1. Fertilization Zuzana Kollarova

2. Fertilization The process by which the nucleus of a sperm fuses with the nucleus of an egg. Occurs in the fallopian tubes. Approximately 300 million sperm are released into the vagina, however only one can fertilize the ovum (egg).

4. 1. Arrival of sperm Sperm are attracted by a chemical signal to swim up the oviduct to reach the egg. The sperm try to push through the layers of the follicle cells (corona radiata) around the egg. Fertilization is only successful if many sperm reach the egg.

5. 2. Binding The first sperm to break through the follicle cells binds to the zona pellucida, a layer of gel composed of glycoprotein that surrounds the egg. This then triggers the acrosome reaction.

6. 3. Acrosome reaction The acrosome is a large membrane-bound sac of enzymes in the head of the sperm. When the sperm makes contact with the egg, the contents of the acrosome are released, by the separation of the acrosomal cap from the sperm. The enzymes loosen the zona pellucida, allowing the sperm to reach the plasma membrane of the egg.

7. 4. Fusion The plasma membrane of the sperm and egg fuse. The sperm nucleus enters the egg and joins the egg nucleus. The tail and mitochondria of the sperm usually remain outside of the egg. This is the moment of fertilization. Fusion causes the cortical reaction.

8. 6. Mitosis The nuclei from the sperm and egg do not fuse together. Instead, both nuclei carry out mitosis, using the same centrioles and spindle of microbules. A two-cell embryo is produced. (Two- haploid nuclei from the sperm and the egg)

9. 5. Cortical reaction The sperm causes activation of the egg. Small vesicles called cortical granules (vesicles located neat the egg membrane) move to the plasma membrane of the egg and fuse with it, releasing their contents by exocytosis. The chemicals in the cortical granules interact with the glycoproteins of the zona pellucida, turning it into an impernetratable fertilization membrane.