Fertilisation

• When the nucleus of a sperm and egg fuse together, the egg is fertilised and it develops into a fetus.

• The fetus grows in the mother’s uterus. The mother's body provides all the baby needs until birth.

Play this game to explore human reproduction and the stages of pregnancy.

Humans typically reproduce by sexual fertilisation and produce offspring that are genetically unique. Fertilisation is the process in which the nucleus of a sperm cell fuses with the nucleus of an egg cell to produce a zygote which will eventually grow into offspring.

Gametes are the sex cells:

• Sperm are male gametes
• Eggs are female gametes

Gametes have adaptations to increase the chances of fertilisation and successful development of an embryo.

• A tail to move them towards an egg cell.
• Many mitochondria to release energy for movement.
• Part of the tip of the head of the sperm releases enzymes to digest the egg membrane to allow fertilisation to take place.
• The haploid nucleus contains the genetic material for fertilisation.
• Produced in large numbers to increase the chance of fertilisation.

• The egg cell’s cytoplasm contains nutrients for the growth of the early embryo.
• The haploid nucleus contains genetic material for fertilisation.
• The cell membrane changes after fertilisation by a single sperm so that no more sperm can enter.

After fertilisation, the zygote will divide by a process called mitosis into a ball of genetically identical cells called the embryo. This embryo will attach itself to the lining of the uterus where it will develop into a fetus and finally into a baby.

The fetus relies upon its mother as it develops. These are some of the things it needs:

• Protection against knocks, bumps and temperature changes.
• Oxygen for respiration.
• Nutrients (food and water).
• The developing fetus also needs its waste substances removing.

The fetus is protected by the uterus and the amniotic fluid, a liquid contained in a bag called the amnion.

The placenta is an organ that grows into the wall of the uterus and is joined to the fetus by the umbilical cord.

Its main function is to allow the exchange of materials such as oxygen and nutrients between the fetus and mother whilst removing waste substances such as carbon dioxide and urea. This relies on a process called diffusion as there is no mixing of maternal and fetal blood in the placenta.

• Oxygen and nutrients such as glucose diffuse across the placenta from the mother to the fetus.

• Carbon dioxide and other waste substances diffuse across the placenta from the fetus to the mother.

Looking for more teaching resources to support your lessons? This short CGI film helps to give a visual description of the process of human fertilisation, following the journey of both the sperm and the egg.

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