Fertilisation

A sperm cell has an acrosome at the tip of its head; this contains hydrolytic (digestive) enzymes that digest the zona pellucida of the oocyte so the sperm can reach the oocyte membrane.

The midpiece contains many mitochondria; these release ATP by aerobic respiration to power the beating of the flagellum.

The flagellum (tail) beats from side to side to give the sperm motility, so it can swim to the oocyte.

(a) The zona pellucida is a glycoprotein coat surrounding the oocyte; the sperm must digest through it during the acrosome reaction, and after fertilisation it hardens to form the fertilisation membrane that blocks further sperm.

(b) The follicle cells form a layer around the oocyte that protects it and helps to nourish it before fertilisation.

(c) The food reserves in the cytoplasm provide energy and raw materials to nourish the zygote / early embryo during the first stages of development.

Gametes are haploid, meaning each contains only one set of chromosomes (in humans, n = 23).

This is important because when two haploid gametes fuse at fertilisation, the diploid number is restored (2n = 46), so the chromosome number is kept constant from one generation to the next.

When the sperm reaches the zona pellucida, the acrosome reaction occurs: the membrane around the acrosome breaks down and releases hydrolytic (digestive) enzymes.

These enzymes digest a pathway through the zona pellucida, allowing the sperm to reach the oocyte cell membrane.

The cell membrane of the sperm then fuses with the cell membrane of the secondary oocyte.

Finally, the haploid nucleus of the sperm passes into the cytoplasm of the oocyte.

When the first sperm fuses with the oocyte cell membrane, this triggers the cortical reaction.

The cortical granules, which lie just beneath the oocyte membrane, move to the surface and release their contents by exocytosis into the zona pellucida.

These released contents cause the zona pellucida to thicken and harden, forming a structure called the fertilisation membrane.

The fertilisation membrane is impenetrable to other sperm, so no second sperm can digest through it or fuse with the oocyte.

In this way the cortical reaction prevents polyspermy — the fertilisation of one oocyte by more than one sperm.

When a sperm reaches the secondary oocyte, the acrosome reaction takes place. The acrosome at the tip of the sperm head releases hydrolytic (digestive) enzymes, which digest a pathway through the zona pellucida surrounding the oocyte.

Having crossed the zona pellucida, the cell membrane of the sperm fuses with the cell membrane of the secondary oocyte, and the haploid nucleus of the sperm enters the cytoplasm of the oocyte.

Membrane fusion now triggers the cortical reaction: the cortical granules beneath the oocyte membrane release their contents into the zona pellucida, which thickens and hardens to form the fertilisation membrane. This membrane is impenetrable to other sperm and so prevents polyspermy.

Finally, the haploid sperm nucleus and the haploid oocyte nucleus fuse. This combines the two single sets of chromosomes and restores the diploid number (in humans, 23 + 23 = 46), forming a diploid zygote that can then divide by mitosis to begin development.

(a) Effect. Preventing the release of cortical granule contents increased the percentage of polyspermic oocytes from 2% to 71% — a large increase of 69 percentage points (the treated value is about 35 times the untreated value).

(b) Explanation. Normally, when the first sperm fuses with the oocyte membrane the cortical reaction occurs: the cortical granules release their contents into the zona pellucida, which thickens and hardens to form the fertilisation membrane. This membrane is impenetrable to other sperm, so it prevents polyspermy — which is why only 2% of untreated oocytes were polyspermic.

In the treated oocytes the cortical granules could not release their contents, so the zona pellucida did not harden and no fertilisation membrane formed. As a result there was no block to polyspermy, so additional sperm were able to enter and a high proportion (71%) of oocytes were fertilised by more than one sperm.

The sperm is a small, streamlined gamete adapted for movement, whereas the secondary oocyte is a large, non-motile gamete adapted to be reached and to support the early embryo.

The sperm has a flagellum (tail) and many mitochondria in its midpiece. The flagellum gives it motility, and the mitochondria release ATP by aerobic respiration to power the beating of the flagellum, so the sperm can swim to the oocyte. At its tip the sperm has an acrosome containing hydrolytic enzymes, which digest the zona pellucida so the sperm can penetrate the oocyte. It has very little cytoplasm, keeping it light and streamlined.

The oocyte, by contrast, is large and non-motile, with a great deal of cytoplasm containing food reserves; these nourish the zygote during the early stages of development. It is surrounded by follicle cells and a zona pellucida, which protect it, and it contains cortical granules beneath its membrane that release their contents after fertilisation to harden the zona pellucida and prevent polyspermy.

A key similarity is that both gametes are haploid, each carrying one set of chromosomes, so that when they fuse the diploid number is restored in the zygote.

Each gamete is produced by meiosis and is therefore haploid, containing only one set of chromosomes (in humans, n = 23). The sperm carries one set from the father and the secondary oocyte carries one set from the mother.

At fertilisation, the haploid sperm nucleus enters the oocyte and the two haploid nuclei fuse. This combines the two single sets of chromosomes, so the diploid number is restored (23 + 23 = 46, i.e. 2n). The single diploid cell formed is the zygote, which divides by mitosis to form the new organism. Because each new generation begins with one haploid set from each parent, the chromosome number is kept constant from generation to generation.

Fertilisation also contributes to genetic variation. The two gametes come from two different parents, and (because of meiosis and random fertilisation) each carries a unique combination of alleles. When they fuse, a new and unique combination of alleles is created in the zygote, increasing variation within the species.

This restoration of the correct, diploid chromosome number depends on only one sperm fusing with the oocyte. If polyspermy occurred — fertilisation by more than one sperm — the zygote would receive too many sets of chromosomes (it would be polyploid) and would not develop normally, usually dying. This is why the cortical reaction, which hardens the zona pellucida into a fertilisation membrane to block additional sperm, is essential: it ensures exactly one haploid sperm set combines with one haploid oocyte set to give the correct diploid number.

(a) During natural fertilisation the acrosome reaction must occur: the acrosome at the tip of the sperm head releases hydrolytic (digestive) enzymes that digest a pathway through the zona pellucida so the sperm can reach the oocyte cell membrane. If the acrosome is defective, these enzymes are not released (or are released in insufficient amounts), so the sperm cannot digest through the zona pellucida and cannot reach or fuse with the oocyte membrane. The sperm is therefore unable to deliver its nucleus, and fertilisation fails.

(b) ICSI bypasses the step that the defective acrosome blocks. The needle physically carries the whole sperm through the zona pellucida and deposits it directly in the oocyte cytoplasm, so the sperm no longer needs to digest its own way in — the acrosome reaction is not required. The sperm still contains a normal haploid nucleus, so once it is inside, its nucleus can fuse with the haploid oocyte nucleus to restore the diploid number and form a zygote.

(c) In ICSI the embryologist injects sperm artificially, so the number entering the oocyte is decided entirely by the technique; if more than one sperm (or sperm nucleus) were introduced, the zygote would receive too many sets of chromosomes (it would be polyploid) and would not develop normally. They must therefore deliberately inject exactly one sperm.

In natural fertilisation this is prevented by the cortical reaction: when the first sperm fuses with the oocyte membrane, the cortical granules release their contents into the zona pellucida, which thickens and hardens to form the fertilisation membrane. This membrane is impenetrable to further sperm, so only one sperm enters and polyspermy is prevented. In ICSI the operator must take the place of this natural single-sperm safeguard.