Preventing Polyspermy During Fertilisation
- Fertilisation is the fusion of one sperm cell and one ovum; this fusion of two haploid nuclei gives rise to a diploid zygote
- During sexual reproduction, many sperm are released, and the sperm cells are attracted towards the secondary oocyte by chemical signals
- Remember that at this point in the oogenesis process, meiosis II has not yet been completed, so the female gamete is still a secondary oocyte and not yet an ovum
- When the sperm cells reach the secondary oocyte, the process that takes place at its cell surface prevents more than one sperm from passing through its cell surface membrane
- The entry of more than one sperm into a single oocyte is known as polyspermy
- The oocyte is surrounded by a layer of follicle cells, as well as a layer of glycoproteins known as the zona pellucida
- The sperm cells need to digest the glycoproteins of the zona pellucida in order to reach the oocyte cell surface membrane; they do this by releasing digestive enzymes from a structure called the acrosome
- This is known as the acrosome reaction
- When the first sperm cell digests its way through the zona pellucida, it reaches the oocyte cell surface membrane; complementary receptors on the head of the sperm bind with proteins on the oocyte cell surface membrane, enabling the cell surface membranes of the two gametes to fuse together and the sperm nucleus to enter the oocyte
- At this point the process of meiosis II continues, leading to the release of the second polar body and the formation of the mature ovum
- The fusion of the two cell surface membranes triggers the movement of a series of vesicles called cortical granules; the vesicles move from the outer layer of the ovum cytoplasm to the cell surface membrane, releasing enzymes that digest the binding proteins on the cell surface of the ovum and that cause the zona pellucida to harden
- This is the cortical reaction
- The enzymes are released by the process of exocytosis
- Once the binding proteins have been broken down and the zona pellucida has hardened, no more sperm cells can enter the ovum; polyspermy is prevented
- Inside the ovum the male and female nuclei fuse together and fertilisation is completed
Preventing polyspermy diagram
Mechanisms during the process of fertilisation prevent polyspermy