Fertilisation of animals

• Sexual reproduction is the fusion of the nucleus of the sperm cell and the nucleus of the egg cell
• External fertilisation occurs outside the body, with the female and male gametes discharged directly into the environment where they meet and fuse. External fertilisation is very wasteful as many of the gametes do not meet and is not an option for organisms that live on land
• External fertilisation is usually seen in aquatic species because spermatozoa and ova are very vulnerable to drying and are rapidly destroyed in the air. These increase the likelihood of fertilisation by ensuring the ova and sperm are released at the same time close to each other
• Internal fertilisation involves the transfer of the male gametes directly into the female. This does not guarantee fertilisation, but makes it more likely. More complex organisms such as insects and vertebrates have developed a system where the male gametes are inserted directly into the female, this make sure that the ova and sperm are kept in a moist environment and are placed as closely as possible to each other, which maximises the chances of successful fertilisation
• The ovum is fully viable and able to receive the male gamete for only two hours. The sperm will survive a day or two in the female reproductive tract. As sperm move through the female reproductive tract, the acrosome region matures so it can release enzymes and penetrate the ovum
• The ovum released at ovulation has not fully completed meiosis. Many sperm touch the surface of the ovum and the acrosome reaction is triggered. Enzymes are released from the acrosome, which digest the follicle cells and the zona pellucida
• Eventually, one sperm will wriggle through the weakened protective barrier and touch the surface membrane of the oocyte. The second meiotic division takes place providing a haploid ovum nucleus to fuse with the haploid male nucleus. If other sperm enter now, this would result in polyspermy (fertilisation by more than one sperm) and would produce a nucleus containing too many enzymes.
• After the fusion of the two haploid nuclei, ion channels in the cell membrane open and close so that inside of the cell, instead of being negative to the outside, is positive. This alteration in charge prevents the entry of any other sperm. It is temporary until the the cortical reaction takes place
• In the cortical reaction, cortical granules in the cytoplasm of the ovum release enzymes in the zona pellucida. This then forms a tough fertilisation membrane around the fertilised ovum. The fertilisation membrane now repels other sperm as the electrical charge returns to normal
• The head of the sperm enters the oocyte, but the tail region is left outside. Once the head is inside the ovum, it absorbs water and swells, releasing its chromosomes to fuse with those or the ovum, forming a diploid zygote. At this point, fertilisation has occurred and a new individual (diploid zygote) has formed