loss of the zona pellucida if the question asks about that structure.
💡 Exam tip
Call the female gamete a secondary oocyte until the sperm has entered. It becomes an ovum only briefly afterwards.
Name the mechanism as exocytosis when describing the cortical granules.
Give the reason polyspermy matters: the zygote must be diploid.
Quote the numbers where you can — around seven days and 125 cells at implantation.
Fertilisation is in the oviduct; implantation is in the uterus. Two organs, one week apart.
Receptor binding is complementary, which is why sperm of another species cannot fertilise the oocyte.
⚠ Common mix-up
Acrosome reaction and cortical reaction are not the same. One is the sperm’s, one is the oocyte’s.
The zona pellucida is not the cell membrane. It is a glycoprotein coat outside it.
Meiosis II is not finished before fertilisation. The sperm entering is what completes it.
A blastocyst is not a foetus. Only the inner blastomeres become the foetus.
Implantation is not fertilisation. They happen in different organs, about a week apart.
Cortical granules do not destroy the sperm inside. They act on the surface of the ovum.
Up next: Hormones in Pregnancy — the hormone the embryo starts making almost immediately, and the feedback loop that eventually delivers a baby.
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